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Aa Lava | John Seach
Aa lava is solidified and clinkery. The surface is sharp and broken into many pieces called clinker.
Walking over aa lava surface can be dangerous because of its unstable and sharp profile.
Size of the clinker in aa lava ranges from a few centimetres to a metre. Some of the fragments show rounding from abrasion during the flow.
In aa lava dust comprises only a small proportion of the flow. Clinker in aa lava is different from cinder or scoria. The main difference is denseness and their spinosity.
The upper clinkery layer of an aa lava flow is underlain into a central massive layer. The percentage of clinker is about 50 % of the aa lava flow, and ranges from 15-65 percent. Some aa lava flows have clinker at the bottom, but this is generally thinner.
Aa lava flows are observed to move with a cooled surface carried over a molten core and to be thicker than pahoehoe flows. Aa flows are common early in eruptions when effusion rates are high, whereas long duration, low effusion eruptions produce pahoehoe flows