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•Litoshpere..Crust & Rigid upper mantle
•Astenosphere – weak and capable of flow
•Oceanic and Continental crust
•Oceanic – 6-12 km thick, dense heavy, basalt, SIMA
•Continental – 40-60km thick, less dense, granite, SIAL
• Made up of 7 major and several minor plates each with indep. movt.
Cause of Movt… Thermal convection currents
hot material rises, cools
& sinks. The lateral movt. drags the plates.
Continental plate…lighter
Oceanic plate….. heavier
Continental Drift..
Wegener 1912..
Pangaea.. One super continent. 225 million yrs ago
200m yrs ago split….
Laurasia.. Europe, Nth America & Asia.
Gondwanaland.. Sth. Am, Africa, India, Australia, Antartica.
1963 (Harry Hess) Theory accepted… discovered Mid Atlantic Ridge which was proof of
ocean floor spreading.
Proof: Continental Fit .. Like a jigsaw,
Matching fossils;
matching rocks (structure & type)
Mt. ranges…Appalachion Mts of U.S. & Caledonian of Irl & Br.
both fold mts.
Matching glacial deposits in S. America, Africa and Australia
Sea floor spreading and Age of ocean floor: oil drilling shows edge of
ocean rock 200m yrs older than the Mid Atlantic ridge.
Satellite measurements show Sth Am. & Africa moving 2.5cm apart each year.
Plate Boundaries:
Constructive
A) Diverge.. Move apart
causes cracks
fills with magma.
Result: (i) New ocean floor…
(ii) Mt. range/ridge e.g. Mid Atlantic Ridge … cont. move apart.
(iii) Islands form….
e.g. Iceland.
East African Rift Valley
6000km long
2 new African plates – Nubian Plates & Somali Plates
Red Sea is a part of the Rift Valley
Western and eastern Rift Valley
Contains some of the deepest lakes in the world
Lake Tanganyika – 1,470 metres deep
Lake Victoria – 2nd largest lake in the world
Eastern Africa- divergent plates
Eurasian & African plates collided 25m yrs ago -- fractures.
Magma plumes warped the surface upwards, making it brittle. It
fractured and moved apart.
Gulf of Eden
This gave rise to 3 active rift zones
East African Rift
Red Sea
The Arabian Peninsula split off from Africa & opened up the Red sea
and the Gulf of Eden.
East Africa in 50m yrs will detach itself and form a new continent &
sea.
Destructive
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1.
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B) Converge.. Meet
Crust destroyed.
one plate sinks = subduction zone….
Oceanic & Oceanic … older heavier plate descends under lighter
plate
(i) Narrow trench e.g Mariana Trench where Pacific & Phillipinnes
plates meet.
(ii) Volcano … Crust melts & produces magma.
(iii) Island arcs…hotspots as magma melts the crust…plates move
eg. Japan.
(iv) Earthquakes… caused by collision stress
2. Oceanic & Continental… collide
Result
oceanic sinks
trench, volcano, earthquake, E.g. Andes Mts. (Sth. Am.)
3. Continental & Continental….. Collide
buckle & fracture.
Result
Mts. & earthquakes e.g. Himalayan mts. Where Indian and
Eurasian plates met…. Contain sea fossils… proof that they were
once in the sea.
Passive
Plates slide past each other (sheer) -- (i) transform fault… San Andreas
Fault… Pacific & American plates.
(ii) Earthquakes
Constructive
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Rising current of magma splits continent
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Move apart – water rushes in – ocean
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Mid-ocean ridge above plume of magma
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Volcanoes along ridge
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Some islands e.g. Iceland might appear
Destructive
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Ocean-ocean – subducted plate melts and
rising magma creates line of volcanoes
called island arc
Ocean-continent – heavier ocean plate
sinks, buckles and folds, islands, extinxt
volcanoes pushed against continent edge
making it wider
Sinking magma creates volcanoes
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Continent- continent- Large Fold Mts
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Passive- Mostly along mid ocean ridges
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Sometimes on dry land – San Andreas
fault
Earthquakes
Irelands past shows evidence of desert & tropical climates.
Geology:
Croagh Patrick, Mayo
Quartzite…
Limestone….
Sugarloaf, Wicklow
formed at opposite ends of an ocean.
Central plain - formed at bed of tropical ocean.
Fold Mts.Nth west they run sw/ne
Munster they run e/w
Why? Different rock types. Plates have drifted from 30˚ south of equator to 54˚
north ... Drifted 9,000 miles in 500 million years.
Million Years ago
400
Nth & sth Irl join(20˚ sth) …. Causing folding and faulting.
380
Move Nth… desert area… only nth. section above sea level
Erosion & deposition in sth. - old red sandstone, Munster
350
Rise in sea level- Irl. submerged in tropical seas.
Result in limestone of the Burren.
250
European & African plates collide - Fold mts. Of Munster.
200
Erosion & marine sediments form into oil & gas.
100 Uplift… land rises above sea level-- more limestone & chalk
65
Volcanic bursts & rifting-- detach from America & form
Atlantic ocean & Antrim Plateau.