hot lava erupts on the ocean floor and cools it not only crystallizes,
but it also becomes magnetic. This is because the magma that seeps
from the Earths mantle has a lot of iron in it. The magnetic
crystals in the cooled lava have become little magnets, all pointing
in the direction of the magnetic field of the Earth. The more of these
little magnetic crystals there are in a rock, the stronger the magnetic
signal of the rock is.
With a magnetometer mounted on Alvin
we can measure small differences in the magnetization of the rocks directly beneath Alvin
. Here we show how Alvins
magnetometer detects weakly magnetic dikes flanked by strongly magnetic lavas beneath the seafloor.
The magnetic field of the Earth. The Earth is like a big magnet with
a North pole and a South pole that lie close to the geographic poles.
The strength of the Earth's magnetic field changes with time, and
about three times per million years it flips so that the field points
completely in the opposite direction. These changes in strength and
direction of the Earth's magnetic field are recorded in
the volcanic rocks beneath the seafloor.