Erosion encompasses a group of processes that
cause Earth material to be loosened, dissolved, abraded, or
worn away and moved from one place to another. These processes
include weathering, dissolution, corrosion, and transportation.
There are two main categories of weathering:
physical and chemical processes. Physical processes break
down bedrock by mechanical action of agents such as moving
water, wind, freeze-thaw cycles, glacial action, forces of crystallization
of ice and other minerals, and biological interactions
with bedrock such as penetration by roots. Chemical
weathering includes the chemical breakdown of bedrock in
aqueous solutions. Erosion occurs when the products of
weathering are loosened and transported from their origin to
another place, most typically by water, wind, or glaciers.
Water is an extremely effective erosional agent, especially
when it falls as rain and runs across the surface in finger-sized
tracks called rivulets, and when it runs in organized streams
and rivers. Water begins to erode as soon as raindrops hit a
surface—the raindrop impact moves particles of rock and soil,
breaking it free from the surface and setting it in motion. During
heavy rains, the runoff is divided into overland flow and
stream flow. Overland flow is the movement of runoff in
broad sheets. Overland flow usually occurs through short distances
before it concentrates into discrete channels as stream
flow. Erosion performed by overland flow is known as sheet
erosion. Stream flow is the flow of surface water in a welldefined
channel. Vegetative cover strongly influences the erosive
power of overland flow by water. Plants that offer thicker
ground cover and have extensive root systems prevent erosion
much more than thin plants and those crops that leave
exposed barren soil between rows of crops. Ground cover
between that found in a true desert and in a savanna grassland
tends to be eroded the fastest, while tropical rainforests
offer the best land cover to protect from erosion. The leaves
and branches break the force of the falling raindrops, and the
roots form an interlocking network that holds soil in place.
Under normal flow regimes streams attain a kind of equilibrium,
eroding material from one bank and depositing on
another. Small floods may add material to overbank and
floodplain areas, typically depositing layers of silt and mud
over wide areas. However, during high-volume floods, streams
may become highly erosive, even removing entire floodplains
that may have taken centuries to accumulate. The most
severely erosive floods are found in confined channels with
high flow, such as where mountain canyons have formed
downstream of many small tributaries that have experienced a
large rainfall event. Other severely erosive floods have resulted
from dam failures, and in the geological past from the
release of large volumes of water from ice-dammed lakes
about 12,000 years ago. The erosive power of these floodwaters
dramatically increases when they reach a velocity known
as supercritical flow, at which time they are able to cut
through alluvium like butter and even erode bedrock channels.
Luckily, supercritical flow can not be sustained for long
periods of time, as the effect of increasing the channel size
causes the flow to self-regulate and become subcritical.
Cavitation in streams can also cause severe erosion. Cavitation
occurs when the stream’s velocity is so high that the
vapor pressure of water is exceeded and bubbles begin to
form on rigid surfaces. These bubbles alternately form and
then collapse with tremendous pressure, and they form an
extremely effective erosive agent. Cavitation is visible on
some dam spillways, where bubbles form during floods and
high discharge events, but it is different from the more common,
and significantly less erosive phenomenon of air entrapment
by turbulence, which accounts for most air bubbles
observed in white-water streams.
Wind is an important but less effective erosional agent
than water. It is most important in desert or dry environments,
with exposed soil or poor regolith. Glaciers are powerful
agents of erosion and are thought to have removed
hundreds of meters from the continental surfaces during the
last ice ages. Glaciers carve deep valleys into mountain ranges
and transport eroded sediments on, within, and in front of
glaciers in meltwater stream systems. Glaciers that have layers
of water along their bases, known as warm-based glaciers,
are more effective erosional agents than cold-based glaciers
that do not have any liquid water near their bases. Coldbased
glaciers are known from Antarctica.
Mass wasting is considered an erosional process in most
definitions, whereas others recognize that mass wasting significantly
denudes the surface but classify these sudden
events separately. Mass wasting includes the transportation
of material from one place to another, so it is included here
with erosional processes. Most mass-wasting processes are
related to landslides, debris flows, and rock slides and can
significantly reduce the elevation of a region, typically occurring
in cycles with intervals ranging from tens to tens of
thousands of years.
Humans are drastically altering the planet’s landscape,
leading to enhanced rates of erosion. Cutting down forests
has caused severe soil erosion in Madagascar, South America,
the United States, and many other parts of the world. Many
other changes are difficult to quantify. Urbanization reduces
erosion in some places but enhances it elsewhere. Damming
of rivers decreases the local gradient, slowing erosion in
upland areas but prevents replenishment of the land in downstream
areas. Agriculture and the construction of levees have
changed the balance of floodplains. Although difficult to
quantify, it is estimated that human activities in the past couple
of centuries have increased erosion rates on average from
five times to 100 times previous levels.
See also DESERT; GLACIER; MASS WASTING; WEATHERING.
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