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 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

Phaneritic texture

Aphanitic 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 Groundmass (phaneritic) Phenocrysts

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 Intermediate Felsic Type of magma Granite Rhyolite Dacite Granodiorite Amphibole

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 (usually biotite) Mafic or ultramafic rocks 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 (including porphyritic aphanitic or porphyritic glassy rocks) are extrusive Except, basalts (which are extrusive) can be 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 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?

Field trip next week Bus departs from outside Huggins (University Avenue side) Promptly at 1:30 – don’t be late Keep an eye on the weather: Colder on shore than in Wolfville (often 10 degrees colder) Bring a jacket Beach is bouldery; wear decent shoes with ankle support (no flip-flops, Birkenstocks, etc.)