Plate Movement

Evaluation of the value of the evidence of continental drift to the plate tectonics theory

It only provided supporting evidence of the once conjoined landmass, and its subsequent movement. It provided no evidence or insight as to the mechanism that moved these land masses. This point is evidenced by Wegener's suggested mechanism by which the continents had moved; he believed the continents 'ploughed through' the sea floor

Climatological Evidence for Continental Drift

Places as far apart as Antarctica, North America and the UK all contain coal deposits of a similar age

Geological Evidence for Continental Drift

Rocks of the same age, type, and displaying the same formations are found in south-east Brazil and South Africa

Evidence for Sea Floor Spreading- Thickness of ocean sediment

Sediment accumulations were thin or absent on the crest of the Mid-Ocean ridge and becomes progressively thicker away from the ridge crest. Which suggests rocks at the ridge ate the youngest (limited time to accumulate a sediment cover)

Biological Evidence for Continental Drift

Similar fossil formations are found on either side of the Atlantic; 280m year-old fossils of the reptile, Mesosaurus, are found only in South America and southern Africa

Continental Fit

Some continents, such as the western seaboard of Africa and the eastern seaboard of South America, seem to fit together if placed beside each other

Evidence Supporting Sea Floor Spreading

1) Age of the sea floor 2) Paleomagnetism (the study of the record of the Earth's magnetic field in sediments) 3) Thickness of ocean sediment

Evidence for Continental Drift (4)

1) Continental fit 2) Geological evidence 3) Climatological evidence 4) Biological evidence

Mid-Atlantic ridge (4)

1) Started to form 200mya 2) Longest mountain chain in the world; stretches 40'000km and is 500-900km wide 3) reaches heights of up to 3,200m 4) Plates are spreading apart at a rate of 2.5cm a year

East African Rift Valley

1) Started to form 35mya 2) It is 4'000km long and up to 50km wide 3) It is 600m deep 4) Plates are spreading apart at a rate of 0.7cm a year

Continental Crust (4)

1) Thickness is between 35-40km, on average, but can reach up to 70km under mountain chains 2) Rock are very old- over 1500 million years 3) Less dense (2.6g/cm3) 4) Rocks contain silica and aluminium; granite is most common rock, but there are numerous types

Oceanic Crust (4)

1) Thickness is between 6-10km, on average 2) Rocks are young- mainly under 200 million years 2) Denser (3.0g/cm3) 4) Rocks contain silica and magnesium; rocks are mainly basalt and there are few types

Mantle

3900km thick located outside of the core. Temperatures range from 2800-1800 degrees, and is where the convection currents that move the earths plates occur. It has 3 layers; lithosphere, asthenosphere and lower mantle.

Plate Tectonics Theory

A geological theory, dating back to 1912 (Alfred Wegener's continental drift theory), which postulates that the Earth's lithosphere behaves as a rigid layer resting on a weaker asthenosphere. It further proposes that the lithosphere is broken up into plates which move in relation to one another by means of slow moving convection currents in the mantle. These convection currents are ultimately driven by the unequal distribution of the Earths internal heat (released by the radioactive decay of isotopes of uranium and thorium in the Earth's core) that causes the lower parts of the mantle to expand and then rise to the asthenosphere.

Evidence for Sea Floor Spreading- Paleomagnetism

A test conducted by two British geologists, Fred Vine and Drummond Matthews in 1963, showed there to be a series of magnetic stripes with rocks aligned alternatively to the north and south pole, in line with the Earth's polarity reversal (every 400'000 years). The striped pattern was symmetrical on either side of the mid-oceanic ridge. This suggested that the ocean crust was slowly spreading away from this boundary.

Evidence for Sea Floor Spreading- Age of the Sea Floor

Core rock samples, obtained from deep ocean drilling missions by the vessel, Glomar Challenger, were used to determine the age of the sea floor at different sites; the age of the rock increased with increasing distance from the ridge crest (rocks nearest the ridge <7m years; those furthest away >160m years).

Lithosphere

Earth's outer zone composed of rigid, brittle rock. it includes both the upper part of the mantle and the crust and its thickness ranges from 60-150km

Evaluation of the value of the evidence of Sea-floor spreading to the plate tectonics theory

It showed that rock was being created and destroyed; thus, providing evidence of the existence of tectonic plates and plate boundaries. It proved the existence of converging plate margins- further evidenced by the existence of rift valleys and the eruption of Surtsey in 1963 near Iceland, which created a new island.

Core

Located at the earths centre, has radius of 3500km and a temperature of around 5000 degrees. Mostly composed of iron with smaller amounts of nickel and other elements. From the way seismic waves change behaviour upon reaching the core, scientists have determined it has 2 parts; inner (solid) and outer (liquid and where the earth's magnetic field is generated)

Crust

Outer layer of the Earth composed largely of igneous rock, with significant amounts of metamorphic rock. Its thickness ranges from 6-70km

Benioff Zone

The melting of a subducted oceanic plate that occurs between 100-700km beneath the surface

Oceanic-Continental Convergence

The, denser, oceanic plate is subducted beneath the, less dense, continental plate. the exact point at which the oceanic plate begins to subduct is marked by a deep ocean trench. Folding of the continental crust combined with the 'scraping off' of sediment on the oceanic slab is incorporated in the accretionary prims and help to create young fold mountain chains.

Sea floor Spreading

Theory proposed by Harry Hess in 1962; it suggested that oceanic crust was formed along mid-ocean ridges by rising magma, and that, the new seafloor then spreads away from the active ridge, and eventually sinks into deep ocean trenches.

Continental Drift

Theory which proposed in 1912 by Alfred Wegener that the continents were once all joined together in a supercontinent (Pangea) and that at some point the land masses had drifted apart until they occupied their current positions on the globe

Theory Behind Paleomagnetism

Volcanic rock of the ocean floor contains iron-rich minerals that lose their magnetism in the mantle due to the high heat. As the magma solidifies at the crest of an oceanic ridge, it is magnetised with the polarity of the existing magnetic field.

Asthenosphere

Weak, plastic layer of rock in the upper part of the mantle. The high pressures in the mantle cause the rock to become plastic-like, allowing the rigid lithosphere to easily move over it.

Divergent Plate Margins

When to plates move apart from each other

Formation of Rift Valleys

When two plates diverge there is a weaker zone in the crust; an upwelling in the mantle partially heats the crusts, which expands and forms a ridge. Lateral tension develops causing a pair of rift faults to emerge. Magma rises and intrudes along the fault lines. As the faults widen, the central part of the crust sinks. As this rifting sequence continues, a rift valley is created, which may eventually be inundated by the sea.