Igneous Rocks Flashcards
IGNEOUS ROCKS formed by?
Solidification of magma
slow cooling = ________ ?
rapid cooling = ________ ?
slow = CRYSTALLIZATION rapid = QUENCHING
Classified by? ( 2 )
1 ) COMPOSITION (chemistry)
2 ) TEXTURE (crystal size/shape)
ULTRAMAFIC
- SiO2 content?
- Major Minerals?
- Rock Types (P/A) ?
- <45% SiO2
- Olivine, Pyroxene (pos amphibole, garnet)
- Peridotite (P)
- Komatite (A)
MAFIC
- SiO2 content?
- Major Minerals?
- Rock Types (P/A) ?
- 45-52% SiO2
- Pyroxene, Olivine, Plagioclase (pos amphibole, biotite)
- Gabbro (P)
- Basalt (A)
INTERMEDIATE
- SiO2 content?
- Major Minerals?
- Rock Types (P/A) ?
- 52-66% SiO2
- Plagioclase, Amphibole (pos biotite, potassium feldspar)
- Diorite (P)
- Andesite (A)
FELSIC
- SiO2 content?
- Major Minerals?
- Rock Types (P/A) ?
- > 66% SiO2
- Potassium feldspar, Quartz (pos biotite, amphibole)
- Granite, Granodiorite (P)
- Rhyolite, Dacite (A)
T A S Diagram
- Total Alkali vs. Silica content evolution
- used to group volcanic rock types
- composition dependent
Dark minerals dominate?
Melanocratic
Light minerals dominate?
Leucocratic
2 Examples of MISLEADING Color
1) Obsidian >66% SiO2 but black
2) Dunite >90% Olivine but pale green
Fine-grained (<1mm) ?
Aphanitic
Coarse-grained (1-30mm) ?
Phaneritic
VERY Coarse-grained (>30mm) ?
Pegmatitic
Mix of large and small grains?
Porphyritic
No crystals present?
Glassy (vitreous)
Texture created by trapped gas bubbles
*More common in FELSIC (more viscous, trap gas more easily)
Vesicular
Property of rocks that indicates ability of fluids/gases to flow through rock
PERMEABILITY
Rocks that are formed in the Earth’s crust
Intrusive / Plutonic
Rocks that are formed at the Earth’s surface
Extrusive / Volcanic
No constraints of on space - clean crystal faces develop
EUHEDRAL
Constraints on space - poor/no crystal faces develop
ANHEDRAL
Mix of some crystal faces and no crystal faces
SUBHEDRAL
Rocks that plot near P on QAPF (type + 3 EX)
MAFIC PLUTONIC ROCKS
- Anorthosite
- Gabbro
- Diorite
Texture controlled by interplay of ? (2)
- Nucleation rates (how quickly new seed crystals are formed from melt)
- Crystal growth rates (how quickly atoms build on a crystal lattice)
Factors in texture controls ? (3)
- viscosity
- ion availability
- volatile composition (H2O, CO2, SO2 - volatiles prevent SiO2 bonding = keeps viscosity low)
Si-deficient feldspar-like minerals ?
Quartz + Feldspathoids
Temperature-depth relationship of the Earth
the GEOTHERM (melting points higher because of higher pressure)
How are MELTS formed? (3)
- Temperature increase
- Decompression
- Volatile addition (H2O, CO2)
________ : As temperature increases in a rock ______ minerals will break down first
PARTIAL MELTING; SILICA-RICH
SiO2, K2O, Na2O
________ : As temperature decreases in a melt, ______ minerals crystallize first and fall out of solution
FRACTIONAL CRYSTALLIZATION;
MAFIC
Edges of the magma chamber cool faster and crystallize first, leaving hot center
MARGINAL ACCRETION
Crystal settling and floating
Gravitational Separation (differentiates magma chamber w/ layering)
When crystals grow on wall of intrusion due to density currents
Convective Fractionation
Melting of rocks around an intrusion, mixing with magma within the intrusion
Assimilation
