How do rocks along the central valley of the mid-ocean ridge provide evidence of sea-floor spreading?
1 Answer
It's primarily due to the dating of the rocks.
Explanation:
The fairly small effusive eruptions that occur at mid-ocean ridges produce a relatively symmetrical flow of rocks that are mostly of basaltic composition. The rocks, as they get further from the rift, get older; alternatively, as the rocks get closer to the rift ("upriver," so to speak), they get progressively younger.
In addition to the age of the rock, there is also generally a thinner layer of sediment covering the rock as it gets closer to the ridge. This is due to the simple fact that rocks that have been around longer will have more time to be covered in "stuff" (in this case the stuff is sediment).
Going one step further, part of the significant evidence of symmetrical seafloor spreading is the matched polarity of the basalt on each side of the rift. This is interesting because it also provides evidence of the directions of the poles during the eruptive event. It's kind of like a ticker tape, which some people call magnetic flip-flops.
