Common rock types

Common Rock Types

Colby Geology




Igneous Rocks
Classification based on composition and texture.
COMPOSITION
Principally the rock-forming minerals
Olivine
Pyroxenes
Amphiboles
Micas
Feldspars
Quartz

Constitute 95% of minerals in
ALL igneous rocks.


COMPOSITION
You can determine basic igneous rock composition
just by the COLOR of the rock.
DARK COLORED igneous rocks

= MAFIC

LIGHT COLORED igneous rocks = FELSIC
GRAY COLORED igneous rocks

= INTERMEDIATE

Color allows an approximation of igneous rock
composition
Related to Bowen’s Reaction Series
(red vs. green)


1200°C

Gabbro-Basalt

900°C

Diorite-Andesite

Granite-Rhyolite
600°C


TEXTURE
TEXTURE reflects cooling rate of magma or lava.
SLOW COOLING

=

LARGE Crystals

RAPID COOLING

=

SMALL Crystals

SLOW COOLING

=

PLUTONIC

RAPID COOLING

=

VOLCANIC


TEXTURE
Types of Textures
Phaneritic
Large crystals > 2 mm
Slow cooling rate =
Plutonic
Granite,
Light
Felsic

Diorite,
Gray

Intermediate

Gabbro
Dark
Mafic


TEXTURE
Types of Textures
Aphanitic
Small crystals < 2 mm
Fast cooling rate =
Volcanic
Rhyolite,
Felsite
Light
Felsic

Andesite,
Gray
Intermediate

Massive Basalt
Scoria
Dark
Mafic


Rhyolite

Felsite


Andesite


Massive Basalt

Scoria


TEXTURE
Types of Textures
Glassy
No crystals, lava cools too fast
Extremely fast cooling rate =
Obsidian,
Pumice
Light
Felsic

Volcanic
Pumice

Obsidian


TEXTURE
Types of Textures
Porphyritic

Two distinct sizes of crystals
Starts cooling slowly, remainder cools quickly
Phenocrysts

Groundmass


TEXTURE
Types of Textures
TEXTURE is related to origin of magma or lava.
Phaneritic =

Coarse-grained

=

Intrusive
Plutonic

Aphanitic =
Glassy

Fine-grained
or smaller

=

Extrusive
Volcanic

Identification/Classification of Igneous Rocks
Key on page 75-77 in lab manual
Chart on page 91 in lab manual


IGNEOUS ROCK CLASSIFICATION
COMPOSITION

PHANERITIC

APHANITIC

GLASSY

Dunite
Ultramafic
(Very Dark) Peridotite
Mafic (Dark) Gabbro
Intermediate
Diorite
(Gray)
Felsic
(Light)

Granite

Basalt
Scoria
Andesite
Rhyolite

Felsite

Obsidian
Pumice



Sedimentary Rocks
Clastic Sedimentary Rocks
COMPOSITION
Derived from pre-existing rocks.
Five main constituents:
Quartz
Feldspar
Rock fragments
Clay (and clay minerals)
Calcite



TEXTURE
CLASTIC – fragments of rock debris
Classified by analyzing grain-size
Sediment Size
(mm)

Particle Name

>256

Boulder

64-256

Cobble

4-64

Pebble

2-4

Granule

0.063-2

Sand

Sandstone

0.004-0.063

Silt

Siltstone

<0.004

Clay

Rock Name

Gravel

Mud

Conglomerate
Breccia

Shale

Mudstone


Particles
>2mm

= coarse-grained

CONGLOMERATE
(rounded clasts)
BRECCIA
(angular clasts)
1/16 - 2mm = medium-grained SANDSTONE
<2mm
= fine-grained
SHALE
Sandstones further classified on the basis of their
mineralogical composition:
Mostly quartz

>25% feldspar
1/3 silt, clay and rock fragments

= QUARTZ SS.
= ARKOSE
= GREYWACKE


Coarse-grained Clastic Sedimentary Rocks

Conglomerate

Breccia


Medium-grained Clastic Sedimentary Rocks

Sandstones
Arkose

Greywacke

Qtz. Sandstone


Fine-grained Clastic Sedimentary Rocks

Shale

Sedimentary Rocks
Chemical Sedimentary Rocks
CHEMICAL SEDIMENTARY ROCKS
precipitated from sea water (evaporites)
initially dissolved in water
very similar in appearance to igneous rocks
except MONOMINERALIC.
texture is crystalline if visible
microcrystalline if not
OOLITIC LIMESTONE
CHERT
DOLOMITE