An intrusive igneous rock that originally fills up a horizontal fracture between sedimentary layers; however, so much magma accumulates, that the laccolith pushes up the overlying sedimentary layers; thus a laccolith is a bubble of magma that separates sedimentary strata.

Laccolith

Explanation:

A laccolith is an intrusion with the shape of a mushroom that grows under the earth's surface. This happens when liquid magma exceeds its way between 2 layers of rock, causing the overlying sediments to puff out as the feature increases. Eventually, this causes the origin of small hills and mountains surrounding the top of the mountain.