Wednesday, 5 September 2012

What is a Pillow Lava?

A pillow lava is a lava erupted under water. They don't always have the classic "tear drop" shape, but they can be spherical or tube like.
The low eruption rate means that the lava can obtain and retain the shapes. The shape occurs when the hot lava erupting, comes in contact with the cool water. The outer most lava cools very rapidly and forms a glassy "skin", and lava keeps filling the inside of the skin, till it breaks away. The inside of the pillow crystallises much slower than the outside so the grains are coarser. Pillow Lavas occur wherever there is mafic to intermediate grade lava erupting in a sub-aqueous environment. Such locations can include, but are not limited to hot spots and mid oceanic ridges. 
Pillow Lavas can be used to determine way-up structures in the geological record using 3 criteria (these criteria are not always accurate and in some cases a pillow that appears to be the right way up has been altered and the last two criteria only relate to the classic "pillow shape")

photo of pillow lava

1- Vesicles in the lava will congregate towards the top of the pillow
2- Pillows will generally show a convex upper surface
3- Pillows will hvae a base that tappers downwards.
The oldest pillow lava's have been found in Archean Green stone Belts.


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