This document last updated on 10-Jan-2011
Prof. Stephen A. Nelson
The branch of geology dealing with the origin, occurrence, structure, and history of
The branch of geology dealing with the description and systematic classification
of rocks, especially by microscopic examination of thin sections. Petrography is a subfield of
In this course, most of the lecture material falls under the field of Petrology, while much of the
laboratory material falls in the field of Petrography.
Introduction to Igneous Rocks
is any crystalline or glassy rock that forms from cooling of a magma.
consists mostly of liquid rock matter, but may contain crystals of various minerals,
and may contain a gas phase that may be dissolved in the liquid or may be present as a separate
Magma can cool to form an igneous rock either on the surface of the Earth - in which case it
extrusive igneous rock
, or beneath the surface of the Earth,
- in which
case it produces a
intrusive igneous rock
Characteristics of Magma
Types of Magma
Types of magma are determined by chemical composition of the magma. Three general types
are recognized, but we will look at other types later in the course:
45-55 wt%, high in Fe, Mg, Ca, low in K, Na
55-65 wt%, intermediate. in Fe, Mg, Ca, Na, K
65-75%, low in Fe, Mg, Ca, high in K, Na
Gases in Magmas
At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but the gas forms a
separate vapor phase when pressure is decreased as magma rises toward the surface.
similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the
gas in solution in the liquid, but when pressure is decreased, like when you open the can or
bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles.
Gas gives magmas their explosive character, because volume of gas expands as pressure is
Page 1 of 14