N o r t h E a st I n d ep en d e nt S c h o o l D i st ri c t
7th Grade Science
Unit 8
Changing Earth
Spring 2006
CHANGING EARTH
The 7th grade student will study weathering, erosion and deposition. Students
first studied the differences between these three processes in the 5th grade.
These concepts were revisited in 6th grade as a part of the rock cycle. The focus
this year will be to analyze the effects of these earth processes.
Weathering
Weathering is the process of breaking down rocks to produce sediment or soil.
The rate at which weathering occurs depends on the climate and type of rock being
weathered. Weathering usually occurs too slowly to notice. Weathering can be one
of two types, physical or chemical.
Physical weathering is the breaking down of rock into smaller pieces. Physical
weathering is also called mechanical weathering. One of the main causes of
physical weathering is abrasion. Wind, water and other rocks rub against a
surface, acting like sandpaper. The pounding of waves against a shoreline also
breaks down rock into soil or sand. In Hawaii, the volcanic flows of lava eventually
break down into soil and waves often play a factor in this process. Another
important cause of physical weathering is ice. If water flows into a crack and then
freezes, it expands, pushing the rock apart, causing it to break. This process is
sometimes referred to as ice wedging. Plant roots can cause physical weathering in
much the same way. A tiny root finds a crack in the rock and as it grows, it causes
the rock to break. In some cases, an animal that digs or burrows into the ground
can help create sediment or soil.
Chemical weathering occurs when rocks are broken down because of chemical
reactions. Water and/or weak acids are usually the causes of chemical weathering.
Caves are formed when water reacts with limestone to produce carbonic acid.
Carbonic acid then begins to dissolve the limestone. Oxygen reacting (oxidation)
with iron found in many rocks is another way the rock can become chemically
weakened.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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N o r t h E a st I n d ep en d e nt S c h o o l D i st ri c t
7th Grade Science
Unit 8
Changing Earth
Spring 2006
Erosion and Deposition
Erosion occurs when weathered rock and soil are moved from one place to another.
Deposition is the actual dropping of the sediment in a new location. These
processes are often caused by moving water. However, gravity, wind, and glaciers
can also cause erosion and deposition. Erosion is usually a slow process but can be
quick if a flood or strong winds occur. Erosion is responsible for wearing down
mountains and creating valleys. Erosion is more common where there is no ground
cover and on slopes that are steep. Gravity can cause erosion by producing mass
movement, one of several processes that move sediment downhill. Some types of
mass movement are landslides, mudslides, and creep. The most destructive type of
mass movement is a landslide. Landslides are usually associated with heavy rains,
snowmelt, and earthquakes. These events loosen weak structures, causing rock and
debris to move down steep slopes. When mass movement is discussed, the
concepts of potential and kinetic energy should be revisited as the TEKS
specifically address energy interactions for geologic events.
Sometimes nutrient and soil carried in rivers will drop out of the flow if for some
reason the rate of the river slows down. Anytime moving water, ice, wind, or
gravity drops earth sediments in a new place, deposition is occurring. This can
create areas of land that are rich in nutrients and very good for growing crops.
These process, however, can also be a serious problem. If too much soil is washed
or blown away, the result can be land that is left infertile because the topsoil is
gone. This is what happened in the United States in the 1930’s. Severe drought
and strong winds blew away the topsoil in parts of Kansas, Oklahoma, Texas, New
Mexico, and Colorado. This area was referred to as the Dust Bowl. At one time
this area was covered with hardy grasses that held the soil in place, but due to
extensive farming, drought and high winds, the fertile topsoil blew away. Soil
conservation practices may help preserve topsoil, but when humans overgraze and
remove excessive vegetation, soil is exposed to erosion.
Clarifying statements are intended to deepen teachers’ understanding of science concepts and serve as a guide for
instructional design. They are not intended to serve as student instructional materials.
This publication is the property of North East Independent School District. Duplication in whole or in part, outside of NEISD, is
prohibited without express written permission from NEISD.
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