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The ocean floor of the central Pacific basin is relatively uniform, with a mean depth of about 4,270m (14,000ft). The major irregularities in the area are the extremely steep-sided, flat-topped submarine peaks known as seamounts. The western part of the floor consists of mountain arcs that rise above the sea as island groups, such as the Solomon Islands and New Zealand, and deep trenches, such as the Mariana Trench, the Philippine Trench , and the Tonga Trench. Most of the trenches lie adjacent to the outer margins of the wide western Pacific continental shelf.
Along the eastern margin of the Pacific Basin is the East Pacific Rise , which is a part of the worldwide mid-oceanic ridge. About 3,000km (1,800mi) across, the rise stands about 3km (2mi) above the adjacent ocean floor. Because a relatively small land area drains into the Pacific, and because of the ocean's immense size, most sediments are authigenic or pelagic in origin. Authigenic sediments include montmorillonite and phillipsite . Pelagic sediments derived from seawater include pelagic red clays and the skeletal remains of sea life. Terrigenous sediments are confined to narrow marginal bands close to land.
Water temperatures in the Pacific vary from freezing in the poleward areas to about 29 degrees Celsius (84 degrees Fahrenheit) near the equator. Salinity also varies latitudinally. Water near the equator is less salty than that found in the mid-latitudes because of abundant equatorial precipitation throughout the year. Poleward of the temperate latitudes salinity is also low, because little evaporation of seawater takes place in these areas.
The surface circulation of Pacific waters is generally clockwise in the Northern Hemisphere and anti-clockwise in the Southern Hemisphere. The North Equatorial Current , driven westward along latitude 15 deg north by the trade winds, turns north near the Philippines to become the warm Japan or Kuroshio Current. Turning eastward at about 45 deg north, the Kuroshio forks and some waters move northward as the Aleutian Current , while the rest turn southward to rejoin the North Equatorial Current. The Aleutian Current branches as it approaches North America and forms the base of an anti-clockwise circulation in the Bering Sea. Its southern arm becomes the slow, south-flowing California Current.
The South Equatorial Current , flowing west along the equator, swings southward east of New Guinea, turns east at about 50 deg south latitude, and joins the main westerly circulation of the Southern Pacific, which includes the Earth-circling Antarctic Circumpolar Current. As it approaches the Chilean coast, the South Equatorial Current divides; one branch flows around Cape Horn and the other turns north to form the Peru or Humboldt Current.
Only the interiors of the large land masses of Australia, New Guinea, and New Zealand escape the pervasive climatic influence of the Pacific. Within the area of the Pacific, five distinctively different climatic regions exist: the mid-latitude westerlies, the trades, the monsoon region, the typhoon region, and the doldrums. Mid-latitude westerly air streams occur in both northerly and southerly latitudes, bringing marked seasonal differences in temperature. Closer to the equator, where most of the islands lie, steadily blowing trade winds allow for relatively constant temperatures throughout the year of 21-27 degrees Celsius (70-81 degrees Fahrenheit).
The monsoon region lies in the far western Pacific between Japan and Australia. Characteristic of this climatic region are winds that blow from the continental interior to the ocean in winter and in the opposite direction in summer. Consequently, a marked seasonality of cloudiness and rainfall occurs. Typhoons often cause extensive damage in the west and southwest Pacific. The greatest typhoon frequency exists within the triangle from southern Japan to the central Philippines to eastern Micronesia. Although more poorly defined than the other climatic regions, two major doldrum areas lie within the ocean, one located off the western shores of Central America and the other within the equatorial waters of the western Pacific. Both areas are noted for their high humidity, considerable cloudiness, light fluctuating winds, and frequent calms.
The Andesite Line is the most significant regional distinction in the Pacific. It separates the deeper, basic igneous rock of the Central Pacific Basin from the partially submerged continental areas of acidic igneous rock on its margins. The Andesite Line follows the western edge of the islands off California and passes south of the Aleutian arc, along the eastern edge of the Kamchatka Peninsula, the Kuril Islands, Japan, the Mariana Islands, the Solomon Islands, and New Zealand. The dissimilarity continues northeastward along the western edge of the Albatross Cordillera along South America to Mexico, returning then to the islands off California. Indonesia, the Philippines, Japan, New Guinea, and New Zealand—all eastward extensions of the continental blocks of Australia and Asia—lie outside the Andesite Line .
Within the closed loop of the Andesite Line are most of the deep troughs, submerged volcanic mountains, and oceanic volcanic islands that characterize the Central Pacific Basin . It is here that basaltic lavas gently flow out of rifts to build huge dome-shaped volcanic mountains whose eroded summits form island arcs, chains, and clusters. Outside the Andesite Line, volcanism is of the explosive type, and the Pacific Ring of Fire is the world's foremost belt of explosive volcanism.
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