1. Common Rock Types
Colby Geology

4. Igneous Rocks Classification based on composition and texture.
Principally the rock-forming minerals
Olivine
Pyroxenes
Amphiboles
Micas
Feldspars
Quartz
Constitute 95% of minerals in
ALL igneous rocks.

5. 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)

6. 1200°C
Gabbro-Basalt
900°C
Diorite-Andesite
Granite-Rhyolite
600°C

7. TEXTURE TEXTURE reflects cooling rate of magma or lava.
SLOW COOLING = LARGE Crystals
RAPID COOLING = SMALL Crystals
SLOW COOLING = PLUTONIC
RAPID COOLING = VOLCANIC

8. TEXTURE Types of Textures
Phaneritic
Large crystals > 2 mm
Slow cooling rate = Plutonic
Granite, Diorite, Gabbro
Light Gray Dark
Felsic Intermediate Mafic

9. TEXTURE Types of Textures
Aphanitic
Small crystals < 2 mm
Fast cooling rate = Volcanic
Rhyolite, Andesite, Massive Basalt
Felsite Scoria
Light Gray Dark
Felsic Intermediate Mafic

10. Rhyolite
Felsite

11. Andesite

12. Massive Basalt
Scoria

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

14. TEXTURE Types of Textures
Porphyritic
Two distinct sizes of crystals
Starts cooling slowly, remainder cools quickly
Phenocrysts
Groundmass

15. TEXTURE Types of Textures
TEXTURE is related to origin of magma or lava.
Phaneritic = Coarse-grained = Intrusive
Plutonic
Aphanitic = Fine-grained = Extrusive
Glassy or smaller Volcanic
Identification/Classification of Igneous Rocks
Key on page in lab manual
Chart on page 91 in lab manual

16. IGNEOUS ROCK CLASSIFICATION
COMPOSITION
PHANERITIC
APHANITIC
GLASSY

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

20. CLASTIC – fragments of rock debris Classified by analyzing grain-size
TEXTURE
CLASTIC – fragments of rock debris
Classified by analyzing grain-size
Sediment Size
(mm)
Particle Name
Rock Name
<0.004
64-256
4-64
2-4
>256
Boulder
Cobble
Pebble
Granule
Sand
Silt
Clay
Gravel
Mud
Conglomerate
Breccia
Sandstone
Shale
Siltstone
Mudstone

21. 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 = QUARTZ SS.
>25% feldspar = ARKOSE
1/3 silt, clay and rock fragments = GREYWACKE

22. Coarse-grained Clastic Sedimentary Rocks
Conglomerate
Breccia

23. Sandstones Medium-grained Clastic Sedimentary Rocks Arkose Greywacke
Qtz. Sandstone

24. Fine-grained Clastic Sedimentary Rocks
Shale

25. 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

26. Oolitic Limestone

27. Chert

28. Dolostone

29. CHEMICAL SEDIMENTARY ROCKS
ROCK SALT or HALITE
composed of halite (NaCl)
microcrystalline to crystalline
precipitates from sea water
forms economic deposits

30. 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)

31. Lithographic Limestone or Micrite
Very fine-grained limestone
Formed from calcareous
algae
Fizzes in HCl

32. Fossiliferous Limestone

33. NONCARBONATE ORGANIC SED. ROCKS
CHERT
microcrystalline
composed of skeletons, microscopic in size
made of SiO2
hardness = 7
may exhibit conchoidal fracture

34. NONCARBONATE ORGANIC SED. ROCKS
PEAT
composed of compressed plant debris
basic building block of coal
used as a fuel source in some areas

35. Identification/Classification of Sedimentary Rocks
Key on page in lab manual
Chart on page 93 in lab manual

36. 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.

40. Metamorphic Rocks Types of Foliation Slatey cleavage
very fine-grained rocks
Usually found in slate and phyllite

41. Metamorphic Rocks Types of Foliation Schistosity
platy minerals visible
Usually found in schist

42. Metamorphic Rocks Types of Foliation Gneissic banding
minerals segregated into bands
Usually found in gneiss

43. Metamorphic Rocks Foliated Rocks SLATE Have slatey cleavage
PHYLLITE Rock splits along parallel planes
SCHIST Has schistosity
GNEISS Has gneissic banding

44. Metamorphic Rocks Nonfoliated Rocks
NO structure is apparent in the rock
Rock is typically MASSIVE (monomineralic)

45. Metamorphic Rocks MARBLE Is composed of calcite (CaCO3), soft H = 3
Nonfoliated Rocks
MARBLE
Is composed of calcite (CaCO3), soft H = 3
Will react to dilute hydrochloric acid

46. Metamorphic Rocks QUARTZITE Nonfoliated Rocks
Metamorphosed quartz sandstone
Hardness = 7

47. Metamorphic Rocks ANTHRACITE COAL Nonfoliated Rocks
Formed from organic material (plant debris)
Contains no minerals

48. Metamorphic Rocks GREENSTONE Nonfoliated Rocks
Composed of metamorphosed mafic igneous rocks

49. Identification/Classification of Metamorphic Rocks
Key on page 82 in lab manual
Chart on page 95 in lab manual