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Another category of ejecta far more common than volcanic bombs is scoria or
cinder, which refers to lapilli- or bomb-size irregular fragments of frothy lava.
If the cinder contains abundant vesicles (gas-bubble cavities), it is called
pumice, which can be light enough to float on water if the vesicles are closed to
rapid filling by water. In Hawaii, these fragments share a common mode of origin:
all result from sudden chilling of frothy lava from which gases were escaping
during fountaining. During the exceptionally high fountaining episodes of some
eruptions, such as at Kilauea Iki in 1959 or at Pu'u 'O'o (all episodes, 1983 to
present), an extremely vesicular, feathery light pumice, called reticulite or
thread-lace scoria, can form and be carried many miles downwind from the high
lava fountains. Even though reticulite is the least dense kind of tephra, it does
not float on water, because its vesicles are open and interconnected.
Consequently, when it falls on water, it becomes easily waterlogged and sinks.
Some common Hawaiian fragmental volcanic products (top to bottom): reticulite; Pele's tears; volcanic bombs; and accretionary lapilli, spherical accumulations of volcanic ash, generally formed during violently explosive eruptions. (Top two photographs by J.D. Griggs, bottom two photographs by John P. Lockwood.) |
If the scoria or pumice clots are sufficiently soft to flatten or splash as they strike the ground, they are called spatter. The still-molten character of spatter fragments can cause them to stick together to form welded spatter or agglutinate. Drops of lava ejected in very fluid condition and solidified in flight can form air-streamlined spherical, dumbbell, and irregular shapes. Drop-shaped lapilli are called Pele's tears, after the Hawaiian Goddess of Volcanoes. In streaming through the air, Pele's tears usually have trailing behind them a thin thread of liquid lava, which is quickly chilled to form a filament of golden brown glass, called Pele's hair. Pele's hair can form thick mats downwind from high lava fountains near a vent; it also can be blown many miles from the vent. |
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Updated 05.01.97