Igneous Rocks (Right Side Question)

Transcription

1 Igneous Rocks (Right Side Question) Record the question in your notebook! Using your understanding of rock texture (grain size, grain shape, grain pattern), explain the process that you would use to determine if a rock sample was intrusive or extrusive.

2 When trying to determine if a rock sample were intrusive or extrusive igneous rock, it would be most important to look at the grain size. Grain size gives information about how quickly or slowly the molten material cooled. Intrusive rock are rocks that are formed within Earth s crust and are not exposed to the cooler air of the atmosphere or the cold water of the oceans. Because of this, the magma cools very slowly, allowing for the formation of large, easy to see crystals with a texture called coarse grained. A rock is considered to be coarse grained if you can see the reflections of the mineral crystals when you turn the rock in the light, or when you can see areas of different colors. Extrusive rocks form when lava erupts from the crust and is exposed to air or water, thus cooling very quickly. When the lava cools quickly, there is little to no time for crystallization to occur. As a result, the rock will have either a fine-grained or glassy texture. Fine grained rocks are those with crystals that are too small to be seen without a hand lens or microscope, most often they appear to be all one color. Glassy rocks are those that have no mineral crystals, even when viewed under a microscope. To determine if a rock were intrusive or extrusive, you could ask the following question: Can I see reflections off mineral crystals OR clear areas of different colors? If yes, the rock is coarse grained and must be INTRUSIVE. If no, the rock is either fine grained or glassy and must be EXTRUSIVE.

3 Metamorphic Rocks (Right Side Question) Record the question in your notebook! Using your understanding of rock texture (grain size, grain shape, grain pattern), explain the process that you would use to determine if a rock sample was foliated or non-foliated.

4 When trying to determine whether a rock sample was foliated or non-foliated, the most important property of texture to look at would be grain pattern. Grain pattern classifies rocks based on whether the grains are stacked like pancakes into layers (layered), arranged in bands of colors or thin sheets (foliated), or have a random arrangement (non-foliated). To determine if a rock were foliated or non foliated I would first ask: If yes, the rock is FOLIATED. Are there clear, definite bands of color? Does the rock look like it could be flaked apart or separated into thin sheets? If yes, the rock is FOLIATED. If yes, the rock is NON-FOLIATED. Are the crystals randomly arranged with no clear pattern?

5 Sedimentary Rocks (Right Side Question) Record the question in your notebook! Using your understanding of rock texture (grain size, grain shape, grain pattern), explain the process that you would use to determine if a rock sample was clastic, organic, or chemical.

6 To determine if a rock were clastic, organic, or chemical, it would require looking at all three grain properties because sedimentary rocks include those with such a wide range of characteristics. First I would ask: Can you tell that the rock is composed of shells or shell fragments? Does the rock look like it may have been a piece of coral? If yes, the rock is ORGANIC, as it is composed of things that were once living. Is the rock composed of grains that are visible with the naked eye or with a hand lens? Do the grains appear to have been deposited in layers? If yes, the rock is CLASTIC, as it is composed of sediments that were compacted and cemented together. Does the rock sample appear to be crystalline (have a crystal structure)? If yes, the rock is CHEMICAL, because chemical rocks form when water evaporates, leaving behind the crystals of dissolved minerals. Because sedimentary rocks are one of the most diverse category of rocks on Earth, it would be impossible to positively identify all of them based on rock texture alone, however looking at other properties (ability to dissolve in water, whether or not they react with an acid, etc) combined with rock texture would allow for classification.