Sentence:  We...saw ‘pahoihoi' or solid lava forming, and also ‘<WSD>aa</WSD>' or clinkers. 
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.)