igneous

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Century College *

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1020

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Geology

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Feb 20, 2024

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Igneous rocks (30 points) Igneous rocks are those formed as molten material (magma or lava) cools and solidifies. The process of cooling and crystallization imparts distinctive characteristics on the rock that allow it to be identified and described. In addition, the tectonic setting in which an igneous rock forms has strong controls on its chemical and mineral composition. The objective of this lab is to learn to recognize the textural, cooling, and compositional characteristics of igneous rocks and gain an understanding of how texture is related to cooling environment. **This lab involves looking at samples on display in the classroom or in D2L. Numbered samples (1- 12) are those in the large green rock boxes in the classroom. Lettered (A-G) samples are on display in other areas of the classroom and in D2L. 1. Magma is molten material that solidifies ___________ the surface. Lava is molten material that solidifies ___________ the surface. TEXTURES The texture of a rock refers to the sizes, shapes, and arrangements of crystals or grains within the rock. Whether dealing with an igneous, sedimentary, or metamorphic rock, the texture gives important clues to the processes that occurred during the formation of that rock. For igneous rocks, texture is a description of the sizes of crystal (or lack thereof) within the rock. These crystal characteristics are based on the rate that molten material cools. The following are five of the most common textures of igneous rocks: Phaneritic —A phaneritic igneous rock is one in which crystals are coarse-grained (but less than 1 cm in diameter) and visible to the naked eye. Crystals within the rock are roughly equal in size. Aphanitic —An aphanitic igneous rock has crystals that are fine-grained and microscopic in size. Porphyritic —An igneous rock with a porphyritic texture has both coarse, visible crystals called phenocrysts and small, microscopic crystals called groundmass. Igneous rocks with this texture show crystals with two distinctly different sizes. Glassy —Glassy texture refers to an igneous rock having no crystals. Atoms were unable to bond and create the interlocking atomic structure required for a mineral. Vesicular —Vesicular textures are characterized by vesicles or voids within the rock. These voids are created when gas bubbles are trapped as the molten material cooled and solidified. Rocks with a vesicular texture are often fine-grained or glassy also. 27

Use lettered samples A-G to answer the following questions. 2a. List the letter of one sample that is coarse-grained (large, visible crystals). b. What is the texture name for igneous rocks that are coarse-grained? 3a. List the letter of one sample that is fine-grained (microscopic crystals). b. What is the texture name for igneous rocks that are fine-grained? 4a. List the letter of one sample that contains both coarse-grained and fine-grained crystals. b. What is the texture name for igneous rocks that are both coarse-grained and fine-grained? 5a. List the letter of one sample that exhibits many voids. b. What texture is used to describe this appearance? 6. List the letter of one sample that shows a glassy texture. COOLING ENVIRONMENT AND RATE OF COOLING Igneous rocks can cool and crystallize either below the surface or above. An igneous rock that solidifies below the surface is called an intrusive igneous rock. An igneous rock that cools above the surface is called an extrusive igneous rock. The cooling environment (intrusive or extrusive) influences that rate of cooling (slow, fast, etc.). These factors are the most important influences on the texture of an igneous rock. Rocks that cool slowly have a coarse-grained appearance. Rocks that cool rapidly have a fine-grained appearance. 28

7a. Fill in the blanks in the statements below using the choices given in parentheses.  Rocks that solidify at or above the surface of the Earth are called ______________ igneous rocks. (intrusive/extrusive)  These rocks form from______________. (lava/magma)  The surface of the Earth is ______________ than Earth’s interior. (warmer/colder)  As a result, molten material that cools at the surface will solidify ______________ than molten material that cools within the Earth. (faster/slower)  Rocks that form at the surface will have ______________-grained appearance (fine/coarse) and a/an ______________ texture. (phaneritic/aphanitic) b. List one lettered sample (A-G) that fits the description you completed above. 8a. Fill in the blanks in the statements below using the choices given in parentheses.  The interior of the Earth is ______________ than Earth’s surface. (warmer/colder)  As a result, molten material that cools below the surface will solidify ______________ than molten material that cools at the surface. (faster/slower)  Rocks that solidify within the Earth are called ______________ igneous rocks (intrusive/extrusive).  These rocks formed from______________. (lava/magma)  Rocks that form beneath the surface will have ______________-grained appearance. (fine/coarse) and a/an ______________ texture. (phaneritic/aphanitic) b. List one lettered sample (A-G) that fits the description you completed above. 9. Describe the cooling environment and rate that would produce a porphyritic igneous texture. 10. Fill in the blanks in the statements below using the choices given in parentheses.  Igneous rocks with a vesicular texture cool ______________. (rapidly/slowly)  Igneous rocks with a glassy texture cool ______________. (rapidly/slowly) CHEMICAL COMPOSITION The chemical composition of an igneous rock usually gives the rock a characteristic color. Igneous rocks contain mostly silicate minerals—ferromagnesian and non-ferromagnesian. Ferromagnesian minerals are those that contain abundant iron and magnesium, while non-ferromagnesian minerals contain lesser amounts of iron and magnesium and greater silica, aluminum, and potassium. The following are the four terms used to describe the chemical composition of igneous rocks: 29

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Felsic —Felsic igneous rocks contain mostly non-ferromagnesian minerals such as quartz, potassium feldspar, and muscovite. These minerals make up at least 85% of the rock. Since rocks made of these minerals lack large quantities of iron and magnesium, they are typically lighter in color. Felsic rocks generally represent those formed in continental settings. Intermediate —Igneous rocks of intermediate composition have a more equal proportion of light and dark minerals. Some intermediate rocks will have a “salt and pepper” appearance. These types of rocks usually form in environments where oceanic and continental crust meet at subduction zones. Mafic —Mafic igneous rocks are composed mostly of ferromagnesian minerals such as olivine, pyroxene, and calcium plagioclase, with lesser amounts of non-ferromagnesian minerals. The abundance of iron and magnesium make mafic igneous rocks dark in color. Mafic rocks tend to form in oceanic settings. Ultramafic —Ultramafic compositions are almost entirely ferromagnesian. This type of rock lacks quartz and feldspar entirely and is almost exclusively composed of the minerals olivine and pyroxene. Silica content is the lowest of any type of igneous rock. Ultramafic rocks are rare on the Earth’s surface, but make up the bulk of rocks in the mantle. 11. Examine the samples on display labeled granite and rhyolite . These two rocks have the same (or very similar) mineral chemical composition yet look different. In what ways are they different? What caused this difference? Use samples A-G to answer the following questions. 12a. List one sample that is primarily light in color. b. What is the name given to this chemical composition? c. Would all samples with this chemical composition have the same texture and cool at the same rate? d. Did this sample most likely form in an oceanic, coastal, or continental setting? 30

13a. List one sample (other than sample F) that is primarily dark in color. b. What is the name given to this chemical composition? c. Did this sample most likely form in an oceanic, coastal, or continental setting? 14a. List one sample that is a near-even mix of light and dark minerals. b. What is the name given to this chemical composition? c. Did these samples most likely form in an oceanic, coastal, or continental setting? NAMING IGNEOUS ROCKS Names of igneous rocks are based on the texture and chemical composition. You will be using the charts on the pages that follow to identify igneous rock samples. 15. Using the information in the “Characteristics” column, fill in the “Texture” column on the “Igneous rock classification” chart on page 32. 16. Identify the characteristics and names of igneous samples 1-7, 9, 10, and 12 . Identify the color, chemical composition, texture, cooling environment, rate of cooling, and rock name for each sample. Record your answers on the “Igneous rock identification” chart provided on page 33. 31

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33 Sam ple # Col or Chem ical comp ositio n Textu re Coolin g enviro nment Rate of coolin g Name Igneous rock identification

igneous

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