Lab 4 Geology

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.

B. Basalt and granite C. Dark/light tone D. Aphanitic texture Lava slows down turning solid or into crystal to create a field formed with granite. Could potentially be a part of a dike. 9) 15 Specimen Collection: Examine and study the identified igneous rock collection provided. Compare the specimens with the information in the Class Notes and AGI Data Sheet 23.1 (next page). Fill in the information in the table below. Specimen No. Rock Name Igneous Texture Color Tone % Dark Minerals Feldspar Visible? Quartz Visible? Other Features? 1 Pegmatite Phaneritic Light 10 yes yes Feldspar Quartz, mica 2 Granite Phaneritic Light 10 yes no Feldspar quartz 3 Syenite Phaneritic Light 7 yes no feldspar 4 Anorthosite Phaneritic Med 15 yes no Small quartz trace 5 Gabbro Phaneritic Dark 50 no no Speckled black 6 Rhyolite Porphyry Porphyritic Med 5 yes yes feldspar 7 Trachyte Porphyry Porphyritic Light 5 yes no feldspar 8 Basalt Aphanitic Dark 40-50 no no quartz 9 Rhyolite Aphanitic Light 2 yes no quartz 10 Andesite Aphanitic Med 5 yes no feldspar 11 Tuff Aphanitic Light 2 no no Brown streaks 12 Obsidian Aphanitic Dark 100 no no Glassy

13 Pitchstone Aphanitic Dark >50 no yes quartz 14 Pumice Aphanitic Med 10 yes yes Porous 15 Scoria Aphanitic Dark 5 no no Porous 10) Unidentified Specimens: Using your available references (Table 3.1.1, Figure 3.1.19, Class Notes), identify the unknown specimens displayed at the front of the classroom. Describe each specimen thoroughly with regard to color, color tone, texture, and visible minerals. Present your findings in the following format (landscape): IGNEOUS ROCK ANALYSIS SUMMARY Specimen No. Luster Color Color Tone Igneous Texture Any minerals you can see? Hardness Rock Name Intrusive or Extrusive 1 dull gray light Phan quartz 6.0 granite intrusive 2 glassy black dark glassy None visible 5.5 obsidian extrusive 3 dull gray med Phan Quartz/mica 7.0 diorite intrusive 4 earthy black dark Aph quartz 6.0 basalt extrusive 5 porous red/ brown dark Aph/ porous None visible 5.0 scoria extrusive 6 earthy bright pink light Aph feldspar 6.0 rhyolite extrusive 7 vitreous pink light glassy None visible 7.0 rose quartz intrusive 8 earthy brown med Aph None visible 6.0 Pumice extrusive 9 dull black dark Phan quartz 6.0 gabbro intrusive