The oldest fossils in the Pacific Basin are only of Middle Cretaceous age. They consist of coral reefs on the tops of extinct volcanoes which have more than 2 miles of local relief. Consequently, the basin must have existed long enough for the accumulation of an ocean more than 2 miles deep prior to Cretaceous time. In all probability the basin is of the greatest antiquity.

Fossils occur on enough atolls and guyots to permit the construction of a geological history of the Pacific during the last 100 million years. Vulcanism was widespread and volcanic islands were relatively common during early Tertiary time. Shortly after they reached full development, the volcanoes began to subside and an area of several million square miles in the central western Pacific apparently sank as a unit.

Tectonic events of the first magnitude have occurred in the eastern and southern Pacific but their geologic history is little known. Present seismicity and high heat flow suggest that the events may be continuing at present. Vertical displacements of 1 to 2 miles, and horizontal displacements of 200 to 400 miles have torn the sea floor into distinct blocks.

Unconsolidated sediment has filled the basin to a depth of only about 1,000 feet. Most of the sediment has been deposited very slowly by the accumulation of pelagic organic remains and continental dust. In a few places, notably off the United States, Canada, New Zealand, and Japan, turbidity currents are separated from the open basin by sediment traps in the form of island arcs and trenches.

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