recent advances in volcanological and hydrothermal ...rmorton/ronshome/volcanology/... · •...
TRANSCRIPT
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Calderas
5 km
1400 m
500 m
Myojin Knoll
Submarine
Caldera
(after Kennedy and Stix, 2003)
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Outline• Definition
• Relationships to Eruption Volume
and VEI
• Structural Components
• Types
• Caldera Genetic Models and the
Caldera Cycle
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Types of Volcanoes
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Definitions: Caldera• A large volcanic depression, more or less circular or
elongate in shape, the diameter of which greatly exceeds that of any included vents. Calderas are formed by the eruption and evacuation of a near-
surface magma chamber (Lipman, 2000).
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Sturgeon
LakeVandever
Mtn.
Santorini
Taupo
Kuwai
Krakatau
Pinatubo
Yellowstone
Tambora
Myojin
Knoll
Crater Lake
Bald Mtn
Locations of Famous Caldera Complexes
Noranda
Wawa
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Calderas and Cauldrons• Two separate features
• Calderas are formed by catastrophic collapse associated with large volume (>5 km3) pyroclastic eruptions
• Cauldrons form from passive foundering of the roof of a static subsurface magma, often due to effusive eruption of magma on flanks of volcano (common on shield volcanoes)
• Large-volume effusive eruptions may form cauldrons that are on the scale of calderas
Noranda Cauldron
Sturgeon Lake
Caldera Complex
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Caldera Forming Eruptions
• Generally large-volume explosive eruptions (e.g. pyroclastic flow
forming eruptions), but large-volume effusive eruptions may also
form calderas (cauldrons)
• May occur in both subaerial and submarine environments (water
depths are generally shallow, < 1200m water depth)
• Different explosive eruption styles (effusion rate, volatile content,
interaction with external water) will form different types of
pyroclastic deposits
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Caldera-Associated Eruption Volumes
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Toba Caldera, Indonesia
• Eruption occurred ~75,000 years ago
• Eruption volume estimates range from
2500km3 to 2800km3
• Caldera measures 40km x 105 km
• Geneticists estimate possibly as few as 5,000
humans survived the eruption
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Caldera Eruptions
and the Volcanic
Explosivity Index
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Structural Elements of Calderas
(after Lipman, 1997, 2000)
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Caldera Classification• Based on Subsidence Style and Geological Environment
• Subsidence Styles Include:
– Plate (Piston)
– Piecemeal
– Trapdoor
– Downsag
– Funnel
• Geological Environments Include:
– Subaerial (e.g. Crater Lake, Yellowstone)
– Subaerial to Submarine (e.g. Santorini, Krakatau,
Kuwae, Sturgeon Lake)
– Submarine (e.g. Myojin Knoll, Bald Mountain)
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Subsidence Geometry of Calderas
Caldera geometries are related to the:
– size of the pyroclastic eruption
– depth of the magma chamber
– width of the magma chamber
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Models of Caldera Development
• Williams, 1941
– Caldera collapse as the result of rapid
eruption from a shallow magma
chamber
• Smith and Bailey, 1968
– Caldera cycle in which voluminous
eruption occurs prior to caldera
collapse
• Druitt and Sparks, 1984
– Caldera formation occurs
simultaneously with voluminous
eruption
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Mechanisms of Caldera Collapse
• Druitt and Sparks (1984)
– Caldera collapse occurs
simultaneously with voluminous
explosive eruptions
• Branney (1995), Kennedy (2000),
Kennedy et al. (2000)
– Caldera collapse-associated
faults are outward-dipping; near
vertical inward-dipping faults
located at the margins of the
caldera are developed after
caldera collapses as a result of
continued sagging – space
problem solved!
(after Kennedy and Stix, 2003)
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Caldera Subsidence
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The Caldera Cycle
• Smith and Bailey, 1968
– Calderas go through a systematic series of developmental
steps related to intrusion, eruption, and crystallization of the
subvolcanic intrusion
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Stage 1:Regional Tumescence and Ring fractures
• Doming of the pre-caldera rocks
• This due to intrusion of a magma into shallow levels of
the earth’s crust.
• Extension of crust over the magma chamber leads to
formation of ring fractures
• Minor pyroclastic eruptions or lavas along “leaky” ring
fractures
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Stage 2: Ignimbrite Eruption
• Eruption of pyroclastic material lowers the pressure in the magma chamber and sets stage for collapse.
• Eruptions occur along ring fractures
• This stage usually occurs with stage 3
• In a subaerial setting pyroclastic eruptions may be relatively continuous lead to formation of relatively thick sequences of ash fall, pyroclastic flow and surge deposits. May get welding. Eruptions are magmatic
• Subaqueous settings pyroclastic eruptions may be episodic and produce relatively thick sequences of bedded pyroclastic flow, mass flow, and ash fall deposits. Welding can occur with high volume eruptions. Eruptions are dominantly magmatic with secondary hydromagmatic activity.
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Columnar Jointed Welded Tuffs, Valles Caldera, NM
Welded Tuff
Deposits
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Welded Tuffs, Valles Caldera
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Thick Non-welded Tuffs, Valles Caldera, NM
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Rhyolite Tuffs, Golden Gate
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Rhyolite Tuff
Ash
Pyroclasts
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Partially Welded Rhyolite Tuffs
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Welded Rhyolite Tuff
Fiamme
Fiamme are
flattened pumice
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Stage 3: Caldera Collapse• The most dynamic event in development of caldera complex
• Accompanied by formation of coarse, heterolithic breccias called meso-and megabreccias
• Mesobreccias-fragments less than 1m in diameter
• Megabreccias- > 1m in diameter (some individual fragments are 500m to >1km in size)
• Products of mass wasting during collapse.
• May represent substantial part of caldera fill
• In places get interlayering of meso- and megabreccias and pyroclastic flows: eruption and collapse
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Stage 3 – Simultaneous Eruption and Caldera CollapseNote interlayering of
ignimbrite and caldera-
collapse breccias
Mesobreccias – chaotic, unsorted caldera collapse-associated breccias with clasts
that have an average diameter less than one meter
Megabreccias – chaotic, unsorted, generally polymict caldera collapse-associated
breccias with clasts that have an average diameter greater than one meter (Lipman,
1988)
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Stage 4: Pre-Resurgent Volcanism / Sedimentation
• Eruption of lava flows and domes along ring fractures or fissures
that bound the caldera.
• Associated with formation of lots of sedimentary/debris flow material
as the walls are extensively eroded.
• Stage 4 to 6- continuous with no Stage 5
• Dome-Moat Complexes; Epithermal Gold
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Valles Caldera, New Mexico
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Valles Caldera, New Mexico
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Caldera
Margin
Lava
Domes
Air Photo,
Valles
Caldera, New
Mexico
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Small Lava Dome, Valles Caldera, New Mexico
Lava Domes
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Low- and High
Sulfidation Mineral
Deposits Associated
with Caldera
Complexes
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Stage 5: Resurgent Doming• This is uplift and doming of the caldera floor due to an
influx of new magma into the subvolcanic pluton (magma chamber).
• May not happen
• This will lead to resettling of the caldera floor (uplift of center, down faulting of edges) and thus development of new basins; these fault bounded basins then become traps for sediments and lavas.
• Intrusion of extensive sill/dyke complexes within intra-caldera strata may also occur at this time- ring dikes
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Creede Caldera,
Colorado
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Yellowstone Caldera - Resurgence
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Resurgent
Dome
Lava
Domes
Air Photo,
Valles Caldera,
New Mexico
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Stage 6: Major Ring Fracture Volcanism
• Eruption of lava flows and domes along ring fractures or fissures
that bound the caldera.
• Associated with formation of lots of sedimentary/debris flow material
as the walls are extensively eroded.
• Stage 4 to 6- continuous with no Stage 5
• Dome-Moat Complexes; Epithermal Gold
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Ring-Fracture Large Volume Rhyolite Lava Flows (Yellowstone)
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Stage 7: Terminal Fumerolic and Hot Spring activity
• Centered on ring faults or basin faults
• Across caldera floor but dome-most complexes often
centers
• Subaqueous- iron formations, epithermal vein deposits,
limestone-skarn,
• Subaerial- epithermal vein, native sulfur mercury, etc.
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Yellowstone Thermal Features
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Yellowstone Geothermal Features
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