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.
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. I often stumbled upon it unintentionally while looking for something else. It was even easy to get bored of pillows, but I tried to remind myself that as soon as I will go home there are pillows nowhere near, so I took many photos. Here are some of the pillows I saw in Cyprus:
