Observing Igneous Rocks Integrated Science Name ______________________ Per. _____ Introduction Igneous rocks are classified according to their color, which indicates chemical composition, and their crystal size, which indicates where they were formed. Intrusive rocks form beneath the Earths crust when magma cools slowly. Extrusive igneous rocks form on the surface when lava cools very rapidly. Differences in cooling rate determine the texture of igneous rocks. When magma cools slowly, it forms large crystals and coarse-grained rocks. When magma cools rapidly, such as in water, it forms extremely small crystals and glassy rocks. The color of igneous rocks indicates their mineral composition. Dark rocks usually contain large amounts of iron and magnesium and are called mafic. Light rocks contain large amounts of silicon and aluminum and are called felsic. Andesitic rocks are a mixture of light and dark colors, and are also intermediate in terms of iron, magnesium, aluminum and silicon levels. In this laboratory, we will observe color and crystal size in igneous rock types. We will use these observations to infer chemistry and depth of formation. Consider the following focus questions as you complete this activity: • What are the observable differences amongst common igneous rocks? • What are the differences in chemical composition and depth of formation amongst common igneous rocks? Procedure 1. Using the Introduction and any supplemental resources pertaining to igneous rocks, complete the unshaded sections of Data Table 1. • First, consider the mineral composition of each igneous rock type: felsic, mafic, and andesitic. Record the characteristic Colors of each rock type in the appropriate row in Data Table 1. • Next, consider the chemical composition of each rock type, felsic, mafic, andesitic. Record the characteristic Chemistry in the appropriate row in Data Table 1. • Then, consider the depth of formation for each igneous rock type (extrusive and intrusive). Based on this, determine and record the relative Crystal Sizes of each rock type in the approprate column in Data Table 1. 2. Using the background information and any supplemental resources pertaining to igneous rocks, complete the shaded sections of Data Table 1. To do this, write the Name of each rock sample in the appropriate box in Data Table 1. The igneous rock names we will be observing are: andesite, basalt, diorite, gabbro, granite, obsidian, pumice, rhyolite, and scoria. Note: the color and crystal size observations should match for each rock type. Although there are 9 types of rocks listed, we will only be observing 7 types. You need to identify the two missing rock types. 3. Gather the actual rock samples. Observe each rock’s characteristics and place them into the appropriate box in Data Table 1. After completing Data Table 1, have your teacher initial your work. 4. With your naked eye and hand lenses or stereo microscopes, make thorough observations of each rock. Record all crystal sizes, colors, and a sketch (labeled or colored, if possible) for each sample in Data Table 2. After completing Data Table 2, have your teacher initial your initial work. Data Table 1: Classification of Common Igneous Rocks (Color, Chemistry and Crystal Size Trends) ROCK TYPE: Andesitic Felsic Mafic Colors: Colors: Colors: Chemistry: Chemistry: Chemistry: Name: Name: Name: Relative crystal size: Name: Name: Name: Relative crystal size: Name: Name: Name: DEPTH OF FORMATION: Relative crystal size: Extrusive Intrusive (Shallow to moderate depth) Intrusive (Moderate to great depth) Data Table 2: Observations of Common Igneous Rocks (Listed in alphabetical order) Andesite Color(s) and sketch: Basalt Color(s) and sketch: Diorite Color(s) and sketch: Crystal Size(s): Crystal Size(s): Crystal Size(s): Gabbro Color(s) and sketch: Granite Color(s) and sketch: Obsidian Color(s) and sketch: Crystal Size(s): Crystal Size(s): Crystal Size(s): Pumice Color(s) and sketch: Rhyolite Color(s) and sketch: Scoria Color(s) and sketch: Crystal Size(s): Crystal Size(s): Crystal Size(s):