Science - 6 December 2019

the volcano. If the conditions are right when

the magma body under a caldera fails, an

unusually large amount of magma may flow

into rifts (see the figure). One example of this

is when tectonic stretching over a long time

has built up sufficient stress. At Kı ̄lauea, ini-

tial magma injection into the rift may have

caused stress to increase, triggering a large

earthquake, on 4 May 2018, on a

subhorizontal basal fault under

Kı ̄lauea’s south flank ( 1 ) that in

turn caused additional stretch-

ing across the rift zone.

The amount of magma trans-

ported into a rift zone can in-

duce slip on caldera ring faults

if it is large enough. Anderson

et al. reveal, in detail, the de-

formation of Kı ̄lauea before

the onset of slip on the caldera

faults and conclude that calde-

ras may begin to collapse after

removal of only a small fraction

of stored magma.

Forming new, active parts of

the volcano plumbing system

occurs by way of a laterally ad-

vancing dike in a rift zone. The

dikes may hit crystal-rich layers

called magma mush or pockets

of magma residing either under

the central part of the volcano

or in the rift zone. These pock-

ets of magma become part of

the plumbing system as magma

continues to advance. For

Kı ̄lauea, Gansecki et al. inferred

that the rift zone had at least

two separately stored magmas

that the advancing dike inter-

sected, which erupted first dur-

ing the event. At Bárðarbunga,

one magma type erupted, but

crystals from crystal mush were

incorporated into the magma

( 11 , 12 ). Both observations sug-

gest an important role for these

magma pockets in the chemistry

of the dike magma and the tim-

ing for when different magma

compositions are erupted.

After the plumbing system is

stabilized as the rift eruption

and central caldera collapse is

ongoing, pressure governs the

system evolution to a large ex-

tent. The central block within

a caldera drives out a large vol-

ume of magma as it subsides.

At Bárðarbunga, the mass flow

rate and caldera collapse were

found to decline almost expo-

nentially throughout the erup-

tion ( 2 , 13 ). Modeling indicated

that the effective cross-sectional area of the

flow path during the eruption ( 2 ) was only

a small fraction of the cross-sectional area

of the previously established dike ( 6 ). At

Kı ̄lauea, magma surges at the eruption site

were linked to steps in the down-sagging of

the caldera. At Bárðarbunga, such steps also

occurred and influenced earthquake activity

SCIENCE sciencemag.org

GRAPHIC: N. DESAI/

SCIENCE

Ring fault failure

Rifting

Recharge

Regulated collapse and rift eruption

Magma

domain

Magma

mush

Large stress change owing

to magma withdrawal

Stress reaches a critical level;

caldera ring faults begin to slip

Magma fow in a laterally

propagating dike

Magma bodies

Rift zone eruption

Isolated

magma bodies

Main recharge Secondary

recharge

Caldera

Caldera

collapses

Rift zone (faults

and fssures)

Inactive part of dike

in the dike. These effects are similar to the

water-hammer effects in fluid-filled pipes,

resulting from sudden changes in pressure

and hydraulic transients that travel in pipes

( 14 ). The observations show that variable

pressure under a collapsing central block

translates directly into eruption activity.

The recent observations of collapses have

serious implications for haz-

ards and volcano monitoring.

Eruptions in rifts require moni-

toring of conditions at calderas

far away from eruption activity

that may provide crucial infor-

mation on eruptions. This is-

sue is more relevant at basaltic

calderas than at other volcanoes

because basalt is less viscous

and flows more easily over long

distances than other magma

types. Although the next caldera

formation may not have a large

flank eruption, the recent de-

tailed monitoring demonstrates

that magma under collapsing

calderas is under pressure, and

copious amounts of magma can

be driven toward far-away ef-

fusive eruptions. This sort of

long-distance volcanic plumb-

ing system appears to be a far

more common occurrence than

previously believed. j

REFERENCES AND NOTES

1. C. A. Neal et al., Science 363 , 367
   (2019).


2. M. T. Gudmundsson et al., Science 353 ,
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3. K. R. Andersen et al., Science 366 ,
   eaaz1822 (2019).


4. C. Gansecki et al., Science 366 ,
   eaaz0147 (2019).


5. M. R. Patrick et al., Science 366 ,
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6. F. Sigmundsson et al., Nature 517 , 191
   (2015).


7. G. Pedersen et al., J. Volcanol.
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8. T. Ágústsdóttir et al., J. Geophys. Res.
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9. J. Wo o d s et al., Earth Planet. Sci. Lett.
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10. M. M. Parks et al., Earth Planet. Sci.
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11. M. E. Hartley et al., Contrib. Mineral.
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12. S. A. Halldórsson et al., Contrib.
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13. D. Coppola et al., Geology 45 , 523
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14. B. S. Jung, B. Karney, Urban Water J. 14 ,
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15. F. Sigmundsson, Geophys. Res. Lett.
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ACKNOWLEDGMENTS

Support comes from the Horizon 2020

project EUROVOLC, funded by the European

Commission (grant 731070).

10.1126/science.aaz7126

6 DECEMBER 2019 • VOL 366 ISSUE 6470 1201

Caldera collapse coupled to rift zone eruption

Progressive steps in caldera formation in relation to lateral transfer of magma into

rifts (generalized cross-section along a volcano rift). Recharge into the magma

domain ( 15 )—a complex of liquid magma bodies and crystal mush—occurs before

an eruption. Rifting is followed by ring-fault failure and finally caldera collapse at

the center of the volcano, regulated by a major eruption in the rift.

Published by AAAS

on December 12, 2019^

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