All rock exposed at the
Earth's surface is weathering, undergoing a combination of chemical and
physical changes. Weathering alters the rock's texture, mineralogical
assemblage and mechanic properties, breaking it down into smaller clasts and dissolved content. Once produced, these clasts are separated from the source rock and transported
away. An important form of transport is mass wasting, the downslope
transport of rock, water and soil by gravity, as opposed to flowing water or
wind. Mass wasting is an active sculptor of the landscape. It can also be a
natural hazard, one that man often triggers or aggravates.
This lecture is about mass
wasting and about the weathering of rock that produces the debris that moves.
We have already seen
weathering in the context of sedimentary rocks. Weathering produces the clasts and dissolved content that are the building blocks
of sedimentary rock. Weathering occurs, that is rocks break down, because rocks
produced at high temperatures and pressures are not in equilibrium with surface
conditions. At the surface, temperatures are low, often low enough to freeze
water. Pressure is low. There is abundant free water full of acids and other
dissolved content. There are plants and animals that root or roost in the rock.
And all of these things act in concert to tear rock down, breaking the rock
apart physically and altering it chemically.
Mechanical weathering: Simply put, mechanical weathering
is the process of producing smaller clasts from large
parent rocks by the creation of joints and fractures
in the rocks. This happens in several ways:
Cooling: As igneous rocks cool, they contract (recall that melt is less dense than solid). This produces stresses that can crack the rock into a system of regularly spaced, near parallel joints. A prime example is columnar basalt. Here the rock is broken into long pilings with hexagonal cross sections.
Uplift: As pressure is released with uplift, rock masses expand, creating joints and fractures. The joint systems produced are quite variable, but they often form sets oriented at right angles to one another. Exfoliation occurs when the joints form thin sheets parallel to the surface of the rock mass. The onion-skin appearance of domes in Yosemite is the result of exfoliation.
Frost wedging: Water is denser than ice by roughly 9%. When water in small cracks and rock pores freezes, it expands and wedges open the crack or pore. Through repeated freeze/thaw cycles, frost wedging can break apart massive rocks. Frost wedging is also responsible for a familiar observation: cobble and boulder size rocks outcropping in soil. The cobbles are lifted from below when frost expands the water-bearing soil. When the ice melts, the soil falls in under the cobble. Repeated many times, the cobble eventually erupts onto the surface.
Salt wedging: Similar to frost wedging, salt wedging occurs when salt crystallizes in cracks and rock pores during evaporation/dissolution cycles.
Fires: Fires heats the outer skin of the rock, which expands, but because rock is a poor conduction, the inside doesn't. The differential expansion produces spall-thin flakes of rock that flake off.
Plant roots: Plant roots will grow into cracks, sometimes wedging them further open, sometimes propping them open between freeze/thaw cycles.
Jointing
increases the ratio of surface area to volume of a rock body. Imaging cutting a
cube of rock into 8 equal sized cubes. The volume of rock obviously does
not change, but the summed surface area of the 8 cubes is twice that of the
original rock. Cutting each of the 8 cubes into 8 smaller cubes results in 64
cubes whose total volume still equals the original, but whose surface area is
four times greater. Chemical weathering attacks the surfaces of rocks. The more
surface area it has to work on, the faster it proceeds.
Chemical weathering: Minerals formed at high pressure
and temperature are unstable at the surface and break
down. This process would be slow if not for water and the dissolved content in
water. Water and weak acids in water act on rocks in several important ways:
Hydrolysis: Carbon dioxide dissolved in water produces carbonic acid:
H20 + CO2 = H2CO3
= H(1+) + HCO3(1-)
The free H(1+) ion substitutes for cations in some
minerals, releasing the cation into solution and
changing the crystalline structure of the product. Example: Potassium Feldspar
reacts, losing K, and gaining H to form Kaolinite, free
K and Silica.
4KAlSi3O8 + 4H(1+) + 2H20
= 4K(1+) + Al4Si4O10(OH)8 + 8SiO2
Kaolinite is a clay that is very stable under
surface conditions and will persist. Like all clays, it is a sheety mineral that easily cleaves to form silt and clay
sized clasts. As water flows through the rock, the
dissolved K and silica are leached away. The silica may end up as cement in a
sedimentary rock, or, like K, in the ocean.
Dissolution: Calcium carbonate is dissolved by carbonic acid.
CaCO3 + H2CO3 =
Ca(2+) + 2(HCO3)(1-)
This reaction is responsible for the limestone karsts found on campus. Run in
the opposite direction, this reaction is used by foraminifera to produce hard
parts.
Oxidation: Chemical weathering of Fe-bearing rocks releases Fe. In the presence of O it is quickly oxidized (meaning it goes from Fe(2+) to Fe(3+)). This plus hydration makes Goethite, a yellowish, earthy mineral:
4FeO + 2H2O + O2 = 4FeO *
OH
Goethite can be dehydrated to produce Hematite, a brick-red mineral whose color is the source of the reddish hue of most weathered
rocks and soil
2FeO *OH = Fe2O3 + H2O
Chemical weathering hits
some minerals harder than others. The survivability of minerals is a function
of many things. For instance, covalently bonded minerals are more resistant
than ionically bonded minerals, and minerals with low
melting temperatures are more robust than minerals with higher melting points. Clay
and quartz are the end products of chemical weathering because they are the
most stable minerals at the surface. The presence of clays and quartz in high
abundance is indicative of intense chemical weathering. Climate is important
also. Hot and moist climates promote intense chemical weathering. Cold and dry
climates turn it off. Physical weathering proceeds most quickly when
temperatures dip below freezing for part of the year, enabling frost wedging to
operate.
Once created, weathering
products are subject to downslope transport. Mass
wasting is one of the agents.
Mass wasting is the movement of rock and regolith by gravity, where regolith
is defined as the irregular blanket of unconsolidated rock and soil on the
surface. There is an important distinction to be made between mass wasting and
transport of sediment by water. In mass wasting, water moves along with the
sediment. In aqueous transport, sediment moves along with the water. The
physics of transport and the landforms they produce are very different.
What controls the movement
of masses during mass wasting? Three are three important controls: shear
stress, shear strength and triggering. Let's look at each.
Shear stress: Shear stress is the force acting
parallel to the slope promoting downslope movement. It
is a function of gravity which can be decomposed into slope-parallel and slope-perpendicular
components. The slope-parallel component increases as the angle of the slope
increases, increasing the shear stress. The slope-perpendicular component
decreases with slope. This lowers the strength of the material which, if
unconsolidated, is held together by its own weight.
Shear strength: Shear strength is the resistance of
a material to shear failure, or sliding. It is a function of the
slope-perpendicular component of gravity and of material properties. For
unconsolidated materials, it can be expressed as the angle of repose: the
steepest angle that debris can maintain. For any greater angle, the shear
stress (slope- parallel component of force) exceeds the shear strength of the
material and it moves downslope until the slope is
regains the angle of repose (or less). In general, coarse aggregates can
maintain steeper slopes than fine aggregates. Ditto wet, but
not saturated, aggregates. When debris carries some water, capillary
action of water pulls it into tight spaces between grains where surface tension
actually holds grains together. This ceases to be true when the material
becomes saturated. Then water between the grains lubricates them and lessens
the extent of solid-solid contact, weakening the material. Saturated debris is
weaker than dry debris. Consolidated materials behave differently. Cemented
sediment and vegetated soils can maintain very steep slope angles since the
cement or root material must be broken to produce downslope
motion.
Trigger: Virtually all loose slopes are engaging
in some sort of downslope movement, but massive and
fast motions are usually triggered. Triggers can be an increase in water
content resulting in saturation; an earthquake which nudges debris downhill or
causes shaking of an unstable debris pile; and devegetation,
either through fire or through human actions.
Geologists recognize a
number of different forms of mass wasting. They base the divisions between them
on the material that moves, the nature of motion and the velocity it moves at. This
allows for lots of combinations and lots of names. Our goal is to understand
the range of mass wasting processes and how they operate: processes come first,
names come second!
Material: We distinguish between granular (rock) and slurry (unconsolidated) materials. Slurries are water saturated; granular masses are supported by grain to grain contacts.
Type of motion: We distinguish between flows and slides (or falls). Flows move as if the material were a fluid and exhibit lots of mixing. Slides move coherently with little mixing or internal deformation.
Velocity: Flows can be as slow as cm/yr or as fast as hundreds of km/hr.
Flows in both materials, in
both types of motion occur over the entire range of velocity. We will discuss
only a few combinations, covering the different physical processes at work and
the most important manifestations of mass wasting.
Rock avalanches are high velocity flows of rock debris that breaks up as it moves downslope. The material is granular, consisting of coarse and angular clasts. The motion is a flow with individual clasts flowing relative to one another. They can move very fast, increasing with downslope distance, sometimes reaching velocities of hundreds of kilometers an hour. When the debris reaches the bottom of the slope, it often forms a conical pile of unsorted debris known as a talus.
Mudflows are flowing masses of fine clasts (sand and smaller) with some rock debris and lots of water. The material is a slurry and the type of motion is a flow. Mudflows range in velocity from 1 to 100 km/hr. Water is very important, both as a trigger and as a means of lowering the viscosity of the slurry allowing it to flow rapidly. Mud flows are common in hilly and semi-arid regions where rains are infrequent, but often torrential. They are also common in volcanic regions where ash and lava-melted snow mix and move rapidly downslope.
Debris flows are downslope movements of unconsolidated regolith, typically coarser than sand. The material is a slurry, the motion a flow and the velocity moderate (1 km/hr or so). Water reduces the strength of the slurry, but gravity moves it. Debris flows are often referred to as landslides in the media.
Slumps are slow slides of unconsolidated material that travel downslope as a unit . The material is granular, the type of motion is a slide and the velocity is moderate to slow, on the order of m/hr. Water is important as a trigger or by lubricating clay-rich strata that serve as sliding boards. Slumps often leave crescent-shaped head scarps and have back-tilted surface layers and are commonly triggered by earthquakes. They are also referred to as landslides in the media.
Creep is the coherent downhill movement of soil or other debris at rates of cm/yr. The material is granular, usually consisting of unsorted aggregates of angular clasts and soil referred to as colluvium. Almost all unconsolidated slopes undergo creep, explaining the nearly ubiquitous inclined trees, telephone posts and fences seen on loose slopes.
Solifluction (or gelifluction) is the very slow downhill movement of regolith in cold climates. The material is a slurry produced when water released during the warm season saturates the upper layer and is unable to percolate downward through the permafrost. The motion is a slide, producing lobate sheets of several feet to tens of feet in thickness that often override one another. Solifluction also occurs in very humid regions where the ground remains saturated year-round.
Mass wasting is most
efficient where steep slopes and event triggers exist. Plate boundaries provide
both. Convergent boundaries build mountains and volcanoes. Earthquakes there
frequently act as triggers. Divergent boundaries produce rifts whose
steep-walled sides fail and fall into the rift valley. In California, the San
Andreas is the transform boundary between the Pacific and the North American
Plates. Mountains uplifted along it, such as the San Gabriel and the Santa Cruz
mountains, are locations of frequent rock avalanches,
debris and mud flows, and creep.
Mass wasting occurs
underwater as well. A prominent local example is the
Rivers dump 20 billion tons
of sediment into Earth's oceans each year and are the dominant agent of
sediment transport. In addition to clastic sediment,
rivers carry dissolved rock content that is responsible for the saltiness of
ocean water. Where water is scarce, wind too is an important carrier of
sediment. Together, rivers and wind act to sculpt Earth's surface. Deriving
their energy from the Sun, they contribute to both the hydrologic and rock
cycles.
The next one and a half
lectures are about rivers and wind; about the characteristics of fluid flow;
the transport and deposition of sediment by fluids; and the landforms produced
by flowing water and air. We will see that the effects of rivers are much more
pronounced than wind with one exception: deserts. Wind is important in these
arid, sparsely vegetated regions, leading to unique landforms and surface
processes. We will talk about these and about the origins of deserts and their
relation to wind.
We begin with fluid flow.
Fluid flow can be broken up
into two regimes: laminar and turbulent. Laminar flow has parallel streamlines
(the lines traced out by water or air molecules as they flow) and little mixing
of crossing of particle trajectories. Turbulent flow is characterized by
crossing streamlines and significant mixing. A familiar example of flow that
varies between the two regimes is waves crashing onto a beach and the return
flow of water down the beach. Water and suspended sand within a breaking wave
flows in a turbulent fashion, mixing and turning. After the wave breaks and
rolls up onto shore, flow within the thin layer of water back down the beach is
laminar, with water and a little sand moving in parallel lines.
In general, which regime a
flow is in depends on three factors: velocity of the fluid, geometry of the
channel it flows in, and its viscosity. Faster flows
are always more turbulent than slower moving flows. In a channel, this results
in laminar flow near the boundaries (since flow velocity at the boundary goes
to zero) and turbulent flow in mid- channel. Winds are laminar near the ground
(where velocity goes to zero) and turbulent at greater heights (a few centimeters off the ground). Lastly, viscous fluid flows
are more laminar than inviscid fluids. Thus water is
more laminar than air, and ice more laminar than water. Transition between the
two regimes can occur whenever flow conditions change. For instance, a sudden
increase in wind speed can drive wind from a laminar flow to turbulent flow. Sheets
that were being blown horizontally by a laminar flow can suddenly be uplifted
or pulled down by turbulent flow.
The properties of fluid
flow determine the type and amount of sediment it can carry. We call the total
sediment transported by the fluid the sediment load. Load comes in two
varieties: suspended and bed load. Suspended load is all material temporarily
or permanently in the flow. In water, this usually includes clays, silts and
sand, but can include gravel if the flow is fast and turbulent. Air usually can
only suspend silts and clays, picking up sand only at high velocities. Bed load
is material carried downstream along the stream channel bottom or along the
ground by rolling or sliding. Bed load always constitutes the coarse fraction
of load, composed of clasts that are two large
(heavy) to pick up in the flow.
When load is deposited from
water, it becomes alluvium. Load deposited from air can be sand or dust. Deposits
of the latter are referred to as loess (German for loose).
The way in which load is
transported depends on whether it is held in suspension or carried as bed load.
Permanently suspended load is carried along streamlines in the flow. Although
the clasts are denser than water, they do not fall
out of suspension in a turbulently flowing fluid. An important concept here is
the settling velocity of a sediment clast. This is the
terminal (highest) velocity the clast would reach in
falling through standing water (or air). Settling velocity is a function of
fluid viscosity, the size of the particle and its density. We know from
experience that large objects fall at the same speed regardless of their shape
or density. This isn't true of small particles. Small particles fall through
standing water more and more slowly as their size decreases. At some point, the
settling velocity drops falls so low that random turbulent perturbations in
fluid flow are enough to push the clast back up
faster than it can drop. At that point, the clast
becomes permanently suspended and will not settle out until the water or air
slows down.
A major component of the
suspended load is only temporarily in suspension. These clasts
move along in short hops, and are said to saltate.
Saltating grains are picked up by turbulent flow or
ejected by the energy of other grains landing on the bed. Once in the water
column they are transported downstream by the current, falling out of
suspension through the action of gravity. This is an important component of
load in both water and air.
Bed load, also known as traction load, has
high settling velocity and is too massive to saltate.
Traction load moves by being bumped along by other clasts,
by rolling or by sliding when friction is low. Although traction load moves
slower than saltating grains and suspended load, it
still moves and is an important component of sediment transport.
The ability of a stream to
transport sediment is characterized by two properties. The first is capacity,
which is the maximum amount of sediment that can be transported by a stream. Capacity
is a function of total stream flow or discharge, the product of velocity
and stream cross-sectional area. Discharge is the volume of water passing a
point per unit time. The greater the discharge, the greater
the load that can be transported. The second property is competence,
which is the maximum size of clast that can be
transported. The ability of a fluid (water or air) to pick up grains is a
function of grain size and the velocity and turbulence of the fluid. For water,
this can be summarized in a single diagram.
Insert Hjulstrom
diagram here
For larger clasts, the relation between water velocity and competence
is straightforward. Larger clasts require faster
water. In general, the size (radius) of a clast that
can be picked up by the flow increases with the square of fluid velocity. Doubling
velocity quadruples the size of clast that can be
entrained in the flow. This relation breaks down for smaller clasts, where the cohesiveness of clay size sediment makes
entrainment more difficult. The competence of air is much less than water at
equal velocity because air is less dense and has lower viscosity.
Water or air flowing over a
bed of loose sediment will form bedforms, regular
topographic patterns with internal structure. The formation of bedforms reflects feedback between fluid flow and sediment.
The patterns that form depend upon flow velocity, the fluid itself and the
sediment supply. In water, ripples (few centimeters
wavelength) and dunes (longer wavelengths) form at slow to moderate flow
velocities. These migrate and grow as grains roll or saltate
up the upstream face. Turbulence built up around the ripple crest
preferentially transports grains to top of lee face. This causes the lee side
of the crest to aggrade until an avalanche returns
the slope to the angle of repose, depositing a foreset
bed or strata. The build up of lee-side layers creates cross bedding within the
sand, silt or gravel layer. These dip in the direction of fluid flow. At higher
flow velocities, very fast currents plane off ripples, producing flat beds. Grains
are kept in suspension or rapid saltation and no bedforms develop.
Dunes formed by wind are
similar, but demonstrate a range of forms reflecting the sediment supply and
shifting wind directions. Barchans are horn-shaped dunes with the horns
directed downwind. These dunes are common in low-sand supply regions and
migrate downwind with little apparent change. Transverse dunes are most like
the dunes formed by flowing water. They have long, linear crests aligned at
right angles to the wind that migrate downwind. They require ample sediment
supply and can be roughly viewed as many barchans aligned into rows. Linear
dunes have their crests aligned with the wind. They form in settings where the wind shift frequently. Beach dunes tend to be transverse or
linear dunes. The last common eolian (wind produced)
dune type is the parabolic or blowout dune. Parabolic
dunes resemble reversed barchan dunes with their
horns directed upwind. These do not appear to migrate. They form by piling sand
on the leeward and lateral margins of depressions and are often vegetated.
At this point we need to
break the discussion up because streams are confined to channels, wind is not
Stream channels are the
passageways in which water normally runs except at flood stage (high water
level). In actively uplifting areas, the stream channel is coincident with the
lowermost part of the stream valley, which is defined as the area between
topographic highs to either side of stream. Where little or no uplift is
occurring, the edge of the channel coincides with the floodplain, a large, flat
area covered by silt and clay deposited during floods. The boundary between a
stream's channel and its floodplain is often marked by a natural levee,
consisting of a broad, low ridge of alluvium deposited by water exiting the
stream channel. As water moves from the deep, fast, turbulent channel to
shallow, slow, laminar flow regime floodplain, its competence drops and it
deposits most of its coarse load. Finer suspended load is carried out onto the
floodplain.
Stream channels are seldom straight, rather they meander, producing a series of smooth,
sinuous bends. Meanders are most characteristic of rivers on low slopes cutting
through unconsolidated sediment, but they can also be found cutting into steep
bedrock slopes. In fact, meanders are a common feature of virtually all flowing
fluids: the
Through a channel bend,
water tends to be deeper and velocity fastest on outer side. High velocities
and greater turbulence result in erosion as the stream eats into its stream
bank and scours its bed. The inner side is shallower, has slower moving water
and is the site of deposition. Sediment accumulates on the inner bank, forming
curved sand deposits known as point bars.
When a meander is chopped
off, the abandoned channel becomes silt filled (through floods and surface
runoff), forming an oxbow lake. Cutting off a meander shortens a stream's
channel. Meanders are sometimes cut off by man to engineer a stream's channel
for flood control or to avoid erosion of land bordering the river.
Some streams occupy many
channels instead of one. They are called braided streams because the multiple cris-crossing channels resemble hair braids. Braided
channels are common where rivers have highly variable flow, high sediment load
and erodible banks, conditions typically associated
with glacier drainage. Channels within a braided stream are not static. Through
the year, the stream will span the entire channel system, while individual
channels meander and shift.
Channels that are actively
incising abandon former floodplains when high water levels can no longer reach
them. The resulting stranded floodplain forms a terrace. It is not uncommon to
observed streams with two, three or more abandoned floodplain terraces. Their
presence is indicative of tectonic uplift and/or climatic change varying the water
and sediment supply to the stream.
Stream channels change in
many ways downstream. Tributaries join the main trunk. This increases the
rivers discharge. Discharge can be increased by an increase in cross-sectional
area of the stream channel, by an increase in stream velocity, or both. When a tributary joins a river, discharge increases by both
mechanisms since water moves faster through larger, deeper channels.
If velocity increases
downstream, then why do transported sediments generally fine downstream? Competence
increases with flow velocity, so the river should be able to transport coarser
material downstream in velocity increases downstream. The reason we don't see
coarser load downstream is two-fold. (1) Coarser load isn't supplied to them.
The upstream portion of the river is slower and can't transport coarse sediment
to the lower reaches. Added to that is the finer load of tributaries. (2)
Physical and chemical weathering downstream reduce clast
size resulting in progressively finer sediment.
Downstream changes in
channel shape and profile are a response to sediment supply, discharge,
gradient and base level. These are related by the concept of a graded stream. This
notion is important and not immediately obvious, so we will spend some time on
it. Let's begin by breaking down the important elements. The longitudinal
(long) profile of a stream is the downstream change in channel elevation. Long
profiles are always concave upwards with slope (or gradient) decreasing
downstream. The profile terminates at base level, which can be sea level or
lake elevation depending on what body of water the streams flows into. Streams
flow downhill, thus they can never cut below base level (if they did, water
would have to flow uphill to reach the base). Streams start out at high
elevation, where they can incise and cut down into bedrock or sediment. The downcut, eroded sediment is transported downstream where it
is deposited. Through downcutting and deposition, a
stream will achieve grade, at which point it is in dynamic equilibrium where
sedimentation balances erosion and the long profile stops changing.
Streams seldom achieve
grade. The reasons why include changes in climate that result in more or less
water and sediment supply, tectonic uplift or drop that change stream gradient
(slope), and changes in base level. The latter can be natural or man made.
Examples of base level changes are changes in sea level or lake level, damming
by avalanches, lava flows or earthquakes, and damming by man for hydroelectric
power. When base level changes, streams erode and deposit sediment in an
attempt to regain grade (equilibrium). For example, emplacement of a dam raises
the base level of the upstream portion of a stream, lowering the slope. Decreased
slope results in lower water velocity and reduced capacity and competence,
causing the stream to deposit sediment upstream of the dam. Below the dam, the
river is sediment starved, causing it to erode its channel to regain grade.
During the last ice age,
extremely low sea level resulted in streams downcutting
into the continental shelf, producing vast shelf channels that are well below
sea level now that water formerly held in ice has returned to the oceans.
Abrupt drops in stream
velocity result in sediment deposition. This occurs in two settings. The first
is where steep mountain channels empty onto flat valleys. This produces a rapid
drop in competence as the slope decreases and the channel widens. The result is
deposition of alluvium onto the valley floor. The migration of braided channels
back and forth on the valley floor produces a fan-shaped, concave upward
deposit known as an alluvial fan. Sediment with the fan fines outward from the
source as competence decreases. Within the fan there is little stratification.
Deltas form when streams
empty into lakes or oceans. Here competence drops as grade vanishes and
velocity drops to near zero. The deposits assume a triangular shape, with the
apex at the stream mouth. Deltas, like alluvial fans, are concave upwards.
Where the stream enters standing water, it deposits its bedload,
forming foreset layers. Suspended load is deposited
further, producing layers that dip and fine seaward known as bottomset layers. With continued deposition, the stream
channel builds outward. Viewed in stratigraphic
column, fine sediments underlie coarse sediments, as foreset
layers build out onto bottomset layers. At the
surface, natural levees are built on the coarse sediment embankment, resulting
in a floodplain of sorts. The internal stratification and presence of channel
bank deposits within deltas distinguish them from alluvial fans.
Rivers drain specific
geographic regions defined by divides, the borders between drainage basins. Divides
are topographic highs; water runs to one side of the other, but doesn't cross,
such that rain that falls on one side of the divide is always drained by rivers
on that side of the divide. Within a drainage basin, drainage networks form to
drain water. Streams connect downstream, with small streams joining other small
streams and/or emptying into main trunks. The network resembles the branches of
a tree with the trunk assuming the role of the trunk stream and the branches
being tributaries.
Drainage patterns can
reflect geologic history of a stream. For instance, streams that cross
mountains must precede the mountain. These antecedent streams were able to
incise faster than the mountain uplifted, such that they now flow through the
mountain belt rather than around it.
Now let's focus on wind.
Wind gets its energy from
the sun. Sunlight at the equator is incident at right angles to the Earth,
whereas sunlight at the poles is incident at grazing angles. Thought of another
way, light striking the equator is incident on less surface area than light at
the poles, thus air at the equator is heated more than air at the poles. Warm
air is less dense and rises. Cold air is dense and sinks. Rising air at the
equator coupled with sinking air at the poles produces wind. This simple
picture predicts north-south winds, however, which are seldom seen. So what's
missing? The answer is the Coriolis effect.
Coriolis effect:
Earth revolves eastward. Viewed from the north pole
toward the south, Earth revolves counterclockwise. Although
the rotation of the Earth is constant, the surface of the Earth moves more
slowly eastward near the poles than near the equator. You can picture this by
imagining running around the equator in a day versus running around the north pole in a day. To circumnavigate the equator in a day,
you have to move fast. To run around a flag posted on the north
pole, you don't.
To see how this affects
wind patterns, picture a packet of air moving over
Circulation cells: Each hemisphere has three
circulation cells aligned parallel to the equator. The trade winds occur in the
two cells that straddle the equator. Here hot air produced at the equator rises
to the top of the cell and flows away from the equator. Cold air moves toward
the equator at the base of the cell (near the surface) and is deflected to the
west, producing the Easterlies. North and south of the trade-winds belts lie the Westerlies. Here cold
surface air moves north in the northern hemisphere and south in the southern
hemisphere, and is deflected to the east in both, creating the westerly winds. Further
north and south lie the polar belts, which, like the
trade winds, are easterlies.
These belts figure heavily
in climate (the long term conditions of a region, including temperature,
rainfall, wind speed, etc.). In particular, warm, moist air rising at the
equator cools and drops its rain on the tropics. It descends as cool, dry air
near 30 degrees N and S, resulting in deserts.
Wind erosion occurs in
through two processes: deflation and abrasion. Deflation involves picking up
and removing sand and dust from the surface, lowering, or deflating, the
surface elevation. It is important only where there is sufficient competence of
wind, that is, it it able to pick up sediment or move
it as bed load. Where deflation does occur, it produces hollows or blow outs
that are sauce shaped depression, typically a meter or two deep. The depth of
blow outs is limited by water, either by reaching the water table, or by
forming a sufficient basin to accumulate seasonal rains and maintain
vegetation. Deflation of mixed sediment produces desert pavement. Winds pick
out fines, leaving coarser sediment behind. As the surface deflates, the
percentage of coarse clasts at the surface increases,
eventually paving the surface densely enough that wind can't access fine
sediment, at which point deflation ceases.
Abrasion produces ventifacts, wind-faceted pebbles, cobbles and boulders. The
windward sides of ventifacts are abraded down to a
flat surface by sand blasting. Their occurrence in a deposit is an indicator of
paleo wind direction. Abrasion can also produce yardangs, long linear ridges aligned with the prevailing
wind direction that resemble upside down boat hulls. Yardangs
range in size from meters to kilometers in length. They
are common only in sediment-starved regions.
Two deposits unique to wind
are loess and volcanic ash. Loess deposits are wind-laid dust (loess from
German for loose) formed when winds die down sufficiently for suspended load to
deposit. Loess deposits lack internal stratification and form a blanket mantling
the terrain (unlike alluvial deposits which fill basins). Loess covers as much
as 10% of present land surface. Some deposits can reach 300 m thickness. The
source of dust in loess deposits is though to be deflation of deserts and
floodplains of glacial meltwater streams. Dust is
also deposited in the oceans and in glacial ice.
Volcanic ash produced
during explosive eruptions can be carried around the world by the wind. During
the eruption, hot ash rises high into atmosphere,
sometimes reaching the stratosphere (10 km or more). The finest portions can
remain suspended for years, altering climate by blocking incident sunlight. The
coarser fraction is deposited downwind. The prevailing wind direction and
location of the source are indicated by beds that thin downwind, clast sizes that decrease downwind and areal
coverage that increases with distance from the source. As discussed in a
previous lecture, ash deposits are important key beds in stratigraphic
sections.
Deserts are arid
environments, receiving less than 250 mm (25 cm, or roughly 10 inches) of rain
each year. Right now, 25% of land area outside the polar
regions is desert. The extreme aridity of deserts originates in several
ways. Subtropical deserts form near 30 degrees north and south where dry winds
descend at the poleward extreme of the trade wind
belts. Continental deserts form in continental interiors far from moisture
sources. Rain shadow deserts form on the lee side of mountains. Air flowing
over the mountains rises up, cools and drops its water on the windward side of
the mountains, then flows down and warms on the leeward side. Coastal deserts
form where cold upwelling marine water cools maritime
air and causes it to drop its water content over the ocean, drying out before
it hits land. Lastly, the polar regions are termed
deserts because the cold, dry descending air within the polar wind cell
produces little or no precipitation.
Erosion in deserts: Although you might guess that water
would have little to do with erosion and landform shaping within deserts, it is
quite important. Rain in arid climates often comes as cloudbursts. Rapid
precipitation does not allow water to soak in. The overland flow moves freely
due to the lack of vegetation, and readily erodes sediment that would be held
together by roots in a more humid region. Deflation and abrasion are
unimportant by comparison.
Deposition in deserts: Alluvial deposits formed during
flash floods typically consist of unsorted debris, thinly mantled over broad
areas. Subsequent deflation removes the fines, leaving behind coarse sediment.
The rocky, irregular surface that remains is much more common than sand dunes,
which cover less than 1/5 of desert land area. Because streams within deserts
do not usually flow to sea, evaporation is quite important. Playa lakes form in
desert mountain valleys or basin lowlands. Evaporation leads to evaporite deposits and high salinity in the water. Although
usually ephemeral, playa lakes recur with each year's rains, reactivating and
concentrating salts in the lake bottom deposits.
A unique form of deposition
in deserts is the formation of desert varnish, a thin, dark brown to black
mixture of clay minerals and manganese and iron oxides
coating that accumulates on rock surfaces. This occurs over time scales
of thousands of years. Exactly how it forms is still not known.
Desert landforms: A number of geomorphic features are
unique to deserts. Most, however, are rare or hard to identify in the field. The
ones you see most commonly are mesas, alluvial fans and talus slopes. Mesas are
flat topped buttes capped by resistant rock. They form when streams cut through
resistant rock, exposing easily erodible rock below. Erosion
cuts down into the lower strata, but operates slowly on the islands of
resistant rock between them. With time, only a few islands remain, standing
well above the erosional surface of the desert. Alluvial
fans and talus slopes are common and co- related in arid environments where
there isn't sufficient water to carry coarse fan deposits away. What water
there is often soaks down into the highly porous fan deposits. Cities have been
built on alluvial fans because of the water stored in them. While easy access
to water may be nice, the fans are there for a reason. The near annual mudflows
that parade through
The world's oceans comprise
71% of Earth's surface. Compared to the other terrestrial planets, Earth is
unique in this respect (Mars has ice caps but little or no free water), and it
was in Earth's early oceans that life originated. In fact, the emergence of
land plants and animals are relatively recent events in geologic time. Oceans
throughout geologic time have been a major component of climate and the major
reservoir in the hydrologic cycle. Through feedback between the hydrologic and
rock cycles, the world's oceans have affected the evolution of ocean basins and
continents-they are part of geology.
Coasts are one of the most
active environments found on Earth. The landscape of coasts is constantly
changing, reflecting sensitive balances between sediment influx and outflow. Today,
as through all of recorded history, most people live on or near the coast. Their
interactions with the coast often have profound, frequently unforeseen,
influences-most of them bad.
Besides being important,
active modern environments, the world's oceans and coasts are very well
represented in the rock record. Sedimentary rocks cover most of the land
surface and most were deposited in marine or coastal depositional environments.
To understand the rock record, we must therefore understand the modern
processes active within the oceans on at the coasts.
The next one and a half
lectures are about oceans and coasts. We will cover the chemistry of ocean
water; the different forms of flow in the oceans (currents, tides and waves);
the nature and effects of El Nino; shoreline morphology, including beaches,
deltas and dunes; and the role of plate tectonics in shaping coastlines. We
begin with a physical description of the world's oceans.
The world's oceans are
really one interconnected ocean. Although it is one body of water, it is a
large enough and complex enough body that water within it mixes very slowly. The
volume of the oceans totals about 1.35 x E9 cubic kilometers
of water. That's equivalent to a cube of water with sides 1100 km long! Water
depth within the oceans is highly variable. The mean (average) depth is 3.8 km,
or a little more than 2 and one quarter miles. This shallows to zero on the
coasts, and deepens to as much as 11 km in subduction
zone trenches.
Most ocean basins are
formed by the creation of new oceanic crust at mid-ocean spreading centers. This process imprints a general morphology on
ocean basins which is well demonstrated in the
Basin geometry has varied
through time with the opening and closing of oceans due to plate tectonics. Today's
oceans are just that, today's. Oceans 100, 500 or 1000
Ma in the past looked very different in map view. Within them, however, we
would find the same features: seamounts, hotspot islands, mid-ocean ridges,
etc. We also believe we would have found the similarly salty water.
Salinity, a measure of the dissolved content
in water, is measured in parts per thousand ("per mil") of dissolved
content by weight. The oceans have an average salinity of 35, meaning there are
35 grams of dissolved material for every 1000 grams or sea water. Water is H2O.
The dissolved content is composed primarily of Cl
(Chlorine) and Na (Sodium). Magnesium (Mg) is the next most abundant element at
nearly one tenth the concentration of Cl and Na. Sulfur (S), Calcium (Ca) and Potassium (K) are roughly one
hundredth as concentrated as Cl and Na. All other
elements are at least a factor of 1000 less abundant. Thus the salinity of
ocean water is due almost entirely to Cl and Na, with
Mg, S, Ca and K as the only other important elements. These elements appear in
water as Cl(1-), Na(1+), Mg(2+), SO4(2-), Ca(2+), and K(1+). Carbon,
which is important for life processes, appears as bicarbonate, HCO3 (1-).
These different elements
originate from different sources. The cations (Na,
Mg, Ca and K) are largely leached (chemically weathered) from rocks and
transported to the oceans by streams. The anions are the product of mantle
degassing through volcanoes, which emit CO2, Cl(1-) and SO4(2-). Hydrothermal
metamorphism also releases these anions into ocean water.
Stream supply and mantle outgassing are ongoing processes. Looking at the rock
record, we have reason to believe that ocean salinity (and the elemental mix
responsible for the salinity) has been relatively constant. This evidence comes
from evaporites (chemical sediments precipitated from
water), which follow the same sequence of precipitation as modern sea water
implying the chemistry of the water is similar. This presents a problem. If
streams and volcanoes are constantly injected Cl, Na,
etc. into the water, why hasn't the salinity of sea water increased through
time? To avoid increasing salinity, these must be an equal output of Cl, Na, etc. to match the stream and volcanic gas input. In
other words, what is added by streams and outgassing
must be removed by some other processes to maintain the balance. What are the
processes that remove Cl, Na, etc.?
Many cations
are used by organisms to build shells, bone and soft parts. This includes Si, Ca, P, and HCO3 as used in the production of CaCO3. When
the organism dies, it sinks to the sea floor, thus these elements are converted
from dissolved content to sediment and are removed from the water. Because these
processes work efficiently and are always going, Ca, and carbon have short
residence times in sea water.
K and some Na is absorbed
by clays and deposited as sediment; Cl and Na form evaporites in arid, isolated regions; and some Cl is taken up by rocks during hydrothermal alteration. Because
these processes are slower or episodic, K, Na and Cl
have long residence times in the oceans.
When sea water is
evaporated, the first compound to precipitate is CaCO3. Because CaCO3 is nearly
saturated in average sea water, it is easily secreted by organisms, but not
easily dissolved in sea water, probably explaining why so many organisms use it
to form hard parts. Below a depth of 4 to 5 km, CaCO3 is undersaturated
and will dissolve. Undersaturation here is the result
of the greater solubility of CO2 in water that is cold and at high pressure
(this is why soda releases CO2 when you warm it or uncap it, lowering the
pressure). Below this depth, called the CCD (short for
carbonate compensation depth) shells made of CaCO3 dissolve. For
this reason, we do not find deep-water calcareous sediments, and can rule out
deep-sea deposition as the source of carbonate rocks.
Looking at a parcel of sea
water, salinity is a function of evaporation, the formation or melting of glacial
ice (ice is pure water), and freshwater influx from streams, rain and
groundwater. As these vary, the salinity of sea water varies, ranging from 33
to 41. The salinity and temperature of sea water affect its density. Sea
water density increases with increasing salinity and decreases with increasing
temperature. Thus cold, saline waters are dense, whereas warm, fresh waters
are not. Differences of density result in stratification of ocean water as
dense water sinks to the bottom and warmer, less saline water rises to the top
of the ocean. This stratification produces a form of ocean circulation that we
will discuss later.
The origin of water: All water is ultimately due to outgassing of the planet through volcanism at mid-ocean
ridges, subduction zones and hotspot islands. Each
year about 15 cubic kilometers of new rock is
erupted. This is derived from the mantle by partial melting and carries about
0.5% water by weight. If we assume that this water escapes the rock and finds
its way to the oceans, then over the course of the Earth's 4.55 Ga history, outgassing would
produce about 95% of the current oceanic volume of water. This assumes,
however, that all the water in newly erupted rock ends up in the ocean and
remains there. A more realistic set of assumptions that allows water to be
recycled into the mantle reduces this number to about 25%, implying that outgassing must have been more vigorous in the past. This
is acceptable since we know that the young Earth was warmer and produced much
more volcanism. Today it appears that the rate at which water is outgassed is about equal to the rate at which water is
recycled into the mantle by subduction of water-rich
sediments and oceanic crust. In other words, the oceans don't appear to be
growing or shrinking today (although sea level still rises and drops due to the
volume of water held in glacial ice and changes in ocean basin bathymetry).
Surface currents and
wind: Winds blowing
over the ocean surface produce waves. They also produce surface currents that
mimic the winds. As with winds, the Coriolis
effect causes deflection of flows off the equator.
Deflection is to the right in the northern hemisphere and to the left in the
southern hemisphere. The deflection produces gyres, which are subcircular current systems, in which water flows in a
circular, "down the drain" pattern. There are two major gyres in both
the Pacific and the
Coastal upwelling in
Thermohaline circulation: Cold, saline water is denser than
warm, less saline water and will sink below it. When it sinks, it displaces
water which must flow out laterally to make room. As it sinks, water is sucked
in to replace it, creating a net flow system. This system is driven by
variations in water temperature and salinity, hence the name: thermohaline circulation (haline
for halite, the prime dissolved constituent of sea water). The production of
cold, saline water is related to surface currents, so thermohaline
circulation is tied to surface currents and the winds.
Thermohaline circulation patterns are complex and
very slow (0.0001 m/s). We know they exist by measurements of the salinity and
temperature of ocean waters and by tracking distinctive features within the
water column. Unlike surface currents, thermohaline
circulation involves the entire ocean. A major component of the circulation is
the movement of water in the
El Nino: El Nino is the name given to a
series of climatic effects associated with changes in oceans currents. The name
is derived from Spanish for Christ since the effects of El Nino are felt most
strongly during the Christmas season. El Nino results from a reversal in the
easterly Trade Winds which usually produce upwelling
off
Low pressures zones formed
in
Lastly, El Nino appears to
be semi-periodic, recurring every 4 to 7 years on average. Specific El Nino
years vary in intensity. The last really strong one was 1982. This year's promises to be very severe.
The following two links
contain movies showing the change in sea surface temperatures during the last
year. The tongue of warm water spreading eastward across the Pacific is very
obvious. Near the end of the movie (July of this year), warm water starts to
spread both north and south away from the Peruvian coast, bringing warm water
up the California coast and depressing deep water upwelling
(resulting in some very warm late summer days in Santa Cruz).
El Nino Movie (MPEG) El Nino Movie (QuickTime) (Images copyright by NASA)
Last two years of SSTs (Images copyright by NOAA)
Tides are the twice-daily
(semidiurnal) rise and fall of ocean waters. They are due primarily to the
Moon, specifically the gravitational attraction of the Moon. The Sun, which also
attracts the Earth, creates smaller tides since it is further away (more later). To understand the tides we must understand two
different forces applied to the Earth. The first is known as centrifugal
force. It is the force due to Earth's rotation about the center of the Earth-Moon system. Although you might think
that the Moon revolves about Earth, in fact both bodies revolve around the center of mass of the two which is very near the Earth's center because the Earth is so much more massive than the
Moon (Earth's mass is 80 times the Moon's mass). You can see this force in
action by placing a marble on a spinning turntable. The marble will spin with
the platter, but it will also move outward from the center
and will quickly fall off the edge. Centrifugal force is also why you can swing
a bucket of water over your head without the water spilling. In that case the
centrifugal force due to rotation of the bucket exceeds the gravitation
attraction of the Earth and the water stays in the bucket. In the Earth-Moon
system, Earth's rotation about the center of mass of
the Earth-Moon system balances the Moon's attraction, such that the two bodies
do not move toward one another, but remain a constant distance apart. Centrifugal
force is uniform on the Earth, meaning it points in the same direction (away
from the Moon) and has the same magnitude (i.e., is equally strong) everywhere.
The gravitation attraction of the Moon exactly balances centrifugal force at
the center of the Earth, but elsewhere the two do not
exactly counteract one another. On the side of the Earth facing the Moon, the
Moon's gravity is slightly greater than the centrifugal force and there is a
net attraction toward to the Moon. On the side of the Earth furthest from the
Moon, the Moon's gravitation attraction is slightly less than the centrifugal
force and there is a net push away from the Moon. Water, which flows readily,
responds to this forcing and forms thicker columns on the near and far sides of
the Earth aligned along the Earth- Moon axis. The tides
are semidiurnal, meaning that sea level rises and falls twice a day, because
the Earth rotates such that any point on the Earth passes through the
Earth-Moon axis twice a day. This is an important point since it tells us that
the coasts move, the tides don't.
It is the small differences
between centrifugal force and the force due to the Moon's gravity that raises
tides. The Sun raises much smaller tides despite having a greater force due to
gravity. The reason Sun tides are smaller is that the difference between
centrifugal force and gravity is smaller. This is because the Sun is much
further away from the Earth and its gravitational attraction is nearly constant
over the surface of the Earth (recall that centrifugal force is constant), such
that the difference between gravitational force and centrifugal force is small.
Twice each lunar month (28
days) the Sun, Moon and Earth align along a common axis, creating the highest
high tides, which are called Spring tides, not for the season but rather
for the height of the tide. Also twice a month, Neap tides occur when
the Moon and Sun pull perpendicularly to one another, creating the lowest high
tides. Even during high high tides, tidal amplitude
is only about 0.5 m in open water. Near coasts and in bays, however,
constricted flow of water can amplify the tides, creating tides that rise or
fall as much as 12 m! The height of the tide in estuaries and bays can create
tidal currents as the tide rushes in and out of constricted openings. Tidal
currents are an important erosive agent in tidal flats and some bays.
Waves, like surface
currents, are produced by winds; they don't, however, result in the transport
of water, only relative motion. To create significant waves, wind speeds must
exceed 20 km/hr. The height of waves created by winds
increases as the wind speed, duration of exposure and area of exposure
increase. Wave motion is restricted to the uppermost portion of the
water column. Motion within the uppermost portion is described by three
quantities: (1) the wavelength, (2) the wave height and (3) the wave period. Wavelength
(L) is the distance between subsequent crests (or troughs) of the wave,
typically anywhere between 6 and 600 m. Wave velocity (V) is the rate
(distance/time) that crests of the wave propagate at. Wave period (T) is
the time that elapses between crests passing a point, varying between a few
seconds and 15 or 20 seconds. Wavelength, wave
velocity and wave period are related in one simple equation:
V = L / T
In words, wave velocity is
wavelength divided by wave period. Why? A propagating wave's crests take T
seconds to move one wavelength, the distance between crests. Velocity is
distance divided by time, or wavelength divided by period. Wave velocity varies
between 3 and 30 m/s. This relation will be used again shortly.
With increasing depth, wave
motion decreases, vanishing at a depth of L/2 (one half wavelength),
a depth known as the wave base. The limited depth of wave motion
explains why surfers can dive beneath a wave and not get worked. Within the
upper L/2 (above the wave base), water moves in circles. If you could tag a
small parcel of water, you would see that it moves in a circular orbit, similar
to a point on the rim of a drum rolling in the direction of wave propagation. Within
the orbit, water moves up, then forward, then down, then back, explaining why
surfers move forward then back as a wave passes.
When the bottom shallows to
less than wave base (L/2 depth), friction slows the wave down and compresses
the circular orbits. Although wave velocity V drops, the period T remains the
same, requiring the wavelength L to decrease. This causes wave crests to be
packed closer together. Because wave height doesn't decrease, the water becomes
more steeply crested. Eventually the wave breaks when it becomes too steep.
Breaking waves create surf.
Surf zone: Surf is turbulent white
water. It is high energy and can do work to the beach and backing cliff. Surf
picks up sand; the turbulent motion within surf uses sand to scour the water bottom,
eroding it down. Surf intrudes into cracks and hydraulically wedges them,
physically weathering rock. It can also apply tremendous pressure to cliff
faces capable of shattering rock (the strain rate is high and the rock fails in
brittle fashion). Most of the work accomplished by waves happens within the
surf zone.
Wave refraction: Waves impeding on a shoreline are
refracted. Refraction means that the wave changes direction, turning to
face the shoreline at a more nearly perpendicular angle. This is the result of
bottom friction and the reduction of wave velocity in shallow water, causing
the portion of a wave closest to the beach to travel slower than the portion
further out. Because the portion of the wave further from shore travels faster,
it catches up to the near-shore portion, resulting in a wavefront
at nearly right angles to the shore. Wave refraction also causes waves to focus
on headlands with shallow water projections since the waves bend to hit them
square on. It also directs waves away from deep-water bays, resulting in
sheltered dockage, but also resulting in a lower energy environment and
sediment deposition that fills bays. Either way, headland or bay, wave
refraction works to produce a uniform, linear coastline.
Longshore current: Wave refraction isn't complete,
meaning that waves don't break at perfect right angles to shorelines. When
waves strike at an acute angle they produce longshore
currents. To understand longshore currents, let's
imagine following a parcel of water near the shore. As a wave runs up the beach
(swash) it moves along the same direction as the waves, that is, it
moves at an angle up the beach. When the water runs back down the beach (backwash)
it does so under the force of gravity only, causing it to flow straight down
the beach (perpendicular to the shoreline). The result of repeated run up and
run down is a zig-zag trajectory that moves water
down the shoreline. This is the longshore current.
Sand is moved with the longshore current, resulting
in longshore or beach drift. The longshore current results in the net transport of water
along shore. Longshore drift results in net transport
of sand along shore. Although the distance traversed in a single swash/backwash
cycle is small, longshore currents and drift are
rapid. For instance, in
Beaches: Shorelines can be broadly classed
as beach and rocky shorelines, of which
Beach shorelines have a sand
budget that determines their health. Inputs that grow the beach consist of
sediment eroded from backshore cliffs, sediment brought in by longshore currents and beach drift, and sediments brought
in by rivers. Outputs that shrink beaches consist of sediments blown into
backshore dunes, sediment removed by longshore
current and beach drift, and sediments transported to deep water by currents
and waves. Beach size varies with these inputs and outputs on a variety of time
scales ranging from days to many years. Beaches that have persisted for many
years must be in near steady state balance. Thus if sand is lost during a large
storm it must be replaced, for instance by stream input. Disrupting this
delicate balance are man made perturbations, such as jetties and groins.
These stop longshore current and beach drift,
building beaches on the up current side, but starving beaches on the down
current side. A less obvious but often more important perturbation is the
damming of rivers. River dams trap sediment, reducing the riverine
influx and causing beaches to shrink. As the extent of damming increases (for
energy and agricultural purposes) so will the effect on sandy beaches. And as
we saw with rivers, a beach robbed of sediment influx has greater capacity to
erode.
Rocky shorelines: The elements of rocky shorelines
are: a wave-cut bench, wave-cut notch and the wave-cut cliff. The bench
is a gently dipping bedrock flat created by surf
erosion. It grows in the landward direction by the action of surf cutting a notch
at the base of backing cliffs. When the notch grows deep enough, or when storm
surges raise wave energy levels sufficiently, the wave-cut cliff will
fail (an example of mass wasting). The rock debris is quickly broken down,
becoming grist used to scour the wave-cut bench. Beaches on rocky shorelines
are usually limited to pockets between headlands. Here wave refraction results
in a lower-energy environment where sediment is deposited.
Sometimes in the cutting of
the bench, portions of the backing cliff will be left standing. As the cliff
face retreats, these "stacks" become isolated columns in the
foreshore or offshore.
Moving beyond the scale of
a beach to a coastline we find several unique landforms. Spits are
elongated ridges of sand and/or gravel that project from land and end in open
water. They extend beaches offshore and form from longshore
currents and beach drift.
Barrier islands are long sandbars offshore that
form a barricade between open ocean waves and the main shoreline. They are
common along low lying coasts where sediment is abundant. They are thought to
have formed from the submergence of berms during sea
level rise or by the progradation of spits. They are
maintained by influx of sand eroded by breakers. Like sandy beaches, barrier
islands are subject to a sand budget and can grow and shrink on very human time
scales. Padre and Mustang islands offshore of eastern
We have already talked
about deltas in "Rivers and Wind", but it's worth mentioning
that the existence of a delta is evidence that sediment influx exceeds the
ability of waves and tidal currents to erode sediment.
Atolls are coral reefs arranged in a
circular form, sometimes bounding a volcanic island. They represent the last
stages in the evolution of an oceanic island. In the first stage, the island is
created by volcanic eruptions creating an edifice that extends above sea level,
forming a shoreline. Corals grow in the warm, shallow waters surrounding the
island. The island subsides as it cools and as the plate it sits on cools. The
corals continue to grow, matching the rate of subsidence, resulting in a
circular coral reef growing up from deeply submerged sea floor. This process
was first proposed by Charles Darwin during the cruise of the HMS Beagle.
As with atolls in
particular, the plate tectonic setting of coasts matters generally. Coasts that
are not on a plate boundary are called passive (or trailing) margins. These
tend to feature lowland, sandy beaches with frequent barrier islands. The east
coast of the
Humans are performing a
unique experiment, testing the effects on global climate of releasing vast
amount of carbon dioxide, chlorofluorocarbons and methane into the atmosphere. These
"greenhouse" gasses are known to trap heat in the atmosphere and
their abundance in the atmosphere is growing exponentially. Scientists
monitoring this experiment predict a 1 to 3 degree increase in globally
averaged temperature in the next century. Attendant with higher temperatures
are a reduction in the volume of the water presently stored in glaciers and the
polar ice caps, and changes to the wind and ocean circulation systems that will
affect the distribution of precipitation on land. Residence times of greenhouse
gasses in the atmosphere are on the order of hundreds of years, so even if we
shut down the experiment today, the effects would continue to be felt for some
time.
The predictions
for global warming on based on observations and modeling
of climate. Earth's
climate systems are enormously complex. Unraveling
the manifold interactions between components of the system is difficult and
only partially completed. One way in which scientists examine the system is by
examining the past. Earth's climate has been variable throughout
the planet's 4.55 Ga history. Many aspects of
past climates are recorded in geologic strata and surface deposits. Geologist are reading that record, in the process learning
much more about Earth's climate than could be gleaned from historic records
alone.
This lecture is about
climate change. We begin with the earliest indications of significant climate
change, the glacial ages of
Glaciers, semi-permanent bodies of ice and
snow that show evidence of downslope movement, exist
when the production of ice through snowfall and recrystallization
exceeds the rate of melting. Temperate glaciers grow in winter when
temperatures are low enough that ice does not melt and there is input of new
snow. They shrink in the summer when temperatures are high enough to melt ice. Geologists
often speak of the mass balance of a glacier. A glacier has a positive mass
balance when the production of new ice from snowfall (accumulation) exceeds the
loss of ice to meltwater and evaporation (ablation). Negative
mass balance exists whenever melt exceeds the addition of new ice. Because the
amount of melting and snowfall are functions of climate, glaciers are highly
sensitive to climate change.
Glaciers form above the
snowline: the lower altitude limit of perennial snow. Snowline depends on local
climate, particularly latitude because mean temperature decreases with
increasing latitude. Near the poles, snowline corresponds to sea level. In the
tropics, snowline may be exceed the height of the
highest mountains. On a glacier, the snowline marks the boundary between the
accumulation area and the ablation area. Glaciers can exist below the snowline
because glacial ice flows downslope,
replacing ice lost to ablation.
Within a glacier, the force
of gravity creates a net downslope force. Glaciers
respond to this force and move downslope in two
modes. The first involves ductile flow wherein ice crystals deform almost like
a deck of cards. This internal flow allows the portion of the glacier not in
contact with the bed to move downslope. The further a
packet of ice is from the bed, the further it can flow. Ice close to the
surface behaves in a brittle fashion (much like rocks near the surface of the
crust), cracking and producing crevasses in response to the ductile flow of deeper
ice. The second mode involves basal sliding of ice along its bed, facilitated
by meltwater which acts as a lubricant easing motion.
Ice within a glacier is always flowing downslope-even
if the toe of the glacier is retreating! Ice flowing toward the toe is being
ablated (melting and evaporating). The last bit of ice is ablated at the toe,
marking the downslope end of the glacier. Typical
rates of downslope velocity are on the order of 50 m
a year-slow in human terms but well above the speed limit of geology.
Glacial downslope
growth and retreat leaves behind distinctive deposits, marks and landscapes. Unfortunately
we don't have time to examine these in any detail. Rather, we will focus on
glacial till and moraines, and glacial striations. Till is unsorted sediment
directly deposited by glacial ice. Till can range from fine grained clasts to enormous glacial erratics
(large rocks different from the bedrock the glacier sits on). Moraines are
deposits of till produced on the boundaries of glacial ice. End moraines are
deposits "bull dozed" into place by advancing ice. Lateral moraines
are deposits produced on the flanks of glaciers. Debris in moraines is derived
from rock falls onto glacier ice and by abrasion and plucking of rocks in the
glacier valley. Because an advancing glacier bull dozes
its own end moraine, a terminal moraine not fronting an active glacier marks
the furthest downslope extent of the glacier that
produced it. The scouring action of clasts held in
glacial ice as it slides leaves behind deep grooves in bedrock called glacial
striations. Apart from attesting to the former presence of glacial ice,
striations on rock outcrops are also used to constrain the direction of glacial
flow.
Geologists working in
Milankovitch theory: Earth's climate is driven by solar energy. The
amount of solar energy received is a function of distance from the Sun, the
further the planet lies from the Sun, the less solar energy is receives. There
is, however, another important consideration: the seasons. Earth has seasons,
not due simply to Earth's rotation about the Sun, but rather to the fact that
the axis of rotation (the line connecting the south and north poles and passing
through the center of the Earth) is not perpendicular
to the plane the Earth orbits the Sun in. Right now the axis is inclined 23.5
degrees to the perpendicular. It is this tilt that creates seasonality. In
northern hemisphere winter, the north pole is further
from the Sun than the south pole. More importantly, it is not in direct
sunlight, receiving only light diffracted in the atmosphere, whereas the south pole (which is in summer) receives constant sunlight. Long,
sunless and, hence, very cold winters allow thick snow blankets to accumulate. In
summer, although there is near constant sunlight, the Sun remains low in the
sky and temperatures do not rise much above 50
degrees. Glacial ages in the Pleistocene have been felt most severely in the
northern hemisphere, hence the duration and severity of northern hemisphere
winter is most important.
Earth's tilt is not
constant. It varies between 21.5 and 24.5 degrees, going back and forth. A full
cycle, that is, the time taken for tilt to increase from 21.5 to 24.5 degrees
and then return back to 21.5 degrees, is about 41,000 years. When the tilt is
25 degrees, winters are more severe; when it is 21.5 degrees, winters are
milder. Because tilt changes, winters cycle from mild to severe and back to
mild every 41,000 years. Glacial ages occur when winters are severe.
Earth's tilt varies in
another fashion. With time, the axis of rotation itself rotates. A very
accurate analogy is the precession of a spinning top. Tops spin fast about
their spin axis, just as Earth spins daily about its spin axis. But the spin
axis of a top also rotates, always much more slowly than the top. Earth's spin
axis does the same, completing one rotation every 23,000 years. This affects
when the seasons occur during the year. Right now it's fall. 11,500 years ago
(one half the time needed for the spin axis to
complete one cycle) it was spring. 11,500 years from now it will be spring. We
call this change in seasons the precession of the equinoxes, the equinoxes
being the day of the year in which the length of day equals the length of
night.
As mentioned above, the
distance of the Earth from the Sun dictates how much solar energy the planet
receives. This distance, which averages about 150 million kilometers,
varies throughout the year because the Earth's orbit is elliptical, not
circular. The variation isn't great, just a few percent, but it is enough to
affect climate. This happens in two ways. The first is tied to the Earth's
tilt. When winter corresponds to the portion of the year spent nearest the Sun,
it is less severe. When winter occurs with the planet far from the Sun, it is
very severe. Because the time in which Earth is in winter varies with
precession of the equinoxes, there is a 23,000 year cycle in winter severity. The
second way in which the ellipticity of Earth's orbit
affects solar input is simpler: when the orbit is highly elliptical, Earth
spends some portion of the year much closer to the Sun that when the orbit is
circular. Thus when ellipticity is high, Earth
receives more solar energy than when it is low. Earth's ellipticity
varies over periods of 100,000 years and 400,000 years.
We have identified 4
periodicities in the amount of sunlight Earth receives in winter: 23,000 years
through precession of the equinoxes, 41,000 years through tilt and 100,000 and
400,000 years through ellipticity of the orbit. The
person to first work out these periods and to guess at their importance in
glacial ages was Milutin Milankovitch.
For this reason, these cycles of mild and severe winters are referred to as Milankovitch cycles.
The modern theory of
glacial ages states that glacial ice volume will increase when Earth's orbital
parameters are such that winters are severe. Since the continents are
dominantly in the northern hemisphere, it is winter in the northern hemisphere
that we consider. Ice volume will decrease and glaciers will recede when
winters in the northern hemisphere are mild. The shortest periodicity in the
change of winter severity is due to precession, thus we would expect that
glacial ages would wax and wane on an approximately 23,000 year time scale. However
it is the 100,000 year periodicity of orbital eccentricity that, for the last
million years, has dictated the duration of glacial ages (glacial and interglacials are both about 50,000 years; interglacials are the periods between glacial ages). Evidence
of this is found, of all places, in ocean sediments.
Foraminifera and other planktonic life forms living near the ocean surface
construct hard parts from calcium carbonate (CaCO3) and silica (SiO2). Plankton
live their short lives (less than a month), die and
sink to the ocean bottom, forming sediment. Through time the thickness of
sediment increases with the oldest plankton casts on the bottom and youngest on
the top of the sediment column. During plankton lifetimes, the oxygen in
calcium carbonate and silica contained in their hard parts reflects the
isotopic composition of sea water. Oxygen has three isotopes: 16O, 17O, and
18O. 17O is rare. 16O is the most common. The isotopic ratio 18O/16O varies naturally
as the amount of water held in ice caps and glaciers varies. Why? Most of the
water occurring as ice is derived from the oceans by evaporation and
precipitation. When water evaporates, water molecules with 16O enter the vapor phase faster than water molecules with 18O because
they are lighter. If this water is stored in glacial ice, the 18O/16O ratio of
the remaining sea water increases. The opposite is true when glaciers melt:
water with low 18O/16O ratios is returned to the ocean, lowering the ratio in
ocean water. Plankton casts on the sea floor record variations in the 18O/16O
ratio and thus record variations in ice volume. By examining the sea floor
record, geologist have confirmed the modern (Milankovitch) theory of ice ages by demonstrating variations
in ice volume that occur in unison with variations in winter severity.
Solar constant
variations and the little ice age: The heat source of the oceans and atmosphere is the Sun. If the output
of sunlight from the Sun were to change, we would expect climate to change as
well. Fortunately, the amount of sunlight the Sun emits does not vary much, at
least not in the last 50 years or so that we have been able to measure solar
radiation accurately. There is a theory, however, that the cooler temperatures
that prevailed from the 13th century to the mid 19th century (the "Little
Ice Age") were the result of a reduction in the amount of sunlight emitted
by the Sun. Direct evidence in support of this theory is lacking, but
astronomers did note a strange absence or shortage of sun spots during times of
the coldest climate, suggesting the Sun might somehow be responsible. There is
growing evidence that small variations in solar output affect crop production
and aspects of climate at high latitudes, lending some additional credence to
this idea. Given the excellent agreement between the Milankovitch
cycles and ice volume, there is little reason to suspect that variations in
solar output are responsible for major ice ages.
Repeated measurements of
temperature distributed over the globe exist for about the last 100 years. Averaged
globally on a year by year basis, these measurements reveal a gradual increase
in temperature of about 0.5 to 1.0 degree Celsius (1 to 2 degrees Fahrenheit). That
may not sound like much, but it is. It is enough to reduce the amount of sea
ice sufficiently to raise sea level noticeably. It is enough to reduce the
amount of permafrost soil at high latitudes. And, if the trend continues, it is
enough to radically change climate (the sum total of rainfall, temperature and
winds).
The scientific community is
in agreement that some portion of this increase is anthropogenic (man made). They
point specifically to the role of greenhouse gasses in the atmosphere, and
their release into the atmosphere through the burning of fossil fuels,
agriculture and cattle grazing, and through refrigerants and aerosol can
emissions. But what are greenhouse gasses really? Before we can answer that
question, we must first understand the way in which sunlight is converted to
heat.
Much of the radiant energy
of the Sun is absorbed by water, land and ice on the surface of the Earth (some
is reflected) and then re-radiated as infrared heat rays. This is the same
process that heats up road surfaces in the summer. Most of the infrared heat
radiation leaks out into space and is lost (if it didn't, the Earth's
atmosphere would be far too warm for life), but not all. Some is absorbed by
carbon dioxide, water vapor and other greenhouse gasses
(so named for their ability to absorb infrared radiation). As a result, the
atmosphere is heated, re-radiating heat down to the surface. The process
operates very much like a greenhouse where incoming visible radiation passes
freely through glass but outgoing infrared radiation is reflected.
Earth's atmosphere is
unique among the terrestrial planets in many ways (oxygen rich, abundant water vapor, etc.). The level of greenhouse gasses is another.
Venus, whose atmosphere is 95% carbon dioxide has a surface temperature of 475
degree C. Mars, with little atmosphere, has daytime temperatures of several
tens of degrees and nighttime temperatures of -100
degrees: a range that inhibits life. Earth's atmosphere affords the temperate
climate we enjoy, warm enough to melt ice, cool enough to avoid vaporizing it,
and with little daytime/nighttime variation.
Earth's greenhouse effect
is due largely (80%) to water vapor. The remaining
20% derives from other greenhouse gasses, the most abundant of which is carbon dioxide.
Carbon plays an important role in life processes. In fact, it's
low concentration in Earth's atmosphere as compared to Venus is due to
photosynthetic respiration by plants and algae. Like rocks and water, carbon
has a series of reservoirs that it moves through.
Carbon can be stored in the
atmosphere as carbon dioxide, in the oceans as carbonate, bicarbonate or
carbonic acid, in biomass, in carbonate rocks, and in the mantle. Draw down of
carbon dioxide from the atmosphere by plants is familiar, as is the release of
carbon with the decay of organic matter. Less familiar paths through the carbon
cycle include mantle outgassing (releasing carbon
dioxide), weathering and erosion of carbonate rocks (also releasing carbon
dioxide) and recycling of carbonate into the mantle through subduction.
Carbon in the atmosphere, oceans and biomass has fairly short residence times,
meaning that it moves about freely amongst these reservoirs and that rates of
transfer between them are large. The residence time of carbon in fossil fuel
deposits, however, is very large, reflecting on how slow the incorporation of
organic carbon into rock is. By burning fossil fuels, we return vast amounts of
carbon (in the form of carbon dioxide) to the atmosphere, and we do so much,
much faster than the atmosphere can accommodate at steady state, hence
atmospheric and oceanic carbon levels increase.
This increase can be seen
clearly in modern records, beginning with the industrial revolution and
continuing to the present. The yearly mean value of CO2 in the atmosphere is
following an exponential trajectory, meaning that the rate at which it is
increasing is itself increasing. Although carbon dioxide is not an efficient
greenhouse gas, its abundance in the atmosphere makes it the most significant
anthropogenic contribution to the greenhouse effect.
CFC emissions have tapered
off in the last decade, but still remain much above pre-1950s levels. Chlorine
released from breakdown of CFCs in the stratosphere attacks ozone (O3) which is
an important trace gas that absorbs harmful solar radiation. Ozone depletion is
especially menacing, not as climate change, but rather as a direct health
hazard. At the present time, the most severe ozone depletion is limited to the
Antarctic due to its unique wind circulation patterns and temperatures, but
scientists are noticing small, but measurable and important, drops in
stratospheric ozone levels at lower latitudes. The following two links show
ozone levels above the north and south poles as of
North Pole Ozone, South Pole Ozone .
Watch the hole develop over
Climate modeling: Knowing that the concentration of carbon dioxide is increasing in the
atmosphere, we want to predict how global temperatures will change. To do this
we need to model climate. This is not an easy proposition. To model climate you
have to understand the physics behind all processes that effect
it. You have to describe the entire system: topographic relief, depth of the
oceans, the formation of clouds, the effect of vegetation on winds, rain and
absorption of solar radiation, the list goes on and on. Our knowledge of many
aspects of climate is incomplete. Even if we did perfectly understand the
physics, chemistry, biology and geology of climate, we would not be able to
program that complexity into a computer model. Climate models, therefore, are
only approximations to climate. Their reliability in projecting climate 5 days,
5 years or 5 decades into the future remains suspect. At the present time it
appears that they can meaningfully predict mean temperatures and that computer
generated projections of 1 to 3 degrees of mean global warming in the next
century should be heeded. My personal feeling is that right or wrong, we do not
understand the effects of global warming well enough to risk continuing the
experiment. It is conceivable that a warming planet might be more hospitable to
human life. Maybe. But we don't know that. What we do
know is that the present day climate is hospitable. If our actions threaten it,
we should change our actions.
Environmental impacts of
global warming: One
effect we are already seeing is a rise in sea level as water previously held in
glaciers, ice sheets and polar ice caps melts and is returned to the ocean. Although
the yearly rise is modest (4 mm/yr), it accumulates quickly. Much of the
eastern seaboard lies close to sea level. Port cities are threatened by
continued sea level rise. Global warming will also affect precipitation
patterns. Because warm air holds more moisture, many regions can expect greater
yearly rainfall. But warmer surface temperatures will also affect wind patterns
and ocean surface currents, cutting off the supply of moisture to other
regions. With changes in precipitation levels and temperature will come a change in vegetation. Temperate forests will yield to
grasslands, tundra will melt. Areas presently well suited to cultivation of
important harvest crops could become too hot, too dry or even too wet to
continue production. A potential side effect of continued warming that has
received little attention is the release of gas hydrates from ocean sediments. Gas
hydrates are ice-like organic materials held in ocean floor sediments. As long
as ocean bottom water temperatures remain cold, hydrates remain in the
sediment. Should water temperatures rise to the melting point of the hydrates,
they could be suddenly released into the oceans and atmosphere. Rich in methane
(an efficient greenhouse gas), release of gas hydrates could produce a positive
feedback causing global temperatures to rise still
faster.
Temperatures in the Cenozoic have been cooler than throughout most of the
Mesozoic. This is not a product of Milankovitch
cycles which have periods of less than a million years (recall that the Cenozoic began 66.4 Ma before the present). So what is
responsible for lower temperatures? Some scientists believe the answer lies in
the collision between the Indian subcontinent and
CaSiO3 + CO2 = CaCO3
+ SiO2
Carbonate sediment was washed out to sea and
deposited, thus drawing down atmospheric CO2 levels. Decreasing atmospheric CO2
levels reduced the greenhouse effect, resulting in lower temperatures. This
hypothesis, if correct, ties together plate tectonics, surface processes and
the climate in explaining cool Cenozoic climate. Your
book describes two other examples of interactions between the mantle, crust,
oceans and atmospheres affected global climate.
Dramatic climate changes in the rock record are
associated with massive extinctions of land and marine organisms. The causes
are varied, ranging from massive meteorite impacts, to rapid eruption of
enormous volcanic bodies. How life responded to them and how they shaped the
flora and fauna of the modern Earth is our next subject.
