Bubble development in explosive silicic eruptions: insights from pyroclast vesicularity textures from Raoul volcano (Kermadec arc)
Bubble development in explosive silicic eruptions: insights from pyroclast vesicularity textures from Raoul volcano (Kermadec arc)
Critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation. Here, we present data on densities, and vesicle size and number characteristics, for representative pyroclasts from six silicic eruptions of contrasting size and style from Raoul volcano (Kermadec arc). We use these data to evaluate histories of bubble nucleation, coalescence, and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material. Density/vesicularity distributions show a scarcity of pyroclasts with ?65–75 % vesicularity; however, pyroclasts closest to this vesicularity range have the highest bubble number density (BND) values regardless of eruptive intensity or style. Clasts with vesicularities greater than this 65–75 % “pivotal” vesicularity range have decreasing BNDs with increasing vesicularities, interpreted to reflect continuing bubble growth and coalescence. Clasts with vesicularities less than the pivotal range have BNDs that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. Bubble size distributions (BSDs) and BNDs show variations consistent with 65–75 % representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation, consistent with the closest packing of spheres. We consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit. These processes can act in concert with multiple nucleation events, generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed. Fragmentation preserves the range in vesicularity seen as well as any post-fragmentation/pre-quenching expansion which may have occurred. We demonstrate that differing density pyroclasts from a single eruption interval can have widely varying BND values corresponding to the degree of bubble maturation that has occurred. The modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation.
Bubble size distribution, Vesiculation, Explosive volcanism, Fragmentation, Pumice
826
Rotella, Melissa D.
be18845a-aaf1-4f05-ae92-55447b6774e8
Wilson, Colin J.N.
5953011a-09ee-47de-a41d-b1e81eaf1037
Barker, Simon J.
27644ab9-faf3-4043-ab4a-9220f23196f7
Cashman, Katherine V.
62c3c865-291a-482f-9231-7f098dbf94f7
Houghton, Bruce F.
32fca8af-e616-490e-af5f-99353567ba51
Wright, I.
be2a8931-3932-4f1e-b387-43e3652bf3fc
July 2014
Rotella, Melissa D.
be18845a-aaf1-4f05-ae92-55447b6774e8
Wilson, Colin J.N.
5953011a-09ee-47de-a41d-b1e81eaf1037
Barker, Simon J.
27644ab9-faf3-4043-ab4a-9220f23196f7
Cashman, Katherine V.
62c3c865-291a-482f-9231-7f098dbf94f7
Houghton, Bruce F.
32fca8af-e616-490e-af5f-99353567ba51
Wright, I.
be2a8931-3932-4f1e-b387-43e3652bf3fc
Rotella, Melissa D., Wilson, Colin J.N., Barker, Simon J., Cashman, Katherine V., Houghton, Bruce F. and Wright, I.
(2014)
Bubble development in explosive silicic eruptions: insights from pyroclast vesicularity textures from Raoul volcano (Kermadec arc).
Bulletin of Volcanology, 76 (7), .
(doi:10.1007/s00445-014-0826-6).
Abstract
Critical to understanding explosive eruptions is establishing how accurately representative pyroclasts are of processes during magma vesiculation and fragmentation. Here, we present data on densities, and vesicle size and number characteristics, for representative pyroclasts from six silicic eruptions of contrasting size and style from Raoul volcano (Kermadec arc). We use these data to evaluate histories of bubble nucleation, coalescence, and growth in explosive eruptions and to provide comparisons with pumiceous dome carapace material. Density/vesicularity distributions show a scarcity of pyroclasts with ?65–75 % vesicularity; however, pyroclasts closest to this vesicularity range have the highest bubble number density (BND) values regardless of eruptive intensity or style. Clasts with vesicularities greater than this 65–75 % “pivotal” vesicularity range have decreasing BNDs with increasing vesicularities, interpreted to reflect continuing bubble growth and coalescence. Clasts with vesicularities less than the pivotal range have BNDs that decrease with decreasing vesicularity and preserve textures indicative of processes such as stalling and open system degassing prior to vesiculation in a microlite-rich magma, or vesiculation during slow ascent of degassing magma. Bubble size distributions (BSDs) and BNDs show variations consistent with 65–75 % representing the vesicularity at which vesiculating magma is most likely to undergo fragmentation, consistent with the closest packing of spheres. We consider that the observed vesicularity range may reflect the development of permeability in the magma through shearing as it flows through the conduit. These processes can act in concert with multiple nucleation events, generating a situation of heterogeneous bubble populations that permit some regions of the magma to expand and bubbles to coalesce with other regions in which permeable networks are formed. Fragmentation preserves the range in vesicularity seen as well as any post-fragmentation/pre-quenching expansion which may have occurred. We demonstrate that differing density pyroclasts from a single eruption interval can have widely varying BND values corresponding to the degree of bubble maturation that has occurred. The modal density clasts (the usual targets for vesicularity studies) have likely undergone some degree of bubble maturation and are therefore may not be representative of the magma at the onset of fragmentation.
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More information
Accepted/In Press date: March 2014
Published date: July 2014
Keywords:
Bubble size distribution, Vesiculation, Explosive volcanism, Fragmentation, Pumice
Organisations:
Marine Geoscience
Identifiers
Local EPrints ID: 363136
URI: http://eprints.soton.ac.uk/id/eprint/363136
ISSN: 0258-8900
PURE UUID: 119849bb-21b5-4a8c-b286-c8c6b904f308
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Date deposited: 14 Mar 2014 15:44
Last modified: 14 Mar 2024 16:19
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Contributors
Author:
Melissa D. Rotella
Author:
Colin J.N. Wilson
Author:
Simon J. Barker
Author:
Katherine V. Cashman
Author:
Bruce F. Houghton
Author:
I. Wright
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