Get Grounded in How to Identify 6 Major Igneous Rocks
There are three basic types of rock: Igneous, Sedimentary and Metamorphic. Today we will look more closely at six major types of igneous rocks and how to tell them apart.
Igneous rocks are those created when Magma or molten rock from the interior of the Earth cools and solidifies. This cooling can occur as deep as 50 miles under the surface or as Magma is expelled from the Earth as Lava and solidifies in the air or sea. Those that cool and form underground are called Intrusive and those that form above ground are called Extrusive.
We can tell the difference between intrusive and extrusive rock by noticing the texture of the rock surface. In rapidly cooling magma, the resulting grains and crystals that form the rock are tiny in contrast to the larger grains and crystals found in rocks that cooled much slower deep in the crust. The longer cooling period for intrusive rocks, deeper in the crust and closer to the heat of the earths interior, gave crystals longer to grow. The result is that intrusive rocks have relatively larger grains than those of their extrusive cousins, which are fine grained.
This distinction of fine grained versus relatively larger grain size gives us our first classification clue. Fine grained, to coarse grained and somewhere in between, as the magma that was closer to the surface didn’t cool as fast as the extrusive rocks did, but cooled faster than their deeper down cousins.
The second step to our initial sorting of six major igneous rock types is to note the overall color of the rock. Is it relatively dark or light in color or somewhere in between?
The top row of the above table shows the fine grained extrusive rocks, the bottom row shows coarser grained intrusive rocks.
The rocks on the left are those that are lighter in color, with those on the right darker colored.
The combination of these two characteristics have given us a starting point to understand how to classify and recognize six basic igneous rock types. There is of course much more to the story, which we will explore over time.
The information for this post was based on the 1972 book by Gordon Fay called The Rockhounds Manual which can still be found online at Amazon and is highly recommended for any serious rockhound interested in learning how to identify and find rocks and minerals. Used copies are inexpensive and well worth buying and using. The link below will get you to Amazon where you can order your own copy.