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
(Gray)
Diorite
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
Rock Name
Gravel
Conglomerate
Breccia
4-64
Pebble
2-4
Granule
0.063-2
Sand
Sandstone
0.004-0.063
Silt
Siltstone
Mudstone
Mud
<0.004
Clay
Shale
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
Oolitic Limestone
Chert
Dolostone
CHEMICAL SEDIMENTARY ROCKS
ROCK SALT or HALITE
composed of halite (NaCl)
microcrystalline to crystalline
precipitates from sea water
forms economic deposits
Sedimentary Rocks
Organic Sedimentary Rocks
ORGANIC or BIOCLASTIC SEDIMENTARY ROCKS
can be crystalline or clastic (bioclastic)
all particles are derived from organisms
typically composed of calcite or aragonite (CaCO3)
Lithographic Limestone or Micrite
Very fine-grained limestone
Formed from calcareous
algae
Fizzes in HCl
Fossiliferous Limestone
NONCARBONATE ORGANIC SED. ROCKS
CHERT
microcrystalline
composed of skeletons, microscopic in size
made of SiO2
hardness = 7
may exhibit conchoidal fracture
NONCARBONATE ORGANIC SED. ROCKS
PEAT
composed of compressed plant debris
basic building block of coal
used as a fuel source in some areas
Identification/Classification of Sedimentary Rocks
Key on page 79-80 in lab manual
Chart on page 93 in lab manual
Metamorphic Rocks
Rocks are fundamentally changed by HEAT and/or
PRESSURE
May be derived from ANY pre-existing rock type
CLASSIFICATION
based on the presence or absence of FOLIATION
FOLIATION is the parallel alignment of the tabular
minerals (micas and clay minerals) and by
varying degrees of banding.
Metamorphic Rocks
Types of Foliation
Slatey cleavage
very fine-grained rocks
Usually found in slate and phyllite
Metamorphic Rocks
Types of Foliation
Schistosity
platy minerals visible
Usually found in schist
Metamorphic Rocks
Types of Foliation
Gneissic banding
minerals segregated into bands
Usually found in gneiss
Metamorphic Rocks
Foliated Rocks
SLATE
PHYLLITE
Have slatey cleavage
Rock splits along parallel planes
SCHIST
Has schistosity
GNEISS
Has gneissic banding
Metamorphic Rocks
Nonfoliated Rocks
NO structure is apparent in the rock
Rock is typically MASSIVE (monomineralic)
Metamorphic Rocks
Nonfoliated Rocks
MARBLE
Is composed of calcite (CaCO3), soft H = 3
Will react to dilute hydrochloric acid
Metamorphic Rocks
Nonfoliated Rocks
QUARTZITE
Metamorphosed quartz sandstone
Hardness = 7
Metamorphic Rocks
Nonfoliated Rocks
ANTHRACITE COAL
Formed from organic material (plant debris)
Contains no minerals
Metamorphic Rocks
Nonfoliated Rocks
GREENSTONE
Composed of metamorphosed mafic igneous rocks
Identification/Classification of Metamorphic Rocks
Key on page 82 in lab manual
Chart on page 95 in lab manual