Geology 101
Name:
Igneous Rock ID
Texture:
1A) Check the Rock ID handout or Chapter 3 in your book and define the textures below:
Coarse-grained (a.k.a. phaneritic) =
Fine-grained (a.k.a. aphanitic) =
Porphyritic =
How big is a millimeter (mm) anyways? Draw a square with sides 1 mm long in the box below:
1mm =
Examine the figures in Chapter 3 of your book or the sample of: Andesite, Basalt, Diorite, Gabbro,
Granite, and Rhyolite in room 110 in the boxes labeled Part III: Rocks box 1 and box 2. (Note: all
of these rocks have RED labels).
Identify each rock listed above as either Course-grained, fine-grained, or Porphyritic in the space
provide:
1B) Which of these rocks have a Plutonic or coarse-grained (phaneritic) texture?
1C) Which rocks have a Volcanic ( fine-grained/ aphanitic or porphrytic) texture?
1D) Describe the cooling history of a prophritic rock:
1E) List the approximate percent (%) abundance of black minerals to the nearest 10% in the spaces
provided below:
Granite =
%
Granodiorite=
%
Diorite =
%
Gabbro =
%
Composition: Mafic, Felsic, or Intermediate?
1F) List the rock NAME that best fits in the table below:
Volcanic Texture Rock Name
Plutonic Texture Rock Name
Composition
Felsic
Intermediate
Mafic
1G) According to your Rock ID handout, the composition of Dacite is intermediate between the
composition of the two volcanic rocks:
&
and the composition of Granodiorite is intermediate between the compositions of the two plutonic rocks:
&
Part 2: Obsidian: (Note a sample may be found in SC 110 in Box III, Part 2)
2A) Describe the color and geologic texture of this rock. Can you see any individual mineral grains?
2B) What is the cooling history of obsidian? (circle one of each choice below).
Plutonic or Volcanic?
Fast Cooling or Slow Cooling?
2C) Read the discussion on pages 81 and 82 of your book (or the front page of your rock handout) and
answer the questions below: Obsidian has a
high / low silica (circle one) content, which makes it
a mafic / felsic (circle one) rock.
2D) Explain why you can’t use color to determine if obsidian is mafic or felsic (check page 1 or your rock
handout or pages 81-82 in your book):