GEOL 1301 - Lab 03 - Igneous Rocks (1)

GEOL1301 Name: Jonathan Venegas Igneous Rock Labs Learning and Lab objectives: Igneous rocks form from magma, lava, or pyroclastic material and are classified according to their composition and texture. In this lab, you are learning to identify a set of common igneous rocks, like the way you identified minerals in Lab 1. However, for Lab 2 your samples are rocks, which are formed of several different minerals instead of one isolated mineral. Nevertheless, you will see minerals that you recognize from last week so keep an eye out for familiar names. The categorization of igneous rocks relies on both texture and color of the sample. In the case of igneous rocks, the color of the rock (the grayscale) is an excellent indicator of the history of the molten rock. This is separated into three categories (mafic, intermediate, felsic). The simple classification of the color of an igneous rock can be used to determine critical facets of the history of the molten rock that it crystallized from, such as the plate tectonic setting the magma moved through or the type of volcano that extruded the lava. Terminology and lab instructions: At the beginning of the lab your lab instructor will review the important terminology below, which was gone over in lecture last week. They will also introduce some of the common igneous rocks and their formation mechanisms. Please complete Table 1 on page 2 of this lab and submit it to your instructor. Don’t forget to put your name on the worksheet! List of textures A list of adjectives used to describe the overall composition  Intrusive  Extrusive  Porphyritic  Glassy  Pyroclastic/fragmental  Vesicular/frothy  Felsic (light)  Intermediate (between light and dark)  Mafic (dark) We have also included two good, online identification keys: http://geology.about.com/od/rocks/a/Rock-Tables.htm https://australian.museum/learn/minerals/shaping-earth/classification-of-igneous-rocks/ Samples included in this lab are:  Basalt  Basalt Porphyry  Gabbro  Granite  Obsidian  Rhyolite  Scoria  Volcanic Tuff 1

Name: Jonathan Venegas Table 1: Observations and classifications of rock samples. Sample Number Texture Composition/ Grayscale Other Observations (Visible Minerals, Special Features… Rock Name Origin (How did the rock form?) 1 Glassy Mafic/Dark Deep black with no visible crystals Obsidian Extrusive formed by rapid cooling of lava on the volcano 2 Aphanitic Felsic/Light Light brown no air bubbles or crystals. Rhyolite Extrusive formed by rapid cooling of magma 3 Porphyritic Intermediate Greyish Black, Small crystals, No air bubbles, small fragments Gabbro Intrusive formed by slow cooling of magnesium and iron-rich magma deep in earth's surface 4 Phaneritic Felsic or Light Pink with slight grey and black, large crystals Granite Intrusive Product if melting of continental rocks smoothness is a result of its area being near subduction zones 5 Aphanitic Mafic/Dark Black, no crystals or rock fragments and air bubbles. Basalt Extrusive formed of rapid cooling of basaltic lava formed near the interior of the crust and 2