Glencoe World GeographyChapter 2:
The EarthChapter Overviews
Planet Earth is part of the solar system, which is made up of the sun and the
many objects that revolve around it. Earth's structure has been and continues
to be shaped by powerful forces within the earth as well as exterior forces such
as wind and water.
Planet Earth At least nine planets revolve around the sun. Earth is
the third planet from the sun, one of the four inner planets closest to the
sun. Planets are classified as terrestrial planets, such as Earth, or gas giant
planets. Thousands of other smaller objects—asteroids, comets, and meteoroids—also
revolve around the sun. Earth is the largest of the inner planets. 70 percent
of the earth's surface is water, and about 30 percent of its surface is land.
Seven large landmasses called continents feature landforms of varying shapes
and elevations. Forces of Change The earth is composed of layers—a super-hot,
solid inner core surrounded by a liquid outer core, a layer of dense rock called
the mantle, and a relatively thin, rocky shell at the surface. Both internal
and external forces change the earth. Continental drift, the movement of surface
plates, causes landmasses to move slowly across the earth's surface. The movement
of magma within the earth may cause this plate movement, resulting in mountains,
trenches, folds, and faults that form where plates meet. Earthquakes and volcanoes
also occur along plate boundaries, especially along the Ring of Fire. Weathering
changes the earth's surface by physically breaking down rock or by changing
the chemical makeup of rock. Land is also eroded by wind, glaciers, and moving
water. Earth's Water Although the total amount of water on the earth does not
change, water is constantly moving and changing form. This water cycle begins
with evaporation and continues with condensation and precipitation. Most of
Earth's water is salt water, circling the planet in 4 great oceans which extend
into seas, gulfs, and bays. Only 3 percent of Earth's water is freshwater, most
of which is frozen in glaciers and ice caps. Lakes, streams, rivers, and groundwater
all supply human settlements with needed freshwater.
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