Igneous Flashcards
igneous rock def
cooled from magma
intrusive vs extrusive
intrusive- rock formed from magma that has cooled within the earths crust
extrusive- rock formed from magma that cooled on the earths surface
classification- crystal size
glassy- no crystals present
cools in mins, extrusive, cools so quick no atomic order= no crystals
fine- less than 1 mm
extrusive, cools weeks-months, can occur intrusive only chilled margins
med- 1-5 mm
intrusive, cool over 1000s of years, form in minor hypabyssal and intrusions
coarse- greater then 5mm
intrusive, cool over millions of years, major intrusions at depth + large vol (batholiths and plutons)
larger crystals= slower cooling and so intrusive has larger crystals
when measuring grain size ignore phenocrysts
phenocrysts
large crystals surrounded smaller matrix
forms due to 2 stage cooling
porphyritic texture
mineral comp- silicic minerals examples and info
feldspar and silica mostly
feldspar most common mineral on earth- form 60% of minerals in these rocks
e.g.
Quartz-
only present in large amounts of silica and inter rock
white/grey
vitreous
7-7.5 hardness
streak=colourless
no cleavage
feldspar (plagioclase and orthoclase)-
pink/grey/white
vitreous
6 hardness
cleavage in 2 directions
white streak
Mafic minerals
contain Fe and Mg
word comes from magnesium and ferric
e.g.
Hornblende (amphibole)-
mostly inter
confused with augite
dark+ bits of beige
vitreous
streak= pale grey/white
5/6 hardness
2 cleavages at 60
6 sided crystal
augite-
type of pyroxene
mafic/ultra mafic
dark green to black
vitreous
5.5/6 hardness
streak= fine white
2 cleavages at 90
8 side crystals
olivine-
mostly ultra
green
6.5/7 hardness
vitreous
streak= white/ colourless
no cleavage
silica percentage
more silica lighter rock
silicic >66%
inter 52-66
mafic 45-52
ultra <45
obsidian is misleading
most of si02 is locked up in minerals
igneous rock table
S I M UM
glassy Ob scor
fine pum/rhy ander bas
medium micro G micro d dol
coarse Granite dior gabbro peri
silicic and inter = feldspar and quartz and micas
Mafic and ultra = pyroxene and a little ol
ultra= olivine
glassy+ fine = extrusive
medium+ coarse= intusive
intermediate igneous rocks formation and examples
more grey
most common= andersite
feldspars + pyroxene
less viscous magma and higher temp the silicic
e.g.
andersite- fine and ves/amy/por/equi
diorite- coarse and equi/por
silicic igneous rock formation +examples
formation-
silicic magma/lava
generated by melting of cont crust at converge boundaries
formed from min like quartz and feldspar
light
silicic more viscous + form at low temp 600-900
examples-
Ob- glassy and conchoidal
pum- fine and vesicular/amygdaloidal (lots of gas in magma)
rhy- fine and flow banding
(due to friction)
gran + gran diorite- coarse and equi/por
Mafic igneous rocks formation and examples
dark
lots of fe and mg
contain- pyroxene and olivine
low viscosity
above 1000 degrees
basalt = most common
e.g.
basalt- fine and ves/amy/por/equi
dolerite- med and por/equi
gabbro- coarse and equi
ultra mafic rocks formation and example
nearly entirely ferromagnesium minerals
nearly entirely olivine
from upper mantle- peridotite
very high melting point
e.g.
peridotite- coarse and equi/ophiolite
nucleation
crystals growing from lava or magma
building on the centre of crystals
crystal grain shape
eu
sub
an
euhedral-
well formed with good crystal faces
often form at depth slowly and unimpaired
subhedral-
some well formed faces but some poorly formed
more likely in faster cooling magmas
anhedral- poorly formed faces
in faster cooling magmas
pegmatite
abnormally large crystals
phenocrysts/ porphyritic
formed during final stage of magma crystallisation
from magmas rich in water as makes grow quick as has lots of rare minerals
very rare/ valuable
flow banding
formed by friction as magma slows near interface which aligns minerals as it moves
conchoidal
a fracture results in curved face
vesicular
contains vesicles, bubbles of gas which come out of solution as result of pressure release
porphyritic
large crystals called phenocrysts are surrounded by finer matrix
equigranular
crystals are approx. same size
amygdaloidal
large vesicles that have been filled with a secondary material
lava
molten rock that cools at the surface
ophiolites
sections of the earths oceanic crust that have been tectonically moved onto continental crust
