Igneous rock is formed from magma. It is igneous intrusive, when it forms within a closed internal structure within the earth, where it is not exposed to the atmosphere. Igneous intrusive rock lives under conditions where the crystals and minerals are allowed to grow to much larger size. These rocks can be as small as a mound of compost or large enough to be the underlying structure of entire mountain ranges.
Rock is igneous extrusive when the magma escapes into the atmosphere and forms into rock. Extrusive rocks are referred to as volcanic rocks, because it is a volcanic action that causes the magma to escape to the earth’s surface. They cool quickly, at times so quickly that the crystalline elements contained within the magma do not have the time to form. Volcanic glass and obsidian are examples of igneous extrusive rock that has cooled very quickly.
The other two types of rock are metamorphic, or different types of rocks that have been pressed together under massive pressure over time, and sedementary, or rock that has been formed from layers of fine material, then subjected to pressure over time, may also be subjected to enough heat to return to a liquid state, where the process starts all over.
Igneous extrusive rock undergoes the most dramatic change as volcanic gasses vent off, taking elements away or evaporate, leaving various chemical reactions and elements behind. Atmospheric conditions provide cooling, the introduction of other minerals, and weathering. Other natural forces introduce pressure, crushing, pulverizing, or multiple processes of liquefaction and cooling.
Some igneous extrusive rocks dissolve in the atmosphere.
Although fresh rocks are rare, all rocks are classified as if they were fresh. But few, if any rocks that we are able to observe remain in a state where they reach equilibrium and remain the same, or “fresh” through their existence. There are ever changing pressures, chemical reactions, introductions of new minerals and chemicals which can erode, introduce color, or even the remnants of living material.
And some products of the magma are crystals that continue to grow as long as there is room for them to grow. Crystals can then be subjected to pressures that force them into formation with other rocks. Forces then tear up the earth’s crust and expose the rock and crystals to the atmosphere, where even less equilibrium is likely, and the processes and forces of nature result in continuous change from environmental causes.
The primary concepts in classifying igneous rocks include that they are formed from the cooling magma, whether they are intrusive, or extrusive (exposed to the atmosphere), and how much equilibrium or change occured after the rock cooled and formed.