Changing Earth`s Surface

Changing Earth’s Surface
The solid outer layer of Earth is called the lithosphere. The lithosphere undergoes
constant change through natural geological processes. Forces inside Earth can cause changes to
occur on Earth’s surface through volcanic activity and faulting as tectonic plates move. There
are other factors such as weathering, erosion and deposition that change Earth’s surface.
Weathering, the breakdown of rock into smaller parts, is a process that can be either
mechanical or chemical in nature.
Mechanical Weathering​
-​
In mechanical weathering, larger rocks are physically broken down
into smaller rocks. Water, ice, wind, gravity, and organisms can cause mechanical weathering.
The changes that occur are classified as physical changes of matter because the properties and
identity of the smaller pieces of rock are the same as the original larger rock. Mechanical
weathering does not change the chemical makeup of the rock.
Moving water, wind and glaciers cause a type of mechanical weathering called abrasion.
Imagine a rock traveling down a fast-moving river. As the force of the moving water pushes the
rock along, it bumps into the other rocks and grinds against their surfaces. Over time, the
rock’s sharp edges are rounded and smoothed as it becomes smaller and smaller in size due to
abrasion. Abrasion is the process of wearing away or scraping away a surface.
Another type of mechanical weathering is a process called ice wedging. When the
temperature is above the freezing point of water, rain or melted snow seeps into the cracks of
rocks. When the temperature is below the freezing point of water, the trapped water freezes
to force ice. As you may know, when ice freezes, it expands, or takes up more space.
The expanding ice pushes against the rock with enough force to
break the rock apart.
Plants and animals also contribute to the mechanical weathering of rocks. For example,
trees and other plants can grow inside the cracks of a rock. As the plant grows larger, it
exerts enough pressure to break the rock apart. Other organisms such as earthworms and
gophers burrow into the ground and help break apart rocks.
Chemical Weathering –​
In chemical weathering, rock is broken down through chemical changes
in matter. The actual chemical makeup of a rock is changed into new substances with new
properties. There are three main types of chemical weathering: oxidation, carbonation and
hydration.
1
Changing Earth’s Surface
Oxidation is the chemical change that occurs when the element oxygen (O) combines with
a metal to form a compound. Many rocks contain the element iron (Fe). When iron comes into
contact with oxygen and water, a new substance called iron oxide (Fe​
O​
) forms. A more
2​
3​
common name for iron oxide is rust. Oxidation causes the rock to turn a reddish-brown color
and become soft and crumbly.
Carbon dioxide (CO​
) is a gas found in Earth’s atmosphere. When it rains, carbon dioxide
2​
dissolves in the water and forms a compound called carbonic acid (H​
CO​
). When carbonic acid
2​
3​
comes into contact with some types of rock material, it causes a reaction that breaks the rock
down chemically. This process is known as carbonation. Limestone caves are formed by the
chemical action of carbonic acid.
Hydration is the chemical reaction of water with other substances. When a rock
undergoes hydration, it swells up and crumbles.
Weathering is normally a slow process. However, in some cases, human activity causes
the process to speed up. Burning fossil fuels created pollution, which increases the acidity of
precipitation. The stronger the acid, the faster that weathering occurs.
Erosion ​
–​
Erosion is the process by which weathered rock pieces are carried away. The
fundamental force that is responsible for erosion is gravity. Rain and other forms of
precipitation fall to Earth because of gravity. Rivers and smaller streams flow across the land
and make their way to the oceans because of the force of gravity. Loose materials such as
rocks and boulders are pulled down from cliffs by gravity. Large masses of ice, called glaciers,
flow downward under the force of gravity.
During the process of erosion, sediment is removed from one area on Earth’s surface and
carried to another. Several different agents of erosion can transport sediment from one
location to another. Glaciers, wind, water, and gravity are considered the major agents of
erosion.
As glaciers move across Earth’s surface, they erode the land and collect large amounts of
rock materials. As the rocks carried at the bottom of the glacier grind against Earth’s surface,
they smooth and polish the solid rock layers below. Large scrapes and scratches often are left
in the solid rock layer of bedrock.
The wind also causes changes to Earth’s surface by moving loose sand, silt and clay from
one place to another. Grains of sand carried by the wind can actually sandblast a surface,
wearing it away over time. Wind erosion is most common in dry, desert-like areas of the world.
2
Changing Earth’s Surface
The most important agent of erosion is running water. Running water includes rivers,
streams, creeks, melting ice and surface runoff after a rain. When water falls on a sloping
landform, the water flows downhill, taking sediment with it. The particles carried by a stream
are called the stream’s “load”. Recall that kinetic energy is energy in motion. The faster a river
or stream moves, the more kinetic energy the water possesses and the greater the load it can
carry. Slow streams possess less kinetic energy; therefore, the water carries smaller particles,
or a smaller load.
Deposition ​
– Flowing water carries sediment, but it cannot carry the load forever. Sooner or
later, the kinetic energy of the water decreases and the sediment settles out. The process by
which sediment is laid down in a new location is called deposition. When a river empties into a
lake or ocean, the speed of the water slows down and drops the sediment it was carrying. The
heaviest sediment drops first. Then the lighter sediment settles out, in an order based on the
size of the sediment as the water slows.
1. How are weathering and erosion similar? How are they different?
2. How are weathering and deposition similar? How are they different?
3. How are erosion and deposition similar? How are they different?
4. What role does kinetic energy play in erosion and deposition?
3
Changing Earth’s Surface
Sediment is classified by its size and range, from boulders to clay. Study the graph below. It shows
the Different sizes of sediment and the speed of water that is required to move each one.
Both erosion and deposition change the shape of Earth's surface. Erosion moves materials from place
to place. Deposition builds up new landforms as it lays down sediment in new locations. Weathering,
erosion and deposition from a cycle of processes that wear down and build up Earth's surface.
5. According to the graph, which types of sediment are carried by a stream moving less than 25cm/s?
6. According to the graph, which types of sediment are carried by a stream moving less than 300
cm/s?
7. As a fast moving river slows down, describe what happens to its load.
8. If a river moving at 100 cm/s slows down to 50 cm/s, which size sediment will be deposited?
9. As the speed of a river decreases, what happens to the the kinetic energy of the water?
10. As the kinetic energy of the water decreases, what happens to the amount of sediment the river
can carry?
4