Pillow lava is a lava flow that forms underwater, usually at the bottom of the oceans. Individual pillows are mostly up to one meter in diameter. Pillow lavas are extremely common on the Earth’s surface — they form the upper part of the oceanic crust, but we have a chance to see them relatively infrequently because very few of us have ever visited their natural birthplace.
Pillow lavas are bulbous, spherical, or tubular lobes of lava. They form during eruptions with relatively low effusion rates. Slow extrusion gives enough time for a thick crust to form on all sides of a pillow lobe, and prevents individual pillows from coalescing into a sheet. Internally the pillows are fed via a distributary system of interconnected channels. Pillows are not typically hollow and tend to solidify all the way through. Pillows often have lineations or scrape marks on their sides that form during extrusion. Pillow flows are produced by the piling up of individual pillow lava lobes. As a pillow flow forms, the newest pillows are erupted from the top of the stack and flow outward a limited distance before freezing, a process which tends to produce steep-sided mounds or ridges which can grow to be 10's of meters thick. Within the 1998 eruption area at Axial Volcano, pillow lavas only occur along the edges of the flow, where lobate lava fronts were thin and stalled out. Castle Vent is located on an older pillow ridge that formed prior to the 1998 eruption
Sometimes though they get pushed on top of the continental crust for us to take a look. One very good place to admire pillow lavas is Cyprus. The Troodos ophiolite, which makes up large part of the island, is a complete section of the former ocean floor. Hence, pillows are common in Cyprus.
Pillow lavas may show radial cracks (when you have a chance to see the cross section). These cracks are similar to cracks in columnar basalt. The cracks are perpendicular to the margin of the pillow, that’s why they are radial. Cracks in columnar basalt are parallel because they formed in a lava flow that cooled as a sheet. Another common feature of pillows are chilled margins. This is very fine-grained or glassy outer part of the pillow which cooled very rapidly in cold seawater. This material is called tachylyte. Some of the pillows below show radial cracks and some have chilled margins, which may be altered to palagonite.
Pillow lava in Cyprus is very common. It is often stumbled upon it unintentionally while looking for something else.
Updated on : Aug 13, 2016 View : 208