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Laboratory #2: Igneous Rocks Materials Needed (to be provided by the instructor): 1. Igneous Rocks Set 2. Scratch Glass, Porcelain Plate, Copper Penny, Iron Nail, Magnets, HCl 3. Hand lens Introduction to Igneous Rocks Igneous rocks form by solidification of molten rock (magma). There are two basic types of igneous rocks: Intrusive and extrusive. Magma that solidifies within the Earth’s surface forms intrusive igneous rocks. Extrusive igneous rocks solidify on the Earth’s surface. Molten rock that is erupted from a magma chamber and flows onto the ground is called lava. Sometimes magma is ejected from a volcano and into the air, where it cools before it falls to the ground. These extrusive igneous rocks are called pyroclasts. Mineral Composition Igneous rocks are comprised primarily of silicate minerals, and can be divided into two groups: light colored (or felsic) minerals and dark colored (or mafic) minerals. Common Felsic Minerals Common Mafic Minerals Plagioclase feldspar Alkali feldspar Quartz Muscovite Olivine Pyroxene Hornblende Biotite If only mafic minerals are present, then the rock is said to be ultramafic. Rocks that contain mostly mafic minerals are called mafic, and if only felsic minerals are present, then the rock is said to be felsic. Rocks that contain roughly equal amounts of both felsic and mafic minerals are intermediate. Igneous Rock Textures Intrusive igneous rock textures: The cooling of magma inside of the Earth produces crystals that interlock with one another. The resulting rock is usually quite hard. Coarse-grained intrusive rocks have visible crystals that are all roughly the same size. This texture is typical of plutonic rocks such as granite, diorite, gabbro and peridotite. Pegmatite texture is typical of intrusive rocks that form in dikes. These rocks are very coarse-grained and crystal sizes can be several centimeters in size. Fine-grained intrusive rocks have crystals that are so small you cannot see them with the naked eye. Not many intrusive rocks have fine-grained textures, although it can develop in shallow intrusive bodies such as dikes or sills. Extrusive igneous rock textures: Extrusive igneous rocks such as lavas cool as one massive unit. This produces an interlocking crystal pattern, similar to that seen in intrusive igneous rocks. Pyroclastic igneous rocks, such as ashes and tuffs, were molten prior to being ejected into the air. These pyroclastic fragments cool and solidify before they reach the ground. This can produce a rock that may have individual fragments and layering. Page 1 Aphanitic (fine-grained) textures develop when the molten rock is cooled very rapidly. Individual crystals are not visible to the naked eye. This texture is typical of extrusive igneous rocks such as basalt, rhyolite, and andesite. Porphyritic texture contains large crystals (called phenocrysts) in an aphanitic matrix. The two different grain sizes must be distinct, and not gradual. This type of texture develops in two stages. First, cooling takes place slowly in the Earth, producing the phenocrysts. The magma, with the phenocrysts, is then ejected from the volcano where it cools quickly, forming the aphanitic groundmass. A porphyry is a rock that contains 25% or more phenocrysts; a porphyritic rock contains less than 25% phenocrysts. Glassy textures form from highly viscous lavas that are often felsic in composition. They can also form when the magma cools so rapidly that minerals do not have a chance to crystallize. Obsidian and pumice are two examples of extrusive igneous rocks with glassy textures. Vesicular textures form as gasses are released from the magma as it cools, and are typical of extrusive igneous rocks. Each small cavity is called a vesicle. Rocks that are very vesicular are called scoria; when there are more vesicles than matrix, the rock is called pumice. Sometimes minerals will be deposited in the vesicles after the rock has cooled. These are called amygdules and the rock is said to have an amygdaloidal texture. Pyroclastic textures may have a “powdery” texture or may appear to be composed of individual fragments stuck together. Volcanic ash is comprised of loose sand to silt size fragments; Tuffs are rocks comprised of silt to pebble sized ash and/or pumice; Welded tuffs are tuffs that were deposited while still partially molten, and may appear streaky or glassy; and Volcanic breccia are rocks with large, broken volcanic rock fragments. The Lab The object of the lab is to be able to identify the various types of igneous rocks based upon their color, mineral composition, and texture. First, determine which minerals are present. Then, determine the grain size. Determine what color the rock is (light, dark, gray, pink, etc.). Does the rock have any textures, such as vesicles, phenocrysts, etc.? Use the chart below to help you determine the name of the rock, and whether the rock is extrusive or intrusive. Page 2 Identification of Igneous Rocks Particle size Physical properties Dark gray to black in color; minerals include plagioclase, pyroxene, and olivine. GABBRO Dark gray to black in color; minerals include plagioclase, pyroxene, and olivine. Some minerals may be significantly larger than others. PORPHYRITIC GABBRO Groundmass is dark gray to black in color; commonly contains olivine phenocrysts BASALT PORPHYRY OR PORPHYRITIC BASALT Dark gray to black in color; individual minerals are too small to see. May be vesicular BASALT OR SCORIA Crystals several centimeters across. Minerals may vary. Pegmatites typically form in dikes PEGMATITE Comprised of dark green to black pyroxene and white to gray plagioclase feldspar in roughly equal proportions (salt and pepper appearance). Presence of pyroxene and absence of quartz are diagnostic features. DIORITE porphyritic with coarse grained groundmass As that of diorite, but some minerals are significantly larger than others. PORPHYRITIC DIORITE porphyritic with aphanitic groundmass As that of andesite, but with phenocrysts of plagioclase, pyroxene and/or hornblende. ANDESITE PORPHYRY OR PORPHYRITIC ANDESITE Gray, dark tan, purple or brown in color. Minerals in the groundmass are too small to see. Andesite is the most common lava-flow rock of stratovolcanoes or composite cones. ANDESITE Volcanic glass with conchoidal fracture. Usually black in color and transparent along thin edges. OBSIDIAN Highly vesicular ("frothy") texture, typically light gray in color. PUMICE Crystals several centimeters across. Minerals may vary. Pegmatites typically form in dikes PEGMATITE Two types of feldspars are visible - plagioclase (white) and microcline (pink), in addition to quartz, biotite and hornblende. GRANITE Similar to that of granite, but with less alkali feldspars (such as microcline). Color is more gray than pink. GRANODIORITE porphyritic with coarse grained groundmass As that of granite, but some minerals are significantly larger than others. PORPHYRITIC GRANITE porphyritic with aphanitic groundmass As that of rhyolite, but contains quartz phenocrysts (and sometimes feldspar). The presence of quartz distinguishes it from andesite. May be light tan, pink, beige, yellowish or light gray in color. Yellowstone National Park is named for the yellow rhyolite found there. Volcanic glass with conchoidal fracture. Usually black in color and transparent along thin edges. RHYOLITE PORPHYRY OR PORPHYRITIC RHYOLITE Highly vesicular ("frothy") texture, typically light gray in color. PUMICE PLUTONIC PERIDOTITE coarse grained porphyritic with coarse grained groundmass porphyritic with aphanitic groundmass fine grained / aphanitic fine grained / aphanitic PLUTONIC VOLCANIC coarse grained (MEDIUM COLORED) INTERMEDIATE very coarse grained fine grained / aphanitic VOLCANIC (LIGHT COLORED) coarse grained PLUTONIC very coarse grained FELSIC Rock name Dark green in color; minerals include olivine, pyroxene, and minor amounts of plagioclase. Can be distinguished from gabbro by having a lower proportion of cleavage surfaces that catch the light. VOLCANIC (DARK COLORED) ULTRAMAFIC / MAFIC Color Page 3 RHYOLITE OBSIDIAN Page 4 Page 5 12 11 10 9 8 Pink, buff Coarse Aphanitic porphyritic Plagioclase, microcline, quartz, biotite None visible White with some black Aphanitic / Fine Visible: olivine Dark gray Black and white Coarse Quartz, plagioclase, microcline, hornblende & biotite Lithic fragments May have olivine Like granite, only one feldspar Frothy texture Gray Aphanitic / Fine glassy None visible Quartz, microcline, muscovite 7 Two types of feldspar VERY coarse grained Glassy None (Amorphous silica) 6 Pink, white and black Very small clear crystals Pink, white, silvery Coarse Quartz, plagioclase, microcline, hornblende & biotite 5 Buff, rose Very coarse Aphanitic / Fine None visible 4 Black and white = diorite! Feldspar phenocrysts OTHER (fragments. Vesicular, etc.) Looks like glass Coarse Plagioclase & hornblende 3 Black, dark gray, white Dark gray, white Black and white COLOR Lithic Tuff Adamellite Basalt (vesicular) Granodiorite Pumice Pegmatite Obsidian Granite Rhyolite Diorite Andesite Porphyry Norite NAME Name ______________________________ Black Aphanitic porphyritic Visible: feldspar 2 Coarse GRAIN SIZE (fine or coarse) Plagioclase & pyroxene VISIBLE MINERALS 1 Sample # Igneous Rock Identification Table