Weathering
Mechanical weathering: breaks rocks down
into smaller and smaller pieces through physical means such as frost
wedging, root wedging, unloading, salt crystal growth, abrasion.
Minerals remain unchanged.
Chemical weathering: changes the minerals into new minerals
and dissolved ions.
acidification of water: carbon dioxide from the
atmosphere dissolves in surface water (raindrops, streams, lakes);
carbon dioxide chemically combines with water to form carbonic acid
-> most natural surface waters are slightly acidic!
hydrolysis: carbonic acid in water reacts
with most silicate minerals (except quartz); the silicate mineral
breaks down into 1) a clay mineral, 2) metal cations in solution, 3)
soluble silica
dissolution: carbonate rocks (limestone and
dolomite) react with carbonic acid and totally dissolve (no solid
particles remain)
Soil
Soils are formed as a result of
weathering of bedrock, biological processes that mix organic matter
in with the mineral regolith in the upper horizons, and downward
leaching of fine particles and soluble ions
A typical soil profile contains (below):
O-Horizon: decaying organic matter; upper couple
inches
A-Horizon: organic rich, fines and solubles are leached
out of A into B below
B-Horizon: organic poor, enriched in fines and solubles
leached from A
C-Horizon: mineral soil - regolith - physically
and chemically weathered rock
Bedrock
Mass Wasting
The downhill movement of soil and regolith is
due to the force of gravity and is resisted by the force of friction.
The forces of gravity and friction are in balance at the angle
of repose - the maximum slope angle that unconsolidated
materials can maintain. Water can reduce the friction and increase
the mass (therefore the gravitational force), thereby reducing the
angle of repose and causing mass wasting.
Forms of mass wasting include soil creep, earthflows, mudflows,
slumps, and landslides.
Undercutting slopes for road building and house sites and removal of
vegetation by fires, etc may induce mass wasting.
Groundwater
Resources
In the hydrologic cycle
precipitation = runoff + evapotranspiration + infiltration
which means that the water that falls to the ground in the form of
rain, snow, etc will either soak into the groundwater, runoff into
surface streams, or be evaporated from the surface or transpired
through plant leaves.
The water that infiltrates the ground will percolate (seep) downward
through porous and permeable soil, sediment, and rock until it
reaches an impermeable unit.
Porous means having void spaces between grains;
permeable means the voids are connected so water can
pass through.
Porous and permeable materials include soil (if not too clay rich),
sand, sandstone, limestone, fractured igneous and metamorphic rock,
vesicular basalt and scoria.
Impermeable and/or non-porous materials include clay, shale,
non-fractured igneous and metamorphic rocks.
Porous/permeable layers are called aquifers;
impermeable layers called aquicludes.
In an unconfined aquifer the zone of
saturation (all voids filled with water) lies above an
aquiclude. The top of the zone of saturation is called the
water table. Above this is the zone of
aeration, where the voids are filled with air, though grains
may be wet or coated with water.
Pumping a well in an unconfined aquifer can lower the water table in
a cone-shaped pattern around the well because it takes time for water
to seep between grains. The total amount the water level drops in the
well is called the drawdown. The area affected by the
pumping is called the cone of depression
In a confined aquifer aquicludes or confining units lie
above and below the permeable aquifer units. The level to which water
rises in a well tapping a confined aquifer is called the
potentiometric surface. In most confined aquifers the
water is under pressure (water rises above the top of the aquifer in
a well). This condition is known as artesian. A
flowing artesian aquifer (well) is one in which
the water in a well flows to the surface because the potentiometric
surface is above the land surface.
Near the coast a lens of fresh groundwater lies above more dense
saltwater. Saltwater intrusion occurs where too much
freshwater is pumped out of the ground and is replaced by brackish
and eventually saltwater.
Groundwater pollution may occur where toxic materials
are dumped (eg. at a landfill). Rainwater leaches toxic chemicals
from the dumped materials and percolates down to the water table. The
toxic-laden groundwater may contaminate local wells. Proper landfills
are now designed with impermeable liners and caps.
Steam Processes
Streams carry dissolved ions as dissolved
load, fine clay and silt particles as suspended
load, and coarse sands and gravels as bed load.
Stream velocity is the speed of the water in the
stream.
Stream discharge is the quantity (volume) of water
passing by a given point in an amount of time.
Stream competence is the largest size particle a stream
can carry. Stream competence depends on stream velocity. The faster
the current, the larger the particle that can be moved.
Stream capacity is the maximum amount of solid load
(bed and suspended) a stream can carry. It depends on both the
discharge and the velocity.
Braided Stream patterns are found where there is a very large bed load where there is either a high sediment supply or the stream lies on a loose, unconsolidated bed of sand and gravel. In braided streams the stream does not occupy a single channel but the flow is diverted into many separate ribbons of water with sand bars between.
Meandering Streams
At a bend in a stream the water's momentum carries most of the
force of the water against the outer bank. This excess force gouges
out a deeper channel on the outer bank. The greater depth on the
outer side of the bend leads to higher velocity at the outer bank
(because the greater depth reduces the average friction). The inner
bank remains shallower, increasing friction, thereby reducing the
velocity.
Where the depth and velocity of the water on the outer bank increase
so do the competence and capacity. Erosion occurs on the outer bank
or cut bank.
Where velocity of the water on the inner bank decreases so do the
competence and capacity. Deposition occurs, leading to the formation
of a point bar.
Over time, the position of the stream changes as the bend migrates in
the direction of the cut bank.
As bends accentuate and migrate, two bends can erode together forming
a cutoff and leaving an oxbow lake.
Stream Valley Evolution
Youthful Stream Valleys have steep-sloping, V-shaped
valleys and little or no flat land next to the stream channel in the
valley bottom.
Mature Stream Valleys have gentle slopes and a flood
plain; the meander belt width equals the flood plain width.
Old Age Stream Valleys have very subdued topography and
very broad flood plains; the flood plain width is greater than the
meander belt width.
Shore Processes
The shoreline is effected by waves (produced by
wind at sea) and tides (produced by the gravitational effect of the
moon and sun).
As a wave approaches the shore it slows down from drag on the bottom
when water depth is less than half the distance between two wave
crests. The waves get closer together and taller. Eventually the
bottom of the wave slows drastically and the wave topples over as a
breaker.
As a wave crashes on the shore, the water pushes sediment up the
beach and then pulls it back down the beach as the water slides back
down. If the waves do not come in parallel to the beach
longshore transport (littoral drift) of sand
occurs.
When waves approach the beach at an angle, the part of the wave that
reaches shallow water earliest slows down the most, allowing the part
of the wave that is farther offshore to catch up. In this way the
wave is refracted (bent) so that it crashes on the shore more nearly
parallel to the shore. You will never see a wave wash up on a beach
at a very high angle from the line of the beach accept perhaps at an
inlet or where the shore makes a sudden right angle bend. This
wave refraction focuses wave energy around a
headland and diffuses it in a bay. Headlands are areas with rough
surf and rapid erosion. Bays have quiet water (good for ship
moorings) and are sites of deposition (nice sandy beaches).
Groins are structures built out from the shore at a
right angle to the beach in an attempt to stop longshore sand
transport. They hold the sand on the upcurrent side of the groin but
the downcurrent side of groins faces enhanced erosion because sand
transport from upcurrent is halted.
Seawalls are structures built parallel to the beach to
protect buildings. When storm waves strike a seawall, the unspent
wave energy is reflected back offshore. This is good for the
building, but that extra energy carries sand offshore. Result: the
beach is gone. The seawall will eventually be undermined and the
building washed away if the sand is not replenished. In the meantime
there is no beach for recreation.
Glaciers
Where summer melting is less than the winter
snowfall, the annual addition of snow results in the growth of a
glacier. Snow is "fluffy" but its frilly appendages are broken
through blowing, partial melting and refreezing, and through
compaction, as more layers of snow are added above. Through these
processes snowflakes become ice granules called firn.
As time passes and compaction continues, the firn recrystallizes into
solid ice - an interlocking network of ice crystals (like an igneous
texture). Glacial ice is blue.
Ice is brittle (breaks when under stress) at its surface, but under
pressure (under 50 meters of ice) it behaves plastically (it flows
under stress). Glaciers are flowing streams of ice.
The upper brittle surface of a glacier forms large open cracks known
as crevasses as the glacier bends to flow over a bump
in the bedrock.
Grit and gravel and even large boulders are incorporated into the
base of a glacier which grind away at the bedrock over which the
glacier flows, resulting in glacial
striations. Glacial striations show the direction
the glacier flowed.
At the end of a glacier, where it is melting as fast as it is being
supplies by ice from upstream, large quantities of unsorted sediments
(clay, silt, sand, gravel, boulders) are heaped into
moraines.
During glacial epochs like the last few million years,
continental ice sheets advance and retreat from the
polar regions over time spans of tens of thousands of years. These
glacial cycles are caused by variations of the Earth's orbit around
the sun which changes the amount of solar radiation coming into the
Earth at high latitudes during the summer. Continental ice sheets
leave behind features such as drumlins,
eskers, and kettle lakes.
Apine glaciers descend from high, cold mountain peaks
cutting deep U-shaped valleys. At the head of the glacier a deep
bowl, called a cirque, is cut by the grinding action of
the glacier. Many cirques are filled by small lakes called
tarns when the glaciers melt Hanging
valleys, many with beautiful waterfalls, are formed where
smaller tributary glaciers once fed into large, deep-cutting,
glaciers.
Aeolian (Wind) Processes
and Deserts
In arid regions the soil/sediment is dry so
there is no cohesion between particles and there is little vegetation
to cover and hold the particles in place. The wind is then a very
important agent for transporting and depositing sediments.
The wind can bounce sand along the surface in a process called
saltation. Sand is blown up a shallow incline on the
windward face of a sand dune and then is
deposited on the steep slip face of the dune away from
the wind.
Gravel and rocks are not moved by the wind and remain behind as a
desert pavement. The process of
removing the clay, silt, and sand and leaving behind the rocks is
called deflation.
Sandy deserts are called ergs. Rocky deserts are called
regs.
The wind can suspend fine clay and silt particles as windblown
dust, perhaps as dust storms. Downwind deposits of windblown dust are
called loess.