3.1.3 Plate Tectonics

Question 1

What are the two types of crust?

Answer 1

• Continental crust is thicker (30-70km thick) and less dense.
• Oceanic crust is thinner (6-10km thick) and more dense.

Question 2

What is the crust?

Answer 2

The outer layer of the Earth

Question 3

Mantle

Answer 3

The middle part of the earth’s internal structure.

• The part of the mantle nearest to the core is quite rigid.
• The layer above this, called the asthenosphere is semi-molten (it can flow).
• And the very, very top bit of the mantle is rigid.

Question 4

The lithosphere

Answer 4

The top part of the mantle and the crust.

Question 5

The core

Answer 5

The centre of the Earth, which is split into an inner core and an outer core.

Question 6

Inner core

Answer 6

A solid ball containing lots of iron and nickel.

Question 7

Outer core

Answer 7

Semi-molten and also contains lots of iron and nickel.

Question 8

What is the Earth’s main source of internal energy?

Answer 8

The heat of the core and mantle.

The inner core is about 6000’C and the mantle is around 1000-3500’C.

Question 9

What is the Earth’s heat energy from?

Answer 9

Some of the heat energy is left over from when the Earth formed, and some comes from radioactive decay, of elements such as uranium.

Question 10

The Earth’s internal structure

Answer 10

Crust
Mantle
Outer Core
Inner Core

Question 11

What are tectonic plates?

Answer 11

The lithosphere is divided into lots of slabs called tectonic plates, which move in relation to each other.

Question 12

What are plate boundaries/margins?

Answer 12

The places where plates meet.

Question 13

The theory of plate tectonics?

Answer 13

The idea that the Earth’s lithosphere is made up of plates that move.

Question 14

What are the reasons for plate movement?

Answer 14

Until recently, scientists thought that convection currents were the main process causing plate movement.

Now, slab pull is thought to be the dominant process in most places, with ridge push happening in others.

Question 15

Slab pull

Answer 15

1) At destructive plate margins, denser crust is forced under less dense crust.

2) The sinking of the plate edge pulls the rest of the plate towards the boundary.

Question 16

Ridge Push

Answer 16

1) At constructive plate margins, magma rises to the surface and forms new crust, which is very hot. It heats the surrounding rocks, which expand and rise above the surface of the surrounding crust, forming a slope.

2) The new crust cools and becomes denser. Gravity causes the denser rock to move downslope, away from the plate margin.

3) This puts pressure on the tectonic plates, causing them to move apart.

Question 17

Convection Currents

Answer 17

1) The Earth’s mantle is hottest close to the core, so lower parts of the asthenosphere heat up, become less dense and slowly rise.

2) As they move towards the top of the asthenosphere they cool down, become more dense and slowly sink.

3) These circular movements of semi-molten rock are called convection currents.

4) They create drag on the base of the tectonic plates, causing them to move.

Question 18

Asthenosphere

Answer 18

The semi-molten part of the upper mantle, just below the lithosphere.

Question 19

Sea floor spreading

Answer 19

1) As tectonic plates diverge (move apart), magma rises up to fill the gap created, then cools to form new crust.

2) Over time, the new crust is dragged apart and even more new crust forms between it.

3) When this happens at a plate margin under the sea, the sea floor gets wider.

4) It creates structures called mid-ocean ridges – ridges of higher terrain on either side of the margin.

5) A similar process of spreading occurs at land margins where the plates are moving apart.

Question 20

Evidence we have to tell us what the structure of the Earth is like

Answer 20

• Drilling
• Studying surface rocks
• Using seismic waves
• Meteorites
• Gravitational anomalies
• Lab experiments
• Electromagnetic anomalies

Question 21

Importance of the theory of plate tectonics

Answer 21

Geologists now have one theory that could explain the different evidence they had accumulated. Until then, these ideas were scattered and unrelated.

Question 22

What do geologists use the theory of plate tectonics for?

Answer 22

• Show the relationship among geologic events.
• Explain the formation of oceans and mountain ranges.
• Explain the occurrence of Earthquakes and volcanic eruptions.

Question 23

History of plate tectonic theory

Answer 23

1692: Edmond Halley proposed a theory to describe Earth’s structure. He suggested it was made of hallow spheres – a bit like Russian Nesting Dolls.

1915: Wegener publishes his theory of continental drift.

1914-1918: Sonar technology developed during WW1 begins to reveal details about the ocean floor in a very basic way. It showed there were geological features on the ocean floor – not flat, as once believed.

1919: Arthur Holmes developed a theory that explains how large landmasses of Earth move – convection in the mantle.

1946: Post-WW2 – Scientists make use of the military’s sonar technology to find the mid-oceanic ridge, as well as mountains, volcanoes and deep ocean trenches.

1954: The first world map of volcanic eruptions and Earthquake zones is published.

1960: Harry Hess and Robert Dietz proposed the theory of sea-floor spreading. Molten rock (magma) oozes up from the Earth’s interior along the mid-oceanic ridges, creating new seafloor that spreads away from the active ridge.

1963: A pattern of magnetic-striped rock on the ocean floor that provides evidence to support Hess and Dietz’s theory of sea floor spreading (paleomagnetism)

1965: J. Tuzo Wilson proposed that the entire lithosphere of the Earth is made of separate sections called plates,developing the theory of plate tectonics.

1983: GPS becomes available for public use and is able to track movement of Earth’s surface.

Question 24

Paleomagnetism

Answer 24

The study of past changes in the Earth’s magnetic field (determined from rocks, sediment and archaeological records).

Question 25

What did Wegner base his theory on?

Answer 25

• Continental shelf shapes: The similarity in outline of the coastlines of Eastern South America and West Africa had been noted for some time.
• Matching rocks and fossils: Remains of Mesosaurus, a freshwater crocodile-like reptile are found solely in Southern Africa and Eastern South America. It would have been impossible for it to swim between the continents, suggesting that Africa and South America were joined together at one point.
• Similarity in rock type: Wegener noticed that rocks in South Africa and South East Brazil were very similar in age and structure.
• Presence of coal in temperate regions: Coal has been found in the Antarctic but coal only forms under wet and warm conditions, the present climate of Britain could never have produced coal.

Question 26

Wegner’s theory of continental drift

Answer 26

Alfred Wegener was a German meteorologist. He proposed a theory in 1912 that continents had once all been joined together forming a giant supercontinent, which he named “Pangea” (which means “all lands” in Greek).

He published a paper explaining his theory that continental landmasses were drifting across the Earth, sometimes ploughing through oceans and into each other. He called this movement continental drift.

The scientific community quickly dismissed his theory as he was unable to explain how the continents would have been able to move.

Today the theory of continental drift has been replaced by the science of plate tectonics.