Lava flow tops are crusty and bubbly.  They represent where the lava chills against the much colder air.  This crust of the lava is stiffer than the hotter lava inside the flow, and it sometimes tends to fracture into pieces.  At Frankilin Square, under the flagpole, is a large boulder of basalt which contains broken fragments of lava crust, bubbly pieces of the cooled flow top. If you look carefully at the boulder you can see these pieces clearly. Hawaiians call this kind of surface “aa”.  It is much harder to walk on than smooth lava crusts called “pahoehoe”.  The boulder was broken from outcropping by the glacier and dropped into a moraine deposit.