SPRING 2015

GEOL 285:INTRODUCTORY

PETROLOGY

Dr. Helen LangDept. of Geology & GeographyWest Virginia University

Geol 285Introductory Petrology

The study of rocks:in Greek: petra = rock

logos = discourse or explanation (study)

Petrology is central to Geology,and is based on MineralogyA rock is a naturally-occurring

aggregate of minerals or mineraloids.

What’s 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

(there are many different versions of the rock cycle)

Why study petrology?• to Learn about early history of Earth• to Learn about the interior of the Earth

– Only the upper crust is exposed or accessible to drilling• Crust is less than 1% of Earth’s volume• 66% of crust is sedimentary • the other 34% is mostly igneous

– The Mantle is metamorphic– The Core is liquid and solid metal Fe(Ni)

Thickness of Crust is just 1-2% of Earth’s Diameter

Crust is to scale here

Crustal thickness is really exaggerated here

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

• We must compare modern processes with the ancient rock record to infer processes that formed the rocks we see at the surface

We’ll start with Igneous Rocks:

• They were once liquid. • How can we tell?

– Properties/behavior of liquids?• What are some likely or expected textures

and structures of igneous rocks?– Volcanic– Plutonic

Igneous Rocks:Outcrop Characteristics (Table I-1)

• 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 of 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)

Fels

icM

afic

Simple Classification is based on two important characteristics:

• Percent mafic and felsic mineral content (dark vs. light)– 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

Intrusive(Plutonic)coarse-grained

Extrusive(Volcanic)fine-grained

Pyroclastic(Volcanic,fragmental)

Propertiesof theMagma

Mafic (hi CI*)(CI > 65)

Intermediate

Felsic (low CI*)(CI < 35)

C.I. = color index = % mafic minerals = mafic/(mafic+felsic) * 100

Gabbro Basalt

Andesite

RhyoliteGranite

Diorite

Scoria,Cinder

Pumice,Tuff, Ash

High density,Low Viscosity

Low density,High Viscosity

A very General Igneous Rock Classification

The amount of SiO2 in igneous magmas is quite variable and significant

• SiO2 in magmas varies between 30 and 80%• 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 Undersaturatedwith SiO2

Intrusive(Plutonic)coarse-grained

Extrusive(Volcanic)fine-grained

Pyroclastic(Volcanic,fragmental)

Propertiesof theMagma

Mafic (hi CI*)

Intermediate

Felsic (low CI*)

C.I. = color index = % mafic minerals = mafic/(mafic+felsic) * 100

Gabbro Basalt

Andesite

RhyoliteGranite

Diorite

Scoria,Cinder

Pumice,Tuff, Ash

High density,Low Viscosity

Low density,High Viscosity

General Igneous Rock Classification

Thickness of Crust is 1-2% of Earth’s Diameter

Crust is to scale here

Crustal thickness is really exaggerated here