Igneous Rocks

Igneous rocks

• Form from “magma” (molten rock)

• “Lava” is magma that reaches the surface and looses gas (mostly water and CO2)

• Magma that crystallizes before reaching the surface forms an intrusive (plutonic) rock

• Lava crystallizes to form an extrusive (volcanic) rock

Intrusive igneous rocks• Cool slowly• Crystals grow larger• Texture is phaneritic (lit. “visible grains”)• Must give a grain size:

– 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

Cooling rates and texture

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

Phenocrysts

Groundmass(aphanitic)

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

Amphibole

IntermediateFelsicType of magma

Granite

Rhyolite Dacite

Granodiorite

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:

– 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 (biotite usually)

• Mafic or ultramafic rocks little or no quartz and lots of Fe-Mg 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 are extrusive– Except, basalts (mafic extrusive) are

commonly fine grained (phaneritic)

Name• Use:

– 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

• Don’t confuse colourless and transparent…

• With pumice “experiment” think of density – as a whole.

Questions?