The Identification of Igneous Rocks
(Elementary, Middle, and High School)

Robert S. Smith (updated 07/05/2006)


 
Sunshine State Standards:
  • Elementary-SC.A.1.2.4, SC.A.2.1.1, SC.H.1.2.3, SC.H.1.2.4 Middle- SC.A.1.3.1, SC.D.1.3.1, SC.H.1.3.1 (AA), SC.H.1.3.2High School- SC.D.1.4.1 
Objectives:
  • Students will learn that Igneous rock texture can be used to identify the type of igneous rock 
  • Students will learn that Igneous rock color (light/dark) can be used to identify the type of igneous rock 
Materials:
  • Samples of igneous rocks: * From Ward’s catalog
  • 15 Rhyolite (47 H 6907) 
  • 15 Andesite (47 H 0517) 
  • 15 Olivine Basalt (47 E 1077) 
  • 15 Granite (47 H 3627) 
  • 15 Diorite (47 H 2692) 
  • 15 Gabbro (47 H 3307) 
  • 15 Obsidian (Brown & Black) (47 E 5747) 
  • 15 Diabase Porphyry (47 H 2637) 
  • 30 Magnifiers (24 H 1112) 
  • Igneous Rock Identification key 
  • Zip Lock Bags 
Commit to an Outcome:
  • Do you think you can learn to identify igneous rocks by observing rock samples and using an igneous rock identification key? 
Expose Beliefs:
  • What do you know about igneous rocks? 
  • How do think that igneous rocks are formed? 
  • Do you know of any types of igneous rocks? 
Confront Beliefs:
  • Give the students the zip lock bags with the first 6 rock samples and a magnifier for each student: 
  • Look at these igneous rocks. 
  • What similarities and differences do you observe? 
Accommodate the Concept:
  • Give the students the Igneous rock identification key: 
  • Can you use an igneous rock identification key? 
  • What characteristics of the igneous rocks can you use with the key to identify the rocks? 
Extend the Concept:
  • What does the texture of an igneous rock tell you about how the rock was formed? 
  • What does the color (light/dark) tell you about the composition of the igneous rock? 
Give the students the samples of obsidian and porphyry:
  • What does the texture of these two types of igneous rocks tell you about how they were formed? 
Go Beyond:
  • Do you have any questions? 
  • How do you think that NASA can use what you have learned to determine what types of rocks make up the moon and planets’ surfaces 

What we learned from this activity:

  1. Texture is an important physical characteristic which can be used to identify igneous rocks.
  2. Color (light, dark, intermediate) is an important physical characteristic which can be used to identify igneous rocks.
  3. An igneous rock identification chart can be used to identify igneous rocks.
Amplifying Information:
  1. Texture indicates the rate of cooling of the magma. Coarse grained (intrusive) igneous rocks (granite, diorite, and gabbro) cool relatively slowly inside the Earth. Fine grained (extrusive) igneous rocks (rhyolite, andesite, and basalt) cool relatively quickly as lava at or very near the Earth’s surface. Some extrusive igneous rocks cool so quickly that their texture is glassy (obsidian). Some igneous rocks begin to cool inside the Earth an form crystals (phenocrysts) of a high temperature mineral before rising to the Earth’s surface cooling quickly as lava. This type of texture is called porphyritic Texture.
  2. The color (light, dark, or intermediate) indicates the origin and composition of the magma. Light colored igneous rocks (granite and rhyolite) come from relatively near the Earth’s surface and consist of low density minerals; quartz, potassium feldspar, muscovite mica, and some dark minerals such as biotite mica and hornblende (amphibole). Dark colored igneous rocks (gabbro and basalt) come from the upper mantle (asthenosphere) and consist of higher density minerals such as calcium rich plagioclase feldspar, iron and magnesium rich rocks including pyroxenes, amphiboles, and olivine (no quartz). Intermediate colored igneous rocks (diorite and andesite) consist of some quartz and sodium rich plagioclase feldspar along with a moderate amount of dark colored minerals such as hornblende (amphibole) and pyroxene.
  3. An example of the value of understanding the relationship of the gray scale color volcanic rocks (lava) is that the color of the lava indicates its origin. Light colored lava (rhyolite) originates in the upper lithosphere (crust) and is associated with the more violent (more silica) cinder cone volcanoes. Dark colored lava (basalt) originates from the upper mantle (asthenosphere) and is associated with less violent (little to no silica) hot spot and mid ocean ridge shield type volcanoes. Composite volcanoes generally have a lava of intermediate color and therefore are generally known as andesite volcanoes. These observations are consistent with our observations of the types of volcanoes associated with the sea floor spreading and convergence (subduction) of the Theory of Plate Tectonics.