Smith Rock State Park years ago, a large caldera was formed when overlying rock collapsed into an underground lava chamber. This created a huge amount of rock and ash debris that filled the caldera. That material solidified into rock, becoming Smith Rock tuff. Rhyolite flows intruded along faults in the Smith Rock Tuff. A half million years ago, basalt lava flows from nearby volcanoes covered the older tuff. More recently, the Crooked River cut its way through the layers of rock to create today's geographic features. Smith Rock itself is a -high ridge (above sea level) with a sheer cliff-face overlooking a bend in
Lava buried by the advancing flow. This produces a layer of lava fragments both at the bottom and top of an aā flow. Accretionary lava balls as large as are common on aā flows. Aā is usually of higher viscosity than pāhoehoe. Pāhoehoe can turn into aā if it becomes turbulent from meeting impediments or steep slopes. The sharp, angled texture makes aā a strong radar reflector, and can easily be seen from an orbiting satellite (bright on Magellan pictures). Aā lavas typically erupt at temperatures of . The word is also spelled "aa", "aa", "aa", and "a-aa", and pronounced .