Paleomagnetism

Paleomagnetism

Paleomagnetism is the study of Earth's old magnetic fields that are stored in rocks, soil, or other 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 saw that compass needles moved near some rocks. In 1797, a scientist named Alexander von Humboldt thought this might be because of lightning.

In the 1800s, scientists learned that some rocks had magnetism like Earth's magnetic field, and others pointed the opposite way.

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

In 1956, a scientist named P.M.S. Blackett made a tool to measure small amounts of magnetism. This helped scientists study old rocks. His work showed that continents move, an idea first suggested by Alfred Wegener in 1915. In 1963, scientists found evidence under the ocean that showed how the seafloor spreads apart.

Fields

Paleomagnetism studies Earth's magnetic field from long ago.

One area is called geomagnetic secular variation. This looks at small changes in Earth's magnetic field. The magnetic north pole moves compared to Earth's spinning axis. Scientists measure these changes using magnetic declination and inclination.

Another area is magnetostratigraphy. This uses times when Earth's magnetic field flipped direction, as recorded in rocks, to help figure out how old the rocks are. These flips happened at uneven times in Earth's history. We learn 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 helps scientists understand how Earth's continents move.

There are other ways rocks can keep magnetic records too. Tiny magnetic particles in mud and sand can line up with Earth's field as they settle. Magnetic minerals can also grow during changes in rocks and record the field at that time. Scientists can often find 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 about 200 million years old, but rocks on land can be much older, up to 3.8 billion years. To learn from these rocks, scientists collect samples that contain magnetite, a special mineral that remembers magnetic directions.

There are two main goals when collecting these samples: to get accurate directions and to make sure results are clear. Scientists use a special rock drill with diamond bits. This drill cuts a round space in the rock. 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, so scientists can remember its original position.

Applications

Paleomagnetic evidence helps scientists learn about how Earth's continents move. It shows that Earth's magnetic field changes direction and that the magnetic poles move over time. This supports ideas about how Earth's surface shifts.

Scientists also use paleomagnetism to estimate how old fossils are. By studying the magnetic record in rocks where fossils are found, they can guess when those fossils lived. This helps us learn about ancient environments and conditions.