The grain of a rock tells you how fast its magma cooled
Granite has crystals big enough to see because its magma cooled slowly, deep underground, wrapped in insulating rock. Basalt is fine-grained or glassy because lava froze fast in open air or water. Reading grain size, geologists can tell where a fiery rock was born.
Igneous rock, from the Latin for fiery, is one of the three great rock families alongside sedimentary and metamorphic. It forms when molten material solidifies. That melt starts as partially molten mantle or crust, produced when temperature rises, pressure drops or composition changes. Together with metamorphic rock, igneous rock makes up 90 to 95 percent of the top 16 kilometres of the crust, forms most of the ocean floor, and covers about 15 percent of today's land surface.
Rock that crystallises inside the crust is intrusive. Surrounded by country rock that holds in heat, it cools slowly into coarse grains visible to the naked eye. Deep plutonic bodies form the cores of great mountain ranges, and erosion can expose these batholiths over vast areas; shallower intrusions form sheets such as dikes and sills. Magma that reaches the surface becomes lava and hardens as extrusive, or volcanic, rock. Basalt is the most common kind, building lava plateaus and sometimes cracking into polygonal columns like those of the Giant's Causeway in Northern Ireland.
How lava behaves depends on its viscosity, set by temperature, chemistry and crystal content. Hot basaltic lava pours like thick oil and spreads in long thin flows. Andesite, of intermediate composition, can be as stiff as cold molasses and builds cinder cones of ash and lava. Gas-rich felsic and intermediate magmas often erupt explosively as water vapour and carbon dioxide escape, blasting out tephra and ash that can blanket huge regions.
These rocks are scientific records. Their minerals reveal conditions in the mantle or lower crust where the magma formed, radiometric dating of them helps calibrate the geological time scale, and their features point to particular tectonic settings. Some host valuable ores: granites and diorites are often linked with tin, tungsten and uranium, gabbros with chromium and platinum.
Source: Igneous rock
