Tachylite from the interior surface of the roof of a lava tube of Mauna Ulu lava field at Kilauea volcano in Hawaii, USA. Tachylite is basaltic glass, formed by partial remelting of the roof of a lava tube. This sample was formed during volcanic activity that occurred between 1969 and 1974 at the East Rift Zone of Kilauea. This view is about 86 millimetres across.