Plate Movement (AQA)


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Internal structure of the earth

Litosphere= rigid upper section, 80.90Km. Divided into 7 plates, smallers ones, continental or oceanic.

asthenosphere= anything below the litosphere

core consistst of= inner; 5150-6370Km, made of iron and nickel 5000 degrees

     outer; 2890-5150km, high electrical conductivity, nickel and iron

 posibley the reason for creating magnetic field

mantle = molten and semi molten rock containing silicon and oxygen, liquid propeties under pressure

Crust- light, silicon, oxygen, aluminuim, pottasium and sodium. 6-10km thick. can either be continental./ oceanic.

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Alfred Wegener- Theory of continental drift

1912 he said that 300 million years ago, earth existed as a single continent known as pangea, later its split into Laurasia in the north and Gondwanaland in the south.

Geological evidence for support: South America and Africa fit, glaciation of carboniferous period deposits found in South America and India = must of been formed there and moved apart. Striations in Brazil and west Africa, rock surfaces in Scotland match northeast Canada, must have laid under some condition in one location. 

Biological Evidene: fossils, brachipods in india limestone compared with australia, mesosauras in south america and africa. plants existed when coal found in india and the artic.

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Other Theories that support

Ewings discovery of MAR- 1948 using echo sounding data on ocean floor, found marine mountain chain made up of volcanic rocks, recent in origin. Similar mountains existed on the sea floor of other oceans. deep sea trenches sometimes along continents and long island chains.

Sea floor spreading, 1962 harry Hess found that the age of rocks spread in the MAR outwards towards North america. older rocks were found at the centre of the oceans such as Iceland and older rocks were closer to USA.

Paleomagnitism. 1973, Fredrick vine and drummonds discovered bands of magnetic either side of the MAR. underwater volcanic eruptions of basaltic lava, reaches surfaces and cools. individual minerals such as Iron which lines along the direction of the poles. Earths polarity flips resulting in a member of magnetic stripes- sea floor spreading.

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Continental/Oceanic Crust and Convection currents

Continental: 30-70km thickness, 1,500 mya, 2.6(lighter) density, made up of Silicon, iron, alluminium and oxygen. SIAL

Oceanic: 6-10km thickness, less than 200 mya, 3.0 (heavier) density. Made up of mainly basalt, silicon, magnesium, oxygen SIMA

Convection Currents: 

-radio active decay in core creates heat

-less dense material rises

- cools towards surface and falls, so it gets less dense

- plates spread apart. 

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Constructive Margins

movement of plates away from each other (divergent)

landforms associated: ocean ridges, transform faults, rift valleys, volcanoes and shallow focus earthquakes. 

Ocean Ridges: longest continous features on the surface of the planet

1. slow rate (10-15mm py) wide range axis of 30-50km deep central valley (3000m)

2. intermediate rate (50-90mm) less well marked rift 50-200km, deep with a smother outline

3. rapid rate more than 90mm per year, produces a smoother crest and no rift.

MAR (slow rate) 

extends from arctic ocean to beyond the tip of africa, it seperates eurasian and north amecan in the north atlantic and the african and south american in the south atlantic.

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Average spreading rate of 2.5 cm per year, longest mountain chain in the world 10000 miles, largely submerged but mountains such as the azores reach above sea level.

Submarine volcanoes, fairly gentle sides because of low viscosity lava. frequent and effusive eruptions.

As new crusts forms and spreads, tranform faults occur at right angles to the plate boundry, causes plates to move at different rates leading to friction and shallow focus earthquakes. 

Great African rift Valley: at a constructive plate boundary, brittle crust fractures as sections move away, areas of crust drops down between faults to form rift valleys

extends 4,000km from Mozambique to the red sea. from the sea it extends northwards into jordon. a total distance of 5,500. in some areas, inward facing scarps are 600m above the valley floor

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Oceanic/ Continental convergence

Oceanic is the denser plate so it subducts beneath continental one, the down warping of the oceanic plate forms a very deep part of the ocean known as a trench.

Case study:nazca underneath south american plate

destructive plae margins occure where plates move together with one plate forced underneath, oceanic crust moves down below the cont crust, melting occurs in the magma pool.

oceanic crust pushed downwards, forms deep ocean trench along the plate margin, oceanic crust moves deeper below the oncoming cont plate, temperature rises. pressure between plates forces material to buckle and rise forming young fold mountains, melting crust forms lower density granite to rise as molten plumes, forming batholiths. 

Molting magma rises to surface resulting in fissure along plate margin

litosphere sinks beneath overriding plate, colder more brittle than the asthenosphere, produces stress and triggers EQ. known as the benioff zone

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Features of Oceanic and continental convergence

Fold mountains such as the andes- nazca subducts beneath south america, land bulges and rises.

Ocean trenches such as the peru- chile trench, nazca plate subducts at 45 degrees under south american, 6000km long, 8000m deep.

volcanoes- composite cone such as cotapaxi

magma is proud by the nazca plate subducting beneath the south american plate= steep sides, explosve = viscous lava and CO2

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ones is focred under by subduction 

associated features= oceanic trenches and island arcs

Eg pacific and phillapene 

volcanic island that formed parallel to ocena trenches such as the marianas islands. 

two oceanic plates converge, older more denser plate subducts, plate begins to heat up as it decends into the litosphere. 

When it reached a certain depth it begins to melt and feeds volcanoes, EG guam is a marianas island.

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When 2 continental plates meet there is no subduction

edges and sediments between them are forced up to form mountains

there is no volcanic activity but shallow focus earthquakes. 

Caste study: indo-Australian plate and Eurasian = Himalayas 

130 million years ago, india was part of gonwanaland, started its journey northwards on a collision course with the eurasian plate, the tethys ocean lay between land a sea, sediments deposited

once the collision occured the indo australian plate subducted fully (more boyant)

sediments are forced upwards in a layer over fold mountains to form, himalayas increasing at a rate 5cm per year.

Contains tallest mountain in the world: Everest.

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Conservative Plate Boundries

2 crust plates, sliding past eachother parallel to plate margins. no creation or destruction of crust.  no subduction therefore no tectonic activity. frequent small tremours and occasional severe EQ.

Friction prevents the plate edge from gliding smoothly so movement tends to occur as a series of sudden jerks accompanied by a shallow focus EQ.

Case Study: San Andreas Fault- LA

N.A plate moves northwest by 2-3cm per yer but appears to be moving south east in relationship to the fater moving pacific plate, which moves at 5-9cm per year,

but both plates are moving in the same direction, stress from this causes transform faults to form, running at right angle to the main san andreas fault.

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Hot Spots

A hot spot is a concentration of radio active elements in the mantle, from this a plum of magma destroys the plate above, lava breaking onto the surface forming active volcanoes

An example of this is the emperor sea mount chain (the Hawaiian islands) found at the centre of the pacific pcean, plate move them 3,200 km from the nearest plate boundry, the island are believed to be formed by a volcanic hotspot.

The hotspots is stationary so as the pacific plate moves northwest at around 5-9cm per year its leaves a live of active volcanoes. Right now the spot is directly below hawaii and the active volcano is the mauna loa.

The oldest volcanoes such as kaui has been put under pressure, the crust has subsided with marine erosion has led the volcanic island to become sea mounts below the sea level.

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Global Distribution of Volcanoes

Destructive margins= suduction zones, here they occur in narrow linear belts, 90% of all active volcanoes in the pacific ring of fire such cotopaxi, pinatubo. 

Constructive margins= MAR, ocean ridges such as surtsey. Can form in rift valleys such as mt Kilimanjaro and nyragongo. 

Exceptions to the rule;

over hotsots, middle of the pacific ocean= Emperor sea mount chain.

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Volcanic Eruptions

variables in formation and the frequency, and type is related to plate margin, emission and lava.

Lava is produced by upward movement  of material from the mantle.

basaltic (low silica) mid ocean ridges, hotpots and constructive rift valleys

lava that is the process of subduction, andesitic occur at island arcs, destructive plates. 

Constructive: basaltic, runny lava. Little violence gases escape easily flow long distances before cooling, materials such as little gas, mainly lava. frequent and sometimes can be continuous, type of volcanoes are lava plateaux and shield

Destructive: includes andisetic and ryolitic lava (increasing viscosity). Potentially explosive lava is shattered into pieces. materials emitted include lava bombs,ash and dust. they can lay dormant for long periods.

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Types Of lava and volcanoes

Basaltic- low silica content, more fluid, gas bubbles expand, less viscous and less violent eruptions.

Andesitic- high silica content, acidic, very viscous, solidifies before reaching the surface, leading to a build up of pressure, and violent eruptions.

Fissure- AKA lava platuex, very gentle slopes found at divergent boundaries, basaltic lava, flow over great distances, flat and featureless, Eg giants causeway

Shield- gentle slopes, steeper than fissure, repeated explosions build up of basaltic lava, eg mauna loa 

Cone- symmetrical, acid thick and viscous, cools rapidly, build up of ash and cinder, volcanic bombs ed paricuten, mexico

Composite- large and old, both layers of ash and lava, usually andesitic, mt fuji

Caldera- very violent eruptions,build up of gas, destroyed magma chamber, makes an opening, may become flooded by a sea or lake may form. eg yellowstone.

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Types of eruption

Hawaiin type- lava slowly and easily escapes from vent. runny basaltic lava, gases escape easily

Strombolian type- gas explosions more frequent, small frequent eruptions.

Vulcanian type- gas emissions, voilent but less frequent, plugs of cooled lava

Vesuvian type- extremely strong eruptions, dormant for long periods, gas and ash clouds

Krakatoan type-  exceptionally violent

Plinian type- most viloent, massive amounts of lava, gas, pryroclastic flow, part of the volcano may be removed when an eruption occurs.

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Primary Volcanic Hazards

Lava Flows: threat to propety more than life, oppurtunity for evacuation. more dangerous when released very quickley. usually large but localised economic losses.

Pyroclastic flow: hot rock fragments around 800 degrees. linked with subduction zone volcanoes, moves very quickly from its source.

Ash and Tephra: ash is material below 2mm anythin above is known as tephra. magma is fragmented by explosions, can cause global variation patterns. ash fall does not cause many deaths but leads to chronic illnesses. 

Volcanic Gases: includes water vapour, CO2, SO2, helium and carbon monoxide. rarely cause death but can cause problems, EG lake nayos in cameroon in 1986 CO2 was released and killed 1700 people.

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Case study: Mt Etna

Located in sicily, italy. strato volcano, 3,323 m high. 

Eurasian and african plate, one of the most active     

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