pillow lava

noun Geology.
lava in the form of an agglomeration of rounded, pillow-shaped masses, the result of subaqueous or subglacial volcanic eruption.


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World English Dictionary
pillow lava
lava that has solidified under water, having a characteristic structure comprising a series of close-fitting pillow-shaped masses

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American Heritage
Science Dictionary
pillow lava   (pĭl'ō)  Pronunciation Key 
Lava that forms from an underwater eruption and is characterized by pillow-shaped masses. Pillow lava forms when hot lava is suddenly exposed to cold water, forming a sacklike membrane that is filled with additional cooling and solidifying lava. The outer membrane is usually finer grained than the inner lava.
The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Example sentences
Pillow flows are produced by the piling up of individual pillow lava lobes.
Pillow lava is produced when hot, fluid basaltic lava erupts under water.
Pillow lava, a type of lava found in some of the rocks, is evidence that much
  of the volcanic action took place underwater.
Lobate and pillow lava flows are formed during lower eruption rate, which
  allows the seawater to cool and harden the lava faster.
Images for pillow lava
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