Paleomagnetism

Paleomagnetism is the study of ancient Earth's magnetic fields that are stored in rocks, soil, or old materials. Scientists who study this are called paleomagnetists.

Some minerals in rocks can remember the direction and strength of Earth's magnetic field when the rocks were formed. This helps us learn about how Earth's magnetic field has changed over time and where pieces of Earth's crust, called tectonic plates, used to be located. The records of times when Earth's magnetic field flipped, called geomagnetic reversals, help scientists create a timeline for studying very old rocks.

Discoveries in paleomagnetism helped prove that continents move. Paths showing how the poles seemed to move gave the first strong proof that continents drift, and patterns in the ocean floor showed that new seafloor spreads out from deep underwater ridges. Paleomagnetism also helps us understand the magnetic fields of other objects in space, like rocks from the Moon and meteorites, by studying their magnetic history.

History

Main article: History of geomagnetism

In the 1700s, people noticed that compass needles moved near certain rocks. In 1797, a scientist named Alexander von Humboldt thought this happened because of lightning strikes. In the 1800s, scientists found that some rocks had magnetism that matched Earth's magnetic field, while others pointed in the opposite direction.

In the late 1920s, a Japanese scientist named Motonori Matuyama discovered that Earth's magnetic field had flipped a long time ago.

In 1956, a British scientist named P.M.S. Blackett created a special tool to measure tiny amounts of magnetism. This helped scientists study old rocks better. His work supported the idea that continents move over time, which was first suggested by Alfred Wegener in 1915. Later, in 1963, scientists found evidence under the ocean that showed how the seafloor spreads apart.

Fields

Paleomagnetism looks at Earth's magnetic field from long ago on different scales.

One area is called geomagnetic secular variation. This studies small changes in the direction and strength of Earth's magnetic field. The magnetic north pole moves around compared to Earth's spinning axis. Scientists measure these changes using magnetic declination and inclination, and also study how strong the magnetic field was.

Another area is magnetostratigraphy. This uses the history of times when Earth's magnetic field flipped direction, as recorded in rocks, to figure out how old the rocks are. These flips happened at uneven times throughout Earth's history. We know about them by studying sea floor spreading and dating volcanic rocks.

Principles

The study of paleomagnetism is possible because certain minerals, like magnetite, can remember Earth's magnetic field from long ago. These minerals act like tiny magnets that keep a record of which way north was when the rocks formed.

One way this happens is called thermoremanent magnetization. When rocks like basalt cool down, the magnetic minerals inside can lock in the direction of Earth's magnetic field. This works because these minerals change when heated and can keep a strong memory of the field direction. This type of record has helped scientists understand how Earth's continents move.

There are other ways rocks can keep magnetic records too. Sometimes, tiny magnetic particles in mud and sand can line up with Earth's field as they settle, creating another kind of memory. Magnetic minerals can also grow during chemical changes in rocks and record the field at that time. Even though some magnetic records can be changed by heat or lightning, scientists can often figure out the original direction of Earth's magnetic field from these clues.

Sampling

Scientists study very old rocks to learn about Earth's magnetic field from long ago. The oldest rocks on the ocean floor are only 200 million years old, but rocks on land can be much older, up to 3.8 billion years. To get information from these older rocks, scientists collect samples that contain magnetite, a special mineral that records magnetic directions.

There are two main goals when collecting these samples: to get accurate directions and to reduce uncertainty. One way scientists do this is by using a special rock drill with diamond bits. This drill cuts a round space in the rock, and then a tool with a compass and inclinometer is used to record the direction. A mark is made on the sample before it is collected, helping scientists keep track of its original orientation.

Applications

Paleomagnetic evidence helped scientists understand how Earth's continents move. It showed that the magnetic field of Earth changes direction and that the magnetic poles move over time. This information confirmed ideas about how the Earth's surface shifts.

Scientists also use paleomagnetism to figure out how old fossils are. By studying the magnetic record in rocks where fossils are found, they can estimate when those fossils lived. This helps us learn about the environment and conditions from long ago.