Recently huge accumulations of pillow lava have been viewed by NOAA’s Okeanos Explorer in the deep sea off the coast of the American Samoa (w/c 3rd April 2017). The words ‘pillow’ and ‘lava’ don’t seem to make for a comfortable combination, so what is pillow lava, how does it form and what does it look like in the rock record? This article aims to give some insight into the formation of pillow lava with some images of beautifully preserved pillow lava that I recently visited in Oamaru, New Zealand.
What is pillow lava?
Pillow lava is lava that was squeezed out under water to form blobs and pillow-like structures on the sea floor, or at the bottom of lakes. They may also potentially form when lava is extruded beneath ice sheets and glaciers.
The outer layer of the lava quickly cools when it comes into contact with water forming a chilled rim to the pillow. This chilled rim may appear a different colour to the rest of the pillow (for example the black outline of the pillow in the image above), or they may simply be composed of much finer crystals or glass.
In water volcanic glass rapidly becomes altered and decomposes, a process known as devitrification. Therefore many pillow lavas are surrounded by a rim of devitrified material that originally formed a rind of glass.
The centre of the pillow will cool the slowest so may be dense or contain larger crystals, with crystals getting smaller towards the rim. The presence of gas bubbles (preserved as vesicles) may increase towards the rim of the pillow, and some of these may be elongated and stretched into small tubes.
When a lava is extruded into a watery environment the water will be heated and hot water and minerals will circulate through the pillow lavas and any surrounding sediments. These warm fluids can contain dissolved minerals which often precipitate into the vesicles (gas voids) formed in the lava, when this happens the mineral-filled vesicles become known as amygdales.
Thick piles of pillows
Stacks of pillow lava can be seen all over the world preserved within the rock record, from Wales (UK), to the American Samoa, and right the way round to Oamaru, New Zealand.
Pillow lavas aren’t just found in the rock record though, in several locations around the world they can be observed right now forming at the bottom of the ocean!
https://www.pmel.noaa.gov/eoi/nemo/explorer/concepts/pillows.html – Information, pictures and videos of pillow lava formation provided by NOAA
https://oceanexplorer.noaa.gov/okeanos/welcome.html – Keep up to date with the progress and finding of NOAA’s Okeanos Explorer expeditions
https://volcanoes.usgs.gov/vsc/glossary/pillow_lava.html – Definition of pillow lava and gifs produced by the USGS