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
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
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
