A Very Slow Ride

The surface of theearth may seem very stable to you.But you might be amazed if you knew some ofthe things that are going on under that surface.

The earth has anouter shell of rigid pieces called tectonic plates（地壳构造板块）.The plates include both oceanfloor and dry land.Some have whole continents on top of

them. Thecon-tinents on top of the plates are just going along for a slow ride,movingonly about four inches per year. But even this small movement causes threetypes of big

interactions.

One type is oceanridges.These ridges develop in places where two plates are moving away fromeach other. As the plates separate,hot magma（岩浆）flows up to fill the

space. New crust （地壳）builds up on the plate boundariesand causes ocean ridges. These ridges form long moun-tain ranges,which onlyrise above the ocean surface in a

few places.

Another type ofreaction-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 deepocean trenches. The Marianas Trench off Guam in the western Pacific Ocean has a depth of more than 36,000 feet. This is the lowestpoint on the ocean

floor. If theleading edges of the two colli- ding plates carry continents , then the layersof rock in the overriding plate crumple（变皱）and fold .A plate that carried what is

now Indiacollided with the southern edge of the plate that carried

Europe and most of Asia. This caused the Himalayas,theworld's highest mountains.

The third reactionis transform faults（转换断层）.Thesefaults occur where two plates that are traveling in opposite directions slidepast each other. Severe earthquakes can

occur. The San

Andreas Fault in California is a goodexample of this type of movement.

To explain theeffect of trenches,the writer gives the example of______.

A.the sea floor inthe Atlantic Ocean

B. the Himalayan Mountains

C. Europe

D. India