Rocks Lab 2

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Colorado State University, Fort Collins *

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121

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Geology

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Dec 6, 2023

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docx

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GEOL 121 Name: Lab Section_________ Rocks Lab 2 – Rock Classification and Identification Learning Objectives: 1) Describe the characteristics used to classify igneous, sedimentary, and metamorphic rocks. 2) Identify common igneous, sedimentary, and metamorphic rocks. Part 1: Igneous Rocks 1) Igneous rocks form from the cooling and crystallization of magma. This cooling may happen on Earth’s surface or deeper within the crust. Based on this understanding of how igneous rocks form and what you learned in Rocks Lab 1, what characteristics of igneous rocks might be useful for identifying and classifying igneous rocks? 2) Examine igneous rocks A, B, and C in your sample set: What are some ways these rocks are similar? What are some ways these rocks are different? - Dark in color - They have crystallization - No effervescence - Rock A has larger crystals than the others - Rock B has the smallest crystals - Rock C has medium crydtals 3) Examine igneous rocks E, F, and G in your sample set: What are some ways these rocks are similar? What are some ways these rocks are different? - All dark in color - Harder than fingernail - No reaction to acid - G is vesicular - F is aphanitic - E is phaneritic

Rocks Lab 2 GEOL 121 Igneous rocks are classified according to their composition and texture. Composition refers to the proportion of the different silicate minerals that are present in the rocks, and can be simplified to the major categories: felsic, intermediate, mafic, and ultra-mafic. Igneous textures generally reflect the method and rate at which the magma cooled. 2

Rocks Lab 2 GEOL 121 4) For the unknown igneous rocks describe their composition and texture. Then use the provided classification charts to identify the rock. Sample Composition Texture Rock ID A Felsic Phaneritic Granite B Intermediate Phaneritic Diorite C Mafic Phaneritic Gabbro D Felsic Aphanitic Rhyolite E Intermediate Porphyritic Porphyritic andesite F Mafic Aphanitic Basalt G Mafic Vesicular Vesicular basalt 5) How confident do you feel in your identifications? Pretty confident – 9/10 confidence 6) What is still unclear to you about igneous rock classification? What is technically classified as aphanitic. 3

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Rocks Lab 2 GEOL 121 Part 2: Sedimentary Rocks 7) Sedimentary rocks form from the products of chemical and physical weathering. The sediments and dissolved ions are transported and deposited by processes happening on Earth’s surface. Based on this understanding of how sedimentary rocks form and what you learned in Rocks Lab 1, what characteristics of sedimentary rocks might be useful for identifying and classifying sedimentary rocks? Grain size and effervescence. 8) Examine sedimentary rocks K, O, and P: What are some ways these rocks are similar? What are some ways these rocks are different? - Fine grain - Different colors - O has effervescence 9) Examine sedimentary rocks J, L, and M: What are some ways these rocks are similar? What are some ways these rocks are different? - Detrital - - Different colors - Different grain sizes - Varying textures Sedimentary rocks are classified at the highest level according to the composition of the material and the origins of the rock. Clastic/detrital rocks are the most common sedimentary rocks; these rocks are further classified by their clast size. 4

Rocks Lab 2 GEOL 121 10) For the unknown sedimentary rocks use the classification charts provided to identify the rock and provide a justification. 5

Rocks Lab 2 GEOL 121 Sample Rock ID Reasoning J Shale Very small grain size (<1/16mm), detrital K Quartz sandstone Sand texture, looks like sand (lighter whiter color) L Sandstone Sand texture, looks like sand (reddish) M Conglomerate Detrital, Gravel sized grains, rounded grains N Breccia Detrital, Angular gravel grain size O Fossiliferous Limestone Carbonate mineral precipitate, Effervescence, P Coal Organic, black, has layers 11) How confident do you feel in your identifications? Fairly confident, 9/10 12) What is still unclear to you about sedimentary rock classification? How to ID different mineral precipitates 6

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Rocks Lab 2 GEOL 121 Part 3: Metamorphic Rocks 13) Metamorphic rocks form through the alteration of other rocks by high heat and/or pressure. This can happen during burial to greater depths, tectonic processes, or the movement of heat by magma. Based on this understanding of how metamorphic rocks form and what you learned in Rocks Lab 1, what characteristics of metamorphic rocks might be useful for identifying and classifying metamorphic rocks? Foliation, grain size 14) Examine metamorphic rocks R, W, and X: What are some ways these rocks are similar? What are some ways these rocks are different? - Small crystallization - Different color - Different grain size 15) Examine metamorphic rocks T, U, and V: What are some ways these rocks are similar? What are some ways these rocks are different? - Visible foliation - No effervescence - Dark in color - T has a lot of visible crystals - U, V are not visibly grained Metamorphic rocks are classified primarily based on their texture, most importantly the presence and degree of foliation. Mineral composition and grain size also play a role in classification. 7

Rocks Lab 2 GEOL 121 8

Rocks Lab 2 GEOL 121 16) For the unknown metamorphic rocks use the classification charts provided to identify the rock and provide a justification. Sample Rock ID Reasoning R Gneiss Distinct foliation, medium to coarse grain, alternating compositional layers S Gneiss Distinct foliation, medium to coarse grain, alternating compositional layers T Mica Shist Common minerals aligned, fine to medium grain U Phyllite Grains too small to be seen by naked eye, sheen surface V Slate Grains too small to be seen by naked eye, dull surface W Marble No distinct foliation, vigorously effervesces X Quartzite No distinct foliation, process of elimination  not marble or hornfels 17) How confident do you feel in your identifications? confident 10/10 18) What is still unclear to you about metamorphic rock classification? Finding the difference between medium to fine grains and medium to coarse grains 9

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