Lab 4 Geology

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Rowan University *

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06103

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Geology

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Dec 6, 2023

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pdf

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6

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Lab 4 Igneous Rocks CE 342 – Geology Dr. John Schuring – Spring 2023 1) Explain the difference between lava and magma. Magma is described as a thick liquid with a viscosity that is comparable to oatmeal or cement concrete. Magma is basically molten rock beneath the surface of the Earth while lava breaks through the Earth's surface and begins to cool. 2) The texture of an igneous rock is largely determined by the rate of cooling. Which texture indicates the fastest cooling? The slowest? Which texture indicates there was a change in rate? Aphanitic texture indicates fastest rate cooling. Phaneritic texture indicates slowest rate cooling. In this texture mostly fine grains developed due to the rapid cooling process as many nucleation points developed and diffusion rate decreased. Porphyritic texture indicates there is change in rate of cooling as it's a texture where a large crystal is surrounded by small grains indicating multiple rates of cooling. 3) The color tone of an igneous rock is an indicator of its chemical composition. Light colored igneous rocks have an abundance of which mineral subgroup? How about dark colored igneous rocks? Give two example minerals of each subgroup. Light colored igneous rock has felsic minerals, which then are mostly the mineral group having high silica percentage like quartz, feldspar, and muscovite. The dark colored igneous rock has a mafic mineral group. Mostly the minerals having low silica percentage like olivine, pyroxene, biotite. 4) Briefly describe magmatic differentiation and how it affects the mineral content of an igneous rock. Magmatic differentiation is the complex process where magma undergoes a bulk chemical change during the partial melting process, emplacement, cooling or eruption. Since it imposes a chemical change in magma, which eventually results in change in the mineral content of the igneous rocks. This is because a single melt can produce a wide variety of igneous rocks. Therefore, in this way, the magmatic differentiation affects the mineral content of the igneous rock.
5) You spot the following minerals in a hand specimen of pegmatite granite: quartz, alkali-plagioclase, and muscovite. Using Bowen’s Reaction Series, determine the order of crystallization, i.e., which formed first, second, etc.? Also, which of the three minerals would likely display the best crystal form? 6) What is the most common rock in the oceanic crust that underlies the oceans? The oceanic crust is typically composed of dark-colored rocks such as basalt and gabbro. It is thinner and denser than continental crust, which is made of light-colored rocks like andesite and granite. 7) Granite Close-up: Examine the Granite Closeup photo posted on Canvas. Transfer it to your homework sheet and identify the three basic minerals: feldspar, quartz and mica. Use leader arrows and labels to make the identifications. Identify each mineral at two different locations in the photo, i.e., two leader lines to quartz at various points, etc.
8) Special Igneous Structures/Features: Examine the two special specimens in the front of the classroom. Take a photo of each and then describe the following: (1) Name the igneous structure/feature (2) Rock type/lithology; (3) 1-2 sentences about how the specimen was formed. A. Scoria/Vesicular Basalt Dark color tone Aphanitic porous texture Pyroclastic Scoria is formed from hot compressed gas geysers or lava pushed from the earth.
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