Question

In sea-floor spreading, molten material rises from the mantle and erupts when?

Answer 1

Whenever it feels like it!

Explanation:

Just kidding. Its a complex set of factors that include: the overall density of the magma, due to heat content, has to be light enough for it to keep rising. Once magma start rising it doesn't necessarily keep rising, like say a hot air ballon. It can rise a bit - then pause for a few thousand or millions of years and then go at it again. It can also shift geographically to another location along a spreading centre. Another factor is fractures and strength of the host rock into which it is intruding - usually older basalts.. There is are also quite a number of factors scientist don't understand related to this question.