Weathering and Erosion Outline
Weathering - Mechanical (physical) and chemical processes that change objects on Earth’s
surface over time. Weathering is responsible for limited changes in Earth’s surface – dissolving,
crumbling, or weakening a rock.
Erosion - Process by which rock and soil materials are loosened or dissolved and transported
from one place to another. Erosion is responsible for large scale changes in the form of the Earth.
Rivers, valleys and canyons are created by the erosion of weakened or disintegrated rock.
Together, weathering and erosion interact with each other to cause changes in Earth’s surface.
Weathering can gradually, over very long periods of time, weaken or wear away rocks. It can
take thousands or years. Erosion can happen gradually or rapidly, depending on the conditions
of the materials that are moved.
Deposition – the process whereby weathered and eroded sediments are released by their
transporting agents (dropped).
Water is a major factor in weathering. Rain makes mud splash and changes Earth’s surface in a
tiny way. As rain continues, puddles form, then water starts to flow along the surface, moving
downhill. Little paths started by gravity get deepened by running water into rills. Over time,
more water flows in the rills and the paths deepen into creaks, streams and finally, rivers. Water
continually digs deeper channels and carries more earth and rock from the shores and bottom.
Physical weathering - Disintegration or breaking of rock material into small fragments or pieces.
In physical weathering the chemical components of rock remain unchanged. Physical
weathering requires force.
Causes of physical weathering:
Freeze/thaw – freezing of water in cracks in rocks
Wedging (forcing rocks apart) by plant roots
Day/Night temperature fluctuations that cause rocks to crack
Loss of thin surface of rocks (by exfoliation or abrasion) exposing surfaces underneath
Gravity causing rocks to fall from heights and then breaking into pieces
Animals, including people, digging into rocks
Physical weathering exposes new surfaces and pieces. The new surfaces can be
chemically weathered.
Chemical weathering - Decomposition of rock material due to the chemical reaction of
compounds in the rock with chemicals in the environment. Not all rocks are composed of the
same chemicals or minerals, so chemical weathering affects different rocks differently.
Chemical weathering occurs at the surfaces of rocks. When physical weathering results in
freshly exposed rock surfaces, agents of chemical weathering have new material to
decompose.
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Rocks are made of different elements and minerals in different proportions. This makes
rocks different.
Chemical weathering occurs when the elements and minerals in rocks react with
chemicals from the environment (oxygen, water, carbonic acid, sulfuric acid). Different
elements and minerals react differently so different rocks weather differently. A specific
example of chemical weathering follows. You will not be tested on the specifics of these
reactions.
Carbonic acid is made when carbon dioxide in the air or produced by plant roots react
with water.
H2O
water
+

CO2
carbon dioxide
H2CO3
carbonic acid
Granite is a very common rock. The primary minerals in granite are feldspar and quartz.
Orthoclase found in feldspar reacts with carbonic acid as shown below.
2KAlSi3O8
orthoclase
+ H2CO3 +
carbonic acid
H2O
water

K2CO3 +
potassium
carbonate
Al2Si2O5(OH)4
kaolinite
+ 4SiO2
silica
Kaolinite is a component of clay and is insoluble. It remains as part of the surface layer that
covers the rock layer. Silica is partly soluble and can be partially washed away and deposited
elsewhere the rest stays with the clay. The potassium carbonate is soluble and can be washed
away or absorbed by plant roots. Plants use the potassium for growth.
Quartz crystals that are left when the feldspar reacts with carbonic acid do not react with the
acid. They are loosened and can be carried by wind and water. Quartz can pile up and form
sandstone which can be physically weathered into sand. The grains of sand are still made of
quartz crystals that came from the granite rock.
Marble is a metamorphic rock that made when limestone is subjected to pressure, heat, and time.
It consists mostly of calcium carbonate and includes small amounts of other minerals. such as
quartz.
Four factors affect the rate of chemical weathering.
1. Surface area – exposing more surface area will increase the rate of weathering
2. Particle size – larger particles weather slower and smaller particles weather at a
faster rate (larger particles have less surface area than the same particle broken into
smaller pieces)
3. Chemical composition – certain rocks and minerals are naturally weaker than others,
while others are more resistant to chemical reactions
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4. Climate – warmer, moister climates have the most weathering, heat and water speed
up all chemical reactions
Chemical weathering and physical weathering work together. A rockslide (physical
weathering) is the result of chemical weathering softening and weakening the rock.
Erosion occurs when soil material is moved from one location to another, often by water, wind,
or gravity. Water, wind, or gravity exert a force on the soil material, causing it to move.
When erosion occurs on a slope, soil is washed from the slope. The steeper the slope, the
more erosion can occur. Obstacles like rocks and vegetation can slow the progress of erosion or
divert the path of the erosion.
Soil – top or surface layer of the Earth, consists of rock, gravel, sand, and organic components.
Rock, gravel, and sand are products of chemical and physical weathering of rocks.
Soil is made of different components. Those components are all different sizes. The different
sized particles were made by physical and chemical weathering. The components of soil can be
separated according to their size differences.
The mixture of soil components is dependent on weathering and erosion. Different sized
rock pieces will be found in soil. The organic component of the soil comes from the dead
remains of plants and animals. The organic component is an essential source of nutrition for
plants growing in the soil.
Summary:
Rocks are physically weathered by breaking them into smaller and smaller pieces. The
rocks do not change their chemical composition when they are physically weathered. Rocks are
chemically weathered when compounds in the rock react chemically with other chemicals. The
chemical nature of the rocks changes. Minerals in the rocks are decomposed into other
chemicals because of the chemical reactions.
By physically breaking rocks into smaller pieces, physical weathering exposes more of a
rock’s surface to chemical weathering. The combination of the two speeds the overall process of
weathering.
When erosion takes place, soil is moved from one location to another. If erosion occurs
on a slope, soil will be washed down the slope.
Soil from a steep slope will be eroded more than the soil from a less steep slope.
Obstacles such as rocks and vegetation will decrease the erosion of soil from the slope.
Some components of soil include gravel, organic material, and sand.
The different components of soil are different chemically and in size.
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