Igneous
Structures and Field Relations
(Chapter 4)
Extrusive
(volcanic) Processes and Landforms
last update:09/11/06
1. Properties of
Magmas and styles of eruption
in general, these are determined
by the viscosity and volatile content of the magmas
image source: Winter (2001)
i. Viscosity
(resistance to flow) is related to:
(a) composition of the magma and its effects
on melt polymerization
(b) temperature -
(higher T, lower viscosity)
(c) yield strength - resistance to flow once
crystals form(stuff in
the way)
(d) amount of H2O
and alkalis in the melt(weakens melt structure)
ii. Violent
eruptions are related to viscosity and, particularly, volatile content of
the melt (a) volatile content is generally greater in
Si-rich magmas relative to Si-poor magmas (b) release of lithostatic P results in rapid
expansion of dissolved volatiles (like a carbonated drink)
iii. Evidence for volatiles
in magmas
Volcanic gases - mostly H2O
and CO2, but also SO2, H2, HCl, CL2,
F2
Sulfur dioxide and other volcanic gases
rise from the Pu`u `O`o vent on Kilauea Volcano, Hawai`i. During periods
of sustained eruption from Pu`u `O`o between 1986 and 2000, Kilauea
emitted about 2,000 to 1,000 metric tons of irritating sulfur dioxide gas
(SO2) gas each day
Close view of a fumarole on
Tokachi Volcano, Hokkaido, Japan. Elemental sulfur vapor escaping from the fumarole
has cooled to form yellow sulfur crystals around its margins.
image source: Darrell
Henry, 2006
Spatters from basaltic magmas in which
there is low viscosity and volatile content. Spatters commonly build up to
cinder cones.
Top: Clumps of molten lava (spatter)
hurled above the rim of a spatter cone have already started to cool and
develop a thin black skin on their surface. Width of the image is about 3
m.
Bottom: Close view of cooled, solidified
spatter fragments hurled from an active littoral cone on the south
shoreline of Kilauea Volcano. The impact of the molten spatter hitting the
ground flattened the fragments into roughly circular disks.
Vesiculated porphyritic basalt with
phenocrysts of olivine, pyroxene and plagioclase.
image source: Hamblin and Christiansen
(2001)
Violent eruption of a volatile-rich high
viscosity magma
A plinian-type explosive eruption from
the Crater Peak vent (hidden beneath clouds) on Mount Spurr, Alaska, sent
an eruption column to a height of about 18 km above sea level
2. Landforms
associated with central vent eruptions
Distinctive landforms develop
from vent eruptions issuing from a cylindrical conduit
The most general groupings of
landforms are:
(a) shield volcano
(b) composite volcano
(c) pyroclastic cone
(d) additional forms associated with craters
Landforms associated with central vent
eruptions (no vertical exaggeration). [image source: Hamblin
and Christiansen, 2001]
Shield Volcano.
Rising gradually to more than 4 km above sea
level, Mauna Loa is the largest volcano on our planet. Its long
submarine flanks descend to the sea floor an additional 5 km, and the sea
floor in turn is depressed by Mauna Loa's great mass another 8 km. This makes
the volcano's summit about 17 km (56,000 ft) above its base! (Photograph
by J.D. Griggs on January 10, 1985.)
Composite volcano (stratovolcano). Mt.
Rainier viewed from Tacoma, Washington. Composite cones are primarily
composed of more silica-rich magmas (andesites to rhyolites).
Cinder cone or scoria cone -
loose collection of airborne ash, lapilli and blocks falling around a
central vent that is generally at the angle of repose. These are subject
to rapid erosion.
Tuff ring - cone of pyroclastic
material that dips inward and outward at roughly the same angle. Diamond
Head, Hawaii is a tuff ring.
image source: Winter (2001)
Spatter cone - cone composed of fluid
fragment of lava.The spatter commonly sticks
together, or agglutinates, when it lands and is buried by later spatter.
Spatter cone in Galapagos Islands. image source:
Barb Dutrow, LSU
Maar- Low
cone resulting from explosive interaction of magma with groundwater.Left: Maar is about 300 m in diameter. Eruption column generated by
phreatic and magmatic explosions rises from the larger east maar. Right:
Aerial view toward N of Ukinrek Maars, Alaska; Lake Becharof at top of
photo. Water partially fills the eastern maar and completely covers a lava
dome that was erupted in the 100-m deep crater during a 10-day eruption in
1977. [image source: http://volcanoes.usgs.gov/Products/Pglossary/maar.html
]
Schematic cross-section of a lava dome. These
are typically Si-rich magmas (e.g. dacite or rhyolite) with high
viscosity, and generally form late in the eruptive cycle.
image source: Winter (2001)
Lava dome that developed after the 1981 Mt.
St Helens eruption.
image source: Press and Siever (2001)
Calderas - large-scale collapse features
that typically form at a central vent, generally late in the eruptive
episode.
Distinctive landforms develop
from fissure eruptions issuing from a fracture, or system of dikes - the most
extensive system being at the mid-ocean ridges.
-multiple flows build up to form immense lava plateaus; e.g. Snake River
Plain Basalts, ID; Columbia River Basalts, OR, WA, ID
Flood basalts of the Columbia River Plateau
[from Press and Siever, 2001]
Locations of exposed feeder dikes and
vents in SE portion of the Columbia River Basalts
image source: Winter (2001)
4. Features
associated with lava flows
Basaltic flows are classified based on their surface form:
i. Pahoehoe (ropy) = flow with a smooth or filamented folds due to
continued flow beneath the a glassy surface; forms from less viscous lava
ii. Aa = flow with jagged, rough surface; forms from more viscous
lava that has lost its gas contents -very sharp edges that eat your boots!
iii. flows often have pahoehoe near their source and slow-moving aa further
from source
Lava flows from Kilauea Volcano, HI.
(Top left) Braided
lava flows spread from a lava fountain on the side of Pu`u `O`o cone, located
on the southeast rift zone of Kilauea Volcano (1986).
(Top right) Active lava flow
near a forest (Jan. 4, 2001)
(Left) Pahoehoe toes
encroach on a 1985 aa flow (Jan. 4, 2001)
Lava tube
in the Galapagos Islands (Ecuador). Note the lava flow lines.
[Photo courtesy of Dr. Barb Dutrow - LSU]
v. Columnar joints - ca. 5-6 sided pillars, produced by slow
cooling of basaltic magma
vi. Pillow Lavas - piles of elliptical, sac-like bodies (ca. 1 m)
that form when lava solidifies underwater
• indicates that lava either erupted underwater or flowed into water
(or ice)
• characterized by a glassy rind, commonly vesicular (was gas charged)
with a crystalline interior
Pillow lavas on the seafloor near the
Galapagos Islands
[from Press and Siever, 2001]
Rhyolitic lavas erupt at 800-1000°C, are very viscous, and form
steep-sided, bulbous deposits
Newberry Crater, Oregon. Central pumice cone is in the left-center of
the photo. It is about 6,600-6,700 years old. Big Obsidian flow is in the
bottom part of the photo. Big Obsidian flow is about 1,400 years old.
View of Big Obsidian flow looking south. The south rim of the caldera is
marked by the steep cliff just south (above) the flow.
- formed by accumulation of fragments of volcanic rock (pyroclasts)
scattered by volcanic ejection into the air
• when P is released, the volatiles escape with explosive force ejecting
and fracturing the magma
Violent eruption of a volatile-rich high
viscosity magma
A plinian-type explosive eruption from
the Crater Peak vent (hidden beneath clouds) on Mount Spurr, Alaska, sent
an eruption column to a height of about 18 km above sea level
• formed from gas- and Si-rich lavas
(e.g. rhyolite/andesite)
• types of deposits classified according to size of fragments
- commonly mixtures of sizes (up to house size)
(a) Ash - <2 mm; rock, mineral, glass fragments - if particles reach the
stratosphere it effects the weather
(b) Volcanic bombs - detached masses of lava ejected by volcanoes which, as
they fall, assume a rounded form like bomb shells
Volcanic bombs - fragments of lava that are
erupted into the air in a plastic or liquid state, and attain aerodynamic
shapes as they fly through the air. (photo
from Hamblin and Christiansen (2001))
Ash Cloud and deposits of the 1980 eruption
of Mt. St. Helens
(c) Volcanic breccias – mixtures of large and smaller angular material
(d) Pyroclastic flows – (also known as nuee ardente or glowing avalanche)
i. rapidly-moving (>120 mi/hr) mixture of hot ash, dust and gas 800°C
flowing down the side of a volcano - buoyed up by the gas,
ii. e.g. 1902 eruption of Mt. Pelee in Martinique; flowed at 100 mi/hr,
avalance of hot gas and red-hot ash enveloped the town of St. Pierre
(Martinique) killing 29,000
Mt. Pelee is famous for the May 8, 1902 eruption and pyroclastic flow
which killed 29,000 people and destroyed the city of St. Pierre. This is
the largest number of casualities
for a volcanic eruption this century.
Photograph of the remains of St. Pierre
(Martinique), 1902.
i. Volcanic Tuff - rock composed of pyroclastic fragments and ash
A. Welded Tuff - if fragments are hot, they melt together on deposition;
often maintaining the flow structure
B. Ash Flow Tuff (pyroclastic flow deposits) - Tuffs that form from ash flowing across the surface;
thicker in low spots
C. Air Fall Tuff (Pyroclastic fall tuffs) - Tuffs that form from ash settling from the atmosphere;
uniformly blanket the surface
Very large explosive eruptions have been a feature of the prehistoric
activity of Piton del Teide (Canary Islands). At numerous road cuts around
the volcano, one finds thick pumice layers (white in this image)
interbedded with more locally-derived dark basaltic ash.
ii. Volcanic Breccia - larger fragments of pyroclastic material in a fine
matrix
Volanic breccia (view is 0.3 m)
[from Press
and Siever, 2001]
iii. Lahar - mud flows of wet volcanic material (typically pyroclastic)
- formed when a flow meets heavy rains, glacial ice melt, rivers
- extremely dangerous e.g. Nevado del Ruiz, COL; 25,000 died (1985)
Broad summit of Nevado del Ruiz. An explosive eruption from
Ruiz's summit crater on November 13, 1985, at 9:08 p.m. generated an
eruption column and sent a series of pyroclastic flows and surges across
the volcano's broad ice-covered summit. Within minutes, pumice and ash
began to fall to the northeast along with heavy rain that had started
earlier in the day.
Río Lagunillas, former location of Armero. Within four hours of
the beginning of the eruption, lahars had traveled 100 km and left behind
a wake of destruction: more than 23,000 people killed, about 5,000
injured, and more than 5,000 homes destroyed along the Chinchiná, Gualí,
and Lagunillas rivers.
Extent and character of large pyroclastic deposits
Areal extent of the ash fall of the 6950
ybp eruption of Crater Lake (Mt Mazama)
Areal extent of the Bishop ash fall
700,000 ybp from the Long Valley eruption
Intrusive
(plutonic) processes and bodies
Difficult to directly observe crystallizing intrusive body but we can infer
shapes and sizes from field mapping of exposed intrusive bodies and by various
geophysical methods.
1. Tabular
intrusive bodies
Radial dike around volcanic neck at
Spanish Peaks, CO
SILL - tabular intrusive that are
concordant
i.e. follow bedding/layering of country rock.
DIKE - tabular intrusive that is
discordant i.e. cut across bedding/layering of country rock.
Sill
from Big Bend, Texas
Dike
cutting the shales in the lower Grand Canyon sequence
Types of dikes cutting stratal features:
(a) simple dilation and injection of magma (b) no dilation, but replacing or
stoping.
VEINS -
deposits of minerals foreign to the host rock developed within fractures
• Hydrothermal veins -
irregular
pencil or sheet-shaped intrusions rich in water (fluid).
- usually from igneous plutons. - Important source for deposits of Au, Ag,
Cu, etc.
2. Non-tabular
intrusive bodies
PLUTON
-
large igneous body crystallized at depth in the crust
• magmas rise if they are less dense than the country
rock = host rock
• intrusion takes place by fracturing country rock,
breaking off blocks (stoping) and melting some rocks
BATHOLITHS
>100 km2
Sierra Nevada batholith - granitic
rocks are light and metamorphic roof rocks are dark. [image source: Hamblin
and Christiansen, 2001]
STOCKS - smaller pluton < 100 km2
3. Contact
relationships with plutons
plutons typically involve
juxtaposition of a hot, viscous, fluid-rich liquid against relatively cold,
solid country rocks
depending on the nature of
the intrusive igneous material, the nature of the country rock and the depth
of intrusion the area near the contact can be quite different, and provide
clues to the processes of emplacement.
(a) The border zone may be strictly
mechanical in character (injected) with the country rocks being intruded
by the magma in apophyses. These grade to agmatite zones (network of
spaced injected dikes) and then to xenoliths (country rock
"floating" in pluton).
(b) In plutons that emanate reactive
fluids or are very hot relative to the melting point of the country
rocks, the border zone is gradational (permeated) due to alteration
(reaction) or local melting (less common) of the country rocks.
(c) Border zone is intermediate these
two, generating a mixed or hybrid rock.
Depending on the size and
composition of the pluton and the depth of intrusion, there will typically be
a
- contact metamorphic aureole due
to thermal and chemical effects from the pluton
possibly, a chill zone in
the pluton border
Salsbury Crags in Edinburgh. Teschenite
(Alkali basalt) sill cutting a sandstone.
photo credit: Barbara Dutrow - LSU
Hutton's Section - location in which
James Hutton initially demonstrated features such as the liquid nature
of magma and cross-cutting relations. Note the disruption of the
sandstone layers below and the reddening and fine-grained nature of the
basalt near the sandstone.
photo credit: Barbara Dutrow - LSU
4. Intrusion
timing
Timing is typically inferred by
textures in the country rocks and plutonic rocks and can be
- post-tectonic - after orogenic (deformational) episode such that plutons lack any deformational
fabrics, and cut any fabrics in the country rocks
- syn-tectonic - intrusion
during deformation such that foliations of the country rock converge into
emplacement-related foliations
- pre-tectonic - intrusion
prior to orogenic cycle.
Continuity of foliation in a pre- or
syn-tectonic intrusion.
5. Intrusion
depth
Three depth zone of plutons
(influences pluton and country rock structural and textural features.
1. Epizone - shallow
(<10km) with cool country rocks (<300 C) with brittle behavior that
are generally post-tectonic.
- Sharp, discordant contacts that are commonly
brecciated with roof pendants and rafts.
- These are typically
associated with hydrothermal alteration and ore mineralization.
- Contact metamorphic
aureole is well-developed at the margin of the pluton.
- Miarolitic cavities
(bubbles of fluid) with euhedral crystals projecting inward.
Relationship of top of epizonal pluton
and country rock. image
source: Winter (2001)
2. Mesozone -
intermediate intrusion depth (5-20 km) with country rock temperatures of
300-500 C that is syn-tectonic to post-tectonic.
- the contacts can be
sharp or gradational; and concordant or discordant because of
the more ductile nature of the country rocks
- well-developed
contact metamorphic aureole with possible foliation and development of
"spotted" slates and phyllites
- minor or no chill
zone
Cross-section of the inferred relation of the
metamorphic zones and the Mooselookmeguntic granitoid pluton of W. Maine
3. Catazone - deepest
intrusion (>10 km) with country rock temperatures of 450-600 C that are
generally syntectonic
- typically with
gradational contacts with no chill effects and shearing and rotation
parallel to the contact
- contact effects are minor
due to the high ambient T
- plutons are commonly
domal or sheet-like with foliations passing into the country rocks and
local melting of country rocks
6. Multiple
injection and zoned plutons
Multiple intrusion of magmas
is relatively common with numerous compound intrusions, generally with later
magmas intruding the center of the pluton
Tuolumne Intrusive series (Yosemite) of the
Sierra Nevadas
(a) original intrusion and crystallization of
marginal quartz diorite
(b) surge of magma with later solidification of
non-porphyritic Half Dome Granodiorite
(c) second surge of magma with later
solidification of porphyritic Half Dome Granodiorite
(d) third surge of magma with later
solidification of Cathedral Peak Granodiorite and latest Johnson Granite
Porphyry
image source: Winter (2001)
7. Magma rise and
emplacement
Melts that segregate from depth
are generally less dense than the surroundings and become buoyant, rising
diapirically (in regions of ductile deformation) until reaching neutral
buoyancy or the surface in an eruption
- similar to salt diapirs
For magmas to rise they must
overcome the "room problem" in an environment in which there is
little open fracturing or void space. However, this problem is reduced in
areas of extension.
Mechanisms by which a
pluton can make room.
image source: Winter (2001)
1. Lift the roof by folding or block
elevation - maybe in upper 2-3 km.
2. Assimilation of wall rock - melting
of country rocks, limited
3. Stoping - dislodged blocks are
dense and sink in magma - shallow depths- solution stoping is combo of 2 and 3.
4. Ductile wall rock deformation -
develop at depths where there is a similar viscosity in melt and wall
rock
5. Lateral wall rock displacement -
ballooning
6. Emplacment in extensional
environments - limited by the rate of extension
Diapirs, based on soft putty
experiments, spread laterally to form relatively thin bodies e.g. NW
Maine
