Chapter 2- Igneous Rocks and Processes Flashcards
Igneous rocks
rocks that have cooled from magma
phenocrysts
large crystals in an igneous rock, bigger than the groundmass
essential minerals
minerals used to classify an igneous rock
felsic minerals
light coloured, silica rich minerals
magma
molton rock beneathe the earth’s surface
mafic minerlas
dark coloured, silica poor and rich in magnesium and iron
Silicic rocks
more than 66% silica, typically light in colour
intermediate rocks
silica content between 52-66%, grey in colour
mafic rocks
silica content between 45-52%, dark in colour
ultramafic rocks
have a silica content of less than 45%
leucocratic
rocks that are light coloured and rich in silica
mesocratic
grey coloured rocks with a medium silica content
melanocratic
rocks that are dark inc olour with little silica
coarse crystals
greater than 5mm, can be seen and identified with the naked eye
medium crystals
1-5mm in size, individual crystals can be seen but not identified with the naked eye
fine crystals
less than 1mm in size, crystals cannot be seen with the naked eye
Rock: glassy and silicic, conchoidal fracture
obsidian
Rock: fine crystals and silicic, vesicular or flow banded (2)
pumice and rhyolite
Rock: medium crystals and silicic, pirphyritic
microgranite
Rock: coarse crystals and silicic, porphyritic
granite
key crystals for silicic rocks (5)
quartz, k feldspar, plagioclase(Na) feldspar, biotite, muscovite
Rock: fine crystals, intermediate, vesicular, amygdaloidal, porphyritic, or equigranular
andesite
Rock: medium crystals, intermediate, equigranular or porphyritic
Microdiorite
Rock: coarse crystals, intermediate, equigranular or pophryitic
diorite
Key minerals for intermediate rocks (5)
quartz (rare), k feldspar, plagioclase feldspar(either), biotite, hornblende
Rock: glassy, mafic
scoria
Rock: fine crystals, mafic, vesicular, amygdaloidal, porphryritic, equigranular
basalt
Rock: medium crystals, mafic, porphyritic, or equigranular
Dolerite
Rock: coarse crystals, mafic, equigranular
gabbro
Key minerals for mafic rocks (3)
plagioclase(Ca) feldspar, augite, olivine
Rock: Coarse crystals, ultramafic
peridotite
key minerals for ultramafic rocks (2)
augite, olivine
Flow banding
igneous texture, formed by friction as the magma/lava slows down near an interface, aligning minerals as it moves
conchoidal
fracture resulting in a curved surface
vesicular
igneous texture for a rock containing vesicles- bubbles of gas which came out of solution with pressure release
porphryitic
texture where large crystals (phenocrysts) are completely surrounded by smaller crystals
equigranular
crystals in an igneous rock are all approximately the same size
amygdaloidal
large vesicles have been filled with a secondary mineral
lava
molten rock which cools at the surface
ophiolites
sections of oceanic crust that have been obducted onto the earth’s surface
extrusion
emission of magma onto the earth’s surface where it forms a lava flow
intrusion
igneous rock formed below the earth’s surface, magma is forced into pre-existing rocks
hypabyssal
igneous rocks form at shallow depths below the surface
plutonic
igneous rocks form deep below the earth’s surface
euhedral
crystals are well formed with good crystal faces
equant
crystals have axes all the same length
sub-equant
crystals have axes almost all the same length
prismatic
crystals have four or more sides but are elongated in one direction
anhedral
crystals show poorly formed crystal faces
subhedral
crystals have some well formed faces and some poorly formed faces
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
glassy
no crystals, rock resembles a block of coloured glass, due to very rapid cooling
ophitic texture
an elongate crystal is enclosed by another mineral, common in dolerite and gabbro where plagioclase ‘laths’ are enclosed by augite
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
adiabatic process
a thermodynamic process wjere no heat enters or leaves the system during expansion or compression
adiabatic cooling
occurs when crust or mantle material rises, undergoes expansion, temperature falls with no loss or gain of thermal energy
adiabatic heating
crust or mantle material descends and temperature rises as it contracts, no loss or gain of thermal energy
divergent plate margins
two plates are moving apart and magma rises up in the centre
partial melting
some of the minerals in a rock melt to form a magma but not all
hot spots
formed by a fixed mantle plume bringing magma to the surface
convergent plate margins
two plates are colliding and magma is formed above a subduction zone or deep in the crust
batholith
very large igneous intrusion within the earth’s crust
decompression melting
melting that occurs at lower pressures, accounts for most volcanism
flux melting
melting with added water
density of magma
determined by composition, pressure and temperature (mafic magma is more dense than silicic) (pressure increases density) (temperature decreases density)
viscosity of magma
determined by composition and temperature (mafic is less viscous than silicic) (temperature decreases viscosity)
Magma recharge
magma chamber refills with magma from deep within the earth
Discharge rate
= pi * radius of the volcano ^4 * pressure within the magma chamber / 8 * viscosity of the magma * depth of the magma chamber
magma mixing
typically incomplete due to different viscositys and temperature (enclaves or one floats atop the other)
ground water and magma
rising magma can turn water to steam and cause explosive eruptions- ie geysers
exsolution
dissolved gasses come out of solution
concordant
intrusions are parallel to existing beds
discordant
intrusions cut across existing beds
country rock
any rock into which an igneous rock intrudes
dyke
discordant, sheet-like intrusion
sill
concordant sheet-like intrusion
minor intrusion
cool at hypabyssal depth, includes sills and dykes
diapir
body of relatively low density material that pierces and rises up through overlaying material of higher density
contact
where the intrusion meets the country rock
baked margin
part of the country rock that was heated and changed by the intrusion
chilled margin
part of the intrusion that was cooled faster than the rest of the intrusion by the country rock
ring complexes
curved structures formed by dykes, can be ring dykes or cone sheets
volcanic plug
intrusion formed by magma crystallising in the conduits or vent of a volcano
major intrusion
plutonic, cool deep below the surface
plutons
large igneous intrusive bodies, smaller than 100km^2
batholith
large igneous intrusion, larger than 100km^2, may be an aggregate of plutons
metamorphic aureole
large area around a batholith where rocks have been metamorphosed
contact
where the igneous rock meets the country rock
batholith examples
Peru-chile batholith, Andes (4500km), Cornubian batholith, cornwall, (235 km)
stoping
process that accommodates magma as it moves upward through the country rock by mechanical fracturing
xenoliths
clasts/blocks of pre-existing rock contained within an igneous rock
assimilation
melting process that incorporates blocks of country rock, freed by stoping, into the magma
Seismic survey
recording seismic waves to find the location and size of the magma chamber
ground movement
measuring rate of swelling of a volcano to detect magma moving below the surface
gas emissions
changes in gas emissions can indicate magma near the surface as confining pressure is released. ie increasing sulfer dioxide emissions
ground water
measured through boreholes and wells, increased gas pressure will cause water levels to rise and suddenly drop right before and eruption
dyke orientation
radial dykes form due to stress around a pluton or volcano
Tiltmeters
instrument that can measure very small changes in vertical level
GPS
Global Positioning System, radio navigation system that allows determination of an exact position
Fumaroles
openings in or near a volcano through which hot gases emerge
palaeosoil
a soil horizon that was formed in geological age
melt
name given to a magma or lava in the liquid phase
aa
lava flows that have a rough, jagged, blocky surface
pahoehoe
lava flows that have a smooth or ropy surface
scoria
a volcanic rock that is usually mafic but can be intermediate composition
pyroclast
an individual fragment ejected during an eruption
pyroclastic material
all material formed by explosive eruptions and ejected from a volcano- ie bombs, blocks, lapilli, and ash
nuee ardente
a gaseous pyroclastic cloud of magma droplets and ash
ignimbrite
cooled and solidified deposit of a pyroclastic cloud
pyroclastic flow
hot mixture of pyroclastic material and gas, a nuee ardente is an example of one of these
polymerisation
process of building a larger molecule by repeated addition of smaller molecules
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).
Volcanoes
vents at the surface of the earth through which magma and other volcanic materials are ejected
low viscosity
where magma or lava are fluid and flow more easily
shield volcanoes
have gentle slopes of less than 10 degrees and a roughly circular shape around the central vent
fissure eruptions
magma reaches the surface along long, linear cracks/fissures
submarine eruptions
magma comes from a vent or fissure on the sea floor
composite volcanoes
tall, conical shaped, and are composed of alternate layers of lava and ash
caldera
large volcanic crater that has undergone collapse follwing an eruption
effusive
term used to describe the fluid, non-explosive basaltic lava
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
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
Volcanic Explosivity Index
measure of how explosive an eruption is, measured 0-8
Hawaiian Eruption
have large amounts of very fluid basaltic magma from which gases escape but few pyroclasts
stombolian eruption
are explosive with less basalt and andesite. regualar explosions of gas and pyroclastic material
vulcanian eruption
violent with viscous andesitic lava and large quantities of pyroclastic material from large explosions
plinian eruptions
extremely explosive with viscous gas-filled andesitic and rhyolitic lava, and large volumes of pyroclastic material
isopachyte
line joining equal thickness deposits of ie ash.
