"Aa" (pronounced "ah-ah") lava is rubbly and often jagged on the surface. This "texture" forms as somewhat viscous, or pasty, lava flows cool and solidify on their surface, but are disrupted and broken as the still molten interior continues to flow, dragging the exterior along for the ride. This implies that the rubble we see at ground level should give way to more or less solid rock at some depth below. There are some magic asterisks after "solid rock," though. First, as the deeper lava cools and shrinks, it will develop fractures called joints to relieve the tension caused by decreasing volume. Second, lava flows like this often move like a bulldozer or tank tread: the upper surface moves forward and rolls over at the front of the flow, then stabilzes, fixed on the ground where it came to rest. That rollover process further breaks up the cool exterior and creates a bed of broken rock called a "basal breccia." Finally, in an active volcanic area like McKenzie Pass, there are flows on flows on flows, so if you were to dig a shaft or borehole here, you'd expect to see that sequence of {aa-> solid with joints-> basal breccia} repeat many times. I don't know that the aa and basal breccia would necessarily be easily distinguishable, though, so it would probably look more like {breccia->solid with joints} repeatedly.
This is key to understanding all the
gushing water we saw in the upper McKenzie Valley...
Photo unmodified. July 6, 2013.
FlashEarth location.
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