Igneous Rocks Lesson #12

When most people think about igneous rocks they envision a volcano erupting pumice and lava. The term igneous comes to us from the Latin word "Ignis" which means fire. Igneous rocks are produced this way but most igneous rocks are produced deep underground by the cooling and hardening of magma. Magma is molten (melted) rock under the surface of the Earth. It is produced in the upper reaches of the mantle or in the lowest areas of the crust usually at a depth of 50 to 200 kilometers.




The diagram above shows you where magma is produced at a subduction zone. Magma is less dense than the surrounding rock which causes it to rise. When magma reaches the surface it is then called lava and the eruptions of lava and ash produce volcanoes. The lava that reaches the Earth's surface will harden and become igneous rock. When the magma does not reach the surface it produces a variety of geologic structures. When lava reaches the surface of the Earth through volcanoes or through great fissures the rocks that are formed from the lava cooling and hardening are called extrusive igneous rocks. Some of the more common types of extrusive igneous rocks are lava rocks, cinders, pumice, obsidian, and volcanic ash and dust.



This is the volcano Paricutin that is located in Mexico. It is erupting cinders and pumice which are examples of extrusive igneous rocks.



Millions and even billions of years ago molten rock was cooling and thus hardening into igneous rocks deep under the surface of the Earth. These rocks are now visible because mountain building has thrust them upward and erosion has removed the softer rocks exposing the much harder igneous rocks. These are called intrusive igneous rocks because the magma has intruded into pre-exiting rock layers. Types of intrusive igneous rocks are granite and basalt.

The diagram above shows you a large intrusive igneous body called a batholith. A batholith is the largest of the intrusive bodies. They are larger than 100 square kilometers and usually form granite cores.



As you can see in the diagram above a batholith is a very large intrusive igneous body. There are two types of intrusive bodies that we are going to discuss 1)Discordant and 2) Concordant. A discordant igneous rock body cuts across the pre-exiting rock bed. Batholiths and dikes are examples of discordant rock bodies. A dike is a vertical or near vertical intrusive igneous rock body that cuts across rock beds. They frequently form from explosive eruptions that crack the area around a volcano with the magma filling the cracks forming a dike.

A concordant igneous rock body runs parallel to the pre-existing bedrock. Laccoliths and sills are examples of concordant igneous rock bodies. A laccolith is a dome shaped intrusive body that has intruded between layers of sedimentary rock. The rising magma forces the overlying layers to rise up into a dome. A sill is similar to a dike with the exception that sills run parallel to the existing rock bed instead of cutting through it.




The composition of igneous rocks falls into four main categories. They are determined by the amount of silica that the rocks contain. The four categories are acidic, intermediate, basic, and ultramafic. Acidic rocks have a high silica content (65% or more) along with a relatively high amount of sodium and potassium. These rocks are composed of the minerals quartz and feldspar. Rhyolite and granite are the two most common types of acidic rock.

Intermediate rocks contain between 53% and 65% silica. They also contain potassium and plagioclase feldspar with a small amount of quartz. Diorite and Andesite are the two most common types of intermediate rock.



Basic rocks are composed of less than 52% silica and a large amount of plagioclase feldspar and very rarely quartz. The two most common types of basic rocks are basalts and gabbros.

Ultrabasic rocks are composed of less than 45% silica and contain no quartz or feldspar. They are composed mainly of the minerals olivine and pyroxene. The most common ultrabasic rock is periodite. Periodite is a dark green, coarse-grained igneous rock that many scientists believe is the main rock of the mantle.



Basalts are dark colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. They are the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive having cooled inside the Earth's interior.



This is a vertical columnar basalt formation. When basaltic lava cools it often forms hexagonal (six sided) columns. Some famous examples of columnar basalt formations are the Columbia Plateau overlooking the Columbia River near Portland, the Giant's Causeway in Northern Ireland, and the Devils Postpile National Monument in California (Above).



Pumice is a very light colored, frothy volcanic rock. Pumice is formed from lava that is full of gas. The lava is ejected and shot through the air during an eruption. As the lava hurtles through the air it cools and the gases escape leaving the rock full of holes.

Pumice is so light that it actually floats on water. Huge pumice blocks have been seen floating on the ocean after large eruptions. Some lava blocks are large enough to carry small animals.

Pumice is ground up and used today in soaps, abrasive cleansers, and also in polishes.



Rhyolite is very closely related to granite. The difference is rhyolite has much finer crystals. These crystals are so small that they can not be seen by the naked eye. Rhyolite is an extrusive igneous rock having cooled much more rapidly than granite giving it a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende.



Gabbros are dark-colored, coarse-grained intrusive igneous rocks. They are very similar to basalts in their mineral composition. They are composed mostly of the mineral plagioclase feldspar with smaller amounts of pyroxene and olivine.



Obsidian is a very shiny natural volcanic glass. When obsidian breaks it fractures with a distinct conchoidal fracture. Notice in the photo to the left how it fractures. Obsidian is produced when lava cools very quickly. The lava cools so quickly that no crystals can form.

When people make glass they melt silica rocks like sand and quartz then cool it rapidly by placing it in water. Obsidian is produced in nature in a similar way.

Obsidian is usually black or a very dark green, but it can also be found in an almost clear form.



Ancient people throughout the world have used obsidian for arrowheads, knives, spearheads, and cutting tools of all kinds. Today obsidian is used as a scalpel by doctors in very sensitive eye operations.


Write the answers to the following questions in complete sentences on a piece of paper.



In your own words write a definition for magma and lava.


What is the difference between intrusive and extrusive igneous rocks?


What are the most common extrusive and intrusive igneous rocks?


What is the difference between granite and rhyolite and how are they similar?