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Page 420
Processes and Surface Features
All terrestrial planets have layers. Each planet gained energy from the
collisions that formed it. This energy heated and melted the planet’s
materials. The heaviest materials were metals, which sank to the center
and formed a core. Lighter rock formed a mantle around the core.
The lightest rock rose to the surface and cooled into a crust.
Four types of processes then shaped each planet’s rocky crust.
The processes acted to different extents on each planet, depending
on how much the crust and inside of the planet cooled.
reading tip
1
Earth’s crust is split into large pieces called tectonic plates.
These plates are moved by Earth’s hot mantle. Mountains, valleys,
and other features form as the plates move together, apart, or along
each other. The crusts of other terrestrial planets are not split into
plates but can be twisted, wrinkled up, or stretched out by the
mantle. Tectonics is the processes of change in a crust due to the
motion of hot material underneath. As a planet cools, the crust
gets stiffer and the mantle may stop moving, so this process stops.
2
Volcanism
3
Weathering and Erosion
4
Impact Cratering
Compare what you read
about each type of feature
with the pictures and
diagrams on page 421.
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impact craters on Earth
and other space objects.
Tectonics
A second process, called volcanism, occurs when
molten rock moves from a planet’s hot interior onto its surface.
The molten rock is called lava when it reaches the surface through
an opening called a volcano. On Earth, lava often builds up into
mountains. Volcanoes are found on Earth, Venus, and Mars.
Lava can also flow onto large areas and cool into flat plains like
the lunar maria. When the inside of a planet cools enough, no
more molten rock reaches the surface.
You have read about weathering on
Earth and the Moon. Weather or small impacts break down rocks.
The broken material is moved by a group of processes called erosion.
The material may form dunes, new layers of rock, or other features.
On Earth, water is important for weathering and erosion. However,
similar things happen even without water. Wind can carry sand
grains that batter at rocks and form new features. Even on a planet
without air, rock breaks down from being heated in the daylight
and cooled at night. The material is pulled downhill by gravity.
A small object sometimes hits a planet’s surface
so fast that it causes an explosion. The resulting impact crater is
often ten times larger than the object that produced it. On Earth,
most craters have been erased by other processes. Impact craters
are easier to find on other planets. If a planet or part of a planet is
completely covered with impact craters, then the other processes
have not changed the surface much in billions of years.
check your reading
420 Unit 4: Space Science
What processes affect the surfaces of terrestrial planets?
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