4IGNEOUS ROCKS

1
4IGNEOUS ROCKS
form when magma/lava cools and solidifies
2
MAGMA
molten rock Liquid (melt, ions in
solution) Solid (crystals, silicate minerals) Gas
(volatiles, CO2, H2O, etc.) forms by partial
melting in the crust and uppermost mantle to a
depth of 250 km lava magma that reaches
Earths surface
Extrusive (volcanic) igneous rocks form when
lava solidifies at the surface
BASALT Intrusive (plutonic) igneous rocks form
when magma solidifies at depth GRANITE
3
CRYSTALLIZATION of MAGMA
Cooling of magma results in the systematic
arrangement of ions into orderly patterns
(mineral crystals) The silicate minerals
resulting from crystallization form in a
predictable order over a wide temperature range
(Bowens Reaction Series) Igneous rocks are
classified based on TEXTURE - size, shape and
arrangement of mineral grains COMPOSITION -
minerals and their percentage
interlocking mosaic of crystals
4
TEXTURE

• size, shape, and arrangement of interlocking
  minerals
• Factors affecting crystal size
• COOLING RATE
• Slow rate fewer nuclei but larger crystals
• Fast rate many nuclei forming small crystals
• Very fast rate forms GLASS unordered ions
• SILICA (SiO2)
• More silica more linkages (polymerization)
• GASES IN SOLUTION
• More gases less polymerization

5
IGNEOUS TEXTURES
APHANITIC (evenly fine-grained) fast
cooling microscopic crystals
VESICULAR preserved gas bubbles
PHANERITIC (evenly coarse-grained) slow
cooling large, visible crystals
6
IGNEOUS TEXTURES

• PORPHYRITIC
• Large crystals (phenocrysts) are embedded in a
  matrix of smaller crystals (groundmass)
• Indicates two-stage cooling process
• GLASSY
• Very rapid cooling of lava
• Resulting rock is called obsidian

7
IGNEOUS TEXTURES
PYROCLASTIC (Fragmental) consolidation of
pyroclastic particles (ash, lapilli,
bombs/blocks) produced by violent volcanic
eruptions
PEGMATITIC exceptionally coarse grained (huge
crystals) late stage crystallization of felsic
magmas high volatiles enhances ion migration
8
IGNEOUS COMPOSITIONS
All magmas are silicate melts.
9
IGNEOUS COMPOSITIONS

• Igneous rocks are composed primarily of silicate
  minerals
• Dark (ferromagnesian) silicates
• olivine (Mg,Fe)2SiO4
• pyroxene (augite) (Mg,Fe)SiO3
• amphibole (hornblende) Ca2(Fe,Mg)5Si8O22(OH)2
• biotite mica K(Mg,Fe)3AlSi3O10(OH)2
• Light (nonferromagnesian) silicates
• quartz SiO2
• feldspars
• potassium feldspar (orthoclase) KAlSi3O8
• plagioclase feldspar (Ca,Na)AlSi3O8
• muscovite mica KAl2 (AlSi3O10)(OH)2

10
IGNEOUS CHART
11
IGNEOUS CHART
12
FELSIC IGNEOUS ROCKS
Na-rich plagiolcase
13
FELSIC IGNEOUS ROCKS
granite pegmatite
Na-rich plagiolcase
14
INTERMEDIATE IGNEOUS ROCKS
Chemical Composition

Dominant Minerals
Accessory Minerals
Phaneritic
Aphanitic
15
MAFICIGNEOUS ROCKS

scoria
Chemical Composition
Dominant Minerals
Accessory Minerals
Phaneritic
Aphanitic
16
ULTRAMAFICIGNEOUS ROCKS
PERIDOTITE
Chemical Composition

Dominant Minerals
Accessory Minerals
Phaneritic
Aphanitic
17
PYROCLASTICROCKS

18
ORIGIN of MAGMA
CRUST and MANTLE are primarily SOLID rock!

Temperature and pressure INCREASE with depth in
the Earth. Melting temperature 700C -
1200C Pressure causes melting temperature to
increase.
Rate of increase in temperature geothermal
gradient 25C/km (average) Range 10C/km
40C/km 100 km 1200C to 1400C
19
ORIGIN of MAGMA MELTING ROCKS

• Increase the temperature
• at a given pressure
• Decrease the pressure
• at a given temperature
• Add water
• at a given pressure

20
ORIGIN of MAGMA
Heat added rising magma may melt nearby
rocks friction may contribute some descending
rocks heat up a bit

Decompression melting removal of overlying rocks
at spreading ridge lowers the pressure on
underlying hot mantle rocks, inducing melting
Water fluxing water driven out of mineral
structures at subduction zones volatiles lower
the melting temperature in overlying mantle rocks
21
MAGMA EVOLUTION
When mantle rocks melt, the magma that forms is
MAFIC in composition. How do igneous rocks with
other compositions form? The magma that forms
when a rock melts is more silica-rich than the
parent rock due to PARTIAL MELTING.

When magma cools, it crystallizes minerals at
different temperatures FRACTIONAL CRYSTALIZATION
22
Minerals crystallize from magma in a systematic
fashion. Minerals that form at the same
temperature can occur together in a rock.
BOWENS REACTION SERIES
23
The formation of one or more secondary magmas
from a single parent magma.
MAGMA DIFFERENTIATION
CRYSTAL SETTLING
24
The formation of one or more secondary magmas
from a single parent magma.
MAGMA DIFFERENTIATION
ASSIMILATION MAGMA MIXING
25
The incomplete melting of rocks produces most, if
not all, magma. The magma that forms through
partial melting is always more silica-rich than
the parent rock.
PARTIAL MELTING
Partial melting of mantle PERIDOTITE yields MAFIC
magma (primary magma). Most MAFIC magmas form at
depths of 50 - 250 km (30 - 150
mi) Decompression melting at divergent
boundaries Water fluxing at subduction
zones Large outpourings of mafic lavas are common
at Earths surface heat lost during ascent is
offset by the drop in melting temperature
26
MAGMA EVOLUTION
If partial melting of ULTRAMAFIC mantle rocks
generates MAFIC magmas, how do we get
INTERMEDIATE and FELSIC magmas?
Intermediate and felsic rocks are found only
within or near continental margins, suggesting
interactions between primary (mantle-derived
mafic) magmas and crustal rocks ASSIMILATION of
more silica-rich country rocks CRYSTAL SETTLING
in mafic or intermediate magmas PARTIAL MELTING
of mafic or intermediate rocks FELSIC magmas
rarely erupt Because of their high viscosity
(high silica content, low temperature), they lose
mobility during ascent and tend to form large
PLUTONS. When they do erupt, they tend to be
very explosive (ash).
27
Aa
Pahoehoe
VISCOSITY

• Viscosity resistance to flow
• Temperature hotter runnier
• Volatiles more dissolved gas runnier
• Gases in solution inhibit polymerization
• Composition more silica stickier
• Linkages between silica tetrahedra
  (polymerization) inhibit flow
• Silica rich (felsic) magma/lavas are thick,
  viscous and resist flow
• Silica poor (mafic) magma/lavas are thinner, have
  a lower viscosity and flow easily

Felsic high viscosity
Mafic low viscosity
Mafic
Felsic
28
PEGMATITE DEPOSITS VEIN DEPOSITS DISSEMINATED
DEPOSITS KIMBERLITE PIPES
MINERAL RESOURCES and IGNEOUS PROCESSES

• Many important accumulations of metals are
  produced by igneous processes CONCENTRATION
• Igneous mineral resources can form from
• Magmatic segregation separation of heavy
  minerals in a magma chamber
• CHROMITE, MAGNETITE, PLATINUM
• Hydrothermal solutions - Originate from hot,
  metal-rich fluids that are remnants of the
  late-stage magmatic process

Chalcopyrite CuFeS2
Gold Au Silver Ag
Beryl (emerald) Be3Al2Si6O8
29
MINERAL RESOURCES and IGNEOUS PROCESSES