Oceans and continents

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  • Created by: naomi
  • Created on: 14-05-13 13:35

under the oceans

  • if you were to travel from Britain to America over the present sea floor, you would pass across a gently sloping conintental shelf, a steeper continental slope down to the abyssal plain of the deep ocean floor, then up the sides of the mid-ocean ridge with its median rift valley at the top, down to the abyssal plain 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 plain you would pass a number of seamounts rising as submarine mountains 
  • Ocean basins make up 60% of the Earth's surgace
  • 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
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continental shelf

Although below sea level at the present time, the shelf is part of the continent 

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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

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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 skeltons of microschopic planktonic organims (i.e. foraminifera and radiolaria)

Thicker deposits of coarser sediment are carried in by turbidity currents

The area is aseismic, meaning there are no earthquakes 

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Mid-ocean ridge

Elongated submarine ridge in the middle of the ocean, rising 2 or 3km 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 sea floor spreading 

An axial rift valley, a deep valley with steep mountainour sides, splits the summit of the ridge

There are frequent shallow-focus earthquakes due to rising magma and movement along transform faults 

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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 

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Seamount

Submarine basalt volcano rising at least 1000m above the ocean floor without reaching sea level

Seamounts may occur singly, in groups or in schains

Some seamounts are topped by a coral atoll

A seamount with an eroded flat top is called a guyot 

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Over the land

The 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 sea level drops during an ice advance

The view of the world shows large areas of shelf around Britain and much of this was once dry land

You could have walked to France just 9000 years ago!

The continents show a huge variety of rock types, structure and age, unlike the rocks of the oceanic crust

Continents may be dividded into structural areas with similar characteristics 

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continental shelf

This is the area of 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

The 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 

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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 intrusive igneous rocks are common 

The high moutain 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 Andes and the other, the 'Aphine-Himalayan belt', which formed in the last 60Ma

Their width is generally 300-800km and they contain the world's highest mountains, e.g. Everest 8km high 

Earthquakes at all depths are common as fold mountains aare formed on destructive plate margins

Depth of focus increases with distance away from the trench 

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Continental shield/craton

The continental shields are stable blocks of rock that form the ancient cores of the continents, e.g. the Canadian, Scandinavian and Australian shields

They are made of highly deformed crystalline Precambrian metamorphic rocks; contain the worlds oldest rocks, e.g. the Isua gneisses of Greenland, dates at 3950my; are often on a very large scale of thousands of km; have no earthquakes

Aseismic areas as they are in the centre of plates away from plate boundaires; are areas of low relief due to long period of erosion; make up large sections of rigid continental crust, although this may not be obvious due to a cover of younger sedimentary rocks 

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Major rift valley and rift system

A rift valley is a linear strip of crust that has slipped down along normal faultsl which dip towards the valley (graben structure)

normal faults are formed by extension of the crust, which is either 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 splitting to form a new ocean 

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