This cross section of an andesite lava flow shows internal features that are commonly found in thick flows. This flow is about 40 m thick. Thick flows often consist of massive interiors with columnar or blocky jointing, which is related to the slow rate of cooling within the flows. The tops and bottoms of thick flows, however, typically are surrounded by an irregular layer of blocky rubble (note base of flow, left). These rubble layers result from cooled, spiny lava on top of and along the steep front of an advancing flow. As the jagged flow front creeps forward, it steepens until small and large sections break off and roll ahead of the flow; the repeated collapse of the flow front produces a rubbly layer over which the flow moves.