Igneous Rocks

1
Igneous Rocks
2
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

3
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)
• Dont have to give phaneritic

4
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

5
Cooling rates and texture
6
Aphanitic texture
7
Phaneritic texture
8
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.

9
Origin of porphyritic texture
10
Porphyritic texture
11
Porphyritic textures
12
Porphyritic textures
13
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.

14
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

15
Determining texture
16
Determining texture
17
Determining texture
18
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 10-40 Fe-Mg mins.
• Mafic rocks have 40-90 Fe-Mg mins.
• Ultramafic rocks have gt90 Fe-Mg mins.

19
Composition
Amphibole
20
(No Transcript)
21
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.

22
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

23
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.

24
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

25
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).

26
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)

27
Name

• Use
• the charts (on handout),
• poster (back of lab, this powerpoint file),
• book,
• Examples (at back of lab).
• 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.

28
Microscopes and pumice

• Look through microscopes at slides
• Answer questions.
• DONT MOVE THE SLIDES
• Dont confuse colourless and transparent
• With pumice experiment think of density as a
  whole.

29
(No Transcript)
30
Questions?
31
Reminder

• Field trip next week
• Horton Bluff
• Watch the weather it can be windy and cold on
  the beach
• Wear a jacket
• Beach is somewhat muddy (where we are going)
  wear shoes you dont mind getting dirty.