WEATHERING AND EROSION: An Exogenic Processes

Weathering describes the breaking down

or dissolving of rocks and minerals on the
surface of the Earth. Water, ice, acids, salts,
plants, animals, and changes
in temperature are all agents of weathering.

Once a rock has been broken down, a

process called erosion transports the bits of
rock and mineral away. No rock on Earth is
hard enough to resist the forces of
weathering and erosion. Together, these
processes carved landmarks such as the
Grand Canyon, in the U.S. state of Arizona.
This massive canyon is 446 kilometers (277
The Grand Canyon in Arizona, USA. miles) long, as much as 29 kilometers (18
Source: www.grayline.com/ miles) wide, and 1,600 meters (1 mile)
deep.

Weathering and erosion constantly change the rocky landscape of Earth. Weathering happens in
situ (in place), that is, particles stay put and no movement is involved. As soon as the weathered
products start to move (due to fluid flow) the process of erosion happens. Weathering wears away
exposed surfaces over time. The length of exposure often contributes to how vulnerable a rock is
to weathering. Rocks, such as lavas, that are quickly buried beneath other rocks are less
vulnerable to weathering and erosion than rocks that are exposed to agents such as wind and
water.

Weathering, erosion, transportation and deposition are exogenic processes that act in concert,
but in differing relative degrees, to bring about changes in the configuration of the earth’s surface.

As it smoothens rough, sharp rock surfaces, weathering is often the first step in the production
of soils. Tiny bits of weathered minerals mix with plants, animal remains, fungi, bacteria, and other
organisms. A single type of weathered rock often produces infertile soil, while weathered materials
from a collection of rocks is richer in mineral diversity and contributes to more fertile soil. Soils
types associated with a mixture of weathered rock include glacial till, loess, and alluvial sediments.

Weathering is often divided into the processes of mechanical weathering and chemical
weathering. Biological weathering, in which living or once-living organisms contribute to
weathering, can be a part of both processes.

Mechanical Weathering

Mechanical weathering, also called physical weathering

and disaggregation, causes rocks to crumble.

Water, in either liquid or solid form, is often a key agent

of mechanical weathering. For instance, liquid water can
seep into cracks and crevices in rock. If temperatures drop
low enough, the water will freeze. When water freezes,
it expands. The ice then works as a wedge. It slowly
widens the cracks and splits the rock. When ice melts,
liquid water performs the act of erosion by carrying away the tiny rock fragments lost in the split.
This specific process (the freeze-thaw cycle) is called frost weathering or cryofracturing while
others use the term frost wedging.

Temperature changes can also contribute to mechanical weathering in a process called thermal
stress. Changes in temperature cause rock to expand (with heat) and contract (with cold). As this
happens over and over again, the structure of the rock weakens. Over time, it crumbles. Rocky
desert landscapes are particularly vulnerable to thermal stress. The outer layer of desert rocks
undergo repeated stress as the temperature changes from day to night. Eventually, outer layers
flake off in thin sheets, a process called exfoliation.

Exfoliation contributes to the formation of bornhardts, one of the most dramatic features in
landscapes formed by weathering and erosion. Bornhardts are tall, domed, isolated rocks often
found in tropical areas. Sugarloaf Mountain, an iconic landmark in Rio de Janeiro, Brazil, is a
bornhardt.

Changes in pressure can also contribute to exfoliation due to weathering. In a process called
unloading, overlying materials are removed. The underlying rocks, released from overlying
pressure, can then expand. As the rock surface expands, it becomes vulnerable to fracturing in a
process called sheeting.

Another type of mechanical weathering occurs when clay or other materials near rock absorb
water. Clay, more porous than rock, can swell with water, weathering the surrounding, harder
rock.

Salt also works to weather rock in a process

called haloclasty. Saltwater sometimes gets into the cracks
and pores of rock. If the saltwater evaporates, salt crystals
are left behind. As the crystals grow, they put pressure on
the rock, slowly breaking it apart.

Honeycomb weathering is associated with haloclasty. As

its name implies, honeycomb weathering describes rock
formations with hundreds or even thousands of pits formed
by the growth of salt crystals. Honeycomb weathering is
common in coastal areas, where sea sprays constantly
force rocks to interact with salts. Haloclasty is not limited
to coastal landscapes. Salt upwelling, the geologic process
Honeycomb weathering in in which underground salt domes expand, can contribute to
a Cambrian sandstone, Timna weathering of the overlying rock.
Valley, Negev Desert, Israel.
Abrasion is another process of weathering where the
friction caused by scuffing, scratching, wearing down, marring, and rubbing away of materials. The
intensity of abrasion depends on the hardness, concentration, velocity and mass of the moving
particles. Abrasion generally occurs when the glaciation slowly grinds rocks picked up by ice
against rock surfaces; a solid objects transported in river channels make abrasive surface contact
with the bed and walls; objects transported in waves breaking on coastlines cause abrasion; and,
finally, abrasion can be caused by wind transporting sand or small stones against surface rocks.

Plants and animals can be agents of mechanical weathering. The seed of a tree may sprout in
soil that has collected in a cracked rock. As the roots grow, they widen the cracks, eventually
breaking the rock into pieces. Over time, trees can break apart even large rocks. Even small
plants, such as mosses, can enlarge tiny cracks as they grow.
Animals that tunnel underground, such as moles and prairie dogs, also work to break apart rock
and soil. Other animals dig and trample rock aboveground, causing rock to slowly crumble.

Chemical Weathering

Chemical weathering changes the molecular structure of rocks and soil.

For instance, carbon dioxide from the air or soil

sometimes combines with water in a process
called carbonation. This produces a weak acid,
called carbonic acid, that can dissolve rock.
Carbonic acid is especially effective at
dissolving limestone. When carbonic acid seeps
through limestone underground, it can open up
huge cracks or hollow out vast networks
of caves. This process is referred to as
dissolution.

Carlsbad Caverns National Park, in the U.S. state

of New Mexico, includes more than 119 limestone
caves created by weathering and erosion. The Carlsbad Caverns National Park is a United States
largest is called the Big Room. With an area of National Park in the Guadalupe Mountains in
about 33,210 square meters (357,469 square southeastern New Mexico.
feet), the Big Room is the size of six football
fields.

Another type of chemical weathering works on

rocks that contain iron. These rocks turn
to rust in a process called oxidation. Rust is a
compound created by the interaction of oxygen
and iron in the presence of water. As rust
expands, it weakens rock and helps break it
apart.

Hydration is a form of chemical weathering in

which the chemical bonds of the mineral are
changed as it interacts with water. One instance
of hydration occurs as the
mineral anhydrite reacts with groundwater. The Contrast of concave out flared slopes and open mouthed
water transforms anhydrite into gypsum, one of caves. Photo: Shargaljut, Dreamstime.
the most common minerals on Earth.

Another familiar form of chemical weathering is hydrolysis. In the process of hydrolysis, a

new solution (a mixture of two or more substances) is formed as chemicals in rock interact with
water. In many rocks, for example, sodium minerals interact with water to form a saltwater
solution.

Hydration and hydrolysis contribute to flared slopes, another dramatic example of a landscape
formed by weathering and erosion. Flared slopes are concave rock formations sometimes
nicknamed ―wave rocks.‖ Their c-shape is largely a result of subsurface weathering, in which
hydration and hydrolysis wear away rocks beneath the landscape’s surface.

Living or once-living organisms can also be agents of chemical weathering. The decaying remains
of plants and some fungi form carbonic acid, which can weaken and dissolve rock. Some bacteria
can weather rock in order to access nutrients such as magnesium or potassium. Clay minerals,
including quartz, are among the most common byproducts of chemical weathering. Clays make up
about 40% of the chemicals in all sedimentary rocks on Earth.

Weathering and People

Weathering is a natural process, but human activities can speed it up.

For example, certain kinds of air pollution increase the rate of weathering. Burning coal, natural
gas, and petroleum releases chemicals such as nitrogen oxide and sulfur dioxide into
the atmosphere. When these chemicals combine with sunlight and moisture, they change into
acids. They then fall back to Earth as acid rain. Acid rain rapidly weathers limestone, marble, and
other kinds of stone. The effects of acid rain can often be seen on gravestones, making names and
other inscriptions impossible to read.

Factors that affects the type, extent and rate of weathering

There are five factors that affects weathering namely; climate, rock type, rock structure,
topography and time. Areas that are cold and dry tend to have slow rates of chemical weathering
and weathering is mostly physical. However, chemical weathering is most active in areas with high
temperature and rainfall. The rock type especially its mineral composition has different
susceptibility to weathering. Those that are most stable to surface conditions will be the most
resistant to weathering. Thus, olivine for example which crystalizes at high temperature conditions
will weather first than quartz which crystalizes at lower temperature conditions.

The rock’s structure also affects its weathering rate, those with joints, folds, faults, bedding planes
can be more prone to weathering agents. Highly jointed or fractured rocks disintegrate faster than
a solid mass of rock of the same dimension. The topography of the rock also adds vulnerability
since weathering occurs more quickly on a steep slope than on a gentle one. Finally, the length of
time of exposure to agents of weather determines the degree of weathering of a rock.

Erosion is the geological process in which earthen materials are worn away and transported by
natural forces such as wind or water. A similar process, weathering, breaks down or dissolves rock,
but does not involve movement.

Erosion is the opposite of deposition, the

geological process in which earthen
materials are deposited, or built up, on
a landform.

Most erosion is performed by liquid water,

wind, or ice (usually in the form of
a glacier). If wind is dusty, or water or
glacial ice is muddy, erosion is taking place.
The brown color indicates that bits of rock
and soil are suspended in the fluid (air or
water) and being transported from one
place to another. This transported material
is called sediment.
Soil Erosion in Benguet, Philippines
Photo Credit: CNN Philippines Physical and Chemical Erosion

The process of erosion is often broken down into two forms: physical erosion and chemical erosion.
They often work together, as well as with other geological processes such as weathering and
sedimentation.
Physical erosion describes the process of rocks changing their physical properties without
changing their basic chemical composition. Physical erosion often causes rocks to get smaller or
smoother. Rocks eroded through physical erosion often form clastic sediments. Clastic sediments
are composed of fragments of older rocks that have been transported from their place of origin.

Landslides and other forms of mass wasting are associated with physical weathering. These
processes cause rocks to dislodge from hillsides and crumble as they tumble down a slope.

Plant growth can also contribute to physical erosion in a process called bioerosion. Plants break up
earthen materials as they take root, and can create cracks and fractures in rocks they encounter.

Ice and liquid water can also contribute to physical erosion as their movement forces rocks to
crash together or crack apart. Some rocks shatter and crumble, while others are worn away. River
rocks are often much smoother than rocks found elsewhere, for instance, because they have been
eroded by constant contact with other river rocks.

Chemical erosion describes the process of rocks changing their chemical composition as they
erode. Chemical erosion almost always refers to rocks interacting and undergoing a chemical
reaction with water.

The most familiar form of chemical erosion is probably rust, the product of a process
called oxidation. During oxidation, rocks interact with oxygen in the presence of water. The
amount of water required for oxidation is minimal, often the amount of water present in
the atmosphere. Iron is the most familiar mineral to undergo oxidation and rust.

Carbonation is another form of chemical erosion. During carbonation, rocks interact with carbon
dioxide in the presence of water. In rocks such as chalk, carbonation can create a weak acid
(carbonic acid) that erodes the surface of the rock.

Hydration is a form of chemical erosion in which the chemical bonds of the mineral are changed
as it interacts with water. One instance of hydration occurs as the mineral anhydrite reacts
with groundwater. The water transforms anhydrite into gypsum, one of the most common minerals
on Earth.

Another familiar form of chemical erosion is hydrolysis. In the process of hydrolysis, a

new solution (a mixture of two or more substances) is formed as chemicals in rock interact with
water. In many rocks, for example, sodium minerals interact with water to form a saltwater
solution.

Erosion by Running Water

Liquid water is the major agent of erosion on Earth. Rain, rivers, floods, lakes, and the ocean carry
away bits of soil and sand, and slowly wash away the sediment.

Rainfall produces four types of soil erosion: splash erosion, sheet erosion, rill erosion, and gully
erosion.
 Splash erosion describes the impact of a falling raindrop, which can scatter tiny soil
particles as far as 2 feet.
 Sheet erosion describes erosion caused by runoff.
 Rill erosion describes erosion that takes place as runoff develops into discrete streams (rills
- very small stream).
 Finally, gully erosion is the stage in which soil particles are transported through
large channels. Gullies carry water for brief periods of time during rainfall or snowmelt, but
appear as small valleys or crevasses during dry seasons.
There are two factors that affects stream erosion and deposition. The first is velocity which
indicates the ability of the stream to erode and transport, controlled by gradient, channel size and
shape, channel roughness and the amount of water flowing in the channel. Secondly the
discharge, which is the volume of water passing through a cross-section of as stream during
given time, as the discharge increases, the width of the channel, the depth of flow, or flow velocity
increases individually or simultaneously.

Erosion due to running water maybe vertical (down cutting), lateral or headward erosion. Streams
transport their sediments in three ways; in solution (dissolved load) in suspension (suspended
load), sliding and rolling along the bottom (bed load). A stream’s ability to transport solid particles
is described by: competence (size of the largest particle that can be transported by the stream)
and capacity (maximum load a stream can transport under given conditions). Deposition occurs
when a river loses its capacity to transport sediments.

Valley erosion is the process in which rushing streams and rivers wear away their banks, creating
larger and larger valleys. Other erosional landforms are waterfalls, potholes, terraces, gully/rills,
and oxbow lakes. Depositional landforms are alluvial fans/cones, natural levees and deltas.

The ocean is a huge force of erosion. Coastal erosion—the wearing a way of rocks, earth, or sand
on the beach—can change the shape of entire coastlines. During the process of coastal erosion,
waves pound rocks into pebbles and pebbles
into sand. Waves and currents sometimes
transport sand away from beaches, moving the
coastline farther inland. Coastal erosion can
have a huge impact on human settlement as
well as coastal ecosystems.

The battering force of ocean waves also erodes

seaside cliffs. The action of erosion can create
an array of coastal landscape features. For
example, erosion can bore holes that
form caves. When water breaks through the
back of the cave, it can create an arch. The
continual pounding of waves can cause the top
of the arch to fall, leaving nothing but rock
columns called sea stacks. The seven Twelve Apostles Marine National Park, in Victoria,
remaining sea stacks of Twelve Apostles Marine Australia
National Park, in Victoria, Australia, are among
the most dramatic and well-known of these features of coastal erosion.

The erosion caused by oceanic waves can cause wave-cut cliff, wave-cut platform, marine terrace,
headland, stacks and sea arches while the depositional features may include beach, spit, baymouth
bar, tombolo and barrier islands.

Erosion by Wind
Wind is a powerful agent of erosion. Aeolian (wind-driven) processes constantly transport dust,
sand, and ash from one place to another. Wind can sometimes blow sand into towering dunes.
Some sand dunes in the Badain Jaran section of the Gobi Desert in China, for example, reach more
than 400 meters (1,300 feet) high.

In dry areas, windblown sand can blast against rock with tremendous force, slowly wearing away
the soft rock. It polishes rocks and cliffs until they are smooth. Wind can also erode material until
little remains at all.
Wind erodes by deflation (removal of loose, fine particles from the surface), and abrasion
(grinding action and sandblasting). Deflation results in features such as blowout and desert
pavement. Abrasion yields ventifacts and
yardangs. Ventifacts are rocks that have been
sculpted by wind erosion. The enormous chalk
formations in the White Desert of Egypt are
ventifacts carved by thousands of years of wind
roaring through the flat landscape.

Erosion by Ice

Ice, usually in the form of glaciers, can erode the

earth and create dramatic landforms. In frigid
(very cold) areas and on some mountaintops,
glaciers move slowly downhill and across the land.
As they move, they transport everything in their
path, from tiny grains of sand to huge boulders. A
glacier is a moving body of ice on land that moves
A ventifact. Impressively carved mushrooms are the downslope or outward from an area of
centerpiece of White Desertnational park, Egypt accumulation.

There are three types of glaciers namely, (a) valley (alpine) glaciers that are bounded by valleys
and tend to be long and narrow, (b) ice sheets (continental glaciers) that cover large areas of the
land surface and unconfined by topography
(Modern ice sheets cover Antarctica and
Greenland) and (c) ice shelves are sheets of ice
floating on water and attached to the land (they
usually occupy coastal embayment).

Rocks carried by glaciers scrape against the

ground below, eroding both the ground and the
rocks. In this way, glaciers grind up rocks and
scrape away the soil. Moving glaciers scratch
out basins and form steep-sided mountain valleys.
Eroded sediment called moraine (material, such
as earth, sand, and gravel, transported by a
glacier) is often visible on and around glaciers. Ross Sea Drift in Antarctica

All glacial deposits are called glacial drift, and are comprised of two types: (1) till, deposited
directly by ice, unsorted, and composed of many different particle sizes; and (2) stratified drift,
deposited by the glacial meltwater and thus has experienced the sorting action of water. As its
name suggests, deposits are layered and exhibit some degree of sorting.

Several times in Earth’s history, vast glaciers covered parts of the Northern Hemisphere. These
glacial periods are known as ice ages. Ice Age glaciers carved much of the modern northern North
American and European landscape.

Today, in places such as Greenland and Antarctica, glaciers continue to erode the earth. Ice sheets
there can be more than a mile thick, making it difficult for scientists to measure the speed and
patterns of erosion. However, ice sheets do erode remarkably quickly—as much as half a
centimeter (0.2 inch) every year.

Erosion by Groundwater
The main erosional process associated with groundwater is solution. Slow-moving groundwater
cannot erode rocks by mechanical processes, as a stream does, but it can dissolve rocks and carry
these off in solution. This process is particularly effective in areas underlain by soluble rocks, such
as limestone, which readily undergoes solution in the presence of acidic water. Rainwater reacts
with carbon dioxide from atmosphere and soil to form a solution of dilute carbonic acid. This acidic
water then penetrates through fractures and bedding planes, and slowly dissolves the limestone by
forming soluble calcium bicarbonate which is carried away in solution.

Karst topography is a distinctive type of landscape which develops as a consequence of

subsurface solution. It consists of an assemblage of landforms that is most common in carbonate
rocks, but also associated with soluble evaporate deposits.
 Cave/Cavern – forms when circulating groundwater at or below the water table dissolves 76
carbonate rock along interconnected fractures and bedding planes. A common feature
found in caverns is dripstone, which is deposited by the dripping of water containing
calcium carbonate. Dripstone features are collectively called speleothems, and include
stalactites, stalagmites, and columns.
 Sinkholes (Dolines) – circular depressions which form through dissolution of underlying
soluble rocks or the collapse of a cave’s roof.
 Tower karst – tall, steep-sided hills created in highly eroded karst regions.

Other Forces of Erosion

Thermal erosion
describes the erosion
of permafrost (permanently frozen
layer of the Earth's surface) along a
river or coastline. Warm temperatures
can cause ice-rich permafrost to break
off coastlines in huge chunks, often
carrying valuable topsoil and vegetation
with them. These eroded ―floating
islands‖ can disintegrate into the ocean,
or even crash into another piece of
land—helping spread new life to
Karst Topography
different landscapes.

Mass wasting describes the downward movement of rocks, soil, regolith and vegetation under the
direct influence of gravity. Mass wasting incidents include landslides, rockslides, and avalanches
(large mass of snow and other material suddenly and quickly tumbling down a mountain). Mass
wasting can erode and transport millions of tons of earth, reshaping hills and mountains and,
often, devastating communities in its path.

Factors that control mass wasting processes include; (a) as the slope angle increases, the
tendency to slide down the slope becomes greater, and (b) role of water which adds weight to
the slope, has the ability to change angle of repose, reduces friction on a sliding surface and water
pore pressure reduces shear strength of materials.

Difference between the Regolith and Soil

A regolith is a region of loose unconsolidated rock and dust that sits atop a layer of bedrock. On
Earth, regolith also includes soil, which is a biologically active medium and a key component in
plant growth. Regolith serves as a source of other geologic resources, such as aluminum, iron,
clays, diamonds, and rare earth elements. While soil is the biologically active, porous medium that
has developed in the uppermost layer of Earth’s crust. Soil is one of the principal substrata of life
on Earth, serving as a reservoir of water and nutrients, as a medium for the filtration and
breakdown of injurious wastes, and as a participant in the cycling of carbon and other elements
through the global ecosystem. It has evolved through weathering processes driven by biological,
climatic, geologic, and topographic influences.