How does sea-floor spreading account for the age of sea floor?
The sea floor can only be so old. The sea floor is constantly spreading and creating a new floor. In the middle of the Atlantic, a ridge of volcanic mountains that diverge (spread apart) exposes lava. This lava is the new sea floor. As the the floor moves apart it is subducted underneath continents, therefore the ocean floor can only be so old. I believe its maximum age is about 1.8 million years old. There is a lot of information on this on the web. I gave a very basic answer. Look up the mid ocean ridge and how its related to ocean floor age. I hope this helps!
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One method is by looking at the iron crystals in the sea floor. The magnetic poles reverse about every 100,000 years. Looking the crystalline alignment can show the pole alignment. Counting the pole shifts is like looking at tree rings for estimating age.
I have no scientific basis for this, but I assume it has to do with layers of sedimentation.