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What is Paleomagnetism?

Tiffany Leonard, Rebecca Gillaspy
  • Author
    Tiffany Leonard

    Tiffany has worked on science curriculum and lesson writing since 2015. She has her Master's in Geology from the University of Illinois and a Bachelor's in Geology and Physics from Carleton College. She taught geology courses while she was getting her MS and was a TA while at Carleton.

  • 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.

Discover what paleomagnetism is and understand its definition. Explore the paleomagnetic evidence that supports the plate tectonics theory, and learn how hot spots prove the existence of tectonic plates. Updated: 11/25/2021

What is Paleomagnetism?

So, what is paleomagnetism? Paleomagnetism definition is the record of the strength and direction of the Earth's magnetic field in rocks. It is also the term for the study of the earth's past by examining these records. The earth generates a dipole magnetic field with magnetic north and south poles, which are currently located near its geographic North and South poles. However, that is not always the case.

Diagram of Earth

Certain rocks and minerals are susceptible to the magnetic field and will align with it during their formation. This signal remains in the rocks after formation and is called remnant magnetism. Some rocks, such as basalt, are more susceptible due to their higher quantities of magnetite and high formation temperatures.

Paleomagnetic data have been used to help decipher many different things about our planet and how it has changed throughout time. For instance, the polarity of Earth's magnetic field has shifted and reversed many times during geologic history. Records of these reversals are still evident in rocks today.

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.

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Plate tectonics

Paleomagnetism was used to help develop our understanding of plate tectonic theory. Plate tectonics is the theory that the surface of the Earth is covered by several large rigid plates made of crustal and upper mantle material, called the lithosphere. These plates float on top of the hotter fluid mantle asthenosphere below it.

The cold lithospheric plates float on top of the hot fluid mantle asthenosphere

Diagram of tectonic boundaries

Plate tectonic theory was derived from the theory of continental drift developed by Alfred Wegener in the early 20th century. In his now iconic paper, Wegener hypothesized that the continents on the planet were once connected. They "drifted" apart from each other over geologic time. He did not have a mechanism for how this drift occurred, so he suggested the continents could plow through the oceanic plate material.

He named his connected supercontinent "Pangea" and used botany, biology, and geology to connect the different landmasses. Some fossilized species were only found in small regions in both Africa and South America that he insisted confirmed his theory.

Wegener was convinced he could piece together the different continents into one as if putting together a puzzle.

Drawing of Pangea

Wegener's theory was hotly contested during his lifetime, and evidence of plate tectonics was not confirmed and widely accepted until decades after his death in 1930. Today, scientists can use paleomagnetism, hot spots, and other data to point to the existence of plate motion.

Paleomagnetic Evidence of Plate Tectonics

Apparent Polar Wander

In the 1950s, scientists began to study the remnant magnetism in rocks. They were able to determine that rocks from the same area with different ages would be aligned with different magnetic poles, insinuating that either the pole or rocks had moved. Initially, the scientists believed this indicated that the magnetic pole had moved from its current position, close to the geographic poles. The determined path was called the polar wander path.

Now, scientists call it the apparent polar wander path because they know that the poles themselves have not significantly wandered over time. Since the paths from one continent, such as North America, did not match with another, such as Europe. Different paths indicated different movements at the same time. These paths are now used to track the movements of the continents as plates move throughout geologic history.

However, this is not the only, or even the main, paleomagnetic evidence used to support the theory of plate tectonics.

Magnetic reversals

Throughout geologic time, the Earth's magnetic field will occasionally switch or reverse polarity, meaning that the magnetic north pole will become the magnetic south pole, and vice versa. These reversals are recorded in oceanic crust, and the best records are created and preserved by the crust generated at mid-ocean ridges.

Mid-ocean ridges create new crust.

Diagram of a mid-ocean ridge

A mid-ocean ridge is a topographic high in the ocean that consists of a massive chain of underwater volcanoes where new crust is created. These features are found all around the globe, though two of the most studied are the Mid-Atlantic Ridge and the East Pacific Rise.

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.

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Video Transcript

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.

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Frequently Asked Questions

How does paleomagnetism support plate tectonics?

Paleomagnetism supports plate tectonics as it can be used to show that the oceanic crust has experienced seafloor spreading, where new crustal material pushes old crustal material away from the ridge. Paleomagnetic data can also be used to show how the continents have moved over time through apparent polar wander paths.

What is paleomagnetism and why is it important?

Paleomagnetism is the record of geomagnetic data preserved in rocks and minerals. Certain minerals are susceptible to the geomagnetic signal during their formation. This preserved signal or remnant magnetism can be used to support plate tectonic theory and explain how the geomagnetic field has changed over time.

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