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Classroom presentations to accompany Understanding Earth , 3rd edition. prepared by Peter Copeland and William Dupré University of Houston. Chapter 19 Exploring Earth’s Interior. Exploring Earth’s Interior. Structure of the Earth.
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Peter Copeland and William Dupré
University of Houston
Exploring Earth’s Interior
Study of the behavior of seismic waves tells
us about the shape and composition of the
interior of the Earth:
Seismology tells us about the density
Continental ice loads the mantle
Ice causes isostatic subsidence
Melting of ice causes isostatic
Return to isostatic equilibrium
Elements that have unpaired electrons (e.g., Fe, Mn, Cr, Co) are effected by a magnetic field. If a mineral containing these minerals cools below its Currie temperature in the presence of a magnetic field, the minerals align in the direction of the north pole (also true for sediments).
The Earth behaves as a magnet whose poles are nearly coincident with the spin axis (i.e., the geographic poles).
Magnetic lines of force emanate from the magnetic poles such that a freely suspended magnet is inclined upward in the southern hemisphere, horizontal at the equator, and downward in the northern hemisphere
declination: horizontal angle between magnetic N and true N
inclination: angle made with horizontal
It was first thought that the Earth's magnetic field was caused by a large, permanently magnetized material deep in the Earth's interior.
In 1900, Pierre Currie recognized that permanent magnetism is lost from magnetizable materials at temperatures from 500 to 700 °C (Currie point).
Since the geothermal gradient in the Earth is ≈ 25°C/km, nothing can be permanently magnetized below about 30 km.
Another explanation is needed.
A dynamo produces electric current by moving a conductor in a magnetic field and vise versa. (i.e., an electric current in a conductor produces a magnetic field.
It is believed that the outer core is in convective motion (because it is liquid and in a temperature gradient).
A "stray" magnetic field (probably from the Sun) interacts with the moving iron in the core to produce an electric current that is moving about the Earth's spin axis yielding a magnetic field—a self-exciting dynamo!
The theory has this going for it:
• It is plausible.
• It predicts that the magnetic and geographic poles should be nearly coincident.
• The polarity is arbitrary.
• The magnetic poles move slowly.
If the details seem vague, it is
because we have a poor
understanding of core dynamics.
We can now use the magnetic
properties of a sequence of rocks to
determine their age.
Based on determining the magnetic characteristics of rocks of known age (from both the oceans and the continents).
We have a good record of geomagnetic reversals back to about 60 Ma.