GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 3
Volcanic Landforms and Processes
and
Emplacement of Igneous Intrusions
Jan. 24, 2016
PHYSIOCHEMICAL PROPERTIES OF LAVAS
General Magma Type:
Ultramafic
Mafic
Intermediate
Felsic
1550º to 1200º
1250º to 1050º
1150º to 950º
1050º to 800º
Low
Low
Intermediate
High
Very Low (<<1%)
Low (<1%)
Intermediate (1-3%)
High (2-5%)
Typical Composition (wt. %)
SiO2
46.5
50.0
57.7
70.5
TiO2
0.3
1.9
1.0
0.3
Al2O3
3.1
15.9
16.6
14.1
FeO
11.2
10.3
7.2
2.8
MnO
0.2
0.2
0.1
< 0.1
MgO
32.9
7.0
3.7
0.7
CaO
4.8
9.7
6.5
1.7
Na2O
.1
2.9
3.4
3.6
K2O
.01
1.1
1.8
3.9
P2O5
n.a.
0.3
0.3
0.1
Total
99.0
99.3
98.3
97.8
Cr
3000
200
10
2
Ni
1000
150
15
2
Ba
20
40
300
350
Zr
10
35
200
170
Temperature:
Viscosity:
Gas Content:
Trace Elements (ppm)
CONTROLS ON VISCOSITY (RESISTANCE TO FLOW)
Viscosity increases with:
• SiO2 concentration
• decreasing temperature
• increasing crystallinity of magma
• decreasing volatile content (H2O, CO2, SO2, H2, HCl, Cl2, F2)
EFFUSIVE ERUPTIONS
Mafic magma
 Relatively low gas content
(<1%)
 Fountaining followed by flow
as gas content diminishes
 Creates vesicular to massive
lava flows

Photos from USGS
EXPLOSIVE ERUPTIONS
Driven by degassing of magma as it rises up the neck of the volcanic vent
The dramatic increase of volume resulting from degassing causes the
magma to be violently thrust out the neck and shattered into fine fragments –
ash
Creates pyroclastic deposits
Eruption Model
Water solubility (carrying capacity) in rhyolite as
function of pressure; from Yamashita (1999)
http://www.geology.sdsu.edu/how_volcanoes_work/
CENTRAL VENT VOLCANIC LANDFORMS
STRATOVOLCANOES
Steep, conical volcanoes built by the eruption
of viscous lava flows, tephra, and pyroclastic
flows, are called stratovolcanoes. Usually
constructed over a period of tens to hundreds
of thousands of years, stratovolcanoes may
erupt a variety of magma types, including
basalt, andesite, dacite, and rhyolite. All but
basalt commonly generate highly explosive
eruptions.
Mt St. Helens (pre-1980 eruption)
CENTRAL VENT VOLCANIC LANDFORMS
SHIELD VOLCANOES
Built almost entirely of fluid mafic lava
flows. Flow after flow effusively pours out
in all directions from a central summit
vent, or group of vents, building a broad,
gently sloping cone of flat, domical shape.
CENTRAL VENT VOLCANIC LANDFORMS
SCALES
CENTRAL VENT VOLCANIC LANDFORMS
CALDERAS AND DOMES
Lava dome structure
(from Winter, Fig. 4-7)
Lava dome building in Mt. St. Helens crater
FISSURE ERUPTION LANDFORMS
Laki fissure, Iceland – Erupted 1783 creating
the largest lava flow in human history
FISSURE ERUPTION LANDFORMS
PLATEAU (FLOOD) BASALTS
FISSURE ERUPTION LANDFORMS
DIKE SWARMS
Feeder conduits to eroded plateau basalts
FLOOD BASALT LAVA FLOW FEATURES
LAVA FLOW FEATURES
BASALTIC LAVA FLOW CONTACT
Massive
Basalt
Amygdaloidal
Basalt
LAVA FLOW FEATURES
BASALTIC LAVA FLOW SURFACES
AA
AA
Pahoehoe
HAWAII
Pahoehoe
NORTH SHORE
LAVA FLOW FEATURES
TOE LOBES
Modern-day Hawaii
1.1 Ga North Shore Volcanics
LAVA FLOW FEATURES
PILLOW STRUCTURES
(SUBMARINE ERUPTIONS)
LAVA FLOW FEATURES
COLUMNAR JOINTING
Shovel Point, MN
CRB, OR (Winter, 2001)
Gooseberry Falls, MN
LAVA FLOW FEATURES
MISCELLANEOUS
Convoluted Flow
Banding in
Rhyolite
Amygdule
Cylinders in
Basalt
Plagioclase
Porphyritic
Texture in
Basalt
Coarse Ophitic
Texture in
Basalt
PYROCLASTIC DEPOSITS
PRODUCTS OF EXPLOSIVE ERUPTIONS
TUFF
Winter (2001) Fig. 2-5
PYROCLASTIC DEPOSITS
FALL DEPOSITS
Mt. St. Helens
Figure 4-16. Approximate aerial extent and thickness of
Mt. Mazama (Crater Lake) ash fall, erupted 6950 years
ago. After Young (1990), Unpubl. Ph. D. thesis, University
of Lancaster. UK.
Figure 4-17. Maximum aerial extent of the Bishop ash fall
deposit erupted at Long Valley 700,000 years ago. After Miller
et al. (1982) USGS Open-File Report 82-583.
PYROCLASTIC DEPOSITS
FLOW DEPOSITS
Figure 4-18. Types of pyroclastic flow deposits. After
MacDonald (1972), Volcanoes. Prentice-Hall, Inc., Fisher and
Schminke (1984), Pyroclastic Rocks. Springer-Verlag. Berlin. a.
collapse of a vertical explosive or plinian column that falls back
to earth, and continues to travel along the ground surface. b.
Lateral blast, such as occurred at Mt. St. Helens in 1980. c.
“Boiling-over” of a highly gas-charged magma from a vent. d.
Gravitational collapse of a hot dome (Fig. 4-18d).
PYROCLASTIC DEPOSITS
COMPLETE ERUPTIVE
PACKAGE - IGNIMBRITE
Figure 4-19. Section through a typical ignimbrite,
showing basal surge deposit, middle flow, and
upper ash fall cover. Tan blocks represent
pumice, and purple represents denser lithic
fragments. After Sparks et al. (1973) Geology, 1,
115-118. Geol. Soc. America
Graded TuffEpisodic
Eruptions/Surges
Tuff +
Heating +
Pressure 
Welded Tuff
FORMS OF IGNEOUS INTRUSIONS
RING DIKES AND
CONE SHEET
Isle of Mull, Scotland
LACCOLITHS
AND
LOPOLITHS
“The Duluth gabbro complex, for example, is a lopolith over 300 km across”
(Winter, 2001, p. 64)
(Winter, 2010, p. 72)
CONTACT ZONES OF INTRUSIONS
(Assimilated)
Thermal Metamophism of Country Rock
LARGE GRANITIC INTRUSIONS
FOLIATED MARGINS
Alignment of
phenocrysts in felsic
magmas ~parallel to
intrusion margins.
Phenocryst-rich
magmas are commonly
called “Crystal Mushes”
LARGE MAFIC INTRUSIONS
VARI-TEXTURED (TAXITIC) MARGINS
TIMING OF INTRUSION
SYN/PRE-TECTONIC VS. POST-TECTONIC
Vermilion Map M141, Jirsa and Boerboom, 2003
DEPTH OF EMPLACEMENT
Epizonal (<10km, <300oC)
Brittle CR; sharp discordant contacts;
strong metamorphic gradients;
smaller plutons
Mesozonal (5-15 km, 300-500oC)
Ductile CR; sharp-gradational,
discordant-concordant contacts; modstrong metamorphic gradients;
larger syn- to post-tectonic plutons
Catazonal (>10km, 450-800oC)
Very ductile CR, gradational
concordant sheared contacts,
strong metamorphic gradients;
syntectonic plutons
COMPOSITE INTRUSIONS
Tuolumne Intrusive Series, California
Beaver Bay Complex, Minnesota
5 km
MECHANISM OF MAGMA EMPLACEMENT
Diagrammatic illustration of
proposed pluton emplacement
mechanisms.
1- doming of roof
2- wall rock assimilation, partial
melting, zone melting
3- stoping
4- ductile wall rock deformation
and wall rock return flow
5- lateral wall rock displacement
by faulting or folding
6- (and 1)- emplacement into
extensional environment.
After Paterson et al. (1991),
Contact Metamorphism. Rev. in
Mineralogy, 26, pp. 105-206. ©
Min. Soc. Amer.
Winter (2001), Figure 4-34.
NEW IDEAS ON GRANITE BATHOLITH
EMPLACEMENT
HYDROTHERMAL SYSTEMS
From Hudak (2006)