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Plate movement

Plate movement

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Specification

Earth structure, plate tectonics theory: convection currents and sea-floor spreading. Evidence: continental drift and palaeomagnetism.
Destructive, constructive and conservative plate margins. Processes: seismicity and vulcanicity. Associated landforms: young fold mountains, rift valleys, ocean ridges, deep sea trenches and island arcs.
Hot spots associated with plumes of magma and their relationship to plate movement.

What you need to know:

http://www.slideshare.net/mobile/anicholls1234/geography-notes-volcanoes-earthquakes-aqa

Earth structure:

Earth's structure

Earth’s structure

• Core, mantle, oceanic and continental crust and the asthenosphere. Basic differences between the two types of crust.

• Continental crust is made of older and less dense rocks such as granites. The most abundant minerals in continental crust are Aluminium and Silicon. From the chemical symbols of these two elements (si and al ) a new name was created for these rocks, sial. Sial is generally 35 to 70 km thick and mostly over 1500 million years old.

• Oceanic crust is made of younger rocks like the ones found along the Mid Atlantic Ridge. The most common rocks are basaltic ones, which are denser than the ones found in Continental crust. The two main minerals in these rocks are silicon and magnesium ( si and ma ), so this type of rock is called sima. Sima is only about 6 to 10km thick and has an average age of a mere 200 million years.

Plate tectonics theory:
• This suggests that the crust of the Earth is split up into seven large plates and a few smaller ones, all of which are able to slowly move around on the Earth’s surface. They float on the semi-molten mantle rocks, and are moved around by convection currents within the very hot rocks.
• Convection currents: Heat rises through the mantle outward toward Earth’s surface from hot spots deep within the mantle. In a layer of the upper mantle known as the asthenosphere, the solid rock can flow in a plastic manner. The current rises to reach its maximum height at a weakness in the hard uppermost mantle with attached crust known as the lithosphere. At these points, the hot mantle rocks melt from the lowering of lithostatic pressure and rises to form new crust at the mid-ocean ridges. As new material is added at the ridges, the tectonic plates on either side move apart. Over time this causes the continents located on the plates to ‘drift’ apart.

Where the convection current is sinking into the mantle, older, denser crust is being pushed under less dense crustal plates and less dense oceanic plates, in a downward trajectory toward the mantle, where it melts and gradually becomes part of the mantle rock once again.
• Sea floor spreading: In 1962 Hess proved that the newest rocks under the Atlantic were next to the Mid Atlantic Ridge, and that the oldest ones were near the coast of the USA. From this he concluded that the sea floor was slowly moving outwards from the Mid Atlantic Ridge, and moving towards the USA coast. The sea floor was spreading out from the middle, by up to 5cm every year.

Evidence:

Continental drift is a hypothesis proposed by Alfred Wegener. He argued that today’s continents once formed a single landmass, which he named Pangaea.

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It broke into pieces due to the weaknesses in the earth’s crust as they were made up of less dense materials, which drifted centimetre by centimetre over millions of years until they arrived at where they are now. The evidence for this break-up can be found in:

– The fit of the Continents (like a jig-saw puzzle) along the edges of the continental shelves.
– The rocks of the ocean floors are relatively young, no more than 200 million years old. The rocks also get older as one moves away from the mid ocean ridges. This shows that the sea floor is spreading out from the mid-ocean ridges.
Fossil Evidence e.g. the mesosaurus can be found in both southern Africa and southern South America
Rock Type e.g. Similar, age, structure and rock types are found in the Appalachian Mountains in North America and mountains in Scotland and Scandinavia.
Structural Similarities e.g. When the continents are reassembled, the mountain chains from a continuous belt — having the same rock types, structures and rock ages.
Paleoclimatic Evidence e.g. Glacial till of the same age is found in southern Africa, South America, India and Australia — areas that it would be very difficult to explain the occurrence of glaciation.

Palaeomagnetism: When igneous rocks containing magnetic minerals crystallise, the crystals align themselves with the Earth’s magnetic field. The magnetic field of the rock then points toward the magnetic pole that existed when the rock formed. Earth’s magnetic field periodically reverses polarity — the north and south poles switch. Rocks crystallising during one of these periods of magnetic reversal will be magnetised with a polarity opposite of rocks that crystallise today. When scientists studied the palaeomagnetic properties of rocks on the ocean floor they discovered that there were stripes of ‘normal’ and reversed polarity. These showed that the rocks formed at a ridge, moved away from that ridge in both directions.

 

Destructive, constructive and conservative plate margins:
Destructive plate margins occur where two tectonic plates are moving towards one another. One moves beneath the other, being carried down into the mantle on the sinking section of a convection current. There are two types:
– Ocean/continental collision e.g. where the oceanic Nazca plate travels beneath the continental South American plate.

Destructive plate boundary

Destructive plate boundary

Associated landforms include:
• Chains of young fold mountains parallel to the plate boundary, separated from each other by intermontane plateaux.
• A line of volcanoes also parallel to the plate boundary
• A deep sea trench just offshore of the continent.
NB there is also a lot of earthquake activity
– Ocean/ocean boundary e.g. where the Pacific plate converges with the Philippine plate, moving beneath it. Associated landforms include:
• Deep ocean trenches e.g. the Marianas trench
• An island arc formed from the accumulation of volcanic material erupted onto the ocean floor.
NB once again there is a lot of earthquake activity.
– Where two continental plates collide neither is subducted because the continental rocks are relatively light, and like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. The Himalayan Mountains and Tibetan Plateau were formed (and are still forming). There are many associated earthquakes.

Constructive Plate margins occur along spreading centres where plates are moving apart and new crust is created by magma pushing up from the mantle. The best known of the constructive plate margins is the Mid-Atlantic Ridge. This submerged mountain range, which extends from the Arctic Ocean to beyond the southern tip of Africa, is but one segment of the global deep mid-ocean ridge system that encircles the Earth. The rate of spreading along the Mid-Atlantic Ridge averages about 2.5 centimetres per year, or 25 km in a million years.

 

Constructive plate boundary

Constructive plate boundary

Landforms associated with constructive plate margins include:
– Mid-ocean ridge (see above)
– A deep rift valley at the centre of the ridge
– Volcanic islands e.g. Iceland, Tristan da Cunha
– The Great African Rift Valley is at the early stages of becoming a constructive plate margin.

Conservative Plate margins occur where there are two plates sliding horizontally past one another. J. Tuzo Wilson proposed that these large transform faults connect two constructive margins or, less commonly, trenches (destructive plate boundaries). Most conservative plate margins are found on the ocean floor. They commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes. However, a few occur on land, for example the San Andreas fault-zone in California.

Conservative plate boundary

Conservative plate boundary

 

Processes

Seismicity: One of the fundamental ideas behind plate tectonics is that plates are rigid. Global maps of earthquakes ( http://www.angelapplications.co.uk/teachersfirst/webplate1.png ) show that most events, especially in the oceans, are concentrated in relatively narrow bands, with large areas between them where there are no earthquakes. The narrow belts of earthquakes are taken to indicate active plate boundaries, while the areas between are the rigid interiors of the plates.
– The narrowest seismic belts (about 20 km wide) are associated with the crests of the mid-ocean ridges, which contain constructive plate boundaries. Earthquakes at these boundaries are mostly at a depth of no more than 8 km. Almost all earthquakes here are of magnitude less than 6.
– The largest-magnitude earthquakes found on Earth occur within the subduction zones of destructive plate boundaries. They are caused by the friction between the descending plate and the plate above. The 2011 Tohoku earthquake, with a magnitude of 9.0 whose focus was where the Pacific plate is subducted under the Eurasian plate.
– Earthquakes also occur at transform boundaries, where two plates are moving at different speeds or opposite directions. These are rarely more than 15km deep and can be of high magnitude e.g. The Northridge earthquake 1994 had a magnitude of 6.7.
– The broadest zone of earthquakes occurs at collision zones e.g. the Himalayas and Tibet. Here the earthquakes are of varying depth. The Sichuan earthquake of 2008 (magnitude 7.8) was the result of the collision of the Indo-Australian plate and the Eurasian plate.

Vulcanicity: In the diagram found at http://www.angelapplications.co.uk/teachersfirst/webplate2.png , each triangle represents the location of a recently active volcano. There are well defined island arcs such as in the Aleutians, central and western North and South America, and the east Pacific all of which correlate with destructive plate margins. Some volcanic regions such as the Hawaiian Islands are isolated (see hot-spot below). A second line of volcanoes follows the constructive plate boundaries of a mid-ocean ridge. Occasionally this volcanic activity reaches the surface in the form of a volcanic island. One final region of substantial volcanic activity is East Africa, the location of a rift valley that scientists believe is the start of a new constructive plate boundary.

Hot spots: Some geologists believe that there are hot spots deep within the mantle above which there is a huge column of upwelling lava, known as a “plume,”. This hot spot lies at a fixed position. As a plate moves over this hot-spot the upwelling lava creates a steady succession of new volcanoes that migrate along with the plate. This accounts for chains of volcanic islands found far away from any plate boundary. E.g. The Hawaiian Islands are believed to lie over a hotspot in the middle of the Pacific plate. As the ocean floor moves over this “hot spot” at about 12cm a year, the upwelling lava has formed not just the 5 main Hawaiian Islands but a chain of islands and submerged volcanoes almost 3.500 km long.

There is a new alternative theory explaining hotspots. They can be explained as stirrings in the upper 400 km of the mantle caused from above by the movements of the crustal plates and surface-based cooling. Hotspots are given the more neutral name of ‘melting anomalies.’

 

Convection currents and the associated plate boundaries.

Convection currents and the associated plate boundaries.

Questions:

Describe the landforms associated with a constructive plate margin. Explain their formation [7 marks]
There is clear evidence that supports the theory of plate tectonics. Describe this evidence this evidence and explain how it led scientists to propose the theory. [8 marks]

With reference to the theory of plate tectonics, describe the global distribution of major features of the earth’s crust. [8 marks]

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