Some flows issue from the vent as pahoehoe and change to aa as they are stirred, cooled, lose gas, and increase in crystallization. Lava does not convert from aa to pahoehoe. Once a crust has formed on pahoehoe, lava tubes below continue to move the remaining liquid for long periods of time. The size of the tubes range from only a few inches to many feet in diameter. When the source of lava stops, the liquid continues to drain, leaving lava tubes or caves.
Spatter cones are steep sided, conical hills built from the spatter of lava fountains.
Underwater flows produce pillow lava which are ellipsoidal, sacklike blocks about a meter in dimension budding off of lava tongues or tubes. Lavas can have many features depending on the rate in which they cool. The jagged flow front normally creeps forward and steepens until a section becomes unstable and breaks off, revealing the incandescent central core. In this view the leading edge of the flow field is advancing via a series of breakouts of lava from such a tube system. Hawaii In Hawaii, many lava flows form tubes that may extend for several kilometers.