geological oceanography

geological oceanography is the study of the rocks and sediments that compose the seabed. Two hundred million years ago there was a single great ocean surrounding one great continent, Pangea. The theory of plate tectonics describes the mechanism that led to the break-up of Pangea and the development of the ocean basins that we see today. The rocks underlying the ocean floor are totally different from those beneath the continents in both their origin and character. The rocks (or ocean crust) underlying the oceans are mostly less than 200 million years old, whereas on the continents the oldest rocks are 3 billion years old. Along the centre of each ocean lies a mountain ridge with a rift valley along its axis. Within the rift valley new ocean crust is continually being formed and spreads sideways; also within the rift valleys are hydrothermal vents. As the lavas that are erupted along the ridge solidify, the rock crystals align with the prevailing earth's magnetic field, which periodically reverses. This creates parallel lines of small, alternating, positive and negative anomalies in the magnetic field across the seabed, providing a sort of tape recording of the formation and subsequent spread of the ocean crust.

It is impossible to move a flat plate across the surface of a sphere without fracturing (try laying orange peel out flat without breaking it) so inevitably a pattern of faults develops. As the new crust moves away from the ridge, it cools and shrinks, so the ocean basins deepen towards their margins. Where the crust abuts against the continents, one of two processes occurs. At ‘active’ margins the ocean crust buckles down under the continent forming deep trenches, and is destroyed in the molten interior of the earth. The ‘passive’ margins are stable and so continents are shunted sideways, constantly shifting their positions on the surface of the planet.

As the crust spreads away from the ridge it becomes covered by increasing thicknesses of sediments that include dust blown off the continents and myriads of skeletal remains of planktonic species. The sediments eventually fill in the rough topography and create vast tracks of featureless seabed, known as abyssal plains, that underlie about half of the world's oceans. These accumulations of sediment become thicker with time, and contain chemical tracers and microfossils that, like tree rings, record the climate conditions that were prevailing at the time of their deposition. The extensive coring of ocean deposits carried out under the Ocean Drilling Programme, and its predecessor the Deep Sea Drilling Programme, has provided the main evidence for the way climate change has occurred over geological time. In very deep water, particularly in the Pacific, the calcium carbonate and silicate constituents of the skeletal remains dissolve, leaving deposits of red clays that contain no climate record.

Another source of the material that infills the ocean basins are flows of debris derived from underwater landslides, which, triggered by earthquakes, slip off the continental slopes. Along active margins, the deep trenches trap these debris flows, but where the margins are passive, as they are around the Atlantic, massive flows of sediment-laden water flood out unchecked across the abyssal plains. When they reach the foothills of the mid-ocean ridge, they deposit thick layers of mud and debris, known as turbidites. Ash deposited after violent volcanic eruptions also contributes to the infill, and it can form thick local deposits, which can be dated from their content of radioactive isotopes.

Yet another source of infill identified in the North Atlantic are thick layers of stones and gravel, called Heinrich layers, dropped from vast numbers of melting icebergs that have periodically broken out from the Arctic.

The monotony of the abyssal plains is also broken in places by sea mounts, which are volcanic cones truncated by wave action that have developed over hot spots in the ocean floor. These hot spots are created by convection in the underlying magma, the liquid inner core of the earth. These hot spots are often stationary relative to the earth's surface so, as the ocean crust spreads across, a chain of sea mounts develops; thus the Hawaiian chain of islands lies at the end of the 6,000-kilometre (3,725-m.) Hawaiian-Emperor chain of sea mounts. This has a dog-leg in it that resulted from a change in the direction of ocean spreading 43 million years ago.

The geology of the seabed on the continental margins is an extension of the land geology. So surveying the structures underlying seas like the North Sea is based on the same techniques as those used on land, but with the added difficulty of coping with the covering of sea water. The rise in sea level since the last ice age has submerged many oil- and gas-bearing strata, hence the development of the offshore oil and gas industry. In the Mediterranean the constant jostling of the African continental land mass against the Eurasian land mass has resulted in the repeated opening and closing of the Strait of Gibraltar. Each time the strait closed the Mediterranean Sea evaporated to dryness leaving thick deposits of salt. The volume of these salt deposits implies that the sea emptied and refilled at least seven times during the Miocene era, causing fluctuations in sea level globally of around 70 metres (200 ft). The salt is porous, so oil has migrated through it and accumulated in domes, hence the rich oil deposits found in Libya. There are other mineral resources with commercial potential, placers of heavy metals (e.g. gold, tin, and diamonds) that get washed into the sea from rivers; phosphates that accumulate under productive regions of sea; manganese nodules that are precipitated at great ocean depths; and sands and gravels, many dropped in glacial moraines during the glaciations, which are now exploited as aggregates for construction projects.

Bibliography

Allaby, A., and and Allaby, M. , The Concise Oxford Dictionary of Earth Sciences (1991).
Crowley. T., and and North, G. , Paleoclimatology (1991).

http://pubs.usgs.gov/publications/text/dynamic.html

M. V. Angel