Granite Flashcards
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
What type of rocks does low silica magma make most often and why?
volcanic rocks - not viscous, so flows out to make volcanoes easily
How do the cracks of rocks get filled with granite?
hot silicate liquids rise through rock and create and fill fractures in surrounding rock, before freezing there
What is metamorphism?
change in a rock’s minerals and/or texture by increasing temperature
What often occurs at the boundary between bodies of rock?
contact metamorphism
What does contact metamorphism indicate about a pluton?
that it must have been hotter than country rock
What are chilled margins?
smaller crystals at the edges of a pluton
What do chilled margins indicate?
quicker cooling - so the country rock must have been colder than magma
What are xenoliths?
pieces of country rock that fell into magma pool and froze there
How are xenoliths related to the country rock?
same rock
What do xenoliths indicate about time frames?
the country rock must have been there before the pluton
How does xenolith texture change depending on its distance from the contact point between the pluton and country rock?
angular near contact; rounded (and may have baked zone) further from contact
How does cooling rate affect grain size?
big grains indicate slow (underground) cooling; little grains indicate fast (surface) cooling
What are phenocrysts?
big crystals in rocks surrounded by little crystals
How are phenocrysts formed?
cooled slowly underground at first, then magma erupted for fast cooling
What is the difference between a sill and a dike?
sills are parallel to the host rock’s beds (concordant); dikes cross host rock (discordant)
What are 2 shapes plutons can take?
sill and dyke
What are the names for the different sizes of plutons (from smallest to largest)?
plut, stock, batholith
How are deep plutons exposed?
through erosion
What are some plutons associated with?
volcanic rocks
What is a swarm?
many dykes in one area
How big can batholiths get?
more than 1000 km long; about 100 km wide; 5-10 km thick
What are batholiths made of?
many smaller plutons (each a few cubic km) with steep sides, flat roofs, and cross-cutting
What is the geothermal gradient?
rate of increasing temperature with respect to increasing depth in Earth’s interior
Where is partial melting predicted by the geothermal gradient?
depths of 70-250 km
What does a 1-2% partial melt of peridotite yield?
basalt
What is the asthenosphere?
the ductile part of the earth just below the lithosphere, including the upper mantle
Why does magma rise?
due to density contrast
What is stoping?
process by which magma intrudes - blocks of wall rock break off and sink into magma
What is diapirism?
type of intrusion in which a more mobile and ductile material is forced into brittle overlying rocks and rises through buoyance; occurs lower in crust
How does magma relate to the crust and mantle?
separates crust from mantle
What does magma do to heat?
transfers heat from earth’s crust
What moves lithospheric plates, builds continents, supplies hydrosphere/lithisophere, and modifies weather/climate?
magma
What does magma transfer within reach of humankind?
matter and energy
What is differential crystallization?
crystalization changes the composition of remaining magma (since some crystals solidify out of magma at higher temperatures)
How does differential crystallization separate crystals from liquid?
crystals solidify at higher temperatures than other magma components and settle on the bottom
What is partial melting?
magma produced from the first-melted portion of crust and mantle
What does assimilation do to magma?
modifies composition and increases cooling
What is assimilation?
when rocks fall in magma and partially melt/magma’s heat partially melts rock wall
