Igneous Rocks

Igneous rocks are rocks formed through the crystallization of magma either on the surface as volcanic (extrustive) rocks or deep underground as plutonic (intrusive) rocks. Remembering that at one time early in our Earth's history, the Earth was molten. When the Earth cooled, the minerals crystallized into igneous rocks. Therefore, the rock cycle on Earth, begins with magma.

Rocks are classified as igneous, metamorphic, and sedimentary based on origin. Classification schemes are designed to answer a specific question or to organize the objects of the classification scheme in groups for easier identification and understanding. Sometimes classification allows us to see patterns in data which can then lead to interpretations. Oftentimes it is just a system of organization to handle quantities of data that are two large to be examined individually. Within each rock type are further classification schemes to better understand how the rocks were formed.


Igneous rocks are generally classified both on where they formed (deep below the surface or at the surface) and on which minerals are present. Each rock is defined by both its rock texture and the minerals it contains. For example, a granite is a coarse-grained igneous rock that contains primarily quartz, feldspar (either potassium feldspar or plagioclase or both), and biotite with lesser amounts of muscovite and hornblende possible. A rhyolite is a fine-grained igneous rock that contains primarily quartz, feldspar (either potassium feldspar or plagioclase or both), and biotite with lesser amounts of muscovite and hornblende possible. Notice that the only difference between a granite and a rhyolite is the grain size. Granites cool slowly deep underground so the crystals are able to grow to 1 mm or larger as it cools. They are said to be plutonic or intrusive. Rhyolites cool rapidly at the surface so the crystals are not able to larger than 1 mm and are often so small to see with the naked eye. They are said to be volcanic or extrusive as they are erupted onto the surface by volcanoes.

The crystalline igneous rocks that we have in our rock boxes can be organized as such:

Coarse-grained texture Plutonic origin	Granite	Diorite	Gabbro	Peridotite
Primary Minerals for both the plutonic and volcanic end members  Volume % of the light vs dark minerals	One or both Feldspars Quartz Biotite 60-80% light minerals	Plagioclase Hornblende 50/50 dark and light minerals	Pyroxene Plagioclase 80% dark minerals	Olivine 100% dark minerals
Fine-grained texture Volcanic origin	Rhyolite	Andesite	Basalt	Komattiite*

 *Komattiites are no longer forming today. In the earliest of Earth's time, when the temperature of the interior of the Earth was hotter, komattiities were present on the Earth's surface. 

The rocks that contain significant amounts of volcanic glass: obsidian, pumice, and tuff do not have any minerals as is the case with obsidian and pumice or they have just phenocrysts possible as is the case with tuff. 

We can also arrange the rocks according to the amount of silica present by weight (weight %). Elements are always reported as oxides when doing chemical analysis of rocks. There are four catagories of rock chemistry according to SiO2 wt %:

Rock Name	Wt % SiO2	Rock Chemistry Name
Granite (p) Rhyolite (v) Obsidian (v)* Pumice (v)* Tuff (v)*	>65%	Felsic
Diorite (p) Andesite (v) Tuff (v)*	55-65%	Intermediate
Gabbro (p) Basalt (v) Vesicular Basalt (v)	45-55%	Mafic
Peridotite (p)	<45%	Ultramafic

                         *These rocks have the same rock chemistry as the crystalline rocks they are combined with; however, 
                           since they are composed completely or nearly completely of volcanic glass which is not crystalline, 
                           they contain few to no minerals.

Within the field of geology, Bowen's reaction series is the work of the petrologist, Norman L. Bowen who was able to explain why certain types of minerals tend to be found together while others are almost never associated with one another. He experimented in the early 1900s with powdered rock material that was heated until it melted and then allowed to cool to a target temperature whereupon he observed the types of minerals that formed in the rocks produced. He repeated this process with progressively cooler temperatures and the results he obtained led him to formulate his reaction series which is still accepted today as the idealized progression of minerals produced by cooling magma. Based upon Bowen's work, one can infer from the minerals present in a rock the relative conditions under which the material had formed.

This figure shows how it is possible to begin with a magma with magic (aka basaltic) composition and differentiate into a rock with felsic (aka granitic) composition.