ROCKS: Records of geologic processes
A. Rock Classification
- Mineral make-up
- Texture: size, shape, & orientation of minerals
Three rock groups:
- Igneous Rocks: solidification of magma
(molten rock)
- Sedimentary Rocks: lithification of sediments
by compaction and cementation
- Metamorphic Rocks: transformations of
preexisting rocks in the solid state under high
T and/or P.
Applications: oil exploration, mineral and metal
prospecting, environmental impact evaluation
B. Igneous Rocks:
Solidification from magma
Magma that reaches the surface is called lava
Intrusive Igneous Rocks: slow cooling,
interlocking large crystals. a.k.a. plutonic rocks
(Pluto: Roman God of the underworld)
Extrusive Igneous Rocks: rapid cooling, glassy
or fine-grained texture. a.k.a. volcanic rocks
C. Sedimentary Rocks:
- Sediments originated from erosion and
weathering (broken up into smaller fragments)
of existing rocks.
- Sediments could be carried to the buried site
by either wind or water
- Some sediments originated from the
precipitation of material dissolved in water
- They typically show layered structure called
bedding (Fig. 6.4)
D. Metamorphic Rocks:
- Regional Metamorphism: plate collisions that
result in mountain building and folding of
rocks (Fig. 1.18)
- Contact Metamorphism: high T transforms
rocks near and in contact with an intrusive
magma (Fig. 7.13)
- Common textures:
Regional metamorphism: foliation (wavy or
flat planes of mineral grains) (Fig. 7.8)
Contact metamorphism: granular texture (very
high T & P regional metamorphism could also
generate this texture)
Fig. 7.11: marble
The Rock Cycle: Figure 1.22
Igneous Rocks: Solid from melts
Introduction:
- Magma is completely or partially molten:
solid (minerals), liquid (melt), volatiles
(gases)
- As magma cools, the constituents in magma
begin to lose their mobility and arrange into
crystalline structures.
- The composition of crystals  magma
Fig. 3.22
- More and more complex crystalline structures
form during the cooling process:
Single silicon-oxygen tetrahedron  single
chain  double chain, etc. (Fig. 2.23)
A. Base of Classification for Igneous Rocks:
- Texture: size, shape, and arrangement of
crystals
- Mineral Composition
B. Texture:
A result of cooling rate, chemical
composition, and dissolved gases
Intrusive Igneous Rocks: Magma forces its way
into surrounding rock  slow cooling large
crystal.
a.k.a. Plutonic rocks.
Fig. 3.4:
phaneritic (coarse-grained)
pegmatitic (> 1cm)
Extrusive Igneous Rocks: Magma erupts to the
surface rapid cooling  small crystal or
glassy. a.k.a. volcanic rocks
Fig. 3.4: aphanitic (fine-grained); glassy
If magma erupts after some crystals already
formed  large crystals embedded in fine
crystals. The large crystal is called phenocrysts,
and the resulting rock is called porphyry.
Figs. 3.3 & 3.11: porphyritic (phenocrysts vs.
groundmass)
C. Chemical and Mineral Compositions
Bowen’s Reaction Series: As the magma cools,
the minerals with highest melting temperature
solidified first. Mineral with progressively lower
melting temperatures will solidify in sequence
upon further cooling.
The chemical composition changes as a result of
this crystallization sequence.
Figure 3.21 Bowen’s reaction series
Figure 3.10 Felsic(granitic)
Intermediate (Andesitic)
Mafic (Basaltic)
Ultramafic
Silica content
Ca & Mg content
Na & K content
Granite: ~25% quartz, ~65% feldspar
Pegmatite: crystal size > 1 cm
Rhyolite; obsidian; pumice
Diorite: < 5% quartz,
Mostly Plagioclase & amphibole;
salt & pepper appearance
Gabbro: pyroxene & Ca-rich plagioclase
Peridotite: olivine & pyroxene
D. Mineral Resources and Igneous
Processes
Magmatic segregation concentrates selected
metal in an igneous complex.
e.g. chromite (ore of chromium),
platinum in mafic/ultramafic rocks
Late state hydrothermal fluid creates
hydrothermal deposits and pegmatite.
Figure 3.26
Table 3.1 Occurrences of metallic minerals