Calderas. Myojin Knoll Submarine Caldera m. 500 m. 5 km. (after Kennedy and Stix, 2003)

Transcription

1 Calderas Myojin Knoll Submarine Caldera 1400 m 500 m 5 km (after Kennedy and Stix, 2003)

2 Definition Outline Relationships to Eruption Volume and VEI Structural Components Types Caldera Genetic Models and the Caldera Cycle

3 Types of Volcanoes

4 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 nearsurface magma chamber (Lipman, 2000).

5 Locations of Famous Caldera Complexes Crater Lake Yellowstone Vandever Mtn. Noranda Wawa Bald Mtn Sturgeon Lake Santorini Krakatau Tambora Myojin Knoll Pinatubo Kuwai Taupo

6 Calderas and Cauldrons Two separate features Calderas are formed by catastrophic collapse associated with large volume (>5 km 3 ) 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

7 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

8 Caldera-Associated Eruption Volumes

9 Toba Caldera, Indonesia Eruption occurred ~75,000 years ago Eruption volume estimates range from 2500km 3 to 2800km 3 Caldera measures 40km x 105 km Geneticists estimate possibly as few as 5,000 humans survived the eruption

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11 Caldera Eruptions and the Volcanic Explosivity Index

12 Structural Elements of Calderas (after Lipman, 1997, 2000)

13 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)

14 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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16 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

17 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)

18 Caldera Subsidence

19 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

20 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

21 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.

22 Columnar Jointed Welded Tuffs, Valles Caldera, NM Welded Tuff Deposits

23 Welded Tuffs, Valles Caldera

24 Thick Non-welded Tuffs, Valles Caldera, NM

25 Rhyolite Tuffs, Golden Gate

26 Rhyolite Tuff Ash Pyroclasts

27 Partially Welded Rhyolite Tuffs

28 Welded Rhyolite Tuff Fiamme Fiamme are flattened pumice

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30 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

31 Stage 3 Simultaneous Eruption and Caldera Collapse Note interlayering of ignimbrite and calderacollapse 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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34 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

35 Valles Caldera, New Mexico

36 Valles Caldera, New Mexico

37 Lava Domes Air Photo, Valles Caldera, New Mexico Caldera Margin

38 Small Lava Dome, Valles Caldera, New Mexico Lava Domes

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42 Low- and High Sulfidation Mineral Deposits Associated with Caldera Complexes

43 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 intracaldera strata may also occur at this time- ring dikes

44 Creede Caldera, Colorado

45 Yellowstone Caldera - Resurgence

46 Lava Domes Air Photo, Valles Caldera, New Mexico Resurgent Dome

47 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

48 Ring-Fracture Large Volume Rhyolite Lava Flows (Yellowstone)

49 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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51 Yellowstone Thermal Features

52 Yellowstone Geothermal Features

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