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Paleomagnetism and Hot Spots: Evidence for Plate Tectonics

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  • 0:03 Plate Tectonics
  • 1:30 Paleomagnetism
  • 3:54 Hot Spots
  • 5:08 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Paleomagnetism is the study of past magnetic fields. Hot spots are fixed pockets of heat that well up to form volcanic features. Learn how paleomagnetism and the study of hot spots provide evidence that supports the theory of plate tectonics.

Plate Tectonics

Have you ever had what you thought was a great idea, yet when you shared your idea with others, they all thought you were crazy? We have all been there, and I think you would agree that it is not a good feeling to be told that your ideas are silly.

So, you can sympathize with how the German meteorologist, Alfred Wegener, must have felt when he proposed his theory called continental drift, in which he hypothesized that at one time all of the continents were joined and then slowly drifted apart.

You can image how such a radical theory would have been met with much ridicule from the scientific community, especially at the time of its proposal back in the early part of the 20th century. In fact, Wegener's ideas continued to be debated for decades following his death in 1930.

But then new evidence began to surface that supported a related theory called plate tectonics, which states that the earth's crust is broken up into plates that float on top of a hotter and more fluid layer below.

If this were true, then these plates, known as tectonic plates, could carry the continents away from each other as Wegener had proposed. In this lesson, we will look at the study of paleomagnetism, as well as research on hot spots, and show how they provide strong evidence for plate tectonics, and therefore support for Wegener's earlier ideas about continental drift.

Paleomagnetism

Paleomagnetism is the study of the earth's past magnetic field. It may help you to recall this term if you remember that it is the combination of two words: 'paleo,' which means ancient, and 'magnetism,' which means exhibiting a magnetic field. So, paleomagnetism can really be thought of as the study of an ancient magnet field.

Some of the strongest evidence in support of the theory of plate tectonics comes from studying the magnetic fields surrounding oceanic ridges. Oceanic ridges are underwater mountain ranges that contain a rift down their center where magma seeps up, forming new oceanic crust. You can think of them as mountains created by underwater volcanoes. In the middle of the 20th century, scientists started to recognize some interesting magnetic variations, detected from the ocean floor surrounding these ridges.

Now, they already knew that the rocks formed from this underwater volcanic activity were mainly basalt, which is an iron-rich, volcanic rock that makes up most of the ocean floor. Basalt contains magnetic minerals and as the rock is solidifying, these minerals align themselves in the direction of the magnetic field. This basically locks in a record of which way the magnetic field was positioned at the time that part of the ocean floor was created.

The interesting thing is that when paleomagnetists, who are scientists who study past magnetic fields, took a look at the ocean floor going out away from oceanic ridges, they found magnetic stripes that were flipped so that one stripe would be normal polarity and the next reversed.

How could this be? Well, there was only one good explanation and that was that these oceanic ridges were actually boundaries with tectonic plates pulling apart, which we remember is the main gist of plate tectonics. This movement of the plates allowed the magma to rise up and harden into new rock. As the new rock was formed near the ridge, older rock, which formed millions of years ago when the magnetic field was reversed, got pushed farther away, resulting in this magnetic striping.

Hot Spots

Another source of evidence that the earth's crust is made up of plates that move comes from studying hot spots. Hot spots are fixed locations where magma wells up from deep within the earth's mantle to form a volcano. Hot spots occur within tectonic plates, not at their boundaries.

It's like forcing a pencil, which represents a hot spot, up through the middle of a flat sheet of paper, which represents a tectonic plate. After the pencil is removed, the raised fragments of paper around the puncture remain, much like a volcanic mountain remains.

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