Komatiite

Komatiite is one of Earth's weirder rocks. It has the composition of the mantle rock peridotite, but it was clearly a molten lava—something that would have taken several hundred degrees more heat than Earth is capable of today. That's why komatiite is almost exclusively confined to the Archean Eon, from the first half of Earth history when radioactive heating was greater. And yet in many places it displays this hashwork of long olivine crystals known as spinifex texture.

Crystals usually take a long time to grow so big (as much as a meter in length), much longer than a lava flow would allow. The usual story is that the lava must have been both superheated and supercooled, so that these things crystallized in a flash. The story goes on to say that the lava must have been as fluid as water, plus other outlandish details. An outlandish story can be plausible for sound reasons. But recent papers in Nature and in the Journal of Petrology point instead to a very steep thermal gradient that encouraged olivine to grow so oddly: the thin crystals acted like heat pipes, transmitting heat by both conduction and radiation. Maybe the story will become a bit less outlandish—or outlandish in different ways.

More igneous rock types
More igneous rock textures
More on the Archean

Spinifex texture — Photo courtesy Mayla Oliveira of Flickr