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)
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
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.
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.
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.
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.
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.
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.