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Chapter 4: Part 2
Volcanoes and Other
Igneous Activity
PowerPoint Presentation
Stan Hatfield . SW Illinois College
PowerPoint Presentation
Ken
Pinzke. .Southwestern
SW Illinois
College
Stan Hatfield
Illinois
College
Ken
Pinzke . Southwestern
Illinoisof
College
Charles
Henderson
. University
Calgary
Charles Henderson . University of Calgary
Tark
Hamilton . Camosun College
Copyright (c) 2005 Pearson Education Canada, Inc.
4-1
Volcanic Structures and Eruptive Styles
A size
comparison
of
the
three
types
of
volcanoes
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4-2
Volcanic Structures and Eruptive Styles
Types of Volcanoes
• Composite cone (Stratovolcano)
– Most are located adjacent to the Pacific Ocean (e.g., Fujiyama,
Pinatubo, Mt. St. Helens, El Chichon, Mt Hudson)
– Monte Vesuvio destroyed Pompeii & Herculaneum
– Large, classic-shaped volcano (> hundreds of metres high & >
several kilometres wide at base)
– Composed of interbedded lava flows & pyroclastics
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4-3
Living in the Shadow of a Composite Cone
Eruption of Vesuvius in AD 79
Victims of Pompeii & Herculaneum
Casts
of several
victims
theInc.
AD 79 eruption of Mount Vesuvius.
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(c) 2005 Pearson
Educationof
Canada
4-4
Volcanic Structures and Eruptive Styles
Living in the Shadow of a Composite Cone
– The Lost Continent of Atlantis and Santorini
Ammoudi Beach, >60 m ash covers ruins
Thera
blew(c)1627
BCE &
destroyed
Crete. Santorini was rebuilt in the caldera.
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2005 Pearson
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Canada Inc.
4-5
Volcanic Structures and Eruptive Styles
Nueé Ardente: A Deadly Pyroclastic Flow also called
a Welded Ash Flow or Welded Tuff
– Most violent type of activity (e.g., Mount Vesuvius 79
AD, Mt. Pelee 1902 and Mount St. Helens 1980)
– A nueé ardente: (glowing avalanche cloud at night)
– Fiery pyroclastic flow made of hot gases infused with
ash and other debris
– Move down the slopes of a volcano at speeds up to
200 km per hour
– Unlike other ash falls, this one fuses as it collapses
– May also produce a lahar, which is a volcanic mudflow
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4-6
Volcanic Structures and Eruptive Styles
A nueé ardente on Mt. St. Helens 1980 & Pinatubo 1991
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Calderas
• Steep-walled collapse depressions at the summit
(Crater Lake is an example)
• Size exceeds 1 km in diameter
• Hawaiian-Type Calderas
• Yellowstone-Type Calderas
Kilauea Caldera
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Mt. Mazama – Crater Lake Caldera
Sequence
events
formed
Copyrightof
(c) 2005
Pearsonthat
Education
Canada Inc.the caldera at Crater Lake, Oregon
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Fissure Eruptions and Lava Plateaus
• Fluid basaltic lava extruded from crustal fractures
called fissures
• CRB: Columbia River Plateau; ~12.5 Ma flood
basalts also occur in the Chilcotin, S-central BC
• Several Massive Extinction Events – Bio-Geological
Period Boundaries coincide with Flood Basalts
– Deccan Traps, Siberian Traps, Karoo SA, Parana
CRB
Chilcotin
Roza Member
Frenchman Springs
Chasm Provincial Park
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4-10
Other Volcanic Landforms
Lava Domes
• Bulbous mass of slowly extruded, congealed lava
• Most are associated with late stages of an explosive
eruption of gas-rich magma
• These are slow but dangerous and can spawn block
& ash flows
Volcanic pipes and necks
• Pipes are short conduits that connect a magma
chamber to the surface
• Often they remain as buttes after the pyroclastics
& cone erode away
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Other Volcanic Landforms: Lava Domes
A lava dome forms on Mt. St. Helens following the eruption.
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4-12
Other Volcanic Landforms
Volcanic pipes and necks continued
• Volcanic necks (e.g., Shiprock, New Mexico) are
resistant vents left standing after erosion has
removed the volcanic cone since ~27 Ma
• Tow Hill, Graham Island is a ~5 Ma volcanic neck
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4-13
Intrusive Igneous Activity
Nature of Intrusions
• Orientation with respect to the host (surrounding)
rock
– Discordant – cuts across sedimentary rock units, or
other existing structures
– Concordant – parallel to sedimentary rock units
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Intrusive Igneous Activity
Most magma is emplaced at depth in the Earth
• An underground igneous body, once cooled and
solidified, is called a pluton
• There is lots more inside than surface to the crust!
Nature of Intrusions
• Shape
– Tabular (sheetlike: ~horizontal, sill, laccolith) both
concordant to strata
– Tabular (sheetlike: ~vertical to inclined, dyke) cross
cutting as planes, cones, rings
– Massive (irregular mass: pluton) cross cutting
– Cylindrical: Plugs, Pipes, Buttes when eroded
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4-15
Shallow Intrusive Igneous Activity
Laccolith
concordant
Caldera
Dyke
cross cutting
Butte
Stock < 1 km3
1<Pluton<10
10<Batholith<100
Some intrusive igneous structures.
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Intrusive Igneous Activity
Nature of Intrusions
• Dyke – a tabular, discordant pluton
– Forms in brittle rocks or from fast intrusion ~km/sec
• Sill – a tabular, concordant pluton (e.g., Palisades
Sill in New York/Jersey ~205 Ma opening of
Atlantic, rifting of Pangea)
– Forms in ductile rocks, pliable sediments, easier to
inflate than to intrude higher, buoyancy or gas lost
• Laccolith – Shonkin Sag, Montana
– Similar to a sill but domed convex up
– Lens or mushroom-shaped mass
– Arches overlying strata upward
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Intrusive Igneous Activity
A dark-coloured sill made of gabbro in the NWT
4-18 .
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Intrusive Igneous Activity
Nature of Intrusions
• Batholith
– Largest intrusive body
– Surface exposure of 100+ square kilometres (smaller
bodies are termed stocks or plugs)
– Frequently form the cores of collisional mountains
Exfoliation Sheet Joints
Tensional cooling structures
In coarse grained granites
Little Rock Texas
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Intrusive Igneous Activity
Mesozoic Granitic batholiths that occur along the western margin of N. America
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Intrusive Igneous Activity
Emplacement of Batholiths
• Magma at depth is much less dense than the
surrounding rock
– Increased temperature and pressure causes solid rock to
deform plastically
– The more buoyant magma pushes aside the host rock
and forcibly rises in the Earth as it deforms the
“plastic” host rock
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Intrusive Igneous Activity
Emplacement of Batholiths
• At shallower depths, the host rock is cooler and
exhibits brittle deformation
– Movement of magma here is accomplished by fractures
in the host rock and stoping
– Inclusions in the host rock or xenoliths are evidence
supporting the movement of magma through solid rock
– We saw some of these as deformed mafic clasts or
enclaves in the lighter coloured granitic rocks of the
Devonian Tyee Stock at Finlayson Point
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Plate Tectonics and Igneous Activity
Global distribution of igneous activity is not
random
• Most volcanoes are located within or near ocean
basins
• Basaltic rocks are common in both oceanic and
continental settings, whereas granitic rocks are
rarely found in the oceans
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Plate Tectonics and Igneous Activity
Location of some of Earth’s major volcanoes.4-24
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Plate Tectonics and Igneous Activity
Igneous Activity at Convergent Plate Boundaries
• Subduction zones
–
–
–
–
Occur in conjunction with deep oceanic trenches
Descending plate partially melts
Magma slowly moves upward
Rising magma can form either
– A volcanic island arc (island arc) if in the ocean
– A continental volcanic arc if subducted under continental
lithosphere
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Plate Tectonics and Igneous Activity
• Subduction zones
– Associated with the Pacific Ocean Basin
– Region around the margin is known as the “Ring of
Fire”
– Most of the world’s explosive volcanoes are found
here
– Stratocones & Calderas both occur in this setting
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Plate Tectonics and Igneous Activity
Igneous Activity at Divergent Plate Boundaries
• Spreading centres
– The greatest volume of volcanic rock is produced along the oceanic
ridge system
– Mechanism of spreading
– Thermal bulge on top of Mantle from upwards convection
– Lithosphere pulls apart and slides downhill to both sides
– Less pressure on underlying rocks, mantle upwells to fill in
– Results in partial melting of mantle (decompression melting)
– Large quantities of basaltic magma are produced
– The process lasts 10’s to 100’s of Ma
– Older ideas like “Ridge Push” clash with the extensional setting
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Plate Tectonics and Igneous Activity
Intraplate Igneous Activity
• Activity within a tectonic plate
• Associated with mass of hotter than normal mantle called
mantle plumes
• Form localized volcanic regions in the overriding plate
called a hot spot
– Produces basaltic magma sources in oceanic crust (e.g., Hawaii and
Canary Islands)
– Vast outpouring of mafic lava creating large basalt plateaus like
Columbia Plateau and Deccan Plateau in India (latter may have
affected climate in Upper Cretaceous)
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Plate Tectonics and Igneous Activity
Hotspot
Decompression
Hawaii, Iceland
Island Arc, flux
Japan, Aleutians
Continental Arc, flux
Andes, Coast Ranges
Three zones of volcanism with central volcanoes4-29
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Plate Tectonics and Igneous Activity
Mid Atlantic Ridge
Deccan Traps K/T
Flood Basalts
Plume Head
East African Rift
Three
zones
ofPearson
extensional
volcanism from decompression.
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Plate Tectonics and Igneous Activity
Large Igneous Provinces (LIPS) also form Oceanic Plateaux: Caribbean, Ontong Java, Kerguelen
Model
of a mantle plume and associated hot-spot volcanism
.
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Can Volcanoes Change Earth’s Climate?
Explosive eruptions emit huge quantities of gases
and fine-grained debris into the atmosphere
which filter out and reflect a portion of the
incoming solar radiation
In Troposphere Months-Years, Stratosphere >>
Examples of volcanism affecting climate
• Did Deccan Traps 65 Ma end Cretaceous Hothouse
• Mount Tambora & Indonesia – 1815 (caldera)
• Krakatau, Indonesia – 1883 (caldera)
• Mount Pinatubo, Philippines – 1991 (stratocone)
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Can Volcanoes Change Earth’s Climate?
Eruptions can emit great quantities of sulphur
dioxide gases, which combine with water to form
sulphuric acid particles called aerosols; they
reflect solar radiation back to space.
The answer is Yes. They are regarded as an
explanation for some aspects of Earth’s climatic
variability.
When aerosols penetrate the stratosphere with its
rapid jet stream the entire heat balance in the
atmosphere can shift, changing the rest of
atmospheric
convection.
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End of Part 2 of
Chapter 4
Volcanism
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