Rocks Melt

A Lake of Lava
During a 36-day eruption in 1959, the Hawaiian volcano Kilauea produced rivers of red-hot lava that filled the adjacent crater, Kilauea Iki, to a depth of 135 m (440 ft) and created a lake of molten rock. The Kilauea lava lake provided scientists with a natural laboratory and a rare opportunity: a chance to observe lava slowly crystallizing. From 1959 through the early 1990s, they closely monitored the lake as it cooled and solidified.

Race to the Surface
Magma's upward journey is a race: Will it erupt at the surface? Or will it cool and crystallize below ground? Depending on the outcome, two kinds of igneous rocks form.

Volcanic Rocks. If magma ascends faster than it cools, it erupts and solidifies at the surface to form a fine-grained volcanic rock. The magma erupts from volcanoes, which are named after Vulcan, the Roman god of fire. The grains of this volcanic rock are enclosed in a glassy matrix—evidence that the magma cooled within minutes to days, turning the liquid portion to glass.

Plutonic Rocks. If magma cools faster than it ascends it becomes a mush of crystals and liquid that stops in its tracks underground to form a coarse-grained plutonic rock. Plutonic rocks are named after Pluto, the Roman god of the underworld. Like all plutonic rocks, this one is made completely of crystals, with no glass or gas bubbles. The large crystals fit closely together—evidence that the rock cooled underground over tens to hundreds of thousands of years.