Weathering
1. Any process that causes rock to crack or break into pieces is called physical
weathering. Initial product = final product
End Result of physical weathering is increased surface area.
2. Physical weathering of rock may occur as the result of alternate freezing and
melting or water, a process called frost action.
3. Physical weathering may occur when cracks in rocks are widened by the forces
exerted by growing plants.
4. Abrasion is a physical weathering process that occurs when rocks grind against
one another. Abrasion occurs when sediments are moved by ice, gravity, running
water, or wind.
5. Physical weathering may occur when rocks split along planes of weakness when
the pressure of overlying material is removed - unloading. (exfoliation - peeling
away)
6. Chemical weathering changes the chemical composition of the minerals in rocks
and usually weakens the rock as a result of these changes. Initial product does not
equal final product.
7. The agents of chemical weathering include oxygen, water, carbon dioxide, and
acids.
8. Water is a major factor in chemical weathering because it speeds up chemical
reactions, dissolves many materials, and reacts directly with many mineral
substances.
9. Some of the substances produced by chemical weathering are soluble in water
and break down into ions (charged particles) when they dissolve. All surface and
ground water contains the ions of dissolved minerals.
10. The rate of weathering is affected by climate, the size of the particles being
weathered, and the type of material being weathered. Particle size and hardness of
minerals affects weathering rates. Smaller particles will weather faster. Harder
minerals will resist weathering.
11. At high latitudes (and high altitudes) frost action is the major form of
weathering. In warm climates, chemical weathering is most important. However,
low humidity will slow the rate of weathering in both cases.
12. Local climatic conditions - winds, nearness to cities, etc - can affect the
expected weathering rate for a region.
13. The end result of all weathering processes is sediment. A soil profile shows
the distinct layers in the soil, which differ in color, texture and mineral
composition. (Mature soil layers: Topsoil, subsoil, partly weathered, unweathered
rock.)
14. Soil profiles develop as a result of weathering processes and biologic activity.
Erosion
1. Sediments that have been moved into a region from another place by natural
processes are called transported sediments.
2. Sediments that formed in their present location are called residual sediments.
3. The removal and transport of sediments is called erosion.
4. Transported sediments are much more common than residual sediments and are
evidence of erosion over large portions of the earth's surface.
5. Gravity is the main driving force of all erosional processes.
6. Gravity may act alone as an erosional agent, producing such effects as
landslides, slumping, and hillside creep. This is called mass wasting.
7. Running water is the predominant agent of erosion on the earth.
8. Every stream and its branches make up a single system that collects all the
runoff with a definite area called the drainage basin of the system.
9. A stream system consists of running water, the land surface it drains, the
sediment it transports, and the potential energy used to drive it.
10. As the slope of a stream bed increases, the average velocity of the stream
increases.
11. An increase in the discharge of a stream increases its average velocity.
12. Streams transport sediments as ions in solution, as suspended matter, and by
the bouncing and rolling of particles along the stream bed.
13. The size of the sediments that a stream can transport increases as the stream
velocity increases.
14. The total amount of sediment that a stream can transport increases as its
discharge increases.
15. Youthful streams carrying sediments down steep gradients can cut through
solid bedrock.
16. When youthful, streams have V-shaped valleys.
17. In the stage of maturity the valley of a stream widens. The stream ceases to
cut through the bedrock.
18. In old age, the stream develops a wide flood plain, across which it wanders
seeking the lowest elevation in a series of curves, or meanders. The outside curve
of the meander (fastest velocity) is usually a cut bank due to erosion. The inside
curve of the meander (slowest velocity) is usually a sand bar due to deposition. In
straight channels, the stream's velocity is greatest in the middle just below the
surface.
19. Moving ice, called glaciers, can erode rock and transport sediments. Erratics
are rocks that have been transported and do not match the bedrock beneath
them. Glaciers form U-shaped valleys. Valley glaciers move fastest in the middle
top and slowest at the sides and bottom because of friction with the walls.
Glaciers are bodies of ice that form from the accumulation of snow and advance
because of gravity. Snow accumulating on the ground (firn) is a sedimentary rock
that coverts to glacial ice, a metamorphic rock, by compaction (pressure) and
recrystallization (heat).
Glaciers exist as either valley glaciers, occupying old stream valleys at high
elevations, or continental glaciers, ice sheets covering wide continental areas at
lower elevations.
The zone of accumulation lies above the snow equilibrium line and has perennial
snow cover, while ice is lost in the zone of ablation, below the snow equilibrium line,
by evaporation, melting or calving at the terminus. Advancing glaciers have a
positive ice budget (accumulation > ablation - wasting away), while retreating
glaciers have a negative ice budget (accumulation < ablation).
Rock fragments transported at the base of a glacier are effective erosional
agents. Erosion by valley glaciers produces most of the scenery in mountainous
regions. Striated and groove bedrock is a common erosional feature of both alpine
and continental glaciation. Valley glaciers produce U-shaped valleys, truncated
spurs, hanging valleys and rock-basin lakes as they modify old stream valleys.
Cirques, horns and arêtes are formed by valley glaciers in combination with
weathering and erosion.
Glacial deposits are either unsorted, unlayered, angular rock debris called till, or
sorted, layered material called outwash that is deposited by meltwater. Moraines
are ridges of till formed by valley and continental glaciers while the terminus is
stationary. Both can form end, terminal, recessional, and ground moraines. Valley
glaciers also form lateral and medial moraines because of rockfalls along their
edges and convergence into trunk glaciers. Continental glaciers deposit drumlins,
spoon-shaped hills of till with a gentle dip in the direction of ice movement.
Outwash plains form in front of a glacier from meltwater.
Other depositional features associated with continental glaciers are kettles, rock
flour and loess, Much of the current landscape is the result of glacial effects.
Scoured bedrock in Canada and thick tills in the central United States were
directly caused by continental glaciation. The Great Lakes were eroded by
continental glaciers as well. As glaciers advanced during the Ice Ages, sea level
fell, causing fiords along the coastlines of Alaska, Norway and elsewhere. Uplifted
and tilted shorelines around the Great Lakes indicate crustal rebound from removal
of the weight of continental ice sheets.
20. Wind can transport sediment of sand size or smaller and can produce
distinctive features by deflation and abrasion - mostly in arid regions.
Sandblasting occurs when winds blow sand or silt grains against rocks and other
objects. Deflation removes sand and lowers the land surface.
21. Wave action erodes shoreline rocks and beaches - cliffs that are cut by waves
and spits caused by long shore current. Long shore current carries large amounts
of sediment and it always flows parallel to the shore. Most waves strike the
coastline at an angle and these bent waves usually result in the long shore current
flowing along the shore in one direction. Sand bar deposits and spits will grow in
the direction of the long shore current.
Each agent of erosion produces distinctive changes in the material it transports.
Water
Ice
Wind
Gravity
rounded
angular to sub
rounded
sorted - cross
bedding
angular
smooth
unsorted - till
frosted and pitted
unsorted
sorted vertical/horizontal
striations, polished sand and silt
V-shaped valleys
U-shaped valleys
mushroom shaped
rocks
talus