Earth Structure

Can you describe or draw a cross section through the Earth and label the relevant layers?Try?

Earth StructureThe specification states that you need to know about:

1) Layers: Compositions, depths and physical states of:

i) Continental and oceanic crust

ii) Mantle

iii) Inner and outer Core

2) Lithosphere and asthenosphere; Describe the nature of the Lithosphere (rigid), Asthenosphere (rheid), Low Velocity Zone,

3) Discontinuities: Depth and nature of discontinuities: Inner core, outer core, mantle, continental crust and oceanic crust.

4) Direct evidence: Such as deep mines, surface rocks, volcanic vents and ophiolites.

5) Indirect evidence: Such as seismic wave velocities, shadow zones, whole earth and rock density and meteorites.

Turn to page 11 of Chernicoff and look at the diagram showing the structure of the Earth.

Earth StructureCrust: the upper layer which is solid (divided into 2 types).Lithosphere: the crust plus the upper part of the mantle (solid).Mantle:

Upper part is solid and is part of the lithosphere.Below this is the asthenosphere which is partly molten and can flow.Rest of mantle is solid.

Outer Core: Liquid.Inner Core: Solid.

What are (lithospheric) plates?Lithospheric plates comprise of 2 parts:

An upper part which is crust.

A lower part which is a solid part of the upper mantle.

This makes a lithospheric plate.

2 Types of Lithospheric Plate.Continental.

Oceanic.

Classification of Igneous Rocks.They can be classified based on their SiO2 content:

Acid: > 66%

Intermediate 52 – 66 %

Basic 44 – 52%

Ultrabasic < 44%

Oceanic CrustTypically made of basic igneous rocks (basalt).

7 - 10 km thick.

Density of 3.0 g/cm3

Will sink at subduction zones.

Forms at Mid Ocean Ridges and is saturated in water.

Young Jurassic to present.

Continental CrustOn average has Acid/Intermediate composition.On average 30 km thick but can be up to 90km thick in mountain ranges.Density of 2.7 g/cm3

Will not sink at subduction zones.Old: 4 billion (Precambrian) to Present

MANTLEMade of an olivine rich ultrabasic rock called: Peridotite.

Higher density than crust (3.3g/cm3)

Upper mantle area - solid peridotite (part of the Lithosphere).

Low Velocity Zone - (50 - 250). The LVZ is also called the asthenosphere (weak sphere) or rheosphere (flowing sphere).

MANTLEIn this zone a very slow movement or creep is possible due to convection currents being present.The convection currents drive plate tectonics.This area is 5% partially molten peridotite. Upper Mantle - (250 - 400). Solid peridotite again.Transition Zone - (400 - 1050). Solid again and covers much of the Upper mantle. This is a transition from low-density form of peridotite to a higher density peridotite.

MANTLELower Mantle - (1050 - 2900). Contains a higher density version of Peridotite (5.4 g/cm3).

COREOUTER CORE

Liquid

Ni, Fe, S, K, Si

(9.9g/cm3)

 

INNER CORE:Solid

Ni, Fe alloy

(13.5g/cm3).

The Driving ForceConvection Currents

Very slow convection currents flow in this asthenosphere.These currents provide horizontal forces on the plates of the lithosphere much as convection in a pan of boiling soup (must be Heinz tomato) causes a crouton on the surface of the water to be pushed sideways.

                                              

                                

The Driving Force 2The convection currents are driven by heat generated within the earth.

This emanates primarily from the core and is transferred to the surface by the convection currents.