III.

Geological Formation and Structural Features

The Pacific is the oldest of the existing ocean basins, its oldest rocks having been dated at about 200 million years. The major features of the basin and rim have been shaped by the phenomena associated with plate tectonics. The coastal shelf, which extends to depths of about 180 m (600 ft), is narrow along North and South America but is relatively wide along Asia and Australia. The East Pacific Rise, a midocean ridge system, extends about 8,700 km (5,400 mi) from the Gulf of California to a point about 3,600 km (2,235 mi) west of the southern tip of South America, and rises an average of about 2,130 m (7,000 ft) above the ocean floor. Along the East Pacific Rise molten rock upwells from the Earth's mantle, adding crust to the plates on both sides of the rise. These plates, which are huge segments of the Earth's surface, are thus forced apart, causing them to collide with the continental plates adjacent to their outer edges. Under this tremendous pressure, the continental plates fold into mountains, and the oceanic plates buckle downwards, forming deep trenches, called subduction zones, from which crust is carried back into the mantle. The stresses at the areas of folding and subduction are responsible for the earthquakes and volcanoes that give the rim of the Pacific basin the name “ring of fire”. In fact, the Pacific contains more than 80 per cent of the Earth’s active volcanoes and most of its earthquake activity.