EPS 101 Spring 2003– Adrian Brearley – Lectures 27 and 28

INTERIOR OF THE EARTH

Keywords and concepts

General Concept:

The internal structure of the Earth can be determined by the behavior of P (compressional) and S (shear) waves as they pass through the Earth.

Key concepts

Shadow zone. The shadow zone is a region of the opposite side of the Earth from an earthquake focus where no P or S waves are received because of the presence of the Earth’s core. For P waves, no waves are received on the angular distance between 105 and 143 degrees from the earthquake, because of reflection of the waves from the core-mantle boundary and refraction of waves when they enter the core. For S waves the shadow zone covers the entire angular distance from 105 degrees in either direct around the Earth from the earthquake. This is produced because S waves cannot travel through the liquid outer core.

Reflection – some seismic waves that hit a boundary within the Earth are reflected by that boundary and return to the surface of the Earth.They do not pass down through the boundary.

Refraction – some seismic waves that hit a boundary within the Earth pass through the boundary. Because of a change in the properties of the material across the boundary, perhaps a change in density or composition, the wave changes its direction as it enters the new layer. Whether reflection or refraction occurs depends on the angle that the wave it’s the boundary.

Mohorivicic discontinuity (or Moho) – the Moho is a boundary at the base of the crust where the velocity of P waves suddenly increases rapidly from 6 km/s to 8 km/s. This boundary is considered to represent the boundary between the crust and the mantle.

Principle of isostasy – the concept that the crust is lighter than the dense mantle and that continents ‘float’ on the mantle. Large mountain ranges have deep roots that extend deeper in the mantle, because the increased mass of the mountains pushes the crust below downwards so that it can continue to float. As material is removed from the mountain ranges over millions of years the weight pressing down into the mantle gets less and the root of the mountain range will adjust by floating up to adjust.

Paleomagnetism

Earth’s magnetic field – the Earth has a magnetic field that is probably produced by motion of the liquid outer core around the solid inner core, such that is behaves like a dynamo. The magnetic field lines of force that radiate out from the Earth’s core can be considered to be exactly like those surrounding a bar magnet.

Paleomagnetism – the study of the magnetic properties of rocks which crystallized and recorded the direction and strength of the Earth’s magnetic field at the time they formed (e.g. igneous rocks). Also termed fossil magnetization.

Thermoremanent magnetization – as certain iron-bearing minerals (e.g magnetite) cool through 500° C, they are magnetized in the direction of the Earth’s magnetic field at that time. This magnetization can be measured and used to determine the direction of the magnetic field at the time the rock crystallized.

Magnetic reversals. Paleomagnetic studies have shown that every half-million years or so, the direction of the Earth’s magnetic field reverses. Why this occurs is not known, but it means that the direction of the magnetic force changes direction by 180 degrees, such that the present north would become south.

Magnetic stratigraphy. By studying the direction of remanent magnetization in thick sequences of volcanic lavas and dating when these lavas formed using radiometric dating it is possible to establish when in time different episodes of magnetic reversals took place. This is called the magnetic stratigraphy and has been very important in establishing the theory of plate tectonics.