Download Igneous Rocks Notes

NORTH SEATTLE COMMUNITY COLLEGE
INTERNATIONAL STUDENTS OF DENMARK
FALL 2014
GEOLOGY
Linda Khandro, MAT
Notes on Igneous Rocks Identification and Classification
These notes accompany the file called Igneous Rock Classification Chart
A. Terms for Textures and Origins. These are the vertical terms, on either or both sides
of the typical Igneous Rock Classification Charts. Note that with igneous rocks, texture
does not refer to how the rocks or minerals “feel” to the hand; it’s a term that represents the
visibility of minerals.
1. Texture terms are based on the relative sizes of minerals. For example, when
minerals are visible to the naked eye, the texture is called “coarse-grained” (Phaneritic). In
the same way, rocks with minerals that are not visible to the naked eye are called “finegrained” (Aphanitic).
2. Origin terms are based on the locations where magma cools and forms igneous
rocks: at the surface or deep in the crust.
Coarse-grained igneous rocks have formed from magma that stays relatively deep in the
earth’s crust. Thus insulted from the colder surface, they cool slowly and the crystalizing
minerals have time to grow to a visible size. As a result of their depth of cooling and
crystallization, they are also often called “Plutonic” (think of the ancient Greek god of the
underworld, Pluto), and plutons are often granitic in composition. They can frequently intrude
into or between host rock areas of the crust so they are also called intrusive.
Fine-grained igneous rocks have formed at or near earth’s surface, where the temperature is
far colder than the magma itself. Thus this magma cools rapidly and there is far less time for
minerals to grow to visible size. Since this igneous rock texture makes up most of the
volcanoes of the earth, they are called volcanic, and since the magma is often extruded (think
about extruding toothpaste) out upon the surface, they’re called extrusive.
B. Terms for Composition. These are the horizontal terms, either along the top or along
the bottom of the chart, or both.
Composition of igneous rocks ranges from those that are very rich in the Felsic minerals
(quartz, potassium (K) feldspar, sodium (Na) plagioclase feldspar) with lesser percentages of
biotite and hornblende; to those that are very rich in the Mafic minerals (Ca-plagioclase
feldspar, pyroxene and olivine). Since the Felsic minerals are typically light in both color and
density, those properties also become part of the igneous classification scheme. In the same
way, color and density of the Mafic minerals become part of the classification scheme.
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