1.a. There is a variety of evidence for the theories of continental drift and plate tectonics.

Question 1

Outline the basic structure of the Earth.

Answer 1

Made from 3 primary concentric layers.

Studies of earthquake waves being transmitted through the Earth show that there is a concentric structure to the Earth’s interior.

Question 2

Outline the mean, minimum, and maximum thickness of continental crust.

Answer 2

Mean: 35 km.
Minimum: ~30 km.
Maximum: 70 km.

Question 3

Outline the thickness of oceanic crust.

Answer 3

5-10 km.

Question 4

Outline the thickness of the mantle.

Answer 4

From 10-35 km, down to 2900 km.

Question 5

Outline the density of continental and oceanic crust.

Answer 5

Continental: 2.6-2.7 kg/m3.

Oceanic: 3.0 kg/m3.

Question 6

Outline the density of the mantle at Moho, and at the CMB.

Answer 6

Moho: 3.3 kg/m3.

CMB: 5.6 kg/m3.

Question 7

Outline the mineral composition of continental and oceanic crust.

Answer 7

Continental: Granite, silicon, aluminium (sial).

Oceanic: Basaltic, silicon, magnesium (sima).

Question 8

Outline the mineral composition of the mantle.

Answer 8

Rich in magnesium and iron.

Question 9

What makes up oceanic and continental plates?

Answer 9

Lithosphere and crust.

Question 10

Outline the Mohorovicic Discontinuity

Answer 10

The boundary zone between Earth’s crust and the mantle.

This boundary marks a change in seismic-wave velocity from the crust to the uppermost mantle within the lithospheric plate.

Depth varies from 35km below the surface at the continents, whilst just between 5 and 15km below the oceans.

Question 11

Outline the core-mantle boundary.

Answer 11

The core and mantle are separated by a distinct boundary at a depth of about 2900km.

Question 12

What is confining pressure?

Answer 12

Pressure increasing with depth, (towards the core).

Question 13

Outline the upper mantle.

Answer 13

Consists of two layers.

Lithosphere and the asthenosphere.

Question 14

Outline the lithosphere.

Answer 14

A rigid layer sandwiched between the crust and the asthenosphere.

Rocks are cold and brittle.

Varies in thickness, and its boundary with the asthenosphere is difficult to define precisely; this is due to confining pressure and thus plastic flow in the rocks.

Question 15

Outline the asthenosphere.

Answer 15

Extends from 100km down too 300km.

Rocks are put under such pressure that they flow like a liquid, (plastic flow).

Convection currents also exist; due to vast amounts of heat generated deep in the mantle.

As a result, the asthenosphere flows carrying with it the solid lithosphere and crust above.

Question 16

What were the most devastating earthquakes?

Answer 16

Haiti, 2010: 222,570 dead.
Shansi, 1556: 830,000 dead (questionable validity - centuries ago).

Question 17

What were the most devastating volcanoes?

Answer 17

Tambora, 1815: 92,000 dead.

Question 18

What happened to continental theories in the 18th Century?

(Continental Drift and the Theory of Plate Tectonics)

Answer 18

During the 18th Century, maps of the world became more accurate and widely distributed.

The shapes of the continents and their relationships to each other attracted interest.

Question 19

Who was Alexander von Humboldt? What did he write about in 1801?

(Continental Drift and the Theory of Plate Tectonics)

Answer 19

German geographer and explorer.

In 1801, he wrote about the apparent fit between northeast South America and the west coast of Africa.

However, it was not until over 100 years later that ideas concerning the fit of the continents received serious attention.

Question 20

What did Wegener do in 1912?

(Continental Drift and the Theory of Plate Tectonics)

Answer 20

In 1912, he made a comprehensive case for continental drift and worked on his ideas until his death in 1930.

Question 21

Outline Wegener’s 1912 proposal.

(Continental Drift and the Theory of Plate Tectonics)

Answer 21

Based on the theory that in the Carboniferous period, 250 million years ago, a single large continent (Pangaea) existed.

Wegener suggested that Pangea broke apart into two large land masses.

Laurasia in the northern hemisphere, and Gondwanaland in the southern hemisphere.

The theory concluded by stating that this movement continued until the present day, spreading the continents across the globe.

Question 22

What was missing from Wegener’s theory of continental drift?

(Continental Drift and the Theory of Plate Tectonics)

Answer 22

A mechanism and explanation for how the continents drifted.

Wegner couldn’t tell the scientific community how the plates move.

Question 23

Outline 2 pieces of geological evidence from Wegener’s theory of continental drift.

(Continental Drift and the Theory of Plate Tectonics)

Answer 23

The fit of continents such as South America and Africa on either side of the Atlantic.

Evidence from about 290 million years ago of the effects of contemporaneous glaciation in southern Africa, Australia, South America, India and Antarctica, suggesting that these land masses were joined at this time, located close to the South Pole.

Mountain chains and some rock sequences on either side of oceans show great similarity, e.g. northeast Canada and northern Scotland.

Question 24

Outline 2 pieces of biological evidence from Wegener’s theory of continental drift.

(Continental Drift and the Theory of Plate Tectonics)

Answer 24

Similar fossil brachiopods (marine shellfish) found in Australian and Indian limestones.

Similar fossil animals found in South America and Australia, especially marsupials.

Fossils from rocks younger than the Carboniferous period, in places such as Australia and India, showing fewer similarities, suggesting that they followed different evolutionary paths.

Question 25

What is palaeomagnetism?

(Palaeomagnetism)

Answer 25

The ancient record of polarity.

Question 26

How was palaeomagnetism discovered?

(Palaeomagnetism)

Answer 26

When echo sounders were used to locate submarines, and they accidentally discovered the existence of a Mid-Atlantic Ridge.

Following this, a curious pattern of magnetism was discovered in the basaltic rocks on the sea floor; showing mirror images of repeating stripes as you moved away from the Mid-Atlantic Ridge.

Question 27

How do the repeating stripes on the Mid-Atlantic ridge form?

(Palaeomagnetism)

Answer 27

Iron minerals in cooling igneous rock align with where the north pole was at the time.

This alignment gets ‘locked in’ once the rock solidifies.

Rocks that are formed at the same time, each side of the Mid-Atlantic Ridge, pointed to different locations separated by thousands of kilometres.

The stripes of magnetic alignment on the ocean floor reflected the fact that the Earth’s polarity is not constant.

Question 28

How often does the Earth’s polarity changes orientation? How was this found?

(Palaeomagnetism)

Answer 28

Every 400,000 to 500,000 years.

This has been recorded in the rocks on the ocean floor.

Question 29

How does the Mid-Atlantic Ridge distribute material?

(Sea-floor spreading)

Answer 29

Moves material across the ocean floor as a ‘conveyor belt’ operating on each side of the Mid-Atlantic Ridge.

Eventually, the sea floor reaches an ocean trench where material is subducted into the asthenosphere and becomes semi-molten.

Question 30

The width of each strip of ocean bed corresponds to what?

(Sea-floor spreading)

Answer 30

Corresponds with the time scale of each magnetic reversal.

Question 31

What does the symmetrical pattern of geomagnetic reversals either side of the Mid-Atlantic Ridge indicate?

(Sea-floor spreading)

Answer 31

Indicates that fresh molten rock from the asthenosphere reaches the ocean bed and pushes older rock away from the ridge.

Question 32

What was predicted about the age of oceanic rocks on the Mid-Atlantic Ridge?

(The age of sea-floor rocks)

Answer 32

The predication was that oceanic rocks would get older the further one was from the mid-ocean ridge.

However, cores showed that nowhere in the oceans was rock older than 200 million years.

This helped confirm that ocean curst was constantly recycled over this time.

Question 33

When were links made between Wegner’s theory and sea-floor spreading?

(Wegner and seafloor spreading)

Answer 33

Made in the mid-1960s.

Question 34

The combination between Wegner’s theory and sea-floor spreading explains what process?

(Wegner and seafloor spreading)

Answer 34

The process that carries continents found on tectonic plates.

Question 35

Is the link between crustal movement and sea-floor spreading certain?

(Wegner and seafloor spreading)

Answer 35

However, the causes of sea-floor spreading and crustal movement in general remain speculative.

More evidence is needed to definitely prove them.

Question 36

How does sea-floor spreading provide evidence for continental drift theory?

(Wegner and seafloor spreading)

Answer 36

The different locations (if reversed and pulled together by closing the Atlantic ocean), shows evidence (alongside Wegener’s theory) for the theory of continental drift.