(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)
The magnetic field surrounding the Earth protects it and all living things upon it from charged particles ejected by the sun.
What creates this magnetic field? Scientists hypothesize that it is caused by a natural geodynamo deep within the Earth. An incredibly hot, molten metal mixture of iron and nickel is constantly moving around a solid iron core, generating a magnetic field.
As you might guess, a magnetic field produced by moving fluid metal is not static. The magnetic field, which has a magnetic north and south pole, has been changing throughout history — at times reversing directions with the south pole, moving in orientation, and even changing in intensity.
How do scientists know this? The Earth’s magnetic field is actually recorded in many types of rocks when they form. The study of this ancient magnetism is known as paleomagnetism. “Paleo” means old or ancient, so paleomagnetism means “old magnetism.”
By studying paleomagnetism, we can learn more about the Earth’s interior, this geodynamo, and even track the moving continents (plate tectonics) throughout time.
The best recorders of paleomagnetism are volcanic rocks, especially basalt. As a result, Hawaiian volcanoes have been the subject of many paleomagnetic studies. When hot lava pours out of a volcano (or magma flows just under the surface), it contains lots of tiny crystals. Some of these crystals, such as magnetite and hematite, are iron-rich minerals that can be easily magnetized.
For example, using a normal refrigerator magnet, you can pick up tiny pieces of iron, paper clips, and other metal objects, because these objects become magnetized. The same magnet doesn’t really do anything if you place it against a ceramic tile or plastic bottle, however, because the chemistry and structure of these materials don’t easily allow for magnetization.
As the lava or magma begins to cool, these tiny little crystals orient their own north and south poles in line with the Earth’s magnetic field, just as the iron filings become oriented when a refrigerator magnet is near them. These properties include a direction (both declination and inclination) and intensity; collectively these properties are known as the “moment.”
Once the lava has cooled even more—below about 550 degrees Centigrade (1,020 degrees Fahernheit) — the magnetic moments of the crystals are frozen. As long as the rock doesn’t heat up again, these minerals will retain the same magnetic moment from the time they are cooled.
Therefore, if a scientist were to find the magnetic moment and orientation of a rock, he or she is actually measuring the magnetic field that was surrounding the flow when it cooled!
The magnetic properties aren’t visible in a rock, the way olivine crystals or the large air pockets known as vesicles are visible. They can, however, be an incredibly useful tool when identifying flows or studying rocks.
For example, two lava flows next to each other can appear to be very similar.
Sometimes slight changes in color or the amount of vegetation growing on them will help distinguish them, but oftentimes, one needs to look for crystals, the amount of vesicles, or even analyze the chemistry. What if those are very similar, as well?
One more test can be the similarity of their magnetic properties. Since the direction of the magnetic field changes throughout geologic time, different flows, which formed at different times in history, have different magnetic characteristics preserved in them!
Therefore, two lava flows that are similar in all other aspects could be distinguished by the direction of their magnetization.
As paleomagnetic techniques improve, another application has arisen — paleomagnetism as a dating technique. Many scientists have collected massive amounts of data on the paleomagnetic moment throughout time by combining paleomagnetic measurements with dating techniques, such as radiocarbon, lead-lead, or argon-argon dating.
If paleomagnetic measurements can be obtained from a sample, they can be compared to the paleomagnetic properties of dated flows to help constrain the age of the sample.
Paleomagnetism provides one more technique to help scientists study the rocks around them and unravel the secrets of the Hawaiian volcanoes.