LAB4_Igneous_Rocks

LAB #4 — IGNEOUS ROCKS AND PROCESSES - 1 - Name: _________________________ Day & Time: ___________________ PURPOSE AND OBJECTIVES: You have previously learned how to identify rock- forming minerals of the Bowen’s Reaction Series. This week you will see these same minerals but in igneous rocks. This lab introduces you to 1) the mechanisms that allow magma to rise to the surface, 2) how cooling rates affect crystal size, 3) techniques used to identify igneous rocks and interpret their geologic origin, and 4) three types of volcanoes. KEY TERMS AND CONCEPTS: • Igneous Rock • Phaneritic • Diorite • Volcanic Rock • Porphyritic • Andesite • Extrusive • Silica Content • Felsic • Lava • Viscosity • Granite • Plutonic • Ultramafic • Rhyolite • Intrusive • Peridotite • Obsidian • Magma • Mafic • Shield Volcano • Texture/Cooling Rate • Basalt • Composite Volcano • Phenocryst • Gabbro • Caldera • Aphanitic • Intermediate BACKGROUND Classification is an important process in all the sciences, including geology. The two main criteria used to classify rock are texture (the size, shape, and arrangement of the grains or crystals; see Figure 4.1 below) and composition ( the minerals or types of particles present). The texture of an igneous rock is controlled by the rate that magma cools, while its composition, or minerology, is controlled by the magma’s chemistry. Classifying rocks is not the goal in itself, but rather, it allows us to discuss the rocks from a common frame of reference, and to make interpretations about how they formed. All igneous rocks start as molten material called magma . Deep within the Earth’s crust, magma solidifies slowly — over several millions of years; the molecules within the magma having time to arrange themselves into large crystals, forming a rock a rock with a coarse grained texture that is referred to as a plutonic (or intrusive) rock. Magma that erupts from a volcano or a fissure is called lava . Because lava cools quickly, its crystals are small, forming a fine grained texture called volcanic (or extrusive ). If the lava cools too rapidly for any crystals to organize into an orderly atomic arrangement, a volcanic glass known as obsidian forms. Native Americans used obsidian to make arrowheads and knives. Sometimes expanding volcanic gas causes the lava to form voids as it erupts. The rock will be riddled with holes called vesicles ; lack or red vesicular ( basalt ) rock is called scoria , while white or gray vesicular glass is pumice which is so light, it floats on water!

LAB #4 — IGNEOUS ROCKS AND PROCESSES - 2 - Figure 3.1 Igneous Rock Textures Sometimes certain crystals grow significantly larger than others producing a rock that has different sizes of grains. This is called porphyritic texture , and the large crystals are called phenocrysts .

LAB #4 — IGNEOUS ROCKS AND PROCESSES - 4 - CRYSTALLIZATION FROM A MELT Magmas crystallize at various temperatures (600 o to 1100 o C), much too high to demonstrate in this lab. Rocks A & B represent materials from the same magma chamber but were cooled at different rates. 1. Which represents plutonic (intrusive) and which represents volcanic (extrusive) rocks? 2. How do different cooling rates influence crystal nucleation ( nucleation being the initial formation of a microscopic crystal to which other atoms progressively bond)? DEMONSTRATION — EMPLACEMENT OF PLUTONS (LAVA LAMP) The movement of the colored fluid (i.e. hot and cold wax) in a lava lamp simulates the rise of magma through the lower crust (represented by the surrounding clear fluid). Notice that when the colored fluid becomes heated it rises, and when it cools it sinks. The thermally driven rise and fall of ductile-to-liquid materials in the Earth is called convection and contributes to the movement of the tectonic plates. 3. Why do some solids (e.g. wood) or liquids (e.g. oil) float on water, while other substances sink? 4. Why does the colored fluid rise and then sink in the lava lamp? What force pulls the liquid back down? Different nucleation and growth behavior were found for different cooling rates. At slow cooling rate, larger crystals were formed, whereas at fast cooling rate, smaller crystals appeared together. Slowly crystallized samples had a broader distribution of crystal size. The density of an object determines whether it will float or sink in another substance. An object will float if it is less dense than the liquid it is placed in. An object will sink if it is more dense than the liquid it is placed in. A lava lamp is an example of a convection current. Convection currents cause liquids and gases to rise and fall because of changes in their density.

LAB #4 — IGNEOUS ROCKS AND PROCESSES - 5 - INTERPRETING VARIAITONS IN CRYSTAL SIZE Examine the samples of andesite and diorite here . The different textures of these rocks make them look very different, but they have the same chemical composition (see Bowen ’s Reaction Series on page 3). Answer the following questions about these two samples by filling out the table below the questions: 1. What is the textural term that best describes each sample (be sure to reference the Igneous Rock Classification Chart)? 2. What interpretations can you make about the cooling rate of magma from which each rock formed? 3. What is the appropriate term for the origin of each rock? 4. In what tectonic plate setting (plate boundary) did each rock form? Property Definitions Andesite Diorite Textural Term Cooling Rate Origin Term Tectonic Setting BASALT AND GABBRO Examine the samples of gabbro and basalt here . The different textures of these rocks make them look very different, but they have the same chemical composition (see Bowen’s Reaction Series on page 3). These rocks comprise the oceanic crust. Answer the following questions about these two samples by filling out the table below the questions: 1. What is the textural term that best describes each sample (be sure to reference the Igneous Rock Classification Chart)? 2. What interpretations can you make about the cooling rate of magma from which each rock formed? 3. What is the appropriate term for the origin of each rock? 4. In what tectonic plate setting (plate boundary) did each rock form?

LAB #4 — IGNEOUS ROCKS AND PROCESSES - 7 - Sample Texture Visible Minerals Specimen Name Origin (Plutonic, Volcanic, etc.) 4-H 4-I 4-J 4-K 4-L 4-M 4-N 4-O 4-P 4-Q Make observations about samples 4-H through 4-Q and identify each sample based on their texture and mineral composition, using the Igneous Rock Classification Chart.

LAB #4 — IGNEOUS ROCKS AND PROCESSES - 8 -