Tectonic Plates

Plate Tectonics is a theory developed in the late 1960s, to explain how the outer layers of the Earth move and deform. The theory has caused a revolution in the way we think about the Earth. Since the development of the plate tectonics theory, geologists have

had to reexamine almost every aspect of Geology. Plate tectonics has proven to be so useful that it can predict geologic events and explain almost all aspects of what we see on the Earth. Although we have discussed plate tectonics throughout the course, in this chapter and lecture we look at how the theory came to be discovered and some of the implications for the evolution of the Earth.

Alfred Wegner was a German Meteorologist who studied ancient climates in the early 1900s. Like most people the jigsaw puzzle appearance of the Atlantic continental margins, caught his attention. He put together the evidence of ancient glaciations and the distribution of fossil to formulate a theory wherein the continents moved over the surface of the Earth, sometimes forming large supercontinents and other times forming separate continental masses. He proposed that prior to about 200 million years ago all of the continents formed one large land mass that he called Pangea .
The weakness of Wegner’s theory, and the reason it was not readily accepted by geologists was that he proposed that the continents slide over ocean floor. Geophysicists disagreed, stating the ocean floor did not have enough strength to hold the continents and too much frictional resistance would be encountered.
In 1950s and 1960s, studies of the Earth’s magnetic field and how it varied through time (paleomagnetism) provided new evidence that would prove that the continents do indeed drift. In order to understand these developments, we must first discuss the Earth’s magnetic field and the study of Paleomagnetism.

The Earth has a magnetic field that causes a compass needle to always point toward the North magnetic pole, currently located near the rotation pole. The Earth’s magnetic field is what would be expected if there were a large bar magnet located at the center of the Earth (we now know that this is not what causes the magnetic field, but the analogy is still good). The magnetic field is composed of lines of force as shown in the diagram here. A compass needle or a magnetic weight suspended from a string, points along these lines of force. Note that the lines of force intersect the surface of the Earth at various angles that depend on position on the Earth’s surface. This angle is called the magnetic inclination. The inclination is 0o at the magnetic equator and 90o at the magnetic poles. Thus, by measuring the inclination and the angle to the magnetic pole, one can tell position on the Earth relative to the magnetic poles.

In the 1950s it was discovered that when magnetic minerals cool below a temperature called the Curie Temperature domains within the magnetic mineral take on an orientation parallel to any external magnetic field present at the time they cool below this temperature.At temperatures above the Curie Temperature, permanent magnetization of materials is not possible. Since the magnetic minerals take on the orientation of the magnetic field present during cooling, we can determine the orientation of the magnetic field present at the time the rock containing the mineral cooled below the Curie Temperature, and thus, be able to determine the position of the magnetic pole at that time. This made possible the study of Paleomagnetism (the history of the Earth’s magnetic field). Magnetite is the most common magnetic mineral in the Earth’s crust and has a Curie Temperature of 580oC

 

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