Lecture Eleven

Question 1

How do we know what the internal structure of the Earth is?

Answer 1

Drilling.

Volcanic activity.

Deeply eroded mountain belts.

High pressure laboratory experiements.

Meteorites.

Seismic waves - natural and man made.

Question 2

How are we going drilling into the Earths crust?

Answer 2

Deepest ever drilled is a little over 12km deep (continental crust).

20 year effort to do this.

Bottom of the hole is 190 degrees C.

Have drilled 2.111km into the oceanic crust.

Question 3

What have volcanic erruptions told us about the structure of the Earth?

Answer 3

Materials from the lower crust and upper mantle are brought up through volcanoes.

Parts of the mantle at the surface are called “xenoliths” which literally means foreign stones.

Question 4

What have mountain ranges told us about the Earth’s structure.

Answer 4

As mountains erode, the rocks at their base, formed deeper in the crust, are exposed at the surface.

Question 5

What are Gneiss’?

Answer 5

Surface sediments that have been almost melted under mountain belts. They are a type of metamorphic rocks.

Question 6

What are seismic waves?

Answer 6

Caused when an earthquake occurs (or an explosion is set off).

Vibrations travel though the Earth.

The speed they travel at, and the path they take is determined by the rock types, temperature and pressure of the rocks.

Question 7

What are the main types of seismic waves?

Answer 7

P waves = primary waves.

Faster than S waves there for get to you first.

Particle motion in the direction of propagaion.

Travel though solids and liquids.

Typical velocity:

4-6km/h in the crust

8-14km/h in the mantle (because this has higher temperature and pressures).

S waves = Secondary waves (or shear waves).

Particle motion is perpendicular to the direction of propagation.

Slower than P waves.

Travel through solids only.

Typical velocity:

3-4km/h in the crust.

6-8km/h in the mantle.

Question 8

How to P waves and S waves move though the Earth?

Answer 8

Question 9

What is the composition of elements in the Earth?

Answer 9

Question 10

What are the chemical and physical properties of the Earth regarding its structure.

Answer 10

Chemical property layers:

Crust

Mantle

Core

Physical property layers:

Lithosphere

Asthenosphere

Mesosphere (lower mantle)

Outer core

Inner core

Question 11

Describe the characteristics of the continental crust.

Answer 11

35-40km deep on average.

Al, Ca, K-rich sillicate (although all the elements that make up the crust originally came from the mantle).

Granit.

Density ~ 2.7-2.8 g/cm^3.

Buoyant = topography (although some continental crust is below sea level).

Base of the crust is called the MOHO.

Question 12

Describe the characteristics of the oceanic crust.

Answer 12

7-10km thick.

Fe, Mh rich silicate.

Basalt.

Density ~3.0g/cm^3.

We called oceanic crust Ophiolite.

Created at mid ocean ridge due to melting of the asthenosphere.

Recycled intot he mante because it is less buoyant than continental crust.

Question 13

Describe the characteristics of the mantle.

Answer 13

2885km thick.

Fe, Mg rich, silicate poor.

Ultramafic e.g. peridotite.

Density ~ 3.5 g/cm^3 at the top and ~5.5 g/cm^3 at the base.

Increase is not gradual - related to layers

1) Upper mantle down to a depth of 400km.
2) Transition zone down to a depth of 670km.
3) Lower mantle to the core boundary.

Soid and not molten as often percieved.

~2% of the mantle is molten and exists between solid grains.

Hot and soft enough for it to flow a few cm a year.

The mantle convects with hot mantle rising and cold mantle descending to the core boundary.

Question 14

Describe the characteristics of the Lithosphere.

Answer 14

Whole of crust and the upperer most part of the mantle.

Total thickness of 100-150km.

Ridged and relatively strong - its layers do not flow.

Moves as the convection in the lower mantle occurs.

Question 15

Describe the characteristics of the Asthenosphere.

Answer 15

Mantle beneath 100-150km and the bottom of the transition zone.

Relatively soft and flows.

Relatively weak layer.

Weaker than the underlying lower mantle.

Boundary between the lithosphere and the asthenosphere is between 1280 and 1350 degres C.

Question 16

Describe the characteristics of the core. `

Answer 16

Two layers with a total thickness of ~3000km.

Outer core:

From 2900-5155km depth.

Liquid iron-nickle alloy, no silicate.

Density ~ 10-12g.cm^3.

Temperature so hot that even at such high pressure, atoms cannot lock into solid framework.

Flow in the outer core generates the Earth’s magnetic field.

Inner core:

From 5155-6370km depth.

Solid iron-nickel alloy.

Probably has Widmanstatten structre, but we can’t see it. There is immense pressure.

Question 17

Why is the planetary magnetic field important?

Answer 17

Shields us from the solar winds - Charged particles from the sun which would strip away our atmosphere over time.

Also acts as a radiation shield.

Allows life on the surface to exist.

Without it, life would only be able to exist deep in the ocean or under ground.

Question 18

What are the temperatures on Earth?

Answer 18

Temperature increases with depth = geotherm.

Near the surface geothermal gradients: 10-30 degrees C per km, average for crust = 25 degrees C per km.

At the core mantle boundary temperature ~ 2500 degrees C.

Overall the average gradient for the whole Earth ~ 1 degree C per km.

High temperatures are the result of:

Residual heat from the initial formation of the Earth under gravitational collapse.

Heat generated by radioactive decay of elements - concentrated in the crust.

High temperatures result in:

Lower strength.
Lower density.

Hot rocks are lighter and want to rise, this is result in mantle convection. That is, hot mantle rises becasue it is less dense and the cold lithosphere and mantle sinks becasue it is more dense. Lithosphere does not convect - relitively cold and strong.