The Pacific Ocean Basin Essay, Research Paper
Plate Tectonics is theory of global tectonics (geologic structural deformations) that has served as a master key in modern geology for understanding the structure, history, and dynamics of the earth’s crust. The theory is based on the observation that the earth’s solid crust is broken up into about a dozen semirigid plates. The boundaries of these plates are zones of tectonic activity, where earthquakes and volcanic eruptions tend to occur. Plate Tectonics also are the cause of major features that exist in the World ocean today such as ocean basins.
The Pacific is the oldest of the existing ocean basins, its oldest rocks having been dated at about 200 million years. The major features of the basin and rim have been shaped by the phenomena associated with plate tectonics. The coastal shelf, which extends to depths of about 180 m (about 600 ft), is narrow along North and South America but is relatively wide along Asia and Australia. The East Pacific Rise, a submarine ridge-and-trough system, extends some 9650 km (about 6000 mi) from Antarctica to the Gulf of California, and rises an average of about 2130 m (about 7000 ft) above the ocean floor. Along the East Pacific Rise molten rock upwells from the earth’s mantle adding crust to the Pacific and Nazca plates on both sides of the rise. These plates, which are huge segments of the earth’s surface, are thus forced apart, causing them to collide with the continental plates adjacent to their outer edges. Under this tremendous pressure, the continental plates fold into mountains, and the oceanic plates downbuckle, forming deep trenches (subduction zones) from which crust is carried back into the mantle.
Following the early rifting of a continent, narrow basins or primordial oceans such as the Red Sea are formed by incipient seafloor spreading. Having formed from a dome or arch, the lips of the basin may still be uplifted, thus preventing fluvial runoff and deposition of terrigenous clastic debris. Given the right climate, carbonate reefs and evaporites will form.
As the ocean widens, arkoses, sandstones, and shales may accumulate in clastic wedges along the continental margin, and marine limestones may be deposited. The successions on the coasts of Brazil and West Africa record symmetrically the change from nonmarine clastic sandstones and shales, via evaporites (about 120 million to 110 million years old), to marine sandstones and limestones. This sequence records the incoming of seawater into a continental rift valley, widened via a Red Sea-type narrow ocean into a mature ocean.
The mid-oceanic ridge, in the youngest part of a growing ocean basin, is usually free of sediment. Passing off the ridge, calcareous sedimentation, caused by microorganisms, is found. Clays and chert (silica rock) occur in deeper parts.
On the continental margins of mature oceans, the basal clastic wedge may be succeeded, if the climate is appropriate, by a carbonate platform, such as the Bahama Banks off the eastern United States and the Great Barrier Reef off northeastern Australia. Turbidity currents flow down the continental slope and deposit terrigenous sediments in the deeper water of the continental rise. River deltas locally add great thicknesses of clastic sediments to the trailing continental margin.