Updated Igneous Rock Prelab - 2023

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Broward College *

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1010L

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Geology

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Jan 9, 2024

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Lab 5 – Igneous Rock Textures - Prelab DIRECTIONS : After completing the reading and watching the videos, answer the following questions in your own words and provide links to your sources for any Internet information used (do not copy and paste answers directly from the websites, this constitutes plagiarism). Each answer should contain enough detail to address all parts of the question. Each response should contain adequate length and detail to fully describe or explain the concept(s). Use this document as your template. Make space to separate your responses from the questions. 1-2. Describe the difference between extrusive and intrusive igneous rock textures Your answers should include the difference in the rate that the molten rock cools and how that rate impacts the size and extent of mineral crystal formation. Extrusive igneous rock has a fast rate of cooling because it is exposed to the open air, low temperature makes it cool quickly. With little cooling time, crystals can’t grow beyond microscopic size, and make the rock have fine-grained or glassy textures. Intrusive igneous rock has a slow rate of cooling due to being insulated from the air. It allows magma to gradually lose heat over a long period of time, allowing crystals to have enough time to grow to a size that we can see with the naked eye. 3. Explain what is meant by aphanitic texture. Use one of your igneous rock samples as an example in your explanation. aphanitic texture is fine-grained with microscopic crystals that we can’t see with the naked eye, so it makes the rock look uniform in color. For example, we have basalt with aphanitic texture 4. Explain what is meant by phaneritic texture. Use one of your igneous rock samples as an example in your explanation. BC Online – Physical Geology Lab (GLY1010L)

Lab 5 – Igneous Rock Textures - Prelab Phaneritic texture refers to rocks in which the mineral crystals are large enough to be easily seen by the naked eye. Make it have speckle, grainy appearance, multi colors. For example, we have granite and diorite have phaneritic texture. 5-7. Related to the two previous questions, please provide a hand drawn sketch of the respective textures you described and name the sample you used in the boxes below. Aphanitic Texture Phaneritic Texture Name of sample: Name of sample: 8. Explain what is meant by a pyroclastic igneous rock. Describe how a pyroclastic rock formed. During a volcanic eruption, magma, gases, and other materials get thrown out of the volcano into the air, and then quickly cool down by the outside temperature and form rocks that are called pyroclastic igneous rock. 9. Why do most pyroclastic rocks have vesicles ? What is a vesicle and how do they form? A vesicle is a hole-like texture that forms when magma erupts from a volcano and solidifies around the gas bubbles that form from the decreasing pressure when magma comes to the surface, which makes the pyroclastic rocks that are formed from erupted magma have vesicles. BC Online – Physical Geology Lab (GLY1010L)

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