Plate Tectonics 1 Flashcards
Define MOHO (Mohoronicic Discontinuity)
Boundary between the crust and the mantle
Define Sima
Oceanic crust - Silicona nd magnesium
Define Sial -
Continental crust - Silicon and aluminium
Define asthenosphere
Highly vicious, mechanically weak region of the upper mantle
Define geoid
Imperfect sphere. Bulges around the centre and narrows at the poles
Give the 4 layers of the Earth
Crust
Mantle
Outer core
Inner core
Give the key characteristics of the crust
Thinnest - between 0-20km on avergae
Composed of continental and oceanic crust
Give the key characteristics of the Mantle
Thickest section of the Earth at 2900km and makes up 84% of its volume
Made up of iron, silica and magnesium
Over 3000ºC on average
Give the key characteristics of the Outer core
The inner core is so hot that it causes this lyer to melt and become liquid.
Made up of iron and nickle
Reaches tempatures of 4000-5000ºC
This layer moves around the inner core and creates the Eath’s magnetic field
Give the key characteristics of the inner core
Tempatures can exceed 6000ºC
Mainly irona nd nickle - silid dense ball of metal
Engine and internal enegy source of the rocks
Give the key characteristics of the continental crust
Made up of silicon and aluminium
20-30km thick on average
Located on contents and areas close to the shore (continental shelves)
Give the key characteristics of the oceanic crust
Made up of silicon and magnesium
Thinner than continental (between 5-10km)
Much denser
Under oceans - formed via sea-floor spreading at mid-ocean ridges
Why is oceanic crust newer than continental
O = constantly being created and destroyed whereas can’t happen so easily with continental
Oceanic can be subducted because its more dense tahn its sorounding area so can be subducted and destryed. Created where sea floor spreads. COntinetal is not dense enough
Give 3 reasons why the inner core is so hot
Heat from when the planet formed and accreted, which has not yet been lost (Big Bang)
Frictional heating caused by denser core material sinking to the center of the planet
Heat from the decay of radioactive elements
Give the 5 pioneers of plate tectoinics
Alfred Wenger
Harry Hess
Fredrick Vine and Drummond Mathews
John Tuzo-Wilson
Dan Mckenzie
Who was Alfred Wenger
German metreologist
Published ‘The Origin of Continents and Oceans’ in 1915, which outlined his theory if cintinental drift
Continental drift theory met by sceptism by lots of scientists
Although he had lots of evidence to support his thoey, he lacked the mechanics on how the plates moved
Give the 5 lines of evidence
Fossil
Jigsaw
Geological
Glacial
Tectonic
Explain fossil evidence
Many examples of fossils found on seperate continents suggesting they were once joined together
If continental drift hadn’t occured then it woudl cntradct darwin’s theory of evolution
Fossils of Mesocrus and Lystceterous found on South Africa, India and Antarctica suggesting the continents were joined
Explain tectonic evidence
Fragments of an old fold mountain belt 450 million yers ago are found on widely seperate continents
When these land masses are re-assembled, the mountain belt forms a continious linear feature
Explain jigsaw evidence
Any areas where the coast may overlap or leave gaops can be exaplined by costal erosion/deposition and change in sea/land level
Explain geological fi evidence
When the geology of eastern South America and West AFrica were mapped, it revealled that ancient outcrops over 200m years ago were continious from one continent to the other
Explain glacial evidence
Today, glaciar deposits from the Permo-Carboniferous period are found in Antarctica, South America, India and Australia
If the continents hadn’t moved then it would suggest an ice sheet extending from the southpole to the equator which is unlikely seeing as the UK was near the equator at that point and showns no evidence of such galiac deposits
Who was Harry Hammon Hess
Professor of geology at Princeton uni (USA)
Served in the navy in WWII - equiping him with usefull knowlwdge of sonar
What book did Hammond Hess publish in 1962
The History of Ociean Basins - outlined how continental drift worked
Late rbecame known as “sea floor spreading”
Explain the sea floor spreading theory
Oceans were shallower in the middle and identified the presence of Mid Ocean Ridges rising as a high as 1.5km
In addition found that the deepest parts of the ocean trenches were found very close to the continental margins in the pacific
Hess believed that oceans grew from their centres from molten material escaping the mantle along the ridges. This created new seafloor which then spread away form the ridge in both directions
The ocean ridge was thermally expanded and consequently higher than the ocean floor further away.
As spreading continued, the older ocean floor cooled, became more dense and eventually subjected at a deep ocean trench
Give a limmitation to Hess
Hess believed that ocean trenches were the locations where ocean floor was destroyed and recycled - However didn’t have enough evidence to prove it
Couldn’t explain Hawii
Describe the work of Vine and Mathews
British geologists - their work looked at the patterns of magnetic striopes in the sea floor.
Confirmed Hess’ theory
How does magnetic stripes work
When the material from the mantle rises up through mid-ocean ridges and cools, it preserves a record of the Earth’s magnetic field.
This is because the magnetite in the basalts is strongly magnetic, and aligns with the field when it cools
What was Frederick Vine’s and Drummond Mathew’s theory
Hess lacked the evidence to assert that oceanic plates were created at Mid Ocean Ridges
When basaltic lava cools on the sea floor, individual minerals separate - especially iron - and these minerals then align themselves on the sea floor in the direction of the magnetic pole
Earth’s magnetic field swaps direction at points throughout time
Noticed there was a symmetrical pattern of magnetic stripes on either side of the mid ocean ridges. In addition when the basalts of the sea floor were dated, they were found to be the same age at similar distances away from the ridge on each side.
This suggested that the ocean floor was created at the mid ocean ridges, then was split in half by later activity and pushed sideways.
How did Vine and Mathews publish their theory
1963, in a paper called ‘magnetic anomomlies over oceanic ridges’.
Became the first scientiic test for sea floor spreading
But still didn’t explain Hawii
Why is Hawii an anomolie
Most volcanic activity is found at locations of sea floor spreading (Hess) or subduction zones.
Some volcanic locations such as Hawaii are far away from any known sea floor spreading location or subduction zone.
Who is John Tuzo-Wilson
Continental drift theories were accumulating without any explanation for land masses such as Hawii
In 1963, Tuzo-Wilson proposed that plates may move over fixed hotspots in the mantle to create a string of volcanic islands
Hawaii is composed of 19 volcanic islands.
What are hotspots
Radioactive decay in the Earth’s core generates hot temperatures.
If the decay is concentrated, hotspots will form around the core
Heat the lower mantle creating localised thermal currents where magma plumes rise vertically
Burn through the lithosphere to create volcanic activity on the surface
Hotspot remains staionary, the movement of the overlying plate reults in formation of a chain of active volcanoes. Hawii
As oceanic lithosphere oves away, volcanic activity ceases, cools and becomes an island
Other than hotspots, what did Wilson also discover
Conservative plate boundries
Plates move transversely – causes earthquakes but no volcanoes, without destroying or creating any oceanic crust
Who was Dan Mckenzie
There was lots of evidence about continental drift but now evidence to prove why it was happening
Heard Fredrick Vine speak about sea florr spreading and magnetic anomolies
What was Mckenzie’s theory - Mantle Convection Hypothesis
He suggested there are two layers in the mantle, each of which are in motion, controlling the movement and behaviour of the tectonic plates above.
Convection currents in the mantle occur when the hot + less dense magma in mantle rises, cools and sinks becuase it is more dense. This repeast in a cyclical motion forming convection currents
Destructive - convection currents force the continental and oceanic plate together. Oeceanic more dense so subducts
Constructive - two plates are forced apart by convection currents forning a gap between the plates
Give a limmitation to Mckenzie’s theory
Mantle convection being resposnible for the movent of tectonic olates is now largely out of favour, with modern imagaing techniques unable to identify convection cells in the mantle sufficientley large to drive plate movement
What is the most recent theory on plate movements
Gravitational sliding
What is gravitational sliding
Combination of ridge push and slab pull
What is ridge push (Gravitational sliding)
Takes place with plate movement driven because of the higher elevation of plates at ocean ridges.
As fresh magma wells up at mid-ocean ridges to form new young, oceanic lithosphere, a higher elevation is formed at spreading ridges.
The new oceanic crust gradually cools, gets denser and thickens with age and is pushed ‘downhill’ by gravity as new magma emerges from the active zone of divergence behind it (and thus adds distance from the ridge).
What is slab pull
More significant gravitational force acting on the plates
Occurs at destructive plate margins where an oceanic plate subducts under a continental plate, working in tandem with convection currents and gravity
Due to the extreme density and mass, the subducting plate pulls with it the rock behind and forces it to move along as it subducts
