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Roger Weller, geology instructor

Igneous Rocks
by Ryan Rohr
Physical Geology
Fall 2012


                                                                                                                Igneous Rocks                                                       


          Igneous derives from the Latin word meaning fire.   Igneous rocks are a large part of a classification of rocks.   Others being sedimentary, and metamorphic. Igneous rocks are created from the cooling of magma or lava. They can form with or without crystals. They can form either below the surface (intrusive) or above (extrusive). Igneous rocks make up at least 90- 95% of the 16 km of the earthís crust.  The textures of these rocks range from course to fine. Here is a small list of igneous rocks.



Obsidian is an igneous rock. Obsidian can be found all over the world  Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Russia, United States, and many other locations.   Obsidian is created when volcanic materials cool too quickly for the atoms arrange a crystalline structure.   When it cools it becomes glass and if fracture it breaks chonchoidal structure. It is extremely rich in silica with about 70%.


Image from, R.Weller/Cochise College.

            Obsidian is commonly black and has a smooth texture.   Obsidian can also be brown and have trace elements or inclusions which give it streaks of other coloring.   Rarer forms of obsidian can be red, blue, and green.   With its low crystalline structure, obsidian breaks in very sharp edges and is ideal when used as a cutting tool.   Obsidian is relatively a soft substance with a hardness of about 5.5. On the Mohs scale of hardness.



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            Obsidian has been used throughout history by humans.  Since the Stone Age primitive man has used obsidian because itís sharp properties.   Obsidian has been used as knives arrow heads spear tips and used as an edge for wooden Aztec swords called a Macuahuitl.




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            Obsidian is also used in jewelry itís often cut into beads.  Its uses in jewelry are fairly limited though because of its hardness of 5.5 mean it can be scratched relatively easy.

            It has that obsidian has also been used as a surgical tool used by Neolithic man about 7000 B.C. and is still used today because a freshly chipped piece is sharper that steel.  The blades are used in heart bypass society, eye surgery and also cosmetic.

            Obsidian has been a part of human culture all throughout the ages.  Obsidian is a gorgeous material and is highly captivating.   It is an extremely useful tool from spears to knife edges obsidian is deadly.   Some people say that obsidian has mystical healing powers but no scientific proof has ever been found but it still has a place in society. 


Pumice is an extrusive volcanic material, produced when lave with a very high water and gas content, is thrown out of the volcano. The gas and water content is called volatiles.   As the material hardens and cools the gas bubbles escape and creates a frothy texture. It is high in silica and low in iron which classifies it as rhyolite. Pumice and obsidian are usually found together. The name comes from the Latin word pumex meaning foam.  Pumice is the only rock that can float on lava.

            Pumice is found in over 50 countries. Largest producer is Italy and other large deposits found in Chile, Spain, turkey, and United States. Pumice has many uses including used in light weight construction materials such as concrete, many different abrasives, horticulture, landscaping, and for washing jeans.


          Basalt is a dark colored igneous rock made mainly of plagioclase and pyroxene minerals. It usually forms as an extrusive rock out from lava flows it can also be found in igneous dikes. Basalt makes up most of our planetís surface. Largely found in ocean basins. Itís less common on contents. Basalt can also be found on the moon. Large portion of the moonís surface is made of basaltic lava flows. The areas on the moon where flood basalts is called lunar Maria. On mars the composition of Olympus Mons is also basalt. Olympus Mons is the solar systemís largest volcano.



            Gabbro is a cores volcanic rock. Itís a dark igneous rock it is largely abundant in the ocean. Gabbro is mainly composed of calcium rich plagioclase feldspar and clinopyroxene. Small amounts of olivine may be found.  Gabbro and basalt are very similar the difference is grain size and basalt cools quickly as gabbro cools slowly.  It is often said that basalt makes up the ocean floor but is actually a thinner layer than the whole. Gabbro is underneath and much deeper.

            Gabbro can be polished with a black luster. Gabbro can be used as counter tops, floor tiles, and cemetery markers. Gabbro sometimes contains rare metals.


            Andesite occurs in volcanic areas. Andesite is a fine grained rock. This rock is packed with phenocrysts that it makes it look course.