In shape the Pacific Ocean is roughly triangular with its apex at Bering Strait. It is enclosed by mountain barriers studded with scores of active volcanoes arranged in a horseshoe loop around the basin. The coasts of the western Americas are generally regular, except those of southern Chile and of North America from Puget Sound to Alaska, which being drowned have numerous inlets, protected channels, and coastal islands. The Gulf of California is the largest fringing sea. On the north the arc of the Aleutian Islands extends nearly to Siberia and bounds Bering Sea on the south. In general the mountains on the mainland of eastern Asia are lower than those of the Americas, and extensive fluvial plains lie between mountainous regions that have rolling, rather than rugged, surfaces. Major islands rise off this western shore of the Pacific and extend from the Kamchatka Peninsula north and northeast of Australia. Here they join with the island arcs of Indonesia, which are formed by crustal folds entering the region from the Malay Peninsula. All the chief islands contain sedimentary, metamorphic, and deep-seated igneous rocks considered to have originated on Continental land masses, in contrast to the lavas and reef limestones found on the remote oceanic islands. Between the islands and the mainland, or amidst the islands themselves, are a series of fringing and semi-enclosed seas, chief of which are Okhotsk, Japan, Yellow, East China, South China, several seas within and adjoining the East Indies, and the Coral Sea. Festoons of islands, which surmount partially submerged mountain ridges, continue into the western and southern Pacific Ocean far beyond the major groups. The trend of the last two arcs in the southwest Pacific is towards the bifurcation of the North Island of New Zealand, and the uplift that continues into South Island is the last of the big mountain arcs toward the south.
Submarine canyons, which resemble extensions of river systems, are located off the shores of California, Washington, and part of eastern Asia. Geologists are much interested in how they originated, but no proposed theory to account for the features has yet received wide acceptance.
Tides are a factor affecting the use of harbors by shipping. Some coasts show a large range between high and low tides, notably the coast of Korea, where tides range between 15 and 30 feet at different stations, and the coast of Alaska, on which tides of 45 feet have been recorded at Cook's Inlet and 30 feet at Skagway. Most of the islands in the central Pacific have only a small tidal range; for example, at Midway the tide shows only an 11-inch range.