Plate Tectonics

Question 1

The Structure of the Earth - Core

Answer 1

At the centre of the earth is the CORE, which is split into an inner and outer core.

The core is made up of dense rock including iron and nickel, with the temperature of the inner core around 6000 degrees. The heat is produced mainly because of primordial heat left over from the earth’s formation and heat produced by radioactive decay of isotopes.

The outer core is semi-molten and also contains lots of iron and nickel.

Question 2

Mantle

Answer 2

Around the core is the mantle, which is mostly made out of silicate rocks.

• The part of the mantle nearest the core is quite rigid. The temperature of the mantle is around 1000-3500 degrees.
• The upper part of the mantle is called the asthenosphere, which is semi molten (it can flow)
• The very top bit of the mantle is rigid

Question 3

Crust

Answer 3

The outer layer of the earth is called the crust. The rigid part of the mantle and the crust together are called the lithosphere.

There are two types of crust

• Continental - Thicker (30-70km) and less dense
• Oceanic - Thinner (6-10km) and denser

The idea that the earth lithosphere is made up of plates that move is called the theory of plate tectonics.

Question 4

Plate Tectonics - Continental Drift

Answer 4

Continental drift was originally proposed by Alfred Wegener is 1912.

He published a theory that a single continent existed around 200 million years ago called Pangea. This then slowly broke up and drifted apart forming two landmasses called Laurasia and Gondwanaland. Further splitting of the two landmasses has led to today’s current positions of continents.

Wegner had evidence but was unable to explain how large continents could move, and therefore his ideas gained little ground. However, from 1940’s onwards evidence began to accumulate and show that Wegener could have been correct.

Question 5

Evidence for Continental Drift

Answer 5

• The fit of the continents - The east coast of South America and the West coast of Africa.
• Fossils - Fossils of the same species were found on several different continents. Fossils of the Mesosaurus were found in both South America and Southern Africa.
• Rock sequences in Northern Scotland closely agree with those found in Eastern Canada.
• Sea floor Spreading - Harry Hess dated the rocks and realised that the youngest rocks were closest to the ridge. As tectonic plates diverge, magma rises to fill the gap, then cools to form new crust. Over time the, the new crust is dragged apart and even more new crust forms between it.

Question 6

Plate Movement

Answer 6

Convection Currents
- The earth’s mantle is hottest near to the core, so lower parts of the asthenosphere heat up, become less dense and rise.
- As they move towards the top of the asthenosphere they cool down, become less dense and slowly sink.
- This circular movement of semi-molten rock are called convection currents.
- They create drag on the base of tectonic plates, causing them to move.

Slab Pull
- At destructive plate margins, the denser oceanic crust is forced under the less dense continental crust. The sinking of the plate pulls the rest of the plate towards the boundary.

Ridge Pull.
- 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, forming a slope.
- The new crust cools and becomes denser. Gravity causes the dense rock to move downslope
- This puts pressure on tectonic plates, causing them to move apart.