1. Identifying Common Igneous Rocks Igneous rocks are composed of a variety of minerals that contribute important classification characteristics. Click here to see two classification methods Click here to see two classification methods

2. Method #1 is a chart that requires you to identify the minerals that compose each igneous rock. Click here for Method #2 Click here for Method #2

3. Method #2 is a simple table that uses color and texture. Do you understand the concepts of “texture” and “color” in igneous rocks? If not, click here.

4. TEXTURE When classifying igneous rocks, the term “texture” is used to provide a simple description of the size of the individual mineral grains, or pieces of grains, that make up the igneous rock. Geologists use terms such as phaneritic, aphanitic, porphyritic, glassy, pyroclastic and pegmatitic to describe igneous rock texture. If you want to know more about these terms ask your teacher or look them up online where there are plenty of images to help you visualize the differences. This exercise employs the more simplistic approach of coarse grained versus fine grained. For the rest of this presentation, this table will provide the organizational structure behind your efforts to classify and identify these six igneous rocks. When you’re done, you should be able to apply the table to the identification of a few igneous rock specimens provided by your teacher. Coarse grained means the rock’s mineral crystals, or pieces of crystals, are large enough to be easily seen by the naked eye. Fine grained means the rock’s mineral crystals, or pieces of crystals, are too small to be seen by the naked eye. Click here for examples Click here to begin COLOR In this classification system, color is really a misnomer. It’s really all about dark versus light rock surfaces. Color can be tricky, as indicated by the intermediate color in the table below. White, pink, and red are light colors. Rhyolite commonly has a reddish color which, in this system, is considered a light color. Basalt is almost always dark gray or black while gabbro often has a greenish tint. Both would be considered a dark color in this system. By the way, notice the terms associated with light and dark in the chart? You might want to some research to clarify felsic and mafic.

5. Coarse grained, light coloredFine grained, light colored Dime for scale is used throughout this presentation.

6. FineCoarse Intermediate Color Light ColorDark Color Step 1: Describe the rock’s texture Step 2: Describe the rock’s color. Step 3: Where does this place you on the chart and what is the name of this igneous rock? Click on a name in the chart to find out if you are correct.

7. FineCoarse Intermediate Color Light ColorDark Color Step 1: Describe the rock’s texture Step 2: Describe the rock’s color. Step 3: Where does this place you on the chart and what is the name of this igneous rock? Click on the name to find out if you are correct.

8. FineCoarse Intermediate Color Light ColorDark Color Step 1: Describe the rock’s texture Step 2: Describe the rock’s color. Step 3: Where does this place you on the chart and what is the name of this igneous rock? Click on the name to find out if you are correct.

9. FineCoarse Intermediate Color Light ColorDark Color Step 1: Describe the rock’s texture Step 2: Describe the rock’s color. Step 3: Where does this place you on the chart and what is the name of this igneous rock? Click on the name to find out if you are correct.

10. FineCoarse Intermediate Color Light ColorDark Color Step 1: Describe the rock’s texture Step 2: Describe the rock’s color. Step 3: Where does this place you on the chart and what is the name of this igneous rock? Click on the name to find out if you are correct

11. Small CrystalsLarge Crystals Intermediate Color Light ColorDark Color Step 1: Describe the rock’s texture Step 2: Describe the rock’s color. Step 4: Where does this place you on the chart and what is the name of this igneous rock? Click on the name to find out if you are correct Step 3: This rock is known for its salt and pepper appearance.

12. Vesicular Glassy Or This is light colored PUMICE. Which term best describes its texture? These three igneous rocks are not on the texture vs. color chart. Can you pick out the characteristic that helps identify each one? This is dark colored SCORIA. Which term best describes its texture? Vesicular Glassy Pumice and scoria are both full of empty gas voids. In pumice, the tiny voids are not well interconnected. The larger ones in scoria are interconnected. Is this enough of a hint to figure out pumice’s unique characteristic? Vesicular? Click here See answer! This is black OBSIDIAN. Which term best describes its texture? Glassy Or Pumice floats on water!

13. Vesicular is the science term for “full of voids.” Voids are open spaces within the rock. Question: How did the voids form? Answer: Gas bubbles. A vesicular igneous rock is one that was full of dissolved gases. At some point, the pressure on the molten material was dramatically lowered, allowing the dissolved gas to revert to its gaseous state, making bubbles. The most common of these gases trapped in rock is carbon dioxide. As the molten material cooled the bubbles burst allowing the gas to escape. But, the space the bubble took up is preserved within the now hardened igneous rock. Question: Do you think this process more commonly occurred as molten magma was cooling within the Earth or as molten lava was cooling on Earth’s surface? Answer: Do some research.

14. Igneous Rock Texture, Cooling Rate, and Origin Location In most cases, the texture of an igneous rock provides a clue that suggests how quickly, in relative terms of fast vs. slow, the molten magma or lava took to cool into solid rock. The concept is really simple: The slower the molten material cooled, the coarser the texture of the igneous rock. An extension of this idea is that molten magma or lava will cool differently depending on physical location. Lava, molten rock expelled onto Earth’s surface, will cool relatively quickly. On the other hand, magma, molten rock trapped inside Earth, will cool much more slowly. Can you develop a simple conceptual model that explains this phenomena? Think you understand? Complete the following sentence by clicking on the correct response. inside Earth. on Earth’s surface. lava magma You should be able to construct a similar sentence for the coarse grained igneous rocks, such as granite. Basalt, a fine grained igneous rock formed as a molten mass of cooled Click here to check your idea

15. The cooling rate is related to insulation. How does a thermos or insulated cup maintain the temperature of the liquid it contains? It reduces the amount of surface area exposed to a different temperature outside of the container. It insulates! Cooling of molten rock experiences the same process. Magma inside Earth is contained within surrounding solid rock. Geologists refer to this surrounding rock as country rock. The magma will cool slowly because the country rock acts as an insulator, slowing the rate at which the heat energy of the magma can escape. On the other hand, lava does not experience any containing or insulating rock. As it is expelled onto Earth’s surface it encounters the much cooler atmosphere, or, in some localities, ocean water. In this scenario, the heat energy transfer is much more rapid and the lava cools much more quickly. This principle is directly related to the igneous rock’s final texture. Slow cooling provides more time for the growth of larger mineral crystals. Fast cooling inhibits mineral growth. Therefore, texture is a clue to cooling rate and location. For example, when observing basalt, imagine it beginning as a mass of lava expelled onto Earth’s surface and cooling so quickly that there was little time for the growth of mineral crystals large enough to be seen with your eye. In contrast, when observing a coarse grained igneous rock, such as granite, visualize a mass of molten magma cooling slowly in a deep magma chamber. One may have cooled into rock in hours while the other took millions of years. This all leads to an interesting connection: The fact that you ever see a coarse textured igneous rock on Earth’s surface implies what geologic process(es) at work? Weathering and erosion! A great deal of surrounding country rock had to removed to expose the buried coarse grained rock that formed within the surrounding country rock Coarse textured igneous rocks are "intrusive igneous rocks" while fine textured igneous rocks are called "extrusive igneous rocks.“ Do you see the connection of the terms to physical location inside Earth or on its surface? More on Igneous Rock Texture, Cooling Rate, and Origin Location

16. Now for the really good stuff!! The mineral composition and chemistry of rhyolite and granite are the same! The only difference is their texture due to cooling rate. Which is the intrusive version? Click a photograph below to confirm your answer The mineral composition and chemistry of gabbro and basalt are the same. The only difference is their cooling rate. Which is the extrusive version? Click a photograph below to confirm your answer. Obsidian is a well known volcanic rock. Its texture is fine grained and looks like glass. It also breaks with the same kind of unique conchoidal fracture pattern that glass does. Is obsidian intrusive or extrusive? What event(s) had to transpire to turn molten rock into a hunk of black glass? And, what’s conchoidal? Time for you to do some research on your own. Granite Obsidian Rhyolite GabbroBasalt

17. Almost done! Apply your knowledge of igneous rocks. 1.Print the next page. 2.Name the five igneous rocks. Write the name of the rock beside it on your printed page. 3.Ask your teacher to verify your decision. 4.Be prepared to explain the process you used to identify each rock. 5.The pictures are new. Don’t memorize images of rocks, use the process of characteristics. 6.The last page is an expanded version of the igneous rock chart. It may be of help.

18. This is ___________________