The ocean basins account for 76 percent of the ocean floor, and their depths range from 1,500 to 3,000 fathoms. They have a very slight average incline of no more than 1:90 miles. For every 90 miles seaward the bottom slopes no more than 1 mile. However, superimposed on this very flat plain are many rugged relief features, such as seamounts, guyots, atolls, sills, and trenches.

SEAMOUNTS. Seamounts are submerged, isolated, pinnacled mountains rising 3,000 feet or more above the sea floor.

GUYOTS OR TABLEMOUNTS. Guyots are submerged, isolated, flat-topped mountains that rise 3,000 feet or more above the sea floor.

ATOLLS. Atolls are seamounts or guyots that have broken the surface, and coral deposits have built up around the rim. The coral forms a reef around a shallow body of watera lagoon.

VOLCANIC ISLANDS. These islands occur individually and in groups (island arcs). They are formed by volcanic eruptions. About 10,000 volcanoes dot the ocean floor, and they are especially abundant in the western Pacific basin. The Hawaiian Islands are probably the best known example of volcanic islands. In the North Atlantic Ocean, the most recent volcanic island (Surtsey) was formed south of Iceland along the Mid-Atlantic Ridge in 1964. 

SILLS. Sills are elevated parts of the ocean floor that partially separate ocean basins. A sill restricts the movement of bottom water masses and results in their partial, and in some cases nearly total, isolation.

TRENCHES. Trenches are long, narrow, and relatively steep-sided depressions. They comprise the deepest portions of the oceans. The trenches of the Pacific Ocean stretch for as long as 2,500 miles (Peru-Chile Trench), are more numerous than in any other ocean, and have by far the greatest depths in the oceans. For example, the Mariana Trench is 35,600 feet deep; the Tonga Trench, 35,430 feet deep; and the Mindanao Trench, 34,428 feet deep. Trenches are normally found on the seaward side of island arcs, while relatively shallow seas exist on the continental side.

On leaving the abyssal plains, we come to the last of the oceanic provinces, the mid-ocean ridges. The Mid-Atlantic Ridge is the most conspicuous of all ridges. It extends from Iceland southward across the equator to about 55S, forming an eastern and western basin in the Atlantic. The Mid-Atlantic Ridge rises from a depth of 2,500 fathoms and is continuous at depths of less than 1,500 fathoms over the greater part of its length. In several places, this ridge rises above sea level to form islands such as the Azores and Ascension.

Most of the ocean bottom is covered by various types of bottom sediments, deposits of mineral grains and rock fragments from the continents, mixed with dissolved shells and bones of marine organisms. In general, sediment deposits are thin or absent on the newly formed crust of mid-ocean ridges and are thickest on the older crust and near continents. The four major classifications of sediments are terrigenous, pelagic, glacial marine, and volcanic.

Terrigenous means "of land origin". Terrigenous sediments are the land derived silts and clays that are carried to sea by rivers. Winds also carry earth (dust) and sand out to sea and deposit them on the surface, where they eventually sink to the bottom. Terrigenous deposits are mostly found in the region of the continental shelf.

These sediments are also known as ooze. They form in deep water and are most commonly composed of shells and skeletal remains of marine plants and animals.

Glacial Marine Sediments 

The majority of these sediments (mud, rocks, sand, and boulders) were deposited when the glaciers of the ice age melted and retreated toward the poles. These sediments are deposited today by icebergs, since in most cases, icebergs are pieces of glaciers that break off and float to sea and melt. These sediments are found primarily in high latitudes within the continental shelf.

These deposits, primarily pumice and ash, are the result of volcanic eruptions. They are found in both deep and shallow water in all the worlds oceans.

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