Geology 12 – Dalesandro
WebQuest #3 – Igneous Rocks
Instructions: Work alone or with a partner. Answer the following questions using the
igneous rocks home page:
http://www.pitt.edu/~cejones/GeoImages/2IgneousRocks.html
Type your answers on MS Word.
Value: 15 pts. total (1 pt per question, except #13 which is worth 3 pts).
Due Date: Thursday, Oct 8th.
Questions:
1) What is the key to identifying igneous (or other) rocks? Explain.
2) Name the two components which influence crystal size in an igneous rock.
3) a) What is the difference between pumice and scoria? b) Why is scoria red?
4) Besides granite, name two other course-grained igneous rocks. What are the colors of these
rocks, and what minerals do they contain?
5) What is the main difference between granite and diorite? How is this reflected in their
appearance?
6) How would you describe the appearance of amphibole as compared to basalt?
7) What factor enables felsic rock to form volcanic glass like obsidian easily?
8) Look at the close-up of gabbro. Use your own words to describe what this rock looks like.
9) What does welded tuff look like? How does the website say it forms?
10) Describe the andesite sample on the web page. Why might it be mistaken for a coarsegrained igneous rock like granite?
11) What determines the color of granite? List the minerals and the colors they shade the rock.
12) What are pegmatitic rocks? Which term used in class means the same thing?
13) List your three favorite igneous rocks and explain why you find them the most interesting.