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

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.

Stratovolcanoes: a traditional view

Periods of growth punctuated by erosion

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.

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

Climate context

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δ18 O

40Ar/39Ar age (ka)

colder

warmer

Majority ofRuapehu’shistory coincideswith cold, glacialperiods

After Lisiecki & Raymo, 2005, Paleoceanography

2 1347 6 5

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

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

Volcano-ice interactions

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)

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

Post-glacial volcanism

Summit area cones: past and present

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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