Weathering, Erosion & Deposition
Weathering, erosion, and deposition are natural processes that can remove, transport, and/or
deposit sediment on the Earth's surface.
Weathering, Erosion & Deposition
Weathering is the breakdown of rock or soil. Physical weathering occurs when natural
forces, such as wind, running water, and moving ice, physically break up rock and soil into
smaller particles. For example, large moving masses of ice called glaciers grind rock up as
they slowly slide across it.Chemical weathering occurs when rock and soil break down as a
result of chemical reactions. For example, slightly acidic groundwater reacts with limestone to
break the rock down, leaving holes in the rock.
Over long periods of time, biological activity within some weathered materials produces new
soils. Other weathered materials are removed by natural forces and transported to new
locations, where they eventually form new landforms or become sedimentary rock.
Erosion is the transport of weathered materials from one place to another by natural
forces. For example, the force of running water in a river picks up weathered sediment and
carries it downstream. Very often, a natural force both weathers and erodes the same rock or
soil.
Deposition occurs when natural forces lay eroded sediment down in new places. For example,
desert winds that weather and erode sand from one place will deposit it to form sand dunes in
another place.
Natural Agents of Weathering, Erosion & Deposition
A natural agent of weathering, erosion, and deposition is any natural process that removes,
transports, and/or deposits sediment on Earth's surface. These include running water (streams
and rivers), moving ice (glaciers), ocean waves, wind, and mass movements. These agents are
all generally driven by gravity, but each produces distinctive changes in the materials it
transports and results in distinct landscapes.
Streams & Rivers
Overall, a stream or river erodes sediment upstream, carries it some distance, and deposits it
farther downstream. A given grain of sediment may be transported once, several times, or for
the entire length of the stream or river.During transport, the sediment becomes smoother and
more rounded through the process of abrasion (i.e. small pieces of the sediment are broken off
by collisions with bedrock and other sediment particles).
Erosion and deposition vary along the path of a given stream or river as well as between
different streams and rivers, especially those that differ in age.Generally, the source area of a
river or stream, as well as youthful rivers in general, tend to be dominated by erosion. These
settings are characterized by narrow, V-shaped channels, high stream gradients (steep slopes),
and high water velocities. The fast-moving water primarily cuts downward through the
bedrock instead of laterally. This process is called downcutting.
Youthful river with high stream gradient and V-shaped cross-section
On the other hand, deposition is more dominant in mature river systems. This may also be case
for parts of a younger river or stream that are closer to the channel mouth. In these settings,
channels tend to be wider, gradients are lower (gentler slopes), and water velocities are
lower. Mature river channels often form broad curves called meanders as the river traverses
back and forth across a plain. Erosion tends to occur along the outside of meanders, which is
called the cut bank, while deposition tends to occur along the inside of meanders, where it
forms deposits of gravel called point bars. Over time, these patterns cause the positions of
meanders to migrate toward the cut bank.
Deposited sediment sometimes builds up as islands in the slow-moving current as well. During
times of flooding, water flows over the banks of the river and deposits mud and silt-sized
sediment to form flood plains along either side of the river.
Mature, low-gradient river with meanders in broad, flat valley
When a river flows into the ocean or other large body of water, it suddenly loses a great deal
of its kinetic energy. As a result, most of the sediment that the river is still carrying gets
dumped at its mouth. Over time, this sediment builds up above water to form a depositional
feature called a delta.
Glaciers
Erosion by glaciers forms valleys that have a U-shape in cross section. When geologists find
U-shaped valleys, they know that they were shaped by glaciers in the past, even if the glaciers
are no longer present. Glaciers also tend to smooth the bedrock beneath them and along their
sides as they slide downhill due to gravity. Sometimes, rocks trapped within the ice cut
parallel striations (scratches and grooves) into the bedrock as the ice moves along.
Glacial striations in bedrock
Image courtesy of Wikimedia Commons
Depositional features formed by glaciers include end moraines, till, drumlins, outwash plains,
stratified drift, and kettles. An end moraine is a deposit of sediment that forms at the terminus
(front edge) of a glacier when it pauses during advance or retreat. Till is a more general term
referring to sediment deposit directly by glacial ice as it melts. Sometimes, glaciers will later
move over previously deposited till and mold it into streamlined hills called drumlins.These
hills are steeper on the upslope side and are parallel to the glacier's direction of
movement. An outwash plain refers to the broad, flat area in front of a glacier where large
volumes of sediment are deposited by glacial meltwater. The sediment deposited in this area is
called stratified drift.Sometimes, large chunks of ice can get buried in stratified drift. After the
ice melts, a large depression called a kettle is left behind. Freshwater often fills in these
depressions to form kettle lakes.
Ocean Waves
Ocean waves shape the shoreline in many ways. The repeated crashing of waves can erode
rock into cliffs, sea stacks, and other erosional features. A familiar depositional feature is a
beach, which is made of sand deposited by waves. In areas where waves approach the
shoreline at an angle, sediment is gradually transported in a direction parallel to the
shore. This process, calledlongshore drift, can result in landforms such as spits and barrier
islands.
Sea stack along coast of Ontario, Canada
Image courtesy of Wikimedia Commons
Wind
Wind erosion is most common in arid regions, where a lack of vegetation exposes sediment,
and along coastal regions, where winds are particularly active. In some desert settings, the
wind erodes bedrock by sandblasting tiny sand grains onto it. This can produce streamlined
landforms called yardangsas well as a wide variety of textures on the bedrock.
Yardangs and unique rock textures—both from wind erosion
Dunes are hills formed by the deposition of sand by the wind. Dunes come in a variety of
shapes and are common in deserts and along coastal areas.
Dunes (foreground) in Death Valley
Image courtesy of USGS
Mass Movements
A mass movement is a mass of Earth materials moving downslope, predominantly under the
influence of gravity. There are many different kinds of mass movement. Some are dry, while
others have a lot of water inside them.Some move quickly, while others move slowly. Some
mass movements slide downhill as a large chunk, while others have particles that flow
randomly within the mass. Mudslides, rockfalls, avalanches, and soil creep are just a few
examples of the many different kinds of mass movement. As shown in the image below, mass
movements can threaten humans and their property.
Mass movement along coast of California