Granite

Question 1

What is a pluton?

Answer 1

a body of igneous rock crystallized from magma within the earth’s crust

Question 2

What is the most common plutonic rock on continents?

Answer 2

granite

Question 3

What minerals make up granite?

Answer 3

feldspar, quartz, mica

Question 4

What types of feldspar are found in granite?

Answer 4

K-feldspar (>50%) and (Na,Ca)-feldspar (rest)

Question 5

What is the most common accessory mineral in granite?

Answer 5

mica (biotite and muscovite)

Question 6

How do biotite and muscovite mica differ in appearance?

Answer 6

biotite is black; muscovite is white

Question 7

What mineral has the formula SiO2?

Answer 7

quartz

Question 8

What is quartz’s hardness?

Answer 8

7

Question 9

What type of fracture does quartz have?

Answer 9

conchoidal

Question 10

What mineral is glassy?

Answer 10

quartz

Question 11

How hard is feldspar?

Answer 11

6

Question 12

What is a blocky mineral?

Answer 12

feldspar

Question 13

What mineral makes up most of granite?

Answer 13

feldspar

Question 14

What mineral in granite is quite soft?

Answer 14

mica

Question 15

What is the structure of mica?

Answer 15

flexible sheets

Question 16

What 5 characteristics of plutons led geologists to conclude that plutons were created from hot silicate liquids within the crust?

Answer 16

• 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)

Question 17

What rock has the same mineralogy as granite?

Answer 17

rhyolite

Question 18

How do granite and rhyolite differ?

Answer 18

granite has much larger grains

Question 19

Where is granite found?

Answer 19

with the crust beneath volcanoes

Question 20

What is the coarse-grained equivalent to basalt?

Answer 20

gabbro

Question 21

Which is more abundant between gabbro and basalt?

Answer 21

basalt

Question 22

Which is more abundant between granite and rhyolite?

Answer 22

granite

Question 23

Why does granite have larger crystals than rhyolite?

Answer 23

it took longer to cool (within earth’s crust), whereas rhyolite cooled quickly in a volcanic eruption

Question 24

What sort of rocks does high silica magma make most often and why?

Answer 24

plutons - viscous, so barely ever makes it to surface for fast cooling

Question 25

What type of rocks does low silica magma make most often and why?

Answer 25

volcanic rocks - not viscous, so flows out to make volcanoes easily

Question 26

How do the cracks of rocks get filled with granite?

Answer 26

hot silicate liquids rise through rock and create and fill fractures in surrounding rock, before freezing there

Question 27

What is metamorphism?

Answer 27

change in a rock's minerals and/or texture by increasing temperature

Question 28

What often occurs at the boundary between bodies of rock?

Answer 28

contact metamorphism

Question 29

What does contact metamorphism indicate about a pluton?

Answer 29

that it must have been hotter than country rock

Question 30

What are chilled margins?

Answer 30

smaller crystals at the edges of a pluton

Question 31

What do chilled margins indicate?

Answer 31

quicker cooling - so the country rock must have been colder than magma

Question 32

What are xenoliths?

Answer 32

pieces of country rock that fell into magma pool and froze there

Question 33

How are xenoliths related to the country rock?

Answer 33

same rock

Question 34

What do xenoliths indicate about time frames?

Answer 34

the country rock must have been there before the pluton

Question 35

How does xenolith texture change depending on its distance from the contact point between the pluton and country rock?

Answer 35

angular near contact; rounded (and may have baked zone) further from contact

Question 36

How does cooling rate affect grain size?

Answer 36

big grains indicate slow (underground) cooling; little grains indicate fast (surface) cooling

Question 37

What are phenocrysts?

Answer 37

big crystals in rocks surrounded by little crystals

Question 38

How are phenocrysts formed?

Answer 38

cooled slowly underground at first, then magma erupted for fast cooling

Question 39

What is the difference between a sill and a dike?

Answer 39

sills are parallel to the host rock's beds (concordant); dikes cross host rock (discordant)

Question 40

What are 2 shapes plutons can take?

Answer 40

sill and dyke

Question 41

What are the names for the different sizes of plutons (from smallest to largest)?

Answer 41

plut, stock, batholith

Question 42

How are deep plutons exposed?

Answer 42

through erosion

Question 43

What are some plutons associated with?

Answer 43

volcanic rocks

Question 44

What is a swarm?

Answer 44

many dykes in one area

Question 45

How big can batholiths get?

Answer 45

more than 1000 km long; about 100 km wide; 5-10 km thick

Question 46

What are batholiths made of?

Answer 46

many smaller plutons (each a few cubic km) with steep sides, flat roofs, and cross-cutting

Question 47

What is the geothermal gradient?

Answer 47

rate of increasing temperature with respect to increasing depth in Earth's interior

Question 48

Where is partial melting predicted by the geothermal gradient?

Answer 48

depths of 70-250 km

Question 49

What does a 1-2% partial melt of peridotite yield?

Answer 49

basalt

Question 50

What is the asthenosphere?

Answer 50

the ductile part of the earth just below the lithosphere, including the upper mantle

Question 51

Why does magma rise?

Answer 51

due to density contrast

Question 52

What is stoping?

Answer 52

process by which magma intrudes - blocks of wall rock break off and sink into magma

Question 53

What is diapirism?

Answer 53

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

Question 54

How does magma relate to the crust and mantle?

Answer 54

separates crust from mantle

Question 55

What does magma do to heat?

Answer 55

transfers heat from earth's crust

Question 56

What moves lithospheric plates, builds continents, supplies hydrosphere/lithisophere, and modifies weather/climate?

Answer 56

magma

Question 57

What does magma transfer within reach of humankind?

Answer 57

matter and energy

Question 58

What is differential crystallization?

Answer 58

crystalization changes the composition of remaining magma (since some crystals solidify out of magma at higher temperatures)

Question 59

How does differential crystallization separate crystals from liquid?

Answer 59

crystals solidify at higher temperatures than other magma components and settle on the bottom

Question 60

What is partial melting?

Answer 60

magma produced from the first-melted portion of crust and mantle

Question 61

What does assimilation do to magma?

Answer 61

modifies composition and increases cooling

Question 62

What is assimilation?

Answer 62

when rocks fall in magma and partially melt/magma's heat partially melts rock wall