For rock lovers
The word igneous comes from the Latin word igneus meaning ‘fiery’. Igneous rocks make up most of the top 9.9 miles of the earth’s crust and are created when lava or magma solidifies.
When magma cools within the crust of the planet, intrusive igneous rocks are formed, such as granite. The rock cools slowly and thus mineral grains can often be seen with the naked eye such as feldspars, quartz, olivines, amphiboles and mica.
The interiors of many mountain ranges are made up of intrusive igneous rocks which can then be exposed by weathering. Rocks formed really deep below the surface are called plutonic or abyssal rocks.
Extrusive igneous rocks are made from the cooling of molten magma from volcanoes on the earth’s surface. The viscosity of lava, due to its temperature, composition and crystal content, affects how it flows, whether akin to thick oil, treacle or rubber.
This kind of rock can be glassy such as obsidian where the mineral grains cannot be seen with the naked eye. They are termed aphanitic from the Greek word meaning ‘invisible.’
Leave a Reply