igneous rocks Flashcards
igneous rock forming minerals
felsic- quartz, plagioclase & potassium feldspar, muscovite mica
mafic- biotite mica, amphibole, pyroxene, olivine
What determines felsic rock
Less than 25% mafic minerals
Feldspar and silica (quartz)
Almost entirely of light colored silicates with small amts of micas or hornblende
What determines intermediate rock
22-25% mafic (dark) mineral
What determines mafic rock
55-85% mafic minerals
Basaltic
Magnesium and ferrum
Contains substantial dark silicate minerals and calcium rich plagioclase feldspar
What determines ultra mafic rock
Greater than 85% mafic (dark) minerals
Intrusive
Cooled slowly at depth or near surface
Large visible crystals
Plutonic
Extrusive
Cooled rapidly on surface
Volcanic
Too big to grow crystals
Extrusive rocks
Rhyolite Andesite Basalt Obsidian basalt Scoria Pumice
Intrusive rocks
Granite
Diorite
Gabbros
Intrusive igneous features
Dike
Sill
Laccoliths
Batholiths
Dike
Tabular disconcordant pluton
Sill
Tabular concordant pluton
Laccoliths
Similar to a sill
Lens or mushroom shaped mass
Arches overlying strata upward
Batholiths
Largest intrusive body
Surface exposure >100 sq km
Smaller bodies termed stocks
Frequently form the cores of mts
Igneous textures
Phaneritic Aphanitic Porphyritic Vitreous Vesicular Pegmatitic Pyroclastic
Phaneritic
Coarse grained
Crystals greater than 1mm but less thn 1 cm
Slow cooling intrusive
Crystals have time to grow
Aphanitic
Fine grained
Crystals less than 1 mm
Fast cooling extrusive
Crystals don’t have time to grow
Porphyritic
Two distinct crystal sizes
Larger/independent crystals called phenocrysts-slow cooling
Mix of coarse and fine crystals
Indicate a 2 stage history
Vitreous
Compact or frothy glassy Rapid cooling of lava in water or air Obsidian-lava flowing in water Pumice- frothy felsic full of vesicles Scoria- glassy vesicular mafic
Vesicular
Holes formed from gas-charged lava
Rapid rate of cooling
Gas bubbles
Pegmatitic
Crystal size greater than 1 cm
Exceptionally coarse
Made from ions that don’t easily fit into crystals
Some rich in prized minerals
Pyroclastic
Contains volcanic fragments
What determines crystal size
Rate of cooling magma
Slow cooling- large crystals
Fast cooling- microscopic crystals
Very fast cooling- glassy
Bowen’s reactions series
Magma crystallize over a temp range of several 100 degrees
Minerals crystallize in predictable order
Last minerals to crystallize are very diff in composition from the earlier formed minerals