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. The word “stable”(paragraph 1)means