Paleomagnetism and Seafloor Age

Paleomagnetism

Magnetic Stripes Scientists have long known that the magnetic field surrounding the Earth has reversed on multiple occasions in Earth's history. If you were alive when the field flipped, your compass would point towards the South Pole rather than the North Pole. This information has been used to confirm Hess's theory of seafloor spreading.

Imagine that you were watching a bunch of people playing in a swimming pool. If you could instantaneously freeze the pool solid, then the people in the pool would be locked in whatever position they were in when the pool froze. In exactly the same manner, magnetic minerals in liquid magma can spin freely to align with the Earth's magnetic field, but when this magma rises to the surface and solidifies, the magnetic crystals can no longer pivot, even if the Earth's magnetic field shifts again.

Samples of the seafloor, taken from both sides of a mid-ocean ridge, show several interesting characteristics. The first is that, as the distance from the ridge increases, the orientation of magnetic crystals in the rock change - for a while, they point north, then switch to south, then back to north, then south, and so on. The second interesting discovery is that the orientation of the minerals in the rocks on one side of the ridge are essentially a mirror image of the other side.

So, as the magma rises to the surface, the magnetic crystals in it are aligned with Earth's magnetic field. When the magma cools and hardens, they are frozen in place. As more magma rises, this new crust is pushed outward. When the magnetic field flips, all of the magnetic crystals in molten magma realign themselves, and are frozen in this new orientation as they rise, cool, and harden. The alternating stripes that line the seafloor are convincing evidence that Hess's theory is correct.

Seafloor Age

Crustal Age Not all rocks are the same age. Using various techniques, scientists have been able calculate the age of rocks (a "brand new" rock is defined as one that has just cooled and hardened from magma), and by applying these techniques around the globe, geologists have accumulated more evidence to support Hess's theory of seafloor spreading.

The youngest rocks on the Earth are consistently found near the mid-ocean ridges, and the age of the rocks increases the farther they are from the ridges. The oldest oceanic rock is found near the continents - this rock has slowly traveled all the way from the mid-ocean ridges until it reached the less dense continental crust, where it plunges back down into the mantle to be re-melted.

In most regions, this distance takes about 200 million years to cover. There are a few places, such as the region between the Eurasian and African plates, where the oceanic crust is moving even slower. As a result, this part of the ocean floor, beneath the Mediterranean Sea, dates back to 340 million years old - back to when Wegener's supercontinent of Pangaea broke apart.

IMAGES CREATED BY GAVS OR OPENSOURCE