from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. one of the major groups of rock that makes up the crust of the Earth; formed by the cooling of molten rock, either below the surface (intrusive) or on the surface (extrusive)
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. rock formed by the solidification of molten magma
Sorry, no etymologies found.
So geologists, by measuring the ratio between potassium-40 and argon-40 in a piece of igneous rock that they pick up today, can tell how long ago the rock first crystallized out of its molten state.
Mostly the rocks brought back from the Sea of Tranquility were basalt: a dense, dark-gray, fine-grained igneous rock composed chiefly of calcium-rich plagioclase feldspar and pyroxene; on Earth, basalt is the commonest type of solidified lava.