other formerly disparate observations into the unifying backbone of the elephant that geologists had sought for 150 years.

The basic premise of plate tectonics holds that the Earth's crust is broken into a number of pieces, called "plates," that mill around pushing and bumping each other. Each plate is a section of the crust and moves as a unit in one direction, like one of the big flat slabs of ice that come floating down the Connecticut River during spring thaw. Plates may be composed of either land (continental crust) or the rocky ocean floor (oceanic crust) - or both. The North American plate, for example, stretches from San Francisco across North America to the middle of the Atlantic Ocean.

There are many observations drawn from global geology that support the concept of plates. Perhaps most important is the occurrence of most earthquakes in distinct linear zones, with broad areas in between that have few or no earthquakes. Each zone links up with other zones so as to surround the inactive regions with bands of intense earthquake activity. When the whole business is plotted up on a world map, a patchwork pattern of quiet areas bordered by earthquake zones emerges. Geologists interpret each patch to be a plate and the earthquake zones to

The boundries and direction of movement of the world's great plates. The black dots locate major earthquakes which occurred between 1961 and 1969. Note how most of the earthquakes occurred along plate boundaries as the plates pushed against and pulled away from each other. Also note that several plates include both land and ocean floor.

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