Pangea

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  • Created on: 08-04-15 11:02

Plate tectonics Theory: Alfred Wengners early idea

Plant Fossils

Description

Fossils of warm climate ferns have been found in India and Antarctica.

Discussion

  • The existence of warm currents around Antarctica at some point may have led to a warmer climate
  • The climates could have changed
  • For Antarctica to be hot the equator would have been too hot to sustain life.
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Plate tectonics Theory: Alfred Wengners early idea

Coal and Oil in Antarctica

Description

Massive coal and oil reserves under Antarctica suggest large amounts of vegetation and animal life were once here.

Discussion

  • Warm water currents may have heated the climate.
  • Climates may have been different
  • Antarctica as a land mass may have been located closer to the equator.
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Plate tectonics Theory: Alfred Wengners early idea

Glacial Striations

Description

These have been found in Brazil and West Africa. Glacial deposits have been found in India and South America suggesting that these were once cooler climates

Discussion

  • Different climate
  • Can only have been formed by Glaciers so these places must have been located in a cooler climate.
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Plate tectonics Theory: Alfred Wengners early idea

Continental Drift

This is the theory that the continents were once a super continent called Pangea.

It was first noticed by Francis Bacon in the 17th Century - 'Jigsaw fit'

It is largly attributed to Alfred Wegner who wrote a paper in 1912 containg evidence to support the theory.

Modern Evidence:

  • Seismic, volcanic and geothermal activity on plate boundaries
  • Ocean ridges
  • Mountains increasing in height
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Kevin Mansfield's Earth formation hypothesis

This hypothesis states that the Earth was formed by the collision of two planets:

  • PreEarth 
  • Old Moon

Old Moon (the smaller one) was 'swallowed' by PreEarth and the combined gravity of the two planets held Old Moon in the centre of PreEarth.

Evidence:

  • The hole in the North West Pacific - where Old Moon entered

It is an area of the sea bed which is much deeper than avarage. When impact happend it forced apart the Americas and Europe/Africa which explains the 'Jigsaw fit'

  • The Ring of Fire - Impact mountains

These were originally thought to have made a whole circle but the impact caused the PreEarth to expand and forced the circle apart across the Phillipines, Japan, Alaska, South America, Antarctic Peninsula, Southern Alpes of New Zeland and the submerged Colville and Kermadec.

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Expanding Earth theory

This theory tries to explain continental drift by suggesting that the Earh is constantly expanding as shown by the Ocean ridges. 

Evidence:

  • The Ocean ridge off the coast of australia is the same shape as the West coast of South America. 
  • 'Jigsaw fit'  - but mechanism for movement is the expanding Earth
  • Insinuates that there is only convergent plate boundaries so new land is always created.
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Shock Dynamics theory

This suggests that all landforms on the planet were made in the same event. Suggests that a block of land made upof India, Australia and South East Asia hit Asia and caused it to shatter into smaller pieces such as the Phillipines and Borneo. 

It also suggests that Ocean trenches were made in this event. These trenches then caused down drafts which further pulled land into the places that it can be found in now. 

This theory has flaws and is unable to sugest a mechanism for the movement of the plates.

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Sea floor mapping

Land is created at conservative plate boundaries such as at the Mid atlantic Ridge.

In 1984 Maurice Ewing examind a group of islands in the Atlantic ocean.

He discovered hat the islands were the highest points along a mountian range hidden under the sea (a chain)

The range was made up of young rock.

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Palaeomagnatism

Rock record the polarity of the Earth. 

The magma rises and the metalic elements face North, then hardens.

As the poles reverse, the rock changes to fae the new North creating strips of rock facing each North adn recording it. 

metalic, iron rich elements are magnatised in the direction of the field.

It reverses every 400,000 years

The patterns formed relate to the alternating field.

palaeomagnatism of basalts suggests that rock has been moving away from the ridge.

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Age of ocean floor

In 1962 harry Hess proved that the newest rocks under the Atlantic were next to the Mid Atlantic Ridge.

The oldest ones were nearest to the coast of the USA.

He concluded that the sea floor was slowly moving outwards from the ridge.

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Evidence for Continental drift

Sea floor spreading

Palaeomagnatism

Age of ocean floor

Distribution of seismic events

Satelite measuring or continents 

Earth isn't expanding - subduction - marianus trench

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Evidence for Plate Tectonics theory

'Jigsaw fit'

Rock structure

Animal fossils

Plant fossils

Coal and oil in Antarctica

Glacial striations

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Plate Margins: Constructive (Divergent)

Movement of plates

The plates are moving towards each other.

Tectonic features

New crust is formed from upwelling magma. Features:

  • mid-oceanic ridges
  • effusive ridge (shield) volcanoes
  • Shallow focus earthquakes
  • median rift valleys
  • continental rift valleys

Examples

  • Mid Atlantic Ridge
  • East African Rift Valley
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Plate Margins: Destructive (convergent) subduction

Movement of plates

Two plates moving away from each other.

Tectonic features

Oceanic to Oceanic:

  • trenches
  • island arcs
  • explosive volcanoes
  • earthquakes

Examples

On the margins of the Pacific plate, with subduction under other seperate sections of the plate, Tonga trench

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Plate Margins: Destructive (convergent) subduction

Movement of plates

Two plates moving away from each other.

Tectonic features

Oceanic to Continental:

  • trenches
  • fold mountains
  • explosive volcanoes
  • earthquakes

Examples

Andean type: Nazca plate subducting under South American plate

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Plate Margins: Destructive (convergent) Collision

Movement of plates

Two plates moving away from each other.

Tectonic features

Continental to Continental:

  • fold mountains
  • shallow focus Earthquakes

Examples

Himalayan type: Indian plate colliding with the Eurasian plate.

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Plate Margins: Conservative (transform)

Movement of plates

Two plates shearing past each other.

Tectonic features

Shallow focus earthquakes

Examples

San Andreas fault - Pacific plate and the American plate

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Plate Margins: not on a margin

Movement of plates

Hot spots: may be near the centre of a plate

Tectonic features

Plume volcanoes

Examples

Hawaiian islands: Emperor seamount chain.

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Plate Tectonic theory

This term was devised by geologists Mackenzie and Plamer

The earths crust is believed to be split into a number of different plates which consits of a mixture of oceanic and continental crust.

There are over 50 plates moving at different rates in different directions

All plates are in motion.

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Constructive plate margin: East African Rift Valle

This is an example of a constructive margin on an area of continental crust.

eastern Africa is moving in a North Easterly direction, diverging from the main African plate which is moving North.

The valley (consisting of two parallel rifts extends for 4000km from Mozambique to the Red Sea.

How it formed:

As the plates pull apart, the crust is subjected to huge pressures causing fracturing and fault lines

Sections of crust between parrallel fault lines then drop down.

The area experiences volcanic activity suggesting that the lithosphere has been weakend and thinned by tension, resulting in rising magma escaping onto the surface at volcanoes such as mount Kilimanjaro.

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Constructive plate margin: Ocean Ridges

Ocean ridges are the longest continuous uplifted features on the suface of the planet.

There total combine length reaches 60,000km)

In some parts they rise up to 3,000 metres above the sea floor.

Their form depends on the rate at which the plates are pulling apart.

Slow rate (10-15 mm per year):

  • wide ridge (30-50 km)
  • depp central rift valley (3,000 m)
  • Inward facing fault scrapes

Intermediate rate (50-90 mm per year):

  • Galapagos ridge
  • smoother outline
  • less well marked rift (50-200 m deep)
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Constructive plate margin: Ocean Ridges (2)

Rapid rate (> 90 mm per year):

  • smooth crest
  • no rift
  • Pacific Rise

Volcanic activity also occurs along the ridge, forming submarine volcaneos. these sometimes rise above sea level after successive eruptions e.g. Surtsy (South Iceland 1963)

These volcanoes have gentle sides because of the low viscosity of basaltic lava

Eruptions are frequent and gentle (effusive)

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Constructive plate margin: Ocean Ridges (3)

As new crust forms and spreads outwards, transform faults occur at right angles to the plate boundary.

Spreading plates on either side of these faults may move at different rates leading to friction and shallow focus earthquakes.

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