“Subaqueous Pyroclastic Flows”

Syneruptive Products of Pyroclastic Volcanism in Subaqueous Environments

Types of Subaqueous Pyroclastic Flows • Subaerial Pyroclastic Flows that Flow into Water

Group I Deposits

Group II Deposits

– Hot submarine flows – Hot subaerial flows travelling across water

• Subaqueous Pyroclastic Flows Resulting from Subaqueous Explosive Volcanism – Hot high concentration mass flows (Group I) – Hot to cold, low concentration eruption-fed mass flows (Group II) – Hot to cold, high- to lowconcentration lava flow-fed mass flows (Group III)

Group I Deposits Group II Deposits

Group I Deposits

Group II Deposits

Pyroclastic Flows Entering Water • Numerous historical and modern examples of these types of eruptions – Krakatau (1883) – Montserrat (1995 to present)

• Three possible ways of interacting – Mix with water – explosions at coast line and then transfer into eruption-fed turbidity current (Montserrat) – Travel across water (Montserrat, Krakatau) – Remain intact and either push aside shallow water near shore or flow under water until stop or change into eruptionfed mass flow (Krakatau?)

• Deposits range from massive tuffs and lapilli tuffs that locally retain heatretention features (Krakatau) to finesdepleted tuffs that grade laterally into finely bedded tuff (Montserrat)

Flows Water Collapse Pyroclastic of a subaerial lava dome orEntering eruption column with flow of material from land into water.

Krakatau, 1883 • Krakatau has had numerous plinean eruptive events that have led to formation of numerous calderas from pre-existing stratovolcanoes • Most famous is the series of eruptions that culminating in the August 26-27 1883 catastrophic event • The 1883 eruption ejected anywhere from 6-10 cubic kilometers (dry rock equivalent) of tephra • May have produced the loudest sound historically reported – seismic and atmospheric waves circled the earth for several days following the eruption • 165 local villages completely destroyed, 132 seriously damaged with loss of life (officially) at 36,417 • Large loss of live due to combination of pyroclastic flows traveling at large speeds across water, as well as subsequent tsunamis associated with submarine caldera collapse (and pyroclastic flows?)

Krakatau 1883

• Plinean Phase of August 26-27, 1883 – Aug 26, ~1:00pm: Huge explosions began and the eruption column soared to 27 km (~17 miles); major eruptions heard every 10 minutes or so; small tsunami hits Java and Sumatra at 6-7pm (40 km away) – August 27: 4 enormous eruptions at 5:30, 6:42, 8:20 and 10:02 am; each associated with tsunamis over 30 meters (~100 feet) high; sound of last eruption was 180 dB at 100 miles away – Eruption column reached 80km (50 miles) high; Pyroclastic flows over ocean (floating pumice rafts, superheated steam?) reached Sumatra; Ash and pumice fell up to 20m thick within a 20km radius – In 1884, global climate cooling of ~1.2°C

Krakatau 1883 (eruption equivalent to 13,000 Little Boys (Hiroshima, 1945))

Krakatau 1883 • • • • • •

Pyroclastic flows and surges traveled over water: No survivors on Sebisi, 13km away; over 1000 burn fatalities 40 km away at Ketimbang on the north coast of Sumatra Ships 65-80 km away engulfed by ash clouds and some sank due to weight of ash on decks Island vegetation burned Deposits-poorly sorted, massive beds of pumice, charcoal, lithics in an ash matrix Large regions comprising concentrations of large pumice rafts with decomposed human remains found floating in south Pacific months after the eruption

Krakatau 1883 • Submarine units locally show evidence of being hot, suggesting hot flows under water (Mandeville et al., 1996) • Massive, poorly sorted, alignment of magnetic minerals locally • Mix of pumice, shards in a gray ash matrix with some crystals and lithics • Uppermost deposits, and more distal deposits, are laminated, better sorted, finer grained

Types of Submarine Pyroclastic Flows • Perhaps the Krakatau submarine pyroclastic flows (e.g. those with evidence of heat retention) never saw external water except along their margins

What if sea level Were here?

Group I Deposits

Montserrat (1995 – present)

http://www.youtube.com/watch?v=7h5XOS7uaWA

Montserrat (1995 – present)

(Hart et al., 2002)

Montserrat (1995-1998)

0.007 km3 0.048 km3 0.055 km3 0.003 km3 0.015 km3 0.018 km3 0.073 km3

Montserrat (1995-present)

Montserrat (1995-present)

(Trofimovs et al., 2006)

Montserrat (Trofimovs et al., 2006)

Types of Subaqueous Pyroclastic Flows • Recent work (e.g. White, 2000) indicates that subaqueous eruption-fed density currents may be classified into three types based on modes of fragmentation and transport mechanisms • Group I deposits result from explosive fragmentation, with deposition from a hot, gas-supported current • Group II deposits result from explosive fragmentation with deposition from a water-supported current – including submarine fire-fountain deposits • Group III deposits result from fragmentation of flowing lava, with deposition from a water-supported current

Types of Submarine Pyroclastic Flows • Group I deposits result from explosive fragmentation, with deposition from a hot, gas-supported current

Group I Deposits

Group I Deposits

Characteristics of Group I Deposits (from White, 2000)

• Evidence from enclosing beds (bounding facies) of continuous subaqueous deposition • Individual Group I deposits are characterized by poor sorting, massive nature, and may show evidence of heat retention (welding, gas segregation pipes, spherulites) • Evidence in young, non-recrystallized deposits may include aligned thermoremanent magnetic orientations combined with poor sorting and massive nature • Associated features include overlying co-genetic tuffs and lapilli tuffs showing evidence for deposition from turbidity currents and subsequent suspension and soft-sediment deformation of underlying sediments

Examples of Group I Deposits (Vandever Mountain Tuff (Kokelaar and Busby, 1992))

Group I Deposits

Examples of Group I Deposits (Sturgeon Lake Caldera Complex: Hudak et al., 2003)

Examples of Group I Deposits (Sturgeon Lake Caldera Complex: Hudak et al., 2003)

Types of Submarine Pyroclastic Flows

• Group II deposits result from explosive fragmentation with deposition from a water-supported current

Group II Deposits

Group II Deposits

Group II Deposits

Characteristics of Group II Deposits (from White, 2000)

Characteristics of Group II Deposits • Deposits may show the entire range of sedimentary structures exhibited by grainflow deposits, debris flow deposits, high- and low-concentration turbidites; therefore, distinguishing eruption-fed sequences from immediately post-eruptive redeposition may not always be possible • Key features of eruption-fed deposits will include their componentry (which will be entirely derived from the eruption itself), an absence or very minimal amount of grain abrasion, localized larger clasts (large lapilli to block/bomb size) which exhibit heat retention structures (polygonal jointing, marginal hydrothermal alteration) • Consistent gradation in bedding style through a sequence of ash turbidites consisting of unabraded juvenile grains may reflect the evolution of an eruption • Where original morphology can be reconstructed, subaqueous edifices will be constructed largely of eruption-fed deposits, which are the primary deposits of the subaqueous realm

Examples of Group II Deposits

Group II Subaqueous Pyroclastic Deposits, Misema Caldera Complex, Neoarchean Blake River Megacaldera Complex

(from Mueller et al., 2007)

Examples of Group II Deposits (Sturgeon Lake Caldera Complex: Hudak et al., 2003)

Examples of Group II Deposits Neoarchean subaqueous fire-fountain Eruption model for the Hunter Mine Group, Abitibi Greenstone Belt (Mueller and White, 1992)

Types of Submarine Pyroclastic Flows Group III Deposits (from Schneider, 2000)

Types of Submarine Pyroclastic Flows – Group III Deposits • Group III deposits result from fragmentation of flowing lava, with deposition from a water-supported currents • This group includes turbidity currents and related non-turbulent flows such as grainflows and grainfalls that entrain and distribute particles formed and shed along the margins of advancing submarine lava flows • Fragments formed during these processes result from thermal shock granulation, dynamo-thermal spalling, and above the critical depth for seawater (4200m), localized and variably suppressed steam explosions

Characteristics of Group III Deposits (from White, 2000)

• Deposits will be clearly associated with simultaneously deposited lava flows • Deposits may include a large range of broadly autoclastic fragments including glassy curved splinter shards and blocky shards, and pillow fragments which may be segregated into coarse debris-fall deposits and finer-grained turbidites and lava marginal fall deposits • Fine ash is not an important component because few fine-ash sized fragments are generally formed by autoclasis

Examples of Group III Deposits

Seamount Six, eastern Pacific Ocean (from Maicher et al., 2000)

Examples of Group III Deposits Syn-eruptive Submarine Lava Dome Collapse Breccia (“Block and Ash Flow” Deposits) Sturgeon Lake Caldera Complex (after Hudak et al., 2003)

Submarine Mass Flows – Resedimented Pyroclastic Rocks • Syn-eruptive volcaniclastic deposits that are reworked by exclusively sedimentary processes after deposition • Reworking occurs mainly by sediment gravity flows as well as by bottom currents • Two end members exist: – Non-eruptive volcaniclastic gravity flows that occur shortly after an eruption – Submarine epiclastic volcaniclastic sedimentation

(from Schneider, 2000)

Submarine Mass Flows – Resedimented Pyroclastic Rocks • Numerous mechanisms and processes are associated with submarine mass flows • The characteristics of the deposits are directly related to a) the physical characteristics of the materials being transported (e.g. size, density); b) the mode of transportation of the materials; and c) the depositional process of the materials in the submarine environment.

(from McPhie et al., 1993)

Submarine Mass Flows – Summary of Principle Transport and Depositional Processes

(from McPhie et al., 1993)

Submarine Mass Flows – Non-Eruptive Volcaniclastic Gravity Flows • Most of these volcaniclastic gravity flows are turbiditic • It is commonly difficult to make a distinction between turbiditic deposits resulting from eruption fed mass flows and those resulting from remobilization of syneruptive deposits • The best indicator of remobilization is commonly the presence of epiclastic, nonjuvenile fragments (from Schneider, 2000)

(from McPhie et al., 1993)

Examples of Neoarchean non-eruptive submarine Mass flow deposits from Timmins (Abitibi Greenstone Belt) and Sturgeon Lake (Wabigoon Greenstone Belt)

Submarine Mass Flows – Submarine Epiclastic Volcaniclastic Sedimentation • Most of these volcaniclastic gravity flows are turbiditic • These deposits represent mass flows of already lithofied volcanic and/or volcaniclastic rocks, and are therefore epiclastic in origin

(from Schneider, 2000)

Submarine Mass Flows – Submarine Epiclastic Volcaniclastic Sedimentation • Mesobreccias and Megabreccias may form synchronous with voluminous pyroclastic eruptions and caldera collapse…these represent epiclastic sedimentation associated with caldera development Mesobreccia

Megabreccia

Mesobreccia

Megabreccia