Petroleum GeologySome of the best evidence for moving plates comes from the bottom of the sea. In the middle of the Atlantic Ocean is a mountain range 10,000 miles (16,100 kilometres) long that snakes from Iceland to the southern tip of Africa (fig. 1.3). It has a deep rift, or trench, along its crest. Investigation of this Mid-Atlantic Ridge has suggested that it is a place where two great plates are moving apart. Along the rift is a string of undersea volcanoes. Each time one erupts, the pressure of the lava pouring out pushes the sides of the rift farther apart. Lava then hardens into rock and becomes new crust between the two plates.
Figure 1.2 The relative positions of the continents as they looked 200 million years ago (A) and today (B).
If the plates in the Atlantic are moving apart, then the Pacific Ocean basin is presumably becoming smaller. In fact, geologists believe that the westward movement of the North and South American continents (lighter continental crust) is forcing the Pacific plate (heavier oceanic crust) downward into the mantle (fig. 1.4). This collision of an oceanic and a continental plate accounts for the volcanoes and earthquakes common along this zone. Geologists also have evidence of what happens when two continental plates collide. They believe that the tallest mountains in the world, the Himalayas, formed when India smashed into Asia. Like an incredibly slow head-on collision between two cars, the crustal collision buckled and folded the rocks along the edges of the two plates. In fact, the Himalayas are still rising by a measurable amount today.
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