Pillow lava
Pillow or Pillowlava (this term was in 1890 by John Walter Gregory coined) is solidified underwater lava of pillow-like shape and is therefore considered as a sure sign of subaquatic volcanism . It is a common occurrence along the mid-ocean ridges and is therefore an integral part of ophiolites - oceanic crust pushed onto continental crust . Pillow lavas are also formed when subglacial volcanoes erupt .
The solidified lava owes its glazed consistency to the extremely rapid cooling of the hot molten rock when it comes into contact with water , a medium that can absorb large amounts of heat. A tough, plastic mass of rock glass forms on the rapidly cooled surface . Inside the cushion, the lava cools down much more slowly, so that a crystalline mass forms. This mostly creates radially arranged rings of gas bubbles just below the crust, as the lava experiences a massive pressure relief when it emerges from the rock and thus leads to degassing of the melt. If more lava flows in, the cover tears open at one point and more pillows are created. The individual cushions are 0.2 to 1 m in size and form layers up to one kilometer thick on the ocean floor . In the spandrels between the lava cushions there is volcanic tuff , chipped glassy lava splinters, and seabed sediment.
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