GEOL 1301 - Lab 03 - Igneous Rocks
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Apr 3, 2024
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GEOL 1301
Lab 03
Igneous Rocks
Igneous rocks form from magma, lava, or pyroclastic material and are classified according to their composition and texture. Your lab instructor will review some of the common igneous rocks, their formation mechanisms, and the terms mentioned in these instructions. Lists of textures and adjectives used to describe the overall composition of igneous rocks can be found below, as well as links to rock identification keys. Please complete the worksheet (“Examining and Classifying Igneous Rocks”), then upload your completed lab on Canvas.
Textures:
Phaneritic
Aphanitic
Porphyritic
Glassy
Pyroclastic/fragmental
Vesicular/frothy
Composition/Grayscale:
Felsic (light)
Intermediate (between light and dark)
Mafic (dark)
Identification Key:
See http://geology.about.com/od/rocks/a/Rock-Tables.htm
Examining and Classifying Igneous Rocks
Use the following link to identify the igneous rock samples listed in the table:
http://profharwood.x10host.com/GEOL101/Labs/Igneous/index.htm
Sample
Number
Texture
Composition/
Grayscale
Other Observations (Visible
Minerals, Special Features, …
Rock Name
Origin (How did
the rock form?)
1
Aphanitic
Mafic
Very fine grains visible.
Basalt
Rapid cooling of basaltic lava
2
Phaneritic
Intermediate
Coarse, many large fragments on surface
Diorite
Slow cooling of silica rich magma
4
Phaneritic
Mafic
Coarse, many large fragments on surface
Gabbro
Cooling and crystallization of magma under the Earth's surface
5
Phaneritic
Felsic
Coarse, many large fragments on surface
Granite
Cooling and crystallization of magma under the Earth's surface
7
Glassy
Felsic
Very smooth, no fine grains or large fragments on surface.
Obsidian
Rapidly cooling felsic lava 9
Porphyritic
Felsic
Coarse grainy surface
Rhyolite
Formed by magma ejected to earths surface that is cooled rapidly
10
Vesicular
Mafic
Coarse surface, man cavities in surface from gas bubbles during cooling
Scoria
Molten rock solidifies with gas present
12
Pyroclastic
Felsic
Coarse surface, appears to be chalk like
Volcanic Tuff
Volcanic ash falls to surface and then undergoes lithification
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