Why does melting take place beneath the axis of a mid ocean ridge?

As the mantle starts to ascend beneath the mid-ocean ridge, very little rock lies on the surface of it. So in replacement, large pressure changes take place, as it is then lead to melting. The melt is much less dense than solid. Which also rises along the surface which is where it will play it's big part in forming the oceanic crust.

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d. the weight of the book