Granite
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What is a pluton?
a body of igneous rock crystallized from magma within the earth’s crust
What is the most common plutonic rock on continents?
granite
What minerals make up granite?
feldspar, quartz, mica
What types of feldspar are found in granite?
K-feldspar (>50%) and (Na,Ca)-feldspar (rest)
What is the most common accessory mineral in granite?
mica (biotite and muscovite)
How do biotite and muscovite mica differ in appearance?
biotite is black; muscovite is white
What mineral has the formula SiO2?
quartz
What is quartz’s hardness?
7
What type of fracture does quartz have?
conchoidal
What mineral is glassy?
quartz
How hard is feldspar?
6
What is a blocky mineral?
feldspar
What mineral makes up most of granite?
feldspar
What mineral in granite is quite soft?
mica
What is the structure of mica?
flexible sheets
What 5 characteristics of plutons led geologists to conclude that plutons were created from hot silicate liquids within the crust?
• different texture than volcanic rocks of same mineralogy • surrounding rock broken and injected • surrounding rock metamorphosed (contact metamorphism) • grain size finer at margins of pluton (chilled margin) • inclusions of surrounding rock within pluton (xenoliths)
What rock has the same mineralogy as granite?
rhyolite
How do granite and rhyolite differ?
granite has much larger grains
Where is granite found?
with the crust beneath volcanoes
What is the coarse-grained equivalent to basalt?
gabbro
Which is more abundant between gabbro and basalt?
basalt
Which is more abundant between granite and rhyolite?
granite
Why does granite have larger crystals than rhyolite?
it took longer to cool (within earth’s crust), whereas rhyolite cooled quickly in a volcanic eruption
What sort of rocks does high silica magma make most often and why?
plutons - viscous, so barely ever makes it to surface for fast cooling