The Pacific Ocean is the largest single feature on earth, covering 165 384 000 km2, or almost half of the Earth's surface. From north to south it is 15 216 km and from east to west 16 885 km. The average depth is 4300 m, but in the trenches, which lie around the periphery, depths of 11 000 m occur. The eastern margin of the Pacific Ocean lies in the oceanic trenches of Central and South America and there are few islands other than the Galapagos islands and Easter Island in the eastern Pacific. In contrast the western Pacific has the many islands of Melanesia, Micronesia and Polynesia. Its western margins are complicated by the island arcs ranging from the Aleutians in the north to the Bismarck–Solomon islands in the south together with the marginal seas which they enclose.

Geological history

That the floor of the Pacific basin was different from the surrounding areas was known from the early years of the present century; the ‘andesite line’ enclosed rocks of basaltic or sea-floor origin as distinct from the sialic rocks of continental origin (Figure 9.1). It was argued that the present Pacific is the remnant of the ocean which encircled Pangea, but this has since been proved incorrect because it appears that no part of the Pacific floor is older than 200 million years. However, tectonic features of the Andean mountains indicate that the margin of the Pacific may have been formed about 2000 million years ago.