Plate Tectonics

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Working from the very centre outwards, describe the structure of the Earth, including the names of the transiitional areas.
Inner core (solid), Outer core (Liquid), Gutenberg Discontinuity, Mantle, Moho, Crust
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Where does the term tectonic originate from?
Greek: "Tekton"- builder
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What evidence is there for all the continents of the planet once all being joined up in one supercontinent: Pangaea.
Fossil evidence of same species found across many continents= one range, geological rock types are similar+ can matched up like puzzle pieces, Palaeozoic glacial deposits and striations across south going North-connected to one ice sheet, Antarctica
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Where do earthquakes occur?
Along clear lines on the map- plate boundaries
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Where do the deepest earthquakes occur?
Ocean trenches
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What are mid-oceanic ridges?
Massive underwater mountain belts several km high.
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What is the name of the flat areas of the ocean floor? What depths are these found at?
Abyssal plains; 4-6 km deep.
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Looking at a map of global distribution of heat flow, where would the hottest areas be?
Along the plate margins.
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How do marine magnetic surveys work?
Every 100,000 years the magnetic poles switch which changes the direction in which the crystals in rocks are aligned. Therefore rocks can be aged by the number of changes in crystal alignments and compared to other rocks.
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How can marine magnetic surveys provide evidence for rocks being produced at mid-oceanic ridges?
Rock closest to ridges= youngest- newly formed from cooled magma- few changes in crystal alignments. Rocks further away- crystals in layers with different alignments. Bands can be observed on either side of mid-Atlantic ridge. Seafloor spreading.
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What is the asthenosphere?
The top part of the mantle- carries the plate
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What is an accretionary wedge?
A build up of material scrapped off the plate being subducted.
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What type of plate boundaries are there?
Divergent, convergent, transform
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What is the difference in the densities of continental and oceanic crust?
Continental=2800kg m3; Oceanic=3000kg m3
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What is a "hotspot"
Otherwise known as a plume- an anomalously hot area in the middle of a plate caused by a column of hot rock rising through the aesthenosphere to the base of the lithosphere causing melting.
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Examples of hotspots. How many are there?
Hawaii; Yellowstone. 100 identified
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What is the chain of extinct volcanoes that forms in succession called?
Hot spot track
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How does evidence of climatic bands support the theory of Pangaea?
Current North of continents- once lay over the equator=swamps +jungles= coal, reef relicts etc. Sub-tropical=desert climates-salt deposits etc. Exposures align in continuous bands across Pangaea.
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Fossils of which plant species have been found in the south of all the continents?
Glossopteris (extinct order of seed ferns)
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Features of the appalachian mountain belt closely resemble which other mountain belts?
Southern Greenland, UK, Scandanavia, N.W. Africa.
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Why does the Earth have a magnetic field?
Outer-core= circulation of liquid Iron Alloy. Creates a field with 2 ends of opposite polarity. Two poles constantly move and also have a pattern of switching.
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How was a bathymetric image of the sea first produced?
Using echo-sounding- pulse through the water and recording echos returned from bouncing off the underwater surfaces.
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How deep are mid-oceanic ridges?
2-2.5 km deep
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How deep are ocean trenches?
Greater than 5 km but can range between 8 and 10 km deep. Deepest= Marianas Trench, western pacific=10.9km.
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How are seamount chains formed?
Originate from volcanic activity- some would have been islands- but most are now dormant and have been eroded to below sea level. Those with flat tops= guyots- had reef growth before eroded to too great a depth.
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What are fracture zones at mid-ocean ridges?
Narrow bands of vertical cracks and broken-up rocks. Less distinct away from the ridge.
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How did geologists produce a magnetic reversal chronology?
Using isotopic dating of rocks and matching the dates up with the various polarities of crystals in the rocks.
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Why are some polarity anomalies classed as positive and others negative?
-ve anomalies add to the Earth's current dipole force- make it stronger. -ve anomalies are found when the basalt has the opposite polarity to the current position of the Earths's poles so they subtract from this force and make it weaker.
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If rocks of 4.5 million years old lie 45 km away from the ridge axis in the North Atlantic, what is the spreading rate?
V= Distance divided by time. 45km=4500000cm. 4500000 divided by 4500000= 1 cm yr-1.
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Define:lithosphere. Is it more malleable or hard?
Crust &uppermost part of the mantle. Relatively hard and so bends or breaks when forces are applied-does not flow.
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How rigid is the aesthenosphere?
More plastic-can flow when acted upon by a force.
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If both the lithosphere and aesthenosphere are made of ulltramafic rock what defines the boundary between the two?
Temperature difference
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Compare the thickness of continental and oceanic plates.
Continental=150-200 km. Oceanic=
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How does the rock type affect the density of oceanic and continental plates?
Oceanic=mainly dense mafic rock=more dense. Continental=mainly low-density felsic and intermediate rocks.
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Is the lithosphere continous?
No-broken up into 20 pieces
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How many of the plates are described as "major"?
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Why are some plate boundaries described as "passive"?
They are usually older thin continental crust buried beneath an accumulation of sediment 10-15 km thick. This is called a passive-margin basin with the surface a broad area of shallow seafloor (continental shelf).
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How can you find active plate boundaries?
Look at a map showing the location of earthquakes (seismic belts) and sometimes volcanoes (destructive convergent/divergent)
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Where does a fault slip producing an earthquake?
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What are the three main types of plate margin?
Convergent, divergent, transform
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What type of plate margin is found at a site of seafloor spreading? What is formed?
Divergent. New ocean floor is produced at the boundary called the mid-ocean ridge.
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Describe the path of magma from the mantle to becoming new rock formations
Some magma=lower density than surrounding rock-so= buoyant. rises + accumultes in a magma chamber ("mush" of sold mineral crystals + liquid melt 2-7 km below seafloor). Some rises to sea floor surface and spills out a submarine volcano as lava.
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What structures are formed by cooling lava on the at the mid-ocean ridge?
Layers of pillow basalt- piles of metre-wide basalt blobs.
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What happens to magma that dosen't rise to the surface from the magma chamber?
Some solidifies on the chamber sides becoming a coarse grained mafic igneous rock called Gabbro. Some rises further into vertical cracks + solidifies to form wall-like sheets- dykes.
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What are black smokers?
Chimneys spewing hot, mineralised water with suspended tiny mineral grains. These precipitate in seawater as the water cools.
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When is lithosphere at maximum thickness?
80 million years old
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Why is older oceanic rock deeper than younger rock? Can you give an example of this?
As it thickens it cools and becomes more dense sinking down into the asthenosphere. Abyssal plains = deeper than mid-ocean ridges.
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Why is it possible that the surface area of the Earth remains the same over time?
Rate of oceanic plate consumption= amount of seafloor spreading (averaged over time)
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What happens at a convergent destructive boundary?
Two plates move towards each other with one being oceanic and so more dense. When the two meet this plate is bent downwards under the more buoyant rock and sinks into the asthenosphere (subduction).
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What feature in formed by oceanic crust being forced downwards by less dense crust?
Oceanic trench
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Why does old oceanic crust eventually sink into the asthenosphere
After around 10 million years the density exceeds that of the asthenosphere and so sinks down very slowly due to the asthenosphere=viscous.
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What is the oldest oceanic crust on Earth?
200 million years old
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How is it that some continental lithosphere can be 3.8 bn years old?
Can only be subducted a little- 100km.
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What is an accretionary prism?
A build up of sediment scraped off the subducted plate by the overriding plate.
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What occurs at a transform fault?
Two plates move sideways next to each other on a horizontal fault with no rock being created or destroyed.
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Give an example of a triple junction.
Mid Indian Ocean Ridge
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When are most new divergent boundaries formed?
When a continent splits- rifting
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Describe the processes causing continental rifting.
Continental lithosphere is stretched horizontally, causing it to thin in the vertical direction.
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Where do faults develop during rifting? Why?
Upper 15 km. Rock=cold +brittle so blocks of rock break off and slip down forming a low area-rift valley.
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If rifting continues what will form?
A new ocean floor- seafloor spreading`
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What happens to the continental lithosphere that was stretched in the rifting?
Becomes a passive margin
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Define rift.
A scar in the crust consisting of a belt of faults, volcanic rock and thick layers of sediment
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What is a suture?
Where two separate continental plates are converging and have trapped slivers of oceanic crust between them.
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At the end of a collision what form of boundary exists?
There isn't one- convergent plate boundary ceases to exist.
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How thick is the continental crust at a convergent plate boundary such as in the Himalayas or Alps?
60-70 km ( 2 x normal thickness of continental crust)
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What are the ways in which plates can move?
Convection-where aesthenosphere flow exerts force/shear on plate bases; ridge-force push-gravity acting on elevated lithosphere; slab-pull force-once an oceanic plate 10Ma dense starts to sink-pulls rest of plate with it.
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How can you calculate the velocity of plate motion?
Set up GPS receivers and record movement over a number of years. Or measure the distance from a mid ocean ridge and divide this by the known age of anomalous rock there.
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How fast do most plates move?
Between 10 mm-150 mm a year
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Other cards in this set

Card 2


Where does the term tectonic originate from?


Greek: "Tekton"- builder

Card 3


What evidence is there for all the continents of the planet once all being joined up in one supercontinent: Pangaea.


Preview of the front of card 3

Card 4


Where do earthquakes occur?


Preview of the front of card 4

Card 5


Where do the deepest earthquakes occur?


Preview of the front of card 5
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