Igneous Processes and Structures
GLY 2010 – Summer 2013
Lecture 7
1
Definition of Igneous
• Igneous - Said of a rock or mineral that
solidified from molten or partly molten
material, i.e. from a magma
• Etymology: Latin ignis, ''fire”
2
Magma
• Magma is naturally occurring mobile rock
material “molten rock”
• Capable of intrusion and extrusion
• Igneous rocks are derived from magma
through solidification and related processes
3
Viscosity
• Viscosity is the property of a substance to
offer internal resistance to flow; its internal
friction
4
Flow Viscosity
Initial Position
After flow starts
Viscosity increases from left to right
5
Viscosity in Magma
• Video shows a rod being poked in hot, viscous
magma on Kilauea, Hawaii
6
Factors Influencing the Viscosity of
Magma
• Temperature
• Chemical composition
• Gas content
7
Temperature
• The higher the temperature, the lower
the viscosity
• Basaltic magmas at 1200°C or
higher, are much more fluid (less
viscous) than granitic magmas at
800°C
8
Low Viscosity Flow Animation
Typical of basaltic magma
9
High Viscosity Flow Animation
• Typical of Andesitic or Rhyolitic Magmas
10
Chemical Composition
• The higher the silica content of
magma, the higher the viscosity
11
Gas Content
• As gas content increases, the
viscosity decreases
• Gases inhibit silica chain formation,
and lower overall viscosity
12
Effects of Increasing Viscosity
• Volcanic violence related to viscosity
(magma type)
• Magma type related to geologic, and
often, plate tectonic setting
13
Partial Melting
• Different minerals melt at different
temperatures
• As temperature increases, this leads to
partial melting
14
Igneous Structures
• Intrusive –
magma freezes
below the
surface
• Extrusive –
magma erupts
onto or above
the surface
15
Intrusive Structures
• Plutons are large bodies of magma that
solidified well below the surface
• Magma may be injected under pressure
into cracks in the rock
16
Intrusive Structures, Cont.
• Igneous sill – Parallel to existing layers
17
Igneous Sill
• Salt River Canyon, Arizona – the dark band is basalt
intruded into horizontal layers of igneous rock
18
Intrusive Structures, Cont.
• Igneous Dike - intrusion cuts across
the rock layers
19
Igneous Dike
•Thin, pink
aplite dikes cut
the black
basaltic dikes
and the gray
granite
•Photo C.A.
Giovanella
• Location Pender Harbor,
Southwest British Columbia
20
Igneous Structure Diagram
21
Aerial View of Intersecting Dikes
• Intersecting dikes (Tertiary) at Spanish Peaks, Las Animas and Huerfano
Counties, CO. View to east. (10Apr66)
22
Spanish Peak Dikes
• West Spanish Peak (13,623 ft) and dikes (Tertiary), Las Animas
and Huerfano Counties, CO. View to the south. (10Apr66)
23
Intrusive Structures, Cont.
• Laccolith
24
Igneous Laccolith
25
Shiprock, New Mexico
• The neck of an ancient volcano, which has eroded
• Structure in the background is a dike
26
Igneous Vein
• Extensional veins in
a thick carbonate
turbidite from the
Liguride Complex in
the Northern
Apennines, Italy
• Photo David Bice,
Carleton College
27
Extrusive Structures
• Lava Flows
• Obsidian
28
Lava Flow From the Air
29
Lava Flow From the Ground
• Lava flows from
Nyiragongo
volcano
• Eruption January
18, 2002
• City is Gomo,
Congo
30
Newberry Caldera, Oregon
• Obsidian
flow from a
vent along
the south
wall of the
caldera
• Road gives
scale
31
Obsidian Domes
Photo M.L.
Bevier
• Holocene obsidian domes, Long Valley Caldera
California - obsidian is formed from very viscous
magma, which is unable to flow long distances 32
Obsidian
• Upper left: Thin
piece of obsidian,
showing flow
banding
• Lower left:
Thicker piece of
obsidian showing
conchoidal
fracture
33
Classification of Igneous Rocks
•
•
•
•
Mineral content
Chemical analysis
Texture
Geologic Association
34
Mineral Content
• Igneous rocks may
be classified on the
basis of what
minerals they contain
 Essential Minerals
 Accessory Minerals
35
Chemical Content
• Rocks may be analyzed to
see what elements they
contain
• Results are reported as
weight percent oxides
36
Texture and Fabric
• Texture is the general physical appearance or
character of a rock
• Fabric refers to the orientation (or lack of it) in
space of the elements of which a rock is
composed
37
Aphanitic
Texture
• A very fine grain texture, with crystals
invisible to the naked eye
• Photo M.L. Bevier
38
Pheneritic
Texture
• Grains are visible and identifiable using the
naked eye
• Photo M.L. Bevier
39
Porphyritic
Texture
• Large crystals in a
fine-grained or
aphanitic
groundmass Field of view 2 cm
• Photo: E.J.
Tarbuck
40
Pegmatitic –
Black Hills,
South Dakota
• Coarse texture,
resulting from rapid
crystal growth, due to
presence of water in
melt at time of
crystallization
• Arrows
point to the location of spodumenne crystals,
removed during mining
•Spodumene is a source of lithium
41
Relation of Texture to Cooling
History
• Aphanetic - Rapid cooling leads to very fine crystals or to
glass - typical of extrusive rocks
• Phaneritic - Slower cooling leads to medium or coarse
grained rocks, typical of intrusive igneous rocks
• Pegmatitic - Very slow cooling in a water-rich magma leads
to the coarse crystals
• Porphyritic - Slow cooling while the magma ascends through
the earth, followed by rapid cooling after the magma erupts
on the surface
 Crystals are often medium to coarse
 Groundmass texture will be fine-grained, or glassy, depending
on the rate of cooling
42
Geologic Association
• Rocks associated by age, position,
and by characteristically being found
together
• Constant or regular variation of
features within the rock body
43
Bowen's Reaction Series
44
Bowen – Tuttle Experiments
45
Magmatic Differentiation
• Fractional Crystallization (Crystal
settling)
• Partial Melting
46
Fractional Crystallization
47
Evolution of
Magma
• Over time, fractional
crystallization changes
the composition of
magma from “A” to “B”
to “C”
48
Intrusive Vs. Extrusive Rocks
• Intrusive
• Medium to coarse
grained
• 100% Crystalline
• May contain
inclusions of rock
which fell into the
magma (xenoliths)
• Extrusive
• Aphanitic to fine
grain size
• Often glassy
• Often contain gas
bubbles
49
Granite, Rhyolite
• Granite has clearly
visible crystals
• Rhyolite has few
visible crystals, and
patches of glass
• Both rocks are felsic
50
Diorite, Andesite
• Diorite has visible
felsic and mafic
crystals
• Andesite has mainly
felsic crystals visible
• Both rocks are
intermediate
51
Gabbro, Basalt
• Gabbro is composed
of dark, visible
minerals
• Basalt is dark, with no
visible crystals; this
piece has many gas
bubbles
• Both rocks are mafic
52
Obsidian
• Obsidian is volcanic glass, formed by quick
chilling of the magma after eruption
• This specimen is called Snowflake obsidian - the
white crystals are cristobalite, a high-temperature
form of silica, which crystallized before eruption
53
Peridotite
• Ultramafic intrusive rock characteristic of
the earth’s mantle
54
Xenolith
• Xenoliths of metamorphic amphibole in
dunite, an ultramafic igneous rock
55
Andesite Line
• Geographic boundary between the
basalt/gabbro of the Pacific Ocean basin
and the andesites at the subductive margins
of the surrounding continents
56