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Calderas

5 km

1400 m

500 m

Myojin Knoll

Submarine

Caldera

(after Kennedy and Stix, 2003)

Outline• Definition

• Relationships to Eruption Volume

and VEI

• Structural Components

• Types

• Caldera Genetic Models and the

Caldera Cycle

Types of Volcanoes

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).

Sturgeon

LakeVandever

Mtn.

Santorini

Taupo

Kuwai

Krakatau

Pinatubo

Yellowstone

Tambora

Myojin

Knoll

Crater Lake

Bald Mtn

Locations of Famous Caldera Complexes

Noranda

Wawa

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

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

Caldera-Associated Eruption Volumes

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

Caldera Eruptions

and the Volcanic

Explosivity Index

Structural Elements of Calderas

(after Lipman, 1997, 2000)

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)

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

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

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)

Caldera Subsidence

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

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

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.

Columnar Jointed Welded Tuffs, Valles Caldera, NM

Welded Tuff

Deposits

Welded Tuffs, Valles Caldera

Thick Non-welded Tuffs, Valles Caldera, NM

Rhyolite Tuffs, Golden Gate

Rhyolite Tuff

Ash

Pyroclasts

Partially Welded Rhyolite Tuffs

Welded Rhyolite Tuff

Fiamme

Fiamme are

flattened pumice

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

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)

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

Valles Caldera, New Mexico

Valles Caldera, New Mexico

Caldera

Margin

Lava

Domes

Air Photo,

Valles

Caldera, New

Mexico

Small Lava Dome, Valles Caldera, New Mexico

Lava Domes

Low- and High

Sulfidation Mineral

Deposits Associated

with Caldera

Complexes

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

Creede Caldera,

Colorado

Yellowstone Caldera - Resurgence

Resurgent

Dome

Lava

Domes

Air Photo,

Valles Caldera,

New Mexico

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

Ring-Fracture Large Volume Rhyolite Lava Flows (Yellowstone)

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.

Yellowstone Thermal Features

Yellowstone Geothermal Features