LAVA Basaltic lava flows erupt primarily from shield volcanoes , fissure systems , and scoria cones . In addition to the smooth and ropy surfaces shown above, solidified pahoehoe flows can also display shelly to slabby surfaces. Shelly pahoehoe contains a billowy flow top with a frothy vesicular surface skin, only a few centimeters thick, overlying large cavities, generally 5-30 centimeters thick. These shelly surfaces often collapse when walking on the top of the flow. Slabby pahoehoe contains a series of closely spaced slabs, a few meters across and a few centimeters thick, broken and tilted by mass movement, or drainage, of the underlying lava.
Slabby pahoehoe is often gradational to a'a lava.
Pahoehoe lavas are typically the first to erupt from a vent. It will slow and be overrun by a new lobe that propagates downslope until it also chills, and in turn is overrun by another flow. Overridding lavas and breakouts on the flow top and sides thus produce compound flows composed of several lobes cooling against one another. Slower moving pahoehoe flows will advance through the protrusion of small bulbous appendages at the flow front, called pahoehoe toes . The image to the left shows a breakout and the advancement of pahoehoe toes along the sides of a ropy pahoehoe lava flow .