Pillow structure

Pillow lava

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 occur 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.

Pillow lavas are a useful way-up criterion in geology. There are three key ideas that can be used as part of this, and that a pillow lava will show if it is the correct way-up: 1. Vesicles (which are gas pockets found in igneous rocks. Gas is in solution when it is pressurised but as it is depressurised there will be expansion) will be found towards the top of a pillow as the gas will be less dense than the surrounding rock (typically of basaltic composition). 2. The pillow structures will show a convex upper surface 3. The pillows will have a tapered base downwards as they have moulded to the surrounding pillows during their formation.

Pillow lavas can be found in many locations across the world, modern day pillow structures are forming off the coast of Hawaii, and other excellent field locations include places in California such as Avila Beach.