Pillow structure

From Wikipedia, the free encyclopedia

Pillow lava are pillow-shaped structures sometimes seen in lavas and are attributed to the congealment of lava under water, or subaqeous extrusion. A pillow structure in certain extrusive igneous rock is characterized by discontinuous pillow-shaped masses, commonly up to 1 metre in diameter. Pillow lava commonly occuer at Constructive plate boundaries, forming part of a mid-ocean ridge. They are created when molten magma reaches the surface, but as there is a large difference in temperature between the lava and the water, the surface of the globule cools very fast, forming a skin. The skin cools a lot faster that the inside of the globule, so it is very fine grained, with a glassy texture. The magma inside the pillow cools more slowly, so is slightly coarser grained than the skin, but still classified as fine grained. There are often whole areas or lines of pillow lava. This is because when more magma rises from the mid-ocean ridge, the skin can no longer hold it in, so more pillow lavas erupt from the first one. Pillow lava can also formed when lava flows directly from the vent of the volcano into the sea.