Part 5 - Plate Tectonics

Question 1

What are plate tectonics?

Answer 1

The theory that the earths lithosphere is divide into a number of plates of irregular shape and size that move over the weaker interior (the asthenosphere)

Question 2

What is the importance of the asthenosphere in allowing lithospheric plates to move?

Answer 2

It is weak and deforms plastically allowing the lithosphere to slide across it, kind of like tar

Question 3

What is an earthquake?

Answer 3

Vibrations that occur when a sudden fracture allows adjacent rock masses to slip past each other. The ground can shake very violently and cause poorly constructed buildings to collapse but distant earthquakes can be measured

Question 4

What is used to measure an earthquake to analyse the seismic waves that have traveled through the earth?

Answer 4

Seismometer

Question 5

Would you expect most earthquakes to happen in the lithosphere in the asthenosphere?

Answer 5

Sudden fracture is more a characteristic of elastic than of plastic , so most earthquakes happen in the lithosphere

Question 6

What is an earthquakes focus?

Answer 6

An earthquakes focus is the point inside the earth where the initial rupture begins

The greatest energy is released at this point, and this is where the strongest seismic waves originate

Question 7

Where do the strongest seismic waves originate in a earthquake?

Answer 7

In the earthquakes focus

Question 8

What is an earthquakes epicentre?

Answer 8

Often misused in the news

It refers to the point on the earths surface directly above the focus

Question 9

What is an earthquakes magnitude a measure of?

Answer 9

An earthquakes magnitude is the measure of the size of the seismic vibrations at the epicentre

Question 10

What does each one magnitude represent?

Answer 10

Each one magnitude represent ten times greater shaking at the focus

Question 11

What is the magnitude of an earthquake often referred to as?

Answer 11

The Richter scale

Question 12

What is a divergent plate boundary?

Answer 12

It is the line between 2 moving plates, where the asthenosphere wells upwards

Question 13

What is the divergent plate boundary also known as and why?

Answer 13

The constructive plate boundary

Because material is being added to plates

Question 14

When part of the asthenosphere mantle wells up through the lithosphere in a divergent plate boundary, why does it melt?

What is this melting known as?

Answer 14

It is a consequence of the drop in pressure

Decompression melting

Question 15

What happens to peridotite composition (silica, oxygen) begins to melt as it rises up through the crust?

Answer 15

It produces a slightly higher silica content 45% - 49%

This results in a the rock basalt

Question 16

What happens to the residual mantle left behind?

Answer 16

It has reduced silica content as it gave some to the oceanic crust

Question 17

What is partial melting in terms of plate spreading?

Answer 17

It is the ability for a rock to melt in a way that it produces mineral enriched rock

Such as the melting of the mantle which produces enriched silica and makes up the oceanic crust

Question 18

What are the two most abundant gases expelled in volcanic eruptions?

Answer 18

Hydrogen and oxygen

Question 19

What is a subduction zone?

Answer 19

It is when one plate is descending at an angle below another plate

Question 20

How far can a subduction zone extend?

Answer 20

700 km deep

Question 21

Why does a plate only descend 700 km and no more?

Answer 21

Because the heat from the surrounding asthenosphere warms the descending plate until it ceases to be unrecognisable

Question 22

What is a destructive plate boundary?

Answer 22

It is the point where two plates meet and the plate becomes destroyed as it is deep in the earth and heated up by the mantle

Question 23

What plate zones are volcanoes usually found?

Answer 23

Subduction zones