Igneous rocks form by the cooling and hardening of the molten material called magma.

There are two major groups of Igneous Rocks depending on where they form:

Intrusive Rocks - form while the molten material is still below the surface of the Earth.
These rocks cool slowly giving mineral crystals that form in the rocks a chance to grow into larger crystals.

Extrusive Rocks - form after the molten material, lava, forms on the surface of the Earth. These rocks can cool fast forming fine-grained rocks or they cool very fast forming Glass.

Plutonic Rocks – are an intrusive rock that forms deep below the Earth's surface, these rocks form very large mineral crystals; granite is a type of Plutonic Rock. One way that igneous rocks are classified is by the amount of silica in their structure. Rocks that contain more than 50% silica are classified as felsic and are normally light in color and rocks that have less then 50% silica are classified as mafic and are dark in color.

TYPES OF IGNEOUS ROCKS

Andesite              Extrusive igneous rock primarily plagioclase plus pyroxene, hornblende, magnetite, biotite and quartz, equivalent to the plutonic rock diorite.

Basalt                   An extrusive igneous rock similar to the intrusive igneous rock gabbro. Found in many of the volcanoes of the oceanic islands and also in areas of Oregon, Idaho, Washington in the United States and in India, Siberia and other parts of the world. Contains calcium rich feldspars, olivine and many iron rich minerals.

Dacite                   Is an extrusive igneous rock with a composition between rhyolite and andesite.

Diorite                 It is a plutonic igneous rock that contains plagioclase, biotite, hornblende and/or pyroxene.

Gabbro                Is a coarse grained plutonic igneous rock with a composition of basalt.

Granite                Granite is a plutonic igneous rock with a coarse crystalline texture and is the most common igneous rock. The chemistry is the same as the fine-grained igneous rock rhyolite. Granite can contain quartz, mica, feldspars, hornblende, and several other minerals.

Obsidian              This rock is an extrusive rock that cools so fast that crystals do not have a chance to form making it a natural glass.

Pegmatite            This is granite that can contain large gemstone size crystals of minerals like; tourmaline, beryl, zircon and others.

Pumice                Is an igneous rock that has a porosity that is full of air; it is a felsic rock that is similar to rhyolite.

Rhyolite               It is the extrusive equivalent to the intrusive igneous rock granite.

Scoria                  Scoria is similar to pumice except the lava that it formed from was more viscous containing more rock and less air in the structure.

Tuff    Tuff forms explosively as it is ejected from volcanic vents. Tuff samples have several names assigned to them depending on their formation, ash-fall, ash-flow, welded tuff, etc.