Geocentric Science

1. Texture

   • Do you see vesicles (gas pockets)?

     • If yes then, the rock is glassy or fine.
     • If no, then the rock is glassy, coarse, or very coarse.

   • Do you see many different colors or is the rock light, medium, or dark in color?

     • If you see many different colors such as pinks, grays, blacks, etc., you are looking at the crystals (the minerals) and the rock must have a coarse to very coarse texture.
     • If the rock is one color such as light (white, pink, to light gray), medium (light gray to dark gray), or dark (green to black), the rock has a fine or glassy texture.
       

2. Environment of Formation

   • Once the crystal size has been determined, direct your attention to the left-hand side of the chart to the Environment of Formation.
   • Glassy to fine textures indicate the igneous rock is an Extrusive Igneous rock (formed on the Earth’s surface and underwent rapid cooling and crystallization).
   • Coarse to very coarse textures indicate the igneous rock is an Intrusive Igneous rock (formed deep below the Earth’s surface and underwent slow cool and crystallization).

3. Igneous Rock Name

   • Using the color information from 1b and the Mineral Composition Chart, the rock name can be determined.

     • If you can see the individual crystals and if a rock is composed of mostly white, pink, and some black crystals, the rock is Felsic.

     • If you can see the individual crystals and if the rock is roughly half dark minerals and half light minerals, the rock is Intermediate.

     • If you can see the individual crystals and if the rock contains mostly black, green, and white crystals, the rock is Mafic.

     • General rules

       • 75% light minerals and 25% dark minerals = Felsic (Granite or Pegmatite)
       • 50% light minerals and 50% dark minerals = Intermediate (Diorite)
         
       • 25% light minerals and 75% dark minerals = Mafic (Gabbro) Greater than 75% dark minerals = Ultra Mafic (Peridotite or Dunite)

     • If you can’t see the individual crystals, the general rule is: Light (white, pink to light gray) = Felsic (Pumice, Vesicular Rhyolite, or Rhyolite) Medium (light gray to dark gray) = Intermediate (Pumice, Vesicular Andesite, or Andesite) Dark (green to black) = Mafic (Basaltic Glass, Scoria, Vesicular Basalt, or Basalt)

     • Obsidian is typically black and shinny, and has prominent conchoidal fractures

Remember

Coarse does not refer to the feel of the rock, it refers to the sizes of the minerals that the rock is composed of.

Glassy does not mean shinny. Glassy refers to the non-crystalline structure of the minerals within the rock. The rock formed extremely rapidly and therefore the minerals did not have enough time to arrange themselves into an interlocking orderly pattern.