By Robert Hazen, Ph.D., George Mason University
The plate tectonics revolution resulted primarily from the merging of different lines of evidence. The most convincing evidence for the formation of new crust came from sea-floor magnetic data and the age of volcanic islands. While radiometric techniques can be used to date volcanic rocks, sensitive ocean magnetometers can be used to measure the orientation of magnetic minerals on the ocean floor.
Age of Volcanic Islands
One major evidence for moving continents is the age of volcanic islands in the Atlantic Ocean. You can use radiometric techniques; radioisotopes, like uranium-lead isotopes and potassium-argon isotopes, to date volcanic rocks. If you do that, you can date the volcanic rocks from all the islands that occur in the Atlantic Ocean: Iceland, the Falkland Islands, the Canaries, the Azores, and so forth.
In 1964, a Canadian-born geologist, John Tuzo Wilson, working at Princeton University, found a striking trend in the ages of these islands. Islands that lie close to the Mid-Atlantic Ridge are relatively young. For example, Iceland, which is right on the ridge, is still erupting, so some of those volcanic rocks are just a few years old. However, islands far off the ridge are much older. The Falklands, for example, are about 100 million years old.
What Wilson found is that if you plot the distance from the Mid-Atlantic Ridge versus the age of the island, you have a very simple, linear trend. The farther away from the Mid-Atlantic Ridge, the older the island.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
Magnetic Field in Volcanic Rocks
The sea-floor magnetic data has provided the most convincing evidence for the formation of new crust. Sensitive ocean magnetometers were developed by the Navy to find submarines during World War II; they were declassified in the 1950s. They can be used to measure the orientation of magnetic minerals on the ocean floor.
You have a magnetometer, which is just a large device that senses a magnetic field. You put it on a long cable, and you drag it behind your ship, close to the ocean bottom. As you do, you can actually measure, very sensitively, the magnetic field that’s frozen into those volcanic rocks on the ocean floor.
Imagine what happens when hot magma comes out of a volcano and cools. This is a hot molten rock; it contains lots of mineral grains that are gradually going to solidify and form that rock. Some of those mineral grains are magnetic minerals called magnetite.
In a sense, crystals of magnetite act like little compass needles; they actually align themselves with the Earth’s magnetic field. When the volcanic rock freezes, it freezes in the orientation of the Earth’s magnetic field. It’s pretty simple, but what they found was absolutely astonishing.
Learn more about the rock cycle.
Earth’s Magnetic Field gets Flipped
As you drag your magnetometer across the ocean ridge, you’d have the north pole and the south pole aligning with our present North Pole and South Pole very nicely. All of a sudden, everything would switch around 180 degrees, and suddenly what should be magnetic north was magnetic south, and vice versa.
The rocks were magnetized 180 degrees wrong. Now, the rocks didn’t somehow pick themselves up and physically move themselves around; it was pointing to a change in the Earth’s magnetic field itself.
The Earth’s magnetic field, at times in the past, has flipped. We don’t know exactly how this happens; but every so often, perhaps once, on average, every half-million years or so, the North and the South Pole of the Earth flip around.
The Earth itself doesn’t flip; the Earth is still rotating about its axis in the normal way. But magnetic North and South are created by currents deep inside the Earth; sometimes those currents must flip direction through turbulence, or some other thing going on deep inside the Earth.
Learn more about electromagnetism.
Magnetic Reversals Over Ocean Ridges
The rocks record these flips; and what was found was that not only were the magnetic stripes flipping in direction, they were absolutely symmetrically disposed, parallel to the ridge, and symmetrical about it. Right at the ridge, all the rocks that were coming out today had the North Pole in the right orientation.
American oceanographer Drummond Matthews and British geophysicist Frederick Vine reported on these parallel and symmetrical stripes on either side of the ridge in the September 7, 1963, issue of Nature magazine. Their article was entitled “Magnetic Reversals Over Ocean Ridges”, and that 1963 paper was a pivotal point in the history of plate tectonics.
People suddenly realized that there was a mechanism that showed that a new crust was being formed, moving out side-to-side like a conveyor belt. All these ideas were consolidated, then, in a very influential paper by Vine and Wilson, which appeared in Science in 1965. It presented the case for moving continents, in the context of all this accumulated data.
Common Questions about Volcanic Islands and Seafloor Magnetic Data
You can use radiometric techniques; radioisotopes, like uranium-lead isotopes and potassium-argon isotopes, to date volcanic rocks.
A magnetometer is a large device that senses a magnetic field. You put it on a long cable, and you drag it behind your ship, close to the ocean bottom. As you do, you can measure, very sensitively, the magnetic field that’s frozen into those volcanic rocks on the ocean floor.
John Tuzu Wilson was a Canadian-born geologist who lived between 1908 and 1993. While working at Princeton University, he noticed a significant trend in the age of volcanic islands. He realized that the islands near the Mid-Atlantic Ridge are younger and the islands farther away from the Ridge are older.
