lava flows have a rough surface made up of broken blocks of lava.
These broken pieces of lava are called clinkers. It looks like little
lava spines are sticking up from the lava as it moves. The lava is
like paste because it is so thick
and the clinkers travel on top of the lava flow. Below the top layer
of the lava flow there is a core of slow viscous lava. The clinkers
are burried by the front of the lava flow as the lava moves.