The gases vesiculate and accumulate as they rise through the magma conduit.
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In Vulcanian eruptions, highly viscous magma within the volcano make it difficult for vesiculate gases to escape.
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These deeply cracked volcanic chunks form when the exterior of ejected lava cools quickly into a glassy or fine-grained shell, but the inside continues to cool and vesiculate.
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More examples
Become vesicular or full of air cells; "The organs vesiculated"
(vesiculation) the formation of vesicles in or beneath the skin