Because the rocks that formed the dike cooled very slowly, it is more massive and is more difficult to erode than the surrounding lava flows.
(the rocks; formed; the dike) (the rocks that formed the dike; cooled very slowly; ) (the surrounding lava; flows; ) (it; is; more massive)
the rocks the rocks that formed the dike the surrounding lava it
formed cooled very slowly flows is
the dike
more massive