Geol 285, Petrology, Dr. Helen M. Lang - West Virginia University, Spring
2010
Geol 285 - Introduction to Petrology
The study of rocks:
Greek: petra = rock
logos = discourse or explanation (study)
Petrology is central to Geology,
and is based on Mineralogy
A rock is a naturally-occurring aggregate of minerals or mineraloids
What is the difference between a rock and a mineral?
Rocks are conveniently divided into 3 categories

Igneous rocks-rocks that solidified from molten or partially molten material
(magma)


Sedimentary rocks-rocks resulting from the consolidation of loose sediment or
chemical precipitation from solution at Earth’s surface


Metamorphic rocks-rocks formed from pre-existing rocks by mineralogical,
chemical and textural changes in response to change in conditions

The Rock Cycle shows how rocks form and change from one type into another
Why study petrology?

Learn about early history of Earth

Learn about the interior of the Earth

Only a small part of the crust is exposed or accessible to drilling
o
Crust is less than 1% of Earth’s volume
o
66% of crust is sedimentary
o
other 34% is mostly igneous

Mantle is metamorphic

Core is liquid and solid metal Fe(Ni)
Thickness of Crust is 1-2% of Earth’s Diameter
Why study Petrology (continued)

We can learn about the whole Earth only by studying exposed rocks, drill cores
and geophysics

Distribution of rock types at Earth’s surface led to Plate Tectonic Theory

Compare modern processes with ancient rock record; infer processes and explain
differences
We’ll start with Igneous Rocks:
Outcrop Characteristics (see Table I-1 in textbook)

Volcanoes and related lava flows

Cross-cutting relations to surrounding rocks (dikes, veins, stocks and batholiths)

Thermal effects on adjacent rocks

Chilled (fine-grained) borders against adjacent rocks

Lack fossils and stratification

Generally structureless (massive) and composed of interlocking grains
Typical Igneous Textures

Porphyritic

Glassy

Vesicular

Pyroclastic

Interlocking crystalline
Typical Igneous Minerals

Plagioclase Feldspars (Ca,Na)(Al,Si)4O8

Alkali Feldspars (Na,K)AlSi3O8

Quartz SiO2

Olivine (Mg,Fe)2SiO4

Pyroxene (Ca,Mg,Fe)2Si2O6

Hornblende

Micas (biotite and muscovite)

Glass (not a mineral, but a mineraloid)
Initial Classification based on:

Percent mafic and felsic mineral content

reflects chemistry of magma

more FeO, MgO, more mafic minerals, darker color

more SiO2, Na2O, K2O, more felsic minerals, lighter color

Grain size

relates to cooling rate

fine-grained or glassy, cooled rapidly

coarse-grained, cooled slowly
See handout for General Classification of Igneous Rocks and their properties
The amount of SiO2 in igneous magmas is quite variable and significant

Magmas with enough SiO2 to crystallize quartz (pure, free SiO2) are said to be
Oversaturated with SiO2

Magmas with so little SiO2 that they crystallize minerals that are incompatible
with quartz are said to be Undersaturated with SiO2