A detailed geological map and eruptive history for Tongariro …€¦ · A detailed geological map...
Transcript of A detailed geological map and eruptive history for Tongariro …€¦ · A detailed geological map...
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A detailed geological map and eruptive history for Tongariro National Park, New Zealand: Integrating mapping,
geochronology, geochemistry, palaeomagnetism and glaciology
Graham Leonard, Dougal Townsend, Chris Conway, Shaun Eaves, John Gamble, Colin Wilson, Andrew Calvert
Elizabeth Ingham, Gillian Turner
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A brief history of research on Ruapehu Volcano since 1980.The general aim has been to understand volcano architecture and to put it into chronological and petrogenetic context.
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Stratovolcanoes: a traditional view
Periods of growth punctuated by erosion
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Stratovolcanoes: from 2011 a new approach
Recognising the interaction between lava flows and glaciers.• New 40Ar/39Ar analyses and cosmogenic 3He dating.• Magma – ice interactions, their extent and morphology.• Palaeomagnetic observations.
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Geochronology results
020406080100120140160180200220240
40Ar/39Ar age (ka)
Gamble et al., 2003
This work
Summary
Oldest lavas on
Ruapehu edifice are~200 ka.
30 kyr break
between 80kaand ~ 50ka
More or less continuouslava eruptionsfor last 50 kyr
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Climate context
020406080100120140160180200220240
δ18 O
40Ar/39Ar age (ka)
colder
warmer
Majority ofRuapehu’shistory coincideswith cold, glacialperiods
After Lisiecki & Raymo, 2005, Paleoceanography
2 1347 6 5
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What happens to a lava flow if there is ice in the valley?
cool crack repeat
Lescinsky and Sisson, 1998, Geology
Volcano-ice interactions
Polygonal cooling joints on a steep, ice‐bounded flow
End up with:• Perched lava flows• Thickened flow edges• Polygonal cooling joints
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Volcano-ice interactions
Lava-ice interactionGlacier in valley, lava deflected toward ridges
Lava thickest on valley sides
Margins of lava have fractures that indicate cooling against ice
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Volcano-ice interactions
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Ice-contact lava at catchment margin15.1 ± 2.4 ka (40Ar/39Ar)
Moraine ridge13 ± 0.9 ka (3 x 3He exposure ages) on cpx
Ice-bounded lava in glaciated valley20.9 ± 2.8 ka (40Ar/39Ar)
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Post-glacial volcanism
Cone formation and collapse
8 – 4 ka40Ar/39Ar
on lava flows
10.5 kaEaves et al 20153He cosmogenicMurimotu Debris
Avalanche
15–10 ka 18–12 ka
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Post-glacial volcanism
Summit area cones: past and present
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Conclusions
• Lava-ice interactions at TNP volcanoes have had a major influence on the morphology of the volcanoes (see Conway et al, 2015, Bull Volc for Ruapehu).
• Ruapehu has erupted more or less continuously for the last 50,000 years.
• We see very encouraging comparisons between 40Ar/39Ar and cosmogenic isotope dating (see Eaves et al 2015, JQS).
• Our high precision 40Ar/39Ar dating forms a framework for study of the Laschamp excursion ~ 41ka (see Ingham et al, in prep).
• Several summit area cones have collapsed since glaciers retreated ~10,000 years ago.
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