The famously active Hawaiian volcano typically erupts at
existing fissures and triggers minor earthquakes. This latest event has no such
limitations.
In
March and April, geologists at the US Geological Survey Hawaiian Volcano
Observatory took notice of ground inflation at both the summit and the main
vent of Kilauea. Inflation is a sign of pressure building beneath the surface,
often because of an intrusion of magma that culminates in an eruption. Such
readings are not a surprise: Kilauea, which towers 1.2 kilometers above sea
level on the southeastern side of the Big Island, has been erupting nearly
continuously since 1983, with most lava flows originating from the summit
caldera, Halemaʻumaʻu, and Puʻu ʻŌʻō, a vent in the volcano’s East Rift Zone.
But
this time, the magma took a far more destructive path. On 3 May it broke the
surface through multiple fissures that opened along the volcano’s eastern
flank, toward the tip of the Big Island. New vents opened 40 kilometers away
from the crater, in the middle of a residential area. Lava fountains spewed 70
meters into the air and destroyed more than 30 homes over several days. Sulfur
dioxide gas plumes billowed to dangerous concentrations. Nearly 2000 residents
were impacted.
So far
almost everything about the eruption has caught geophysicists by surprise. They
don’t yet understand how and why Kilauea started to erupt in a new place, and
they are struggling to predict just how the eruption will evolve. In the long
term, they’d like to know whether the current event will fundamentally change
where magma is stored and how future eruptions proceed. “We need to learn as
much as possible about the nuances of what happened and why,” says geophysicist
Michael Poland of the US Geological Survey. “This is a terrible tragedy that is
impacting people’s lives.”
The
first major sign that this was not going to be a typical Kilauea eruption
occurred on 30 April, when the crater floor of Puʻu ʻŌʻō began to collapse. The
change indicated to scientists that lava was draining away from Puʻu ʻŌʻō and
moving elsewhere. In the past few decades, new outlets for eruption always
opened within a few kilometers of the main vent, and in each instance the magma
eventually returned to Puʻu ʻŌʻō. This time, magma moved down the East Rift
Zone, with fissures opening much farther afield than before.
Recent
seismic activity supports the hypothesis that magma is traveling tens of
kilometers underground and breaking rock in the process. Magma intrusions tend
to push on the mountain’s heavily faulted south flank, often causing
earthquakes of magnitude 4 or 5 during or just after the intrusion. On 4 May,
the day after the eruption began, the Big Island experienced a magnitude 6.9
earthquake.
Ground
and lava monitoring at the volcano’s summit indicates that magma is migrating
from there to the new eruption site, a radical rerouting that may prove
long-lived. On 2 May the summit deflated as pressure in the subsurface dropped.
By 6 May the surface of the lava lake at the summit eruptive vent, which has
been active since 2008, had dropped 220 meters below the crater rim, a nearly
unprecedented plunge. “What’s remarkable is how fast the established eruptive
plumbing system was reorganized to direct magma away from the sites of
semi-steady eruption,” says geologist George Bergantz of the University of
Washington. US Geological Survey officials warned on 10 May that if the lava
lake continues to drain, the evacuated but still hot conduit at the summit
could mix with cool groundwater and generate high-pressure steam. The result
could be a violently explosive eruption capable of launching massive boulders
hundreds of meters from the vent.
To
find out whether a new lava field is likely to form and permanently displace
people from their homes, geologists are monitoring seismicity, steam and gas
plumes, and ground deformation. They want to find out how signals like rising
magma levels correspond to earthquake activity and how fast those levels could
change. Poland says that analyzing and modeling the geophysical data currently
being collected will “keep people busy for years.”
No comments:
Post a Comment