The Ocean Floor

The Pacific's floor is covered principally by oozes (soft deposits consisting chiefly of the remains of minute organisms). There are also extensive areas of sand, mud, and lava, and deposits of manganese ore and ores of other useful metals. Like the land areas of the world, the floor of the Pacific varies from smooth to rough and extremely irregular. Many of its formations, such as mountains and volcanoes, were built underwater in much the same manner as those on land. Others consist of sediments that accumulated during millions of years. Because exploration is still in the early stages, only the general configuration of the ocean's floor is known.

Bordering the Pacific along the continents is a shallow, gently sloping edge, known as the continental shelf. It varies from narrow along the Americas to wide off the Asian and Australian coasts. In some places, deep valleys, called submarine canyons, cut the shelf. Beyond it, the floor descends rapidly to great depths.

Underwater mountains, called ridges and rises, jut from the floor. Consisting of slowly evolving mountains and volcanoes, they occur where the earth's crust is weakest. One of the largest is the East Pacific Rise, sometimes called the Easter Island Cordillera. It runs parallel to the South American coast, turns southeastward toward Antarctica, continues past Australia into the Indian Ocean, and eventually joins the Mid-Atlantic Ridge to form a world-encircling range. There is no well-defined mid-ocean ridge in the Pacific such as the one that runs the length of the Atlantic.

Some of the underwater ranges are isolated; others branch into complex systems that run for thousands of miles. They are often marked by exposed volcanic summits, which dot the water as islands and island chains. The Hawaiian Islands, a volcanic chain, rise more than 29,000 feet (8,840 m) above the ocean floor. Throughout much of the Pacific, there are submerged, isolated peaks called seamounts and flat-topped peaks known as guyots. Summits that are almost awash are often the foundations of coral atolls.

Slicing the floor of the Pacific are canyon-like trenches, or deeps. They lie mainly in the western Pacific and adjacent to island chains. In the Mariana Trench off Guam, oceanographers have measured a depth of 35,810 feet (10,915 m), the greatest ocean depth in the world. Depths of more than 32,000 feet (10,000 m) are found also in the Kuril, Philippine, Kermadec, and Tonga trenches.