Oceans and Continents

Under the OceansIf you were to travel from Britain to America over the present sea floor, you would pass across a gently sloping continental shelf, a steeper continental slope down to the abyssal plain of the deep ocean floor, the up the sides of the mid ocean ridge with its median rift at the top, down to the abyssal plane on the other side and back up onto a continent again. From South America into the Pacific you would cross a narrow continental shelf and down into a very deep oceanic trench. Beyond this, on the abyssal plan you would pass a number of seamounts rising as submarine mountains. Ocean basins make up 60% of the Earth's surface. At present, sea water fills the basins and overflows onto the continental shelves so that about 70% of the Earth's surface is covered by water. Continental Shelf - although below sea level at the present time, the shelf is part of the continent Continental Slope - steeper (average 4 degrees) slope of the continental margin between the edge of the continental shelf at about 200m and where it merges into the abyssal plain at 1500-3500m deep. Deep submarine canyons cut across the slope in places. Sediments are transported down the slope by submarine avalanches called turbidity currents. Abyssal Plain - this is the deep ocean basin 3-5km deep. It is flat with the mafic rocks of the oceanic crust below it. It is covered by thin beds of fine grained, slowly deposited, pelagic sediment derived from wind blown dust, volcanic ash and the skeletons of microscopic planktonic organisms. Thicker deposits of coarser sediment are carried in by turbidity currents. The area is aseismic, meaning there are no earthquakes. Mid-Ocean Ridge - elongated submarine ridge in the middle of the ocean, rising 2 or 3 km above the abyssal plain and up to 1000km wide. Built up of basalt extruded at the divergent plate boundary where two oceanic plates move apart by the sea floor spreading. An axial rift valley, a deep valley with steep mountainous sides, splits the summit of the ridge. There are frequent shallow focus earthquakes due to rising magma and movement along transform faults. Deep-sea Trench - very deep (up to 11km), narrow (up to 150km), elongated submarine valley occurring at the edges of the oceans alongside fold mountain belts and island arc systems. There are shallow focus earthquakes. Many trenches occur around the edge of the Pacific. Seamount - Submarine basalt volcano rising at least 1000m above the ocean floor without reaching sea level. Seamounts may occur singly, in groups or chains. Some seamounts are topped by a coral atoll. A seamount with an eroded flat top is called a guyot. Over the LandThe continental land surface together with the continental shelf and slope make up 40% of the Earth's surface. The continental shelf is often uncovered when the sea level drops during an ice advance. Continental Shelf - the area of the sea floor around the edge of continental landmasses, which gently slopes to a depth of 150-200m before giving way to the steeper continental slope. This shelf is formed of continental crust and is covered with beds of clastic sands and clays or carbonate (limestone) deposits. Marine organisms thrive in the shallow, sunlit waters. There are few earthquakes. Fold Mountains - Fold mountain ranges or the eroded roots of former mountain chains, form linear belts along the margins of the continents. They are made of very thick sequences of sedimentary rocks, which have been strongly folded and faulted by compressive forces and metamorphosed by the high pressures and temperatures. Extrusive and instrusive igneous rocks are common. The high mountain ranges form two main linear belts several 1000km long; one situated on the western side of North and South America e.g. the Rockies and the Andes and the other being the 'Alpine-Himalayan belt' which formed in the last 60 Ma. Their width is generally 300-800km and they contain the world's highest mountains e.g. Everest 8 km high. Earthquakes at all depths are common as fold mountains are formed on destructive plate margins. Depth of focus increases with distance away from the trench. Continental shield or craton - continental shields are stable blocks of rock that form the ancient cores of the continents e.g. the Canadian, Scandinavian and Australian shields. Made of highly deformed crystalline Precambrian metamorphic and igneous rocks; contain the worlds oldest rocks e.g. the Isua gneisses of Greenland, dated at 3950 my; they are often on a very large scale of thousands of kilometres; have no earthquakes - aseismic areas as they are in the centre of plates away from the plate boundaries; are areas of low relief due to long periods of erosion; make up large section of rigid continental crust, although this may not be obvious due to a cover of younger sedimentary rocks Major rift valley and rift system - a rift valley is a linear strip of crust that has slipped down along normal faults, which dip towards the valley (graben structure). Normal faults are formed by extension of the crust, which is either being pulled apart by tension or gently arched upwards by rising magma. The crust subsides due to gravity, between the fractures, to form the valley. Magma may rise up the faults, leading to volcanic activity alongside and within the rift valley. Shallow-focus earthquakes are common along the fault lines. The East African Rift Valley is part of a major rift system that continues to the Red Sea. This system represents the initial stages of the continent spliting to form a new ocean.

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