All rocks have their origins in the molten lava and are igneous in nature. Over time, the rocks are subjected to various rapid tumults or slow processes of nature and fall into other categories, such as sedimentary, metamorphic, and so on.
Some magma escapes into the atmosphere, where it cools and hardens very rapidly and is exposed to the atmospheric elements of water, wind, existing atmospheric chemicals, and the effects of rapid venting off of gases which create chemical reactions. These rocks are formed under little pressure, and the effects of such gassy processes cause interesting results. Such rocks are called “igneous extrusive”, as they have extruded from their underground source.
In some igneous extrusive rock, the crystals that exist in the cooled magma are prevented from growing large enough to be seen by the naked eye, giving the rock a glassy and smooth appearance. Such rock is called volcanic glass or obsidian. Other igneous extrusive rocks are full of holes from the formation and venting off of gas bubbles or veins of gas. Vesiculated basalt and Scoria are examples of rock that retains the appearance of gassy action. Basalt is another igneous extrusive rock, with it’s fine texture. Some entire mountain ranges are made up of basalt.
When the magma remains confined under the earth’s crust and cools, rocks are formed that are “igneous intrusive” in nature. As the magma intrudes into nooks and crannies, away from the molten source, the crystals that form in them are allowed to grow to great size as the cooling process is vastly slower. The pressures on igneous intrusive rock are enormous, creating other conditions for specialized crystal development and growth.
There is no gaseous process in igneous intrusive rocks. In larger spaces, crystals such as quartz, and the other gem quality stones may grow to impressive size, Other crystals, such as gold and silver grow in smaller interstices and cracks, creating “veins” of valuable minerals.
We must also consider that, over time, some rock is melted back into magma, where it again cools and hardens. Other rock is turned over and exposed, or buried and crushed under the weight of massive trauma and turnover in the earth’s crust, by meteor strikes, and as it is blown out of volcanic craters during eruptions. Other rock is broken down into it’s finest form by glacial, grit laden wind, or water action and builds up in layers that are subject to pressure over time.
As a result, another category of volcanic rock is “clastic” rock. This rock is typically sedimentary or metamorphic rock, but exposure to the heat and explosive activity of reactivating volcanoes gives them special properties that normal sedimentary rocks do not posess.