Littoral Cone Formation

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Bench Formation and Collapse: Stay Away!

Lava from Kilauea volcano travels many kilometers before finally reaching the ocean. The interaction between lava and seawater, while showcasing Madame Pele’s beauty and power, also produces extremely dangerous conditions around the shoreline entry; both scientists and visitors must remain far back for safety.

The Poupou ocean entry and bench featured a  short-lived lava flow in Hawaii Volcanoes National Park. Lava is rarely seen cascading onto a bench; it quickly tubes over, and only steam is visible, along with explosions of spatter thrown into the air. Seen from the east, the Poupou bench of temporary, unstable land is resting on sand, which will eventually be washed away, leading to the bench collapse.

When molten lava first enters the sea, the water flashes to steam, and much of the lava explodes into small fragments of glass. If the entry flow continues long enough, this glassy black sand will accumulate to form a delta and then a beach that will gradually be covered by encroaching lava flows to form what volcanologists call a bench. Once this bench is established, new lava can flow onto it, as seen in the Poupou entry in Hawaii Volcanoes National Park, photos above. Continued flow of lava into the ocean will extend the delta far beyond the stable shoreline. 

If enough water gains access to the tube, it can occasionally produce much larger steam explosions that can throw rocks and molten lava more than 100 meters back from the edge of the bench.  The view is best from a half-mile upwind; close views are obscured by steam, and are potentially deadly.

Lava enters the ocean in 2004. Note the unstable bench of temporary land. Moments later, seawater interacts violently with the lava, hurling rocks far inland. Stay far away from the lava bench!

Although the bench can look quite substantial, experienced lava watchers know that the solid-looking bench lava is supported only by a thick bed of unstable sand and that the larger the bench, the steeper and less stable is the submarine face of the growing sand delta.  The continual wave action will, at some point, undermine the submarine sand until the bench can no longer be supported and, with virtually no warning, the bench will collapse into the ocean. When the bench collapses, seawater rushes into the nearly molten core of the bench, producing enormous amounts of superheated steam that propel rocks and molten material far inland. Bench collapses are unpredictable, and hence, visitors are required to stay far back behind roped-off areas.

Immediately following a partial bench collapse, spectacular fountaining and bubble burst activity may occur, usually for only a few hours. This is sometimes referred to as “pseudo fountaining,” because it is the result of seawater mixing with lava, not volcanic gases expanding. Basically, when a section of bench falls into the sea, it cracks the adjacent rock; seawater can then seep into the cracks, and become trapped in the lava tube. The lava and seawater create an explosive mixture that blasts a hole through the roof of the lava tube, throwing molten lava into the air.

 

In this video, shot in 1994, both fountaining and bubble bursts are seen. The resulting spatter piles up to form littoral cones. The telephoto lens provides details not otherwise observable.

Littoral cone

In the photo at left (T. Mattox, USGS), the 1994 littoral cones are seen by daylight, providing context for the CSAV video shot at night.

Sometimes, bubble bursts expand the molten lava to paper-thin dimensions; the air immediately cools the bubble to glass, which breaks and is blown away by the wind. These thin shards of glass are called “Limu o Pele,” or Pele’s seaweed.

Limu