共用题干 第二篇

A Very Slow Ride

The surface of the earth may seem very stable to you.But you might be amazed if you
knew some of the things that are going on under that surface.
The earth has an outer shell of rigid pieces called tectonic plates（地壳构造板块）.The
plates include both ocean floor and dry land.Some have whole continents on top of them.
The continents on top of the plates are just going along for a slow ride,moving only about
four inches per year. But even this small movement causes three types of big interactions.
One type is ocean ridges.These ridges develop in places where two plates are moving
away from each other. As the plates separate , hot magma（岩浆）flows up to fill the
space. New crust（地壳）builds up on the plate boundaries and causes ocean ridges.
These ridges form long mountain ranges,which only rise above the ocean surface in a few
places.
Another type of reaction-trenches-occurs between two plates that are moving toward
each other.As the plates meet,one bends downward and plunges underneath the other.
This forms deep ocean trenches.The Marianas Trench off Guam in the western Pacific
Ocean has a depth of more than 36,000 feet.This is the lowest point on the ocean floor.
If the leading edges of the two colliding plates carry continents,then the layers of rock in the
overriding plate crumple（变皱）and fold. A plate that carried what is now India collided with
the southern edge of the plate that carried Europe and most of Asia.This caused the
Himalayas,the world"s highest mountains.
The third reaction is transform faults（转换断层）.These faults occur where two plates
that are traveling in opposite directions slide past each other.Severe earthquakes can
occur. The San Andreas Fault in California is a good example of this type of movement. This passage is mostly about