Sentence:  Cooling and solidification frequently takes a different course [...] in lava flows, producing the clinker-like ‘<WSD>aa</WSD>' lava. 
Meaning of the word marked with <WSD> tags:  A form of lava flow associated with Hawaiian-type volcanoes, consisting of basaltic rock, usually dark-colored with a jagged and loose, clinkery surface. Compare (pahoehoe). (From 19th c.)