Unit 9 Weathering, Erosion, and Deposition
Weathering, erosion, and deposition act together in a cycle that wears
down and builds up the Earth’s surface. Weathering is the breaking down
and wearing away of rocks by water, wind, ice, and plants. Erosion is the
movement of sediment by wind, water, or ice. Deposition is the process
by which wind, water, or ice drops (deposits) sediment in a new location
which builds up the Earth’s surface. The changes to the Earth’s surface
are slow changes that occur over a very long period of time.
Weathering
Sediments are naturally occurring material that is broken down by the
processes of weathering into smaller (tiny) and smaller pieces of rock. The
actual weathering of rock does not involve movement of these sediments.
Also weathering must occur before erosion can happen. There are many
agents of weathering including, water, wind, ice, and plant roots. For
example, as a tree grows, its roots grow deeper and spread out more. It
then encounters a rock. On a tiny crack in the rock the root grows and
pushes its way into the crack. It makes the crack bigger and bigger until
the rock splits and becomes two smaller rocks. This rock can go through
this many times as it gets smaller and smaller.
Erosion
Erosion is the movement of weathered earth materials (sediments) by
moving water, wind, or ice. Erosion can occur fast or slow, depending on
the speed and amount of these agents. Also, humans can impact the
erosion of earth’s surface. For example, if trees are cut down on a hill and
not replanted, water from rain will cause erosion on the side of the
hill. Without the roots of the trees, there is nothing to hold the soil on the
hill.
Deposition
Deposition is the final part of the cycle in which the sediments that are
being transported are dropped (deposited) in a new location. This happens
when the agent (wind, water, ice) no longer has the ability to move the
sediment. For example, a river may be fast moving and narrow, but then
widens out and become slower, which then deposits sediments that are too
heavy for the slower moving water to keep pushing.
Agents of Weathering, Erosion, and Deposition
Wind
Changes to earth’s surface can be caused by wind. Wind can erode,
transport, or deposit fine particles of sediment in areas where there is little
vegetation to hold the sediment together. When wind blows particles of
sand or other sediment against larger rocks, it scours them. Over a long
period of time this will cause the wearing away of these larger rocks. The
wind can also transport sediments through the air. How far the sediments
travel will depend on the strength of the wind, how long it blows, and what
type of vegetation is around. Once the wind dies down or if vegetation
stops or slows the wind, the particles of sediment will begin to drop.
Water
Another agent that can erode, transport, or deposit sediment is
water. Flowing water is a major agent of erosion. As water flows, it carries
sediment and pieces of rock. These particles rub against other rocks,
changing their shapes. Eventually, the sediment and rock are deposited as
the water slows. Faster moving water causes more weathering and
erosion than slower moving
water.
A canyon is a valley with very steep sides that usually has been carved by
a fast-flowing river. Gravity causes rivers to flow from a higher elevation to
a lower elevation. The larger the difference in elevations, the more energy
the river expends on its journey. This energy of moving water carves
through the weaker rock to form a canyon.
Deltas are formed when a river carrying sediment reaches a lake, reservoir,
or ocean and the velocity of the water is slowed. The slower water carries
less sediment and the larger particles start to fall to the bottom. The
smaller particles are carried further into the body of standing water. This
deposition forms the delta.
Water also changes the Earth by creating caves. Dissolved minerals in
water seep into the ground, causing natural hollows to form. Dripping
water in caves creates interesting formations, such as stalactites and
stalagmites.
Ice
Ice is the last major agent of weathering, erosion, and deposition. Water
often seeps into the cracks in rocks. When the temperature drops, the
water in the cracks freeze and expand, causing the crack to
widen. Eventually, the rock is broken into smaller pieces.
Glaciers are large, flowing rivers of ice that carve U-shaped valleys. As
they move inch by inch they push many large particles, rocks, and even
mountains out of their way. When these glaciers retreat, they deposit these
same rocks and sediment that have been carried by the glacier. Meltwater
from these glaciers often form lakes.
Unit 9 Vocabulary
canyon - a valley between steep cliff sides formed by running water,
such as a river or stream
cave - a natural, hollow, underground space
delta - soil which is deposited at the mouth or a river, usually in a triangular
shape
deposition - a process by which wind or water drop sediment in a new
location
erosion - the movement of sediment by wind, water, or ice
formation - the way something is structured or created
glacier - a large mass of slowly moving ice and snow that carves new
features and deposits sediments
landform - a physical feature on Earth's surface
sand dune - a hill of sand created by the wind
stalactite - a cave formation which grows downward from the ceiling,
created
by mineral deposits from dripping water
stalagmite - a cave formation which grows upward from the ground,
created
by mineral deposits from dripping water
valley - low lying land between mountains
weathering - the breaking down and wearing away of rocks by water, wind,
and ice