The separating plates release pressure in the mantle, causing the rock below to rise, melt into magma and percolate up to fill the gap.
(the rock below; to percolate up; to fill the gap) (The separating plates; release; pressure; L:in the mantle) (the rock below; to melt; )
the rock below The separating plates the rock below
to percolate up release to melt
to fill the gap
pressure L:in the mantle