1.2 Tectonic Theories

Question 1

What is the structure of the earth?

Answer 1

• the crust is the upper most part comprised of oceanic and continental plates floating on the asthenosphere
• the mantle is made of molten rock and is when convection currents take place that move the plates around the earth
• the outercore heats the mantle with radioactive uranium decay generating heat which causes plumes of hot magma
• the inner core is comprised of solid iron and is 6000C hot

Question 2

Explain Wegners theory of convection currents within the mantle.

What evidence is there to support his theory?

Answer 2

• Wegner theorised that all the continents were originally all one super continent called Pangea. This was because he noticed that all the continents did fit into one another like a puzzle
• he then suggested that they had drifted apart and that mountains foremen when the edge of a drifting continent collided with another causing it crumpled and fold upwards

EVIDENCE

• fossil records of modern day species show very similar bone structure across the globe which is too similar to be a coincidence therefore they must have been from a common ancestor that lived together before the continent was split e.g. pentadactyl limb
• fossil records also show species in deserts that are designed to live in forest biomes therefore the plates must have moved to facilitate this change in climate
• northern mountain ranges had very similar rock compositions and layered strata had a distinct pattern that meant mountain ranges must have been split up

Question 3

Explain Harry Hess’s theory of paleomagnetism and sea floor spreading.

What evidence is there to support his theory?

Answer 3

• using sonar he realised that the sea floor had mountain ranges under the water
• this led to the discovery that the sea floor is getting progressively older as you get closer to the shore and newer rock must be being produced at the centre where new magma is rising and cooling
• as molten rock came to the surface it pushed the crust outwards
• pushing at this divergent margin and pulling at the convergent margins (near the coast) caused sea floor spreading
• because the earth is not growing he knew that rock must me destroyed somewhere else, this takes place at the coastal convergent margins as the oceanic plate is subducted by the Continental plate and melts in the Benioff zone

EVIDENCE
-the magnetic poles of the earth flip backwards and forwards regularly over 100,000s of years (sometimes even millions). When the magma rising from the mantle cools magnetic minerals in the magma cool in the direction of the magnetic field at that time. This means that horizontal bands of cooled magma form parallel to each other and the central ridge. This is because after a series of magnetic reversals a pattern can be seen displaying a series of cooled magma strips with magnetic minerals facing in opposite directions

Question 4

Explain subduction and slab pull.

Answer 4

• a magma plume, heated by 5e core, rises to the asthenosphere
• here it slowly pulls the “slabs” of crust apart as it cools
• in the centre of the plume a ridge is formed
• eventually the slabs meet a less dense slab which subducts it forming a trench
• these currents operate as cells