At convergent boundaries, there is a collision between two plates. Where the plates meet, the cold, denser plate sinks under the warmer, less dense plate in a process called subduction. This process forms deep trenches in the ocean floor and creates chains of explosive stratovolcanoes that form islands rising from the ocean bottom.

Continents, made of less dense material, generally do not subduct under oceanic crust, but ride over them; the oceanic crust sinks and is destroyed. Where a continent overruns oceanic crust, a chain of towering stratovol-canoes may form on the continent, as shown in the figure below. The Cascade Mountain Range, including Mt. St. Helens, formed this way.

Hot material slowly rises like molten plastic from the mantle. Where this hot material (magma) reaches the planet's surface, it cools and forms new oceanic crust, which spreads apart at a rift zone or spreading center. This process forms great ridges on the ocean bottoms and is called sea floor spreading.

Continents may be torn apart if the rift occurs beneath them. As the land separates, water floods in, leaving a sea or ocean where dry land once stood. The Atlantic Ocean formed when the Americas were separated from Africa and Europe by a rift.

When two continents collide, neither sinks because each is too buoyant. Instead, the colliding continents crumple and pile up, forming rugged mountain ranges. Some of the old ocean bottom that once lay between the continents may be shoved up as part of the mountains, leaving fossil seashells high in the mountains. The Himalayas and Alps are well-known examples of collisions between continents.

At transform boundaries, two plates slowly grind past each other; a fault separates them. As the plates move, tremendous strain can build up. The pent-up energy may then be released grad- ually as small earthquakes or suddenly in a single catastrophic earthquake. The plates each continue on their separate ways. The San Andreas Fault is a famous example of a transform fault along the eastern edge of the Pacific plate. This plate, which includes the western edge of California, is moving northwestward relative to the North American plate.

ILLINOIS STATE GEOLOGICAL SURVEY
INSTITUTE OF NATURAL RESOURCE SUSTAINABILITY     |    UNIVERSITY OF ILLINOIS AT URBANA-CHAMPAIGN

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