Some of the most spectacular floods on Earth burst out when a volcano erupts under an icecap or glacier. This happens frequently in Iceland, where Grimsvotn volcano under Vatnajokul icecap was the type location for this phenomenon. Also known as glacial volcanic outburst floods, they can be very destructive, releasing huge volumes of water in a very short time, which can also mix with ash on the surface to become lahars. These resemble a fast flowing thick river of mud, their destructive power magnified by its higher density, often incorporating boulders, trees and floating ice blocks the size of houses. The rock formed by these eruptions resemble those formed under water, with pillow lavas and hyaloclastite (a sand sized sediment formed of shards of volcanic glass spalled off by steam when lava meets water).