The Theory Of Plate Tectonics

Question 1

What is the crust?

Answer 1

The outermost layer is called the crust. There are two types of crust; oceanic crust (which is a relatively thin layer of dense basalt under the oceans) and continental crust (a thick layer of less dense granite rock which makes up continents)

Question 2

What is the mantle?

Answer 2

the mantle is the thickest layer, and is made with molten and semi-molten rock rich in iron and magnesium

Question 3

Describe the structure of the earth?

Answer 3

The earth’s structure is divided into four sections; crust, mantle, outer core, inner core. The crust is divided into a series of plates. These plates are either oceanic (thin and dense) or continental (thick).

Question 4

The core of the earth has a temperature of over 5000 c. What is the origin of this heat

Answer 4

Primordial. Heat left over from the earth’s formation
Radiogenic. Heat produced by the radioactive decay of isotopes

Question 5

What separates the crust from the core?

Answer 5

the mantle (molten and semi molten rocks)

Question 6

What is the Lithosphere?

Answer 6

crust and rigid upper mantle - divided into 7 larger pieces and a few smaller pieces

Question 7

What is the Asthenosphere?

Answer 7

semi molten layer of mantle on which the plates float and move

Question 8

The plates that make up the Lithosphere are either continental or oceanic. How are they different?

Answer 8

1. continental plates are thicker (70km),less dense + older (1500 million years old) and make up the land and the continents.
2. Oceanic plates are thinner (10km), more dense + younger (200 million years old) and make up the floors of the ocean.

Question 9

State the 4 theories of plate tectonics?

Answer 9

1. Continental drift
2. Sea floor spreading
3. Subduction
4. Convection Currents

Question 10

Explain the evidence and explain the theory of continental drift?

Answer 10

Continental Drift, 1910s
The theory is that today’s continents are formed from the splitting of Laurasia and Gondwanaland
The geographical evidence is the apparent fit of South America and West Africa. The biological evidence is that fossils brachiopods found in India limestone is comparable with fossils found in Australian limestone
However the theory can not explain how the continents move

Question 11

Explain the evidence and explain the theory of sea floor spreading?

Answer 11

Sea Floor Spreading, 1940s
The theory is that the creation of new land mid ocean ridges - spreading the oceanic plates away from the centre
The evidence is that magnetic stripes due to paleomagnetism mirrored exactly either side of mid ocean ridges
However it can not explain how space for new landis created + the speed of plate movement is faster than what can be explained by sea floor spreading alone

Question 12

Explain the evidence and explain the theory of subduction?

Answer 12

Subduction, 1960s
The theory is that if land is created at mid ocean ridge, it must be destroyed at ocean margin
The evidence theory is huge oceanic trenches where large areas of ocean floor are being pulled downward
However it doesn’t explain the speed of plate movement is faster than can be explained by sea floor spreading and subduction alone

Question 13

Explain the evidence and explain the theory of convection currents?

Answer 13

Convection Currents, 1980s
The theory is that the circulation of magmas (due to thermal convection currents) makes the crustal plates move
The evidence is that there is unequal temperatures at the surface which indicates where hotter magma is rising and cooler magma is sinking
However this has not been proven as we can’t access the mantle to ‘see’ what is happening

Question 14

What are destructive (convergent) plate margins ?

Answer 14

A plate margin where crust is destroyed as two plates converge

Question 15

What are constructive (divergent) plate margins?

Answer 15

a plate margin where new crust is generated as the plates pull away from one another

Question 16

What are conservative (transform) plate margins?

Answer 16

A plate margin where two tectonic plates are moving past one another with no addition or destruction of plate material

Question 17

What is seismicity?

Answer 17

The occupancy or frequency of earthquakes in a region

Question 18

What is vulcanicity?

Answer 18

The quality or state of being volcanic, the level of power of a volcano

Question 19

What is ridge push/ gravitational sliding?

Answer 19

There are also other forces influencing how convergent boundaries can occur - for example ridge push.
The slope created when plates move apart has gravity acting upon it as it is at a higher elevation. Gravity pushes the plate further away, widening the gap (as this movement is influenced by gravity, it is also known as gravitational sliding)

Question 20

What is slab pull?

Answer 20

There are also other forces influencing how convergent boundaries can occur - for example slab pull.
When a plate subducts, the plate dining into the mantle pulls the rest of the plate (slab) with it, causing further subduction

Question 21

What is the sea-floor spreading ?

Answer 21

New land forming on the ocean floor by lava filling the gaps s known as sea floor spreading (as the floor spreads and gets wider)

Question 22

What is and how do ocean ridges form?

Answer 22

when two oceanic plates are at a constructive plate boundary. Magma rises in between the gap left by the two plates separating, forming new land when it cools. Less explosive underwater volcanoes can form as magma rises. New land forming on the ocean floor by lava filling the gaps os known as sea floor spreading

Question 23

What is and how do rift valleys form?

Answer 23

when two continental plates meet at a constructive plate margin. Any land in the middle of the separation is forced apart, causing a Rift Valley. Volcanoes can form where the magma rises. Eventually the gap will most likely fill with water and separate from the main island

Question 24

What is and how does a young fold mountain form?

Answer 24

When two continental plates collide at a destructive plate boundary. Both plates are not as dense as oceanic so lots of pressure builds up. Ancient oceanic crust is subducted slightly, but there is no subduction of continental crust. The pile of continental crust on the top of the lithosphere due to pressure between the plates. Fold mountains are formed from the piles of continental rust.

Question 25

What is and how do deep sea trenches form?

Answer 25

When either an oceanic and continental or two oceanic I plates meet at a destructive plate boundary. The denser oceanic plate subducts below the other plate. The plate subduction got leaves a oceanic trench

Question 26

What is and how do island arcs form?

Answer 26

They can also form from two oceanic plates at a destructive plate margin. The heavier plate would subduct, leaving an ocean trench. The built pressure causes underwater volcanoes bursting through the oceanic plate. The larva can cool and create new land called island arcs.

Question 27

What is and how do volcanoes form from plate boundaries?

Answer 27

1.when there is a continental and oceanic plate at a destructive plate boundary. The denser oceanic plate subducts below the continental. The plate subduction got below can leave an ocean trench, while built up pressure from the melting plat causes explosive volcanoes bursting through the continental plate
2. They can also form from two oceanic plates at a destructive plate margin. The heavier plate would subduct, leaving an ocean trench. The built pressure causes underwater volcanoes bursting through the oceanic plate. The larva can cool and create new land called island arcs.

Question 28

What are magma plumes and what is their relationship to plate movement?

Answer 28

The theory of magma plumes is that localised heating at the core/mantle boundary caused a plume of magma to rise through the mantle and ‘eat’ into the plate above the so-called ‘hot spot’ where the lava broke through the surface, active volcanoes formed above the hot spot. The Hawaiian island chain is an example of ‘hot spot’ activity.