If enough gas can coalesce to form a bubble, it may begin to rise through the magma. The roar results when the gas breaches the magma-air interface and escapes suddenly. If this boundary is below the ground surface in an open conduit, then the rush of escaping gas may rip rock from the conduit walls and thrust it upward. Pele's hair is spun from the magma as the gas escapes. Seismogram from the Steamcrack seismic station, located roughly 3 km from the vent.
Time proceeds from left to right and from top to bottom.
The few roaring events we've witnessed recently, however, were too small to record a seismic trace, and timing for larger events is too poorly known to allow correlation between seismic signal and gas escape. The flows are encased within lava tubes for most of their length and are visible only through skylights in the roof of the tube. The tubes discharge their lava at the shoreline. The hot lava, about 1,150 degrees Celsius when it reaches the ocean, generates dense plumes of steam upon contact with seawater. The new lava builds benches beyond the low seacliffs that bound the south coast of the Big Island.