Jurassic pillow basalt at Avila Beach, California. Subaqueous explosions result from magma-water reactions in shallow water bodies or beneath ice. Littoral explosions occur where subaerial lava flows or hot pyroclastic flows intersect with water along shorelines. Littoral cones are formed at the edge of the sea from lava entering the water. Various kinds of explosive hydroclastic processes generate maar volcanoes, low-rimmed craters.
Rim beds inside maar crater, Ukinrek volcano, Alaska.
Arrow points to Dr. Michael Ort, Northern Arizona University, rappeling down crater wall to investigate origin of beds. Dr. Wendell Duffield of the U.S. Geological Survey is on the crater rim holding the rope. These parameters reflect aspects of eruption mechanisms such as flux of magma and water within the volcano, the depth of interaction, and the geometry of the vent. Thermodynamics of hydroclastic systems are very complex, but varying the mass ratios of water and magma produces a systematic effect. Mass ratios less than about 0.3 to 0.4 can be expected to be in the dominantly superheated state, depending upon magma composition.