Chapter 2- Igneous Rocks and Processes Flashcards
1
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Igneous rocks
A
rocks that have cooled from magma
2
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phenocrysts
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large crystals in an igneous rock, bigger than the groundmass
3
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essential minerals
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minerals used to classify an igneous rock
4
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felsic minerals
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light coloured, silica rich minerals
5
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magma
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molton rock beneathe the earth’s surface
6
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mafic minerlas
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dark coloured, silica poor and rich in magnesium and iron
7
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Silicic rocks
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more than 66% silica, typically light in colour
8
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intermediate rocks
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silica content between 52-66%, grey in colour
9
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mafic rocks
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silica content between 45-52%, dark in colour
10
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ultramafic rocks
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have a silica content of less than 45%
11
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leucocratic
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rocks that are light coloured and rich in silica
12
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mesocratic
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grey coloured rocks with a medium silica content
13
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melanocratic
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rocks that are dark inc olour with little silica
14
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coarse crystals
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greater than 5mm, can be seen and identified with the naked eye
15
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medium crystals
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1-5mm in size, individual crystals can be seen but not identified with the naked eye
16
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fine crystals
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less than 1mm in size, crystals cannot be seen with the naked eye
17
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Rock: glassy and silicic, conchoidal fracture
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obsidian
18
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Rock: fine crystals and silicic, vesicular or flow banded (2)
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pumice and rhyolite
19
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Rock: medium crystals and silicic, pirphyritic
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microgranite
20
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Rock: coarse crystals and silicic, porphyritic
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granite
21
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key crystals for silicic rocks (5)
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quartz, k feldspar, plagioclase(Na) feldspar, biotite, muscovite
22
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Rock: fine crystals, intermediate, vesicular, amygdaloidal, porphyritic, or equigranular
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andesite
23
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Rock: medium crystals, intermediate, equigranular or porphyritic
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Microdiorite
24
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Rock: coarse crystals, intermediate, equigranular or pophryitic
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diorite
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Key minerals for intermediate rocks (5)
quartz (rare), k feldspar, plagioclase feldspar(either), biotite, hornblende
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Rock: glassy, mafic
scoria
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Rock: fine crystals, mafic, vesicular, amygdaloidal, porphryritic, equigranular
basalt
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Rock: medium crystals, mafic, porphyritic, or equigranular
Dolerite
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Rock: coarse crystals, mafic, equigranular
gabbro
30
Key minerals for mafic rocks (3)
plagioclase(Ca) feldspar, augite, olivine
31
Rock: Coarse crystals, ultramafic
peridotite
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key minerals for ultramafic rocks (2)
augite, olivine
33
Flow banding
igneous texture, formed by friction as the magma/lava slows down near an interface, aligning minerals as it moves
34
conchoidal
fracture resulting in a curved surface
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vesicular
igneous texture for a rock containing vesicles- bubbles of gas which came out of solution with pressure release
36
porphryitic
texture where large crystals (phenocrysts) are completely surrounded by smaller crystals
37
equigranular
crystals in an igneous rock are all approximately the same size
38
amygdaloidal
large vesicles have been filled with a secondary mineral
39
lava
molten rock which cools at the surface
40
ophiolites
sections of oceanic crust that have been obducted onto the earth's surface
41
extrusion
emission of magma onto the earth's surface where it forms a lava flow
42
intrusion
igneous rock formed below the earth's surface, magma is forced into pre-existing rocks
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hypabyssal
igneous rocks form at shallow depths below the surface
44
plutonic
igneous rocks form deep below the earth's surface
45
euhedral
crystals are well formed with good crystal faces
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equant
crystals have axes all the same length
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sub-equant
crystals have axes almost all the same length
48
prismatic
crystals have four or more sides but are elongated in one direction
49
anhedral
crystals show poorly formed crystal faces
50
subhedral
crystals have some well formed faces and some poorly formed faces
51
texture
general character or appearance of a rock shown by the arrangement of minerals with the crystal shape and size, doesn't refer to roughness/smoothness of the surface
52
glassy
no crystals, rock resembles a block of coloured glass, due to very rapid cooling
53
ophitic texture
an elongate crystal is enclosed by another mineral, common in dolerite and gabbro where plagioclase 'laths' are enclosed by augite
54
cumulate
crystals settle out of the magma, typically ontot he floor of the magma chamber and accumulate in mutual contact. they continue growing once settled
55
adiabatic process
a thermodynamic process wjere no heat enters or leaves the system during expansion or compression
56
adiabatic cooling
occurs when crust or mantle material rises, undergoes expansion, temperature falls with no loss or gain of thermal energy
57
adiabatic heating
crust or mantle material descends and temperature rises as it contracts, no loss or gain of thermal energy
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divergent plate margins
two plates are moving apart and magma rises up in the centre
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partial melting
some of the minerals in a rock melt to form a magma but not all
60
hot spots
formed by a fixed mantle plume bringing magma to the surface
61
convergent plate margins
two plates are colliding and magma is formed above a subduction zone or deep in the crust
62
batholith
very large igneous intrusion within the earth's crust
63
decompression melting
melting that occurs at lower pressures, accounts for most volcanism
64
flux melting
melting with added water
65
density of magma
determined by composition, pressure and temperature (mafic magma is more dense than silicic) (pressure increases density) (temperature decreases density)
66
viscosity of magma
determined by composition and temperature (mafic is less viscous than silicic) (temperature decreases viscosity)
67
Magma recharge
magma chamber refills with magma from deep within the earth
68
Discharge rate
= pi * radius of the volcano ^4 * pressure within the magma chamber / 8 * viscosity of the magma * depth of the magma chamber
69
magma mixing
typically incomplete due to different viscositys and temperature (enclaves or one floats atop the other)
70
ground water and magma
rising magma can turn water to steam and cause explosive eruptions- ie geysers
71
exsolution
dissolved gasses come out of solution
72
concordant
intrusions are parallel to existing beds
73
discordant
intrusions cut across existing beds
74
country rock
any rock into which an igneous rock intrudes
75
dyke
discordant, sheet-like intrusion
76
sill
concordant sheet-like intrusion
77
minor intrusion
cool at hypabyssal depth, includes sills and dykes
78
diapir
body of relatively low density material that pierces and rises up through overlaying material of higher density
79
contact
where the intrusion meets the country rock
80
baked margin
part of the country rock that was heated and changed by the intrusion
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chilled margin
part of the intrusion that was cooled faster than the rest of the intrusion by the country rock
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ring complexes
curved structures formed by dykes, can be ring dykes or cone sheets
83
volcanic plug
intrusion formed by magma crystallising in the conduits or vent of a volcano
84
major intrusion
plutonic, cool deep below the surface
85
plutons
large igneous intrusive bodies, smaller than 100km^2
86
batholith
large igneous intrusion, larger than 100km^2, may be an aggregate of plutons
87
metamorphic aureole
large area around a batholith where rocks have been metamorphosed
88
contact
where the igneous rock meets the country rock
89
batholith examples
Peru-chile batholith, Andes (4500km), Cornubian batholith, cornwall, (235 km)
90
stoping
process that accommodates magma as it moves upward through the country rock by mechanical fracturing
91
xenoliths
clasts/blocks of pre-existing rock contained within an igneous rock
92
assimilation
melting process that incorporates blocks of country rock, freed by stoping, into the magma
93
Seismic survey
recording seismic waves to find the location and size of the magma chamber
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ground movement
measuring rate of swelling of a volcano to detect magma moving below the surface
95
gas emissions
changes in gas emissions can indicate magma near the surface as confining pressure is released. ie increasing sulfer dioxide emissions
96
ground water
measured through boreholes and wells, increased gas pressure will cause water levels to rise and suddenly drop right before and eruption
97
dyke orientation
radial dykes form due to stress around a pluton or volcano
98
Tiltmeters
instrument that can measure very small changes in vertical level
99
GPS
Global Positioning System, radio navigation system that allows determination of an exact position
100
Fumaroles
openings in or near a volcano through which hot gases emerge
101
palaeosoil
a soil horizon that was formed in geological age
102
melt
name given to a magma or lava in the liquid phase
103
aa
lava flows that have a rough, jagged, blocky surface
104
pahoehoe
lava flows that have a smooth or ropy surface
105
scoria
a volcanic rock that is usually mafic but can be intermediate composition
106
pyroclast
an individual fragment ejected during an eruption
107
pyroclastic material
all material formed by explosive eruptions and ejected from a volcano- ie bombs, blocks, lapilli, and ash
108
nuee ardente
a gaseous pyroclastic cloud of magma droplets and ash
109
ignimbrite
cooled and solidified deposit of a pyroclastic cloud
110
pyroclastic flow
hot mixture of pyroclastic material and gas, a nuee ardente is an example of one of these
111
polymerisation
process of building a larger molecule by repeated addition of smaller molecules
112
depolymerisation, factors (4)
increasing temperature, increasing pressure(can go either way depending on other factors), composition (mafic lavas are more likely to depolymerise), pressence of volatiles (water depolymerises, CO2 polymerisises the melt).
113
Volcanoes
vents at the surface of the earth through which magma and other volcanic materials are ejected
114
low viscosity
where magma or lava are fluid and flow more easily
115
shield volcanoes
have gentle slopes of less than 10 degrees and a roughly circular shape around the central vent
116
fissure eruptions
magma reaches the surface along long, linear cracks/fissures
117
submarine eruptions
magma comes from a vent or fissure on the sea floor
118
composite volcanoes
tall, conical shaped, and are composed of alternate layers of lava and ash
119
caldera
large volcanic crater that has undergone collapse follwing an eruption
120
effusive
term used to describe the fluid, non-explosive basaltic lava
121
columnar jointing
happens when lavas are more than 3km thick, the inside cools more slowly than the outside. Forms hexagonal columns as it contracts during cooling
122
pillow lavas
during submarine eruptions, the outside of the lava flow cools very quickly whilst the inside is still molten. Characteristic rounded shape with a sagging bottom- way up structure
123
Volcanic Explosivity Index
measure of how explosive an eruption is, measured 0-8
124
Hawaiian Eruption
have large amounts of very fluid basaltic magma from which gases escape but few pyroclasts
125
stombolian eruption
are explosive with less basalt and andesite. regualar explosions of gas and pyroclastic material
126
vulcanian eruption
violent with viscous andesitic lava and large quantities of pyroclastic material from large explosions
127
plinian eruptions
extremely explosive with viscous gas-filled andesitic and rhyolitic lava, and large volumes of pyroclastic material
128
isopachyte
line joining equal thickness deposits of ie ash.
