Describe how paleomagnetism is used to calculate the rate of seafloor spreading....?

Answer 1

Paleomagnetism involves looking at the earth's magnetic field, and a good way to find out about the properties of the magnetic field in the past is by looking at rocks that contain magnetic minerals. The sea floor is made up of lots of basalt, and basalt has iron-titanium oxide minerals which will preserve the direction and intensity of the earth's magnetic field when the rock cools down past around 900 degrees C.

Since the earth's magnetic field tends to wander over time, get stronger/weaker, and flip in orientation -- the basalts on the sea floor contain a record of what the magnetic field was like at the time they cooled (ie, the time that they came out of the spreading center).

In order to calculate the rate of seafloor spreading you just look at the orientation of the magnetic minerals and based on that decide when the rock cooled. Then you measure the distance from where the rock is now to the spreading center where it cooled from a magma. Since you know when the rock was formed, where it was formed, and how far it has moved in that time, you can very acurately calculate the rate of seafloor spreading.

Answer 2

when magma is erupted at mid-ocean ridges the magnetic minerals contained within will allign themselves with the magnetic poles. As was mentioned, the poles reverse (north pole becomes south pole and vice versa) at an approximately constant rate. because of this you can measure the thickness of each polarized unit and infer how fast the sea floor was spreading. thin units would indicate slow spreading, while wide units would indicate fast spreading.