Earth Science 3.2 Igneous Rocks

 Remember from the Rock Cycle that igneous rocks form when magma or lava cools and hardens  Different types of igneous rocks form when this hardening occurs depending on a variety of factors.

Igneous Rocks  The word igneous comes from the Latin ignis, which means fire  Fiery volcanic eruptions are responsible for some types of igneous rocks.  Still others are formed deep inside the Earth’s surface.

Igneous Rocks  Intrusive Igneous Rocks Rocks that form when magma hardens beneath Earth’s surface are called intrusive igneous rocks.

Igneous Rocks  Magma consists mainly of the elements silicon and oxygen, plus aluminum, iron, calcium, sodium, potassium, and magnesium.

Igneous Rocks  Magma also contains some gases including water vapor.  These gases are kept within the magma by the pressure of the surrounding rocks.  Because magma is less dense it slowly works it’s way toward the surface.

Igneous Rocks  As magma rises, it cools slowly, allowing elements to combine and form minerals.  Gradually, the minerals grow in size forming a solid mass of interlocking crystals.  Granite is a common intrusive igneous rock.

Extrusive Igneous Rocks  E xtrusive Igneous Rocks: Remember that when magma reaches the surface, we call it lava. Lava is similar to magma except that most of the gases have escaped. When lava hardens, the rocks that are formed are called extrusive igneous rocks “This is because they are extruded onto Earth’s surface”

Classifications of Igneous Rocks  Many different types of igneous rocks come from these intrusive and extrusive processes.  Texture and composition are the two most important characteristics to look at when we classify igneous rocks.

Classifications of Igneous Rocks  Texture describes the appearance of an igneous rock based on it’s size, shape and arrangement of it’s interlocking crystals.  Composition classes of igneous rocks are based on the proportions of light and dark materials in the rock.

Classifications Igneous Rocks  C ourse G rained Texture The rate of cooling strongly affects the textures of igneous rocks. Slow cooling results in the formation of large crystals. Igneous rocks with large crystals exhibit a course-grained texture.

Classifications Igneous Rocks Fine Grained Texture  If cooling occurs rapidly instead, the ions in the melted material loose their motion and quickly combine.  This results in large numbers of tiny crystals that all compete for the available ions.  Rapid cooling of magma or lava results in rocks with small interconnected grains.  These rocks are said to be fine grained texture.

Classifications Igneous Rocks G lassy texture  When lava spews onto Earth’s surface rapidly, there may not be enough time for the ions to arrange themselves into a network of crystals.  Solids produced this way are made of randomly distributed ions.  Such rocks have a glassy texture.

Classifications Igneous Rocks Porphyritic Texture  A large body of magma located deep within the Earth may take tens of thousands of years to harden.  Minerals that crystallize from the magma do not form at the same rate or at the same time.  It is possible for some crystals to get quite large while others are just starting to form.

Classifications Igneous Rocks P orphyritic Texture  The resulting rock can have large crystals, called phenocrysts, while the surrounding rock has a fine grain.  These rocks have a porphyritic texture.  They are a mixture of large and fine grains intermingled.

Classification of Igneous Rocks Granite Composition:  Igneous rocks in which light colored silicate mineral quartz and feldspar are the main ingredient are said to have a granite composition.  In addition to quartz and feldspar, most granite rock contains about 10% dark silicate minerals.

Classification of Igneous Rocks B asaltic Compositions Rocks that contain many dark silicate minerals and plagioclase feldspar have a basaltic composition. Basaltic rocks are rich in iron and magnesium.  Because of the iron, they are usually darker and denser than granite rocks. The most common basaltic rock is basalt.

Classification of Igneous Rocks R ocks with a composition between granitic and basaltic rock have an andesitic composition. This group of rocks is named after the common volcanic rock andesite. Andesitic rocks contain at least 25 % dark silicate minerals.