Download Igneous Rocks

Igneous Rocks
Classifying rocks
• Rock classification usually involves
determining two parameters:
– Texture: the size, shape, and relationship of the
mineral grains that make up the rock
– Composition: the amount/proportion of various
minerals that make up the rock
• True for all rocks but we’ll start with
igneous rocks
Igneous rocks…
• Form from magma
• Lava is magma that reaches the surface and
loses gasses (mostly H2O and CO2)
• If magma crystallizes before reaching the
surface it forms an intrusive (plutonic) rock
• When lava crystallizes it forms extrusive
(volcanic) rock
• Intrusive and extrusive rocks have different
textures
Intrusive igneous rocks
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Cool slowly
Crystals grow larger
Texture is phaneritic (lit. “visible grains”)
Must give a grain size:
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Visible but less than 1 mm = fine grained (f.g.)
Between 1 and 5 mm = medium grained (m.g.)
Greater then 5 mm = coarse grained (c.g.)
Greater than 2 cm = pegmatitic (see below)
• Don’t have to give “phaneritic”
Extrusive igneous rocks
• Cool rapidly
• Crystals are small (generally too small to
see)
• Texture is aphanitic (lit. not visible grains)
• Where lava cools very quickly (quenches)
the rock will be a glass (non-crystalline)
• Textural term is glassy
Aphanitic texture
Phaneritic texture
Porphyritic rocks
• When magma cooling rate changes the rock will
commonly have two different grain sizes
• Term used is porphyritic
• The larger grains are phenocrysts which sit in a
groundmass
• Must indicate the grain size of the groundmass
(aphanitic, phaneritic or (rarely) glassy) to denote
complete texture
– e.g., f.g. porphyritic, aphanitic porphyritic, etc.
Origin of porphyritic texture
Porphyritic texture
Porphyritic textures
Groundmass
(aphanitic)
Phenocrysts
Porphyritic textures
Phenocrysts
Groundmass
(phaneritic)
Other textural terms
• Volcanic rocks commonly have bubble
holes (vesicles) caused by escaping gas.
Texture is vesicular
• Use different terms for light or dark
coloured vesicular rocks.
– Light coloured vesicular rocks generally have
many tiny holes. Texture is pumaceous, rock is
pumice.
– Dark coloured vesicular rocks have fewer and
larger holes. Texture is scoriaceous, rock is
scoria.
Pumice
Scoria
Other special (textural) terms
• Amygdaloidal: textural term used when vesicles
have became filled with minerals deposited from
solutions percolating through the rock
• Tuff is a pyroclastic rock formed from volcanic
fragments (ash)
• Obsidian is a volcanic glass
• Pegmatite is an intrusive rock with very large
grains (cm size) – typically due to crystallisation
from water-rich magma. Texture is pegmatitic
Determining texture
Determining texture
Determining texture
1 cm
Composition
• Rocks are named according to the composition
(proportion of minerals). The proportion of dark
Fe-Mg (ferromagnesian) minerals is an important
criterion
• Felsic rocks contain less than 10% Fe-Mg mins.
• Intermediate rocks 10-30, 30-40% Fe-Mg mins.
• Mafic rocks have 40-90% Fe-Mg mins.
• Ultramafic rocks have >90% Fe-Mg mins.
Composition
Rhyolite
Granite
Dacite
Granodiorite
Amphibole
Type of
magma
Felsic
Intermediate
F(elsic), I(intermediate), M(afic),
U(ltramafic)
• Is the rock felsic, intermediate, mafic, or
ultramafic?
– Determined on the basis of percentage Fe-Mg minerals
– Chart gives ranges of % Fe-Mg minerals.
– In general one can associate % Fe-Mg mins with
colour. The darker the rock, the higher the % Fe-Mg
minerals.
– Relatively easy to determine for phaneritic rocks
– With aphanitic rocks must go entirely on colour:
felsic rocks are buff, pink or red (felsic glass, obsidian,
is black); intermediate rocks vary from shades of grey
to green; mafic rocks are dark green or grey to black.
Name of the feldspar
• Name the feldspar. Two choices:
– K-feldspar (pink, cream)
– Plagioclase (white, grey or blue), striations on
cleavage surfaces
– Note that in aphanitic rocks you may not be
able to see any feldspar (too fine grained).
Therefore report feldspar as n.d. = not
determined.
This is not the same as saying there is none.
– Glassy rocks have no minerals, i.e. no feldspar
Fe-Mg mineral
• Name the Fe-Mg mineral(s).
• There are four choices:
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Biotite (usually in felsic rocks)
Amphibole (predominant in intermediate rocks)
Pyroxene (in mafic and ultramafic rocks)
Olivine (in mafic and ultramafic rocks)
Note: In the aphanitic rocks you may not be
able to see the Fe-Mg mineral. Report n.d. (not
determined)
– Glassy rocks have no Fe-Mg minerals.
Identifying the Fe-Mg minerals
• Biotite: Black, shiny, flakey
• Amphibole: Black/dark green, shiny
(visible cleavage surfaces), not flakey
• Pyroxene: Black/dark green, dull (cleavage
not readily visible), not flakey
• Olivine, apple green, glassy
% quartz
• Give the percentage quartz
– In phaneritic rocks this is relatively easy; quartz
is the grey vitreous (glassy-looking) mineral
– Note that % quartz varies inversely with the
amount of Fe-Mg mineral.
• Felsic rock contain significant amount of quartz and
a little Fe-Mg mineral (usually biotite)
• Mafic or ultramafic rocks no quartz and lots of FeMg mineral (pyroxene and/olivine)
– In aphanitic rocks report “n.d.” not 0%
– Obsidian has no minerals (i.e. 0% quartz).
I(ntrusive) or E(xtrusive)
• Is the rock intrusive (I) or extrusive (E)?
– Rule of thumb: phaneritic rocks are intrusive,
aphanitic or glassy rocks (including porphyritic
aphanitic or porphyritic glassy rocks) are
extrusive
– Except, basalts (which are extrusive) can be
fine grained (phaneritic)
Name
• Use:
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the charts (on handout),
examples (at back of lab),
poster (back of lab, this powerpoint file),
book.
• Note that the name is, in some respects, the least
important column of this lab.
• This lab is your first introduction to igneous rocks
and it is far more important to become familiar
with them than to name them.
Microscopes and pumice
• Look through microscopes at slides
• Answer questions
• DON’T MOVE THE SLIDES OR
ROTATE THE STAGE
• The drawings on the board show you where
to find points of interest in the field of view
• Don’t confuse colourless and transparent…
• With pumice “experiment” think of density
– as a whole.
One final point…
• It will help to use the hand lens (particularly
to differentiate biotite from amphibole)
• Hand lenses have a very short focal length
so, to be of use, you must hold them close
to your eye (and the sample)
Questions?