Forces Of Change

Chapter two, Section two

Earth's Structure

For millions of years, the surface of earth has been changing slowly but surely. Some forces that change the earth happen on the surface of it, such as wind and water. Other forces that change the earth happen on the interior of it, such as volcanic eruptions and earth quakes.

A Layered Planet

There are three main layers that make up the Earth: crust, mantle, and core. The crust is the rocky shell that ranges from about two miles thick under oceans to about seventy-five miles thick under mountains. The mantle is the thick layer of hot, dense rock that consists of elements such as silicone, iron, and oxygen. The core contains two parts: a super hot yet solid inner core and a really hot liquid outer core.

Plate Movement

Five million years ago, the continents were not separated separated like they are today. They once formed a super continent called Pangaea. Over time, these continents drifted apart and in some places recombined. The theory that they were once together is called Continental Drift. Many of earth's largest features are claimed to be caused by plate tectonics. They push up mountains, create volcanos, and produce earthquakes. Ridges are formed by the plates spreading apart and magma (molten rock) is pushed up.

Internal Forces of Change

Other internal forces involve movements that can fold, lift, bend, or break the rock along the earth's crust.

Colliding and Spreading Plates

When a sea plate collides with a continental plate, it forms a mountain. When the heavier sea plate dives beneath the continental plate, it becomes a subduction zone. In other cases, when continental and sea plates meet, a different, known as accretion. Accretion is when pieces of the earth's crust comes together slowly as the continental plate slides above the sea plate. When two sea plates converge, a new land is created. Sea plates can also pull apart in the process known as spreading.

Folds and Faults

Folds are the layers of rock on the earth's crust. Other times, plates may grind or slide past each other that create cracks in the earth's crust, also known as faults.


Earthquakes occur where different plates meet each other and tension builds up along the fault lines. The strain eventually becomes so intense that the rock suddenly snaps and shifts.

External Forces of Change

There are two different types of external change; weathering and erosion. Weathering is the process that breaks rock down into smaller pieces. Erosion is the wearing away of earth's surface by wind, glaciers, and moving water.


There are two basic kinds of weathering; chemical and physical. Physical weathering is when large masses of rock are physically broken down into smaller pieces. Chemical weathering changes the chemical makeup of rocks, transforming their minerals or combining new elements.


There are three main types of erosion; wind, water, glacier. Wind erosion moves around loess, a fertile yellow-grey soil. Glacier erosion is the erosion of large bodies of ice known as glaciers. When glaciers melt and receed, in some places they keave behind large piles of rock and debris called moraines. Water erosion is the process of fast moving wter, such as rain, rivers, streams, and oceans, move sediments.

The Boundaries


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