Global tectonics: Earth structure Flashcards
Why is the earth an ellipsoid in shape?
The centripedal pressure exerted on the North and South poles, as the earth spins on its axis has squashed its shape.
What is the Polar and equatorial radius of the Earth?
Polar-6357km
Equatorial-6378km
What depths is the Inner Core between?
6371km to approx. 5000km
What metals is the Inner Core believed to be made from?
Iron and Nickel
Why is the Inner Core believed to be solid and what evidence is there to support this?
solid due to the extreme pressure
P and S waves move through the core ( S waves generated by P waves), S waves only move through solids.
What evidence to do with meteorites is there to suggest the Inner Core is made from Nickel and iron?
Meteorite evidence= Believed whole solar system formed same time from a nebula cloud. Nearly all material coalesced to make planets, some remained to make meteorites and asteroids. Meteorites are either iron and nickel or silicate.
How do we use density to infer the nature of the inner core?
There must be a dense material at the centre of the earth because the average density of the earth is 5.5 gm/cm^3 and the average density of the surface rocks is 2.7-2.8 gm/cm^3
What depth is the Lehmann Boundary at?
5000km
What type of Boundary is Lehmann’s Boundary?
It’s a phase boundary between materials of the same composition but in different states (Outer Core and Inner Core)
How big is the Lehmann Boundary?
100km
Describe the Lehmann Boundary?
A phase boundary. 100km Zone where the rocks change from all liquid through to liquid-solid mix to all solid.
What depth is the Outer Core between?
5000km to 2900km.
What is the state of the outer core and how do we know this?
Liquid
S waves cannot pass through liquid and they do not pass through he Outer Core. P waves slow down due to the reduction in rigidity.
How do we know that the Outer Core is made form Iron and Nickel?
The density graphs show the same pattern as that of the inner core and the same meteorite evidence is true
What is the origin of the Earth’s magnetics field?
The temperature difference between the outer and inner core set up convection currents in the liquid outer core. The convecting mass of molten iron generates electricity which induces magnetism.- self exciting dynamo
What depth is the Gutenburg Boundary at?
2900km
Why is the Gutenburg boundary very clear and distinct?
It is at a change of material from metallic to a stony type of material and it also changes state as it goes form liquid to solid.
What happens to the P-wave velocity and S waves at the Gutenburg boundary?
P wave velocity decreases
S waves stop
What depth is the Lower Mantle between?
2900km to 700km
How do we know the lower mantle is solid?
S waves travel through it.
Why does the velocity of P waves increase as they pass through the lower mantle?
The increasing pressure causes the rocks to become more rigid- less compressible.
What is the lower mantle made of?
The same type of silicates material as the stony meteorites.
How does perovskite form in the lower mantle?
The pressure is high, therefore the elements push together to form tight bonds, creating dense silicate minerals- perovskite.
What depth is the Upper Mantle between?
700km to an average of 30km
