Photograph by L. Keszthelyi on 23 February 1996

Littoral cone

A cone of lava fragments built on the surface of a lava flow pouring into a body of water, usually the sea, is called a littoral cone ("littoral" refers to a shoreline). Lava entering the ocean heats and boils seawater, often generating steam explosions that hurl tephra onto the shore, including spatter, bombs, blocks, ash,, lapilli, and, rarely, limu. As the various tephra accumulates on the shoreline, a well-developed cone may be created.

Tephra generated by steam explosions built the small littoral cone, shown here, along the southern shoreline of Kilauea Volcano, Hawai`i. Note pahoehoe flow nearing the ocean below the cone.

Did you know?

• Once thought to be exclusively created by `a`a flows, many littoral cones have been observed recently on the surface of pahoehoe flows at Kilauea Volcano. Water easily penetrates the clinkery surface of an `a`a flow as it enters the ocean, thereby generating explosions that produce considerable tephra and large littoral cones. The smooth crust on most pahoehoe flows, however, keeps water from invading the the hot interior to trigger steam explosions and form the tephra needed to build a cone. But recent activity at Kilauea has shown that littoral cones form easily on pahoehoe flows above a lava tube that delivers lava to the sea. Incoming waves disrupt lava exiting the tube, which increases the surface area of the molten stream that is exposed to seawater by more than 10 to 20 times. Hundreds of small steam explosions at the open end of a lava tube will slowly build a littoral cone over a period of days to weeks.
• Littoral is often mispronounced. The accent is correctly placed on the first syllable of the word, which means "beach".