Science - 06.12.2019

from vegetation fires). Rising lava levels in the
channel between 14:40 and 19:00 HST trig-
gered inflation of the levees and“seeps”of
spiny lava intruded through the levee ( 31 ).

Driving processes
What controls short-term fluctuations (pulses)?

The anticorrelation that we observed between
bulk effusion rate and vent activity during

pulsing behavior (Figs. 2F and 4) can be ex-

plained by variations in outgassing efficiency

of lava at the vent. Stronger fountaining in the

fissure 8 cone was associated with more effi-

cient outgassing of lava at the vent, producing

a denser, lower-volume lava flowing through

the spillway (Fig. 4B). Weaker fountaining

produced less efficient outgassing of lava, re-

sulting in a bulkier, higher-volume, foamy lava

pouring out of the crater into the spillway.

This gas-charged lava then outgassed some-

what amidst the disruption of the spillway

(Figs. 4A and 3D).

We speculate that these fluctuations in out-

gassing and fountaining at the vent are modu-

lated by a“gas piston”process. Gas pistoning

is the cyclic rise and fall of a ponded lava sur-

face, with intense spattering accompanying

the fall phase ( 32 ). The process has been fre-

quently observed at Kīlauea and can be ex-

plained as the periodic growth and collapse

of foam at the top of the lava column or lava

lake ( 33 ). During peak lava levels of the pul-

sing regimes, gas release and seismic tremor

are inhibited at the vent as gas accumulates in

a foamy layer at the top of the lava column,

producing weak dome fountaining at the vent

andlittlebubblebursting.Thefoamy“head”

spills out of the crater into the channel, pro-

ducing a gas-charged, higher-volume flow.

Eventually, the foam at the top of the lava

column breaks down, liberating accumulated

gas and driving a morevigorous fountain at

the vent with extensive bubble bursting and

higher seismic tremor (RSAM). The lava pour-

ing out of the crater is more efficiently out-

gassed and has a lower bulk volume, producing

low levels of lava in the channel. We can ex-

plain the two regimes that fissure 8 under-

went as a transition from periods of steady

outgassing (nonpulsing regime) to oscillatory

gas pistoning (pulsing regime). Similar spora-

dic regimes of gas pistoning were commonly

observed at Kīlauea’s summit lava lake during

2008 to 2018 ( 34 ).

Gas pistoning is only one potential outgassing-

driven model to explain the pulsing. A chal-

lenge to this model is that gas pistoning is

normally observed within a confined pond

( 32 , 33 ), whereas the fissure 8 pond was feed-

ing a substantial outflow. Also, foam buildup

maybedifficulttoreconcilewiththeobserved

low fountaining and surface disruption that

occurred in those phases of the cycles (Fig.

4A). Regardless of the model, the data suggest

that some type of shallow outgassing process

local to the fissure 8 vent modulated the pul-

sing effusion.

Comparable short-term cycles in bulk ef-

fusion rate were observed during the 1984

Mauna Loa eruption and were attributed to

a similar gas-driven process. Study of the 1984

fountains and proximalchannel showed that

during high lava fountaining, the lava was

outgassed more effectively, supplying lower-

volume, denser lava to the channel and lower-

ing the lava level without changing the lava

mass flux ( 17 ).

On the basis of that previous study ( 17 ), we

speculate that the pulses at fissure 8 did not

involveamajorchangeinlavasupplyrate[i.e.,

bubble-free“dense rock equivalent”(DRE) ef-

fusion rate]. We can investigate this idea by

Patricket al.,Science 366 , eaay9070 (2019) 6 December 2019 5of10

Vent: dome fountains,

weak bubble bursting

Vent: agitated surface,

stronger bubble bursting

Channel: low velocity,

extensive

crust coverage

Channel: high velocity,

little crust coverage

Weak gas

plume

from vent

Strong gas plume

from channel

Strong gas

plume

from vent

Weak gas plume

from channel

56

Time, minutes

6

8

10

12

Mean velocity, m/s

10 15 20 25 30 35

Cross-channel distance, m

0

5

10

15

Velocity, m/s

(^0132405)
A
A’
A
A’
A A’
peak
trough
peak
B Trough of pulse
A Peak of pulse
peak
trough
C D
Fig. 4. UAS images of pulsing in the lava channel on 14 July 2018.(A) During the peak of pulses, the vent
activity was subdued (low dome fountains and weak gas plume), but the upper channel (spillway) flow was faster
and more vigorous and emitted a stronger plume. The vent pond dimensions were ~60 × 100 m. (B) During the
trough of the pulses, the vent activity was heightenedwith vigorous bubble bursting and stronger gas plume,
whereas the lava flowing in the channel was slower and more crusted. (C) One cycle of pulsing velocity from the
UAS video. (D) Across-channel velocity profiles showing the change during a pulsing cycle.
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