bob@uhmanoa.UUCP (Bob Cunningham) (07/20/86)
[for volcano-watchers] The character of the Kilaua eruption changed yesterday, as two small fissures opened up and started producing pahoehoe lava. The Kilauea volcano on Hawaii (the `big island' in the Hawaiian chain) has been in more-or-less continuious eruptin since 3 January 1983, producing fountaining---primarily at the Pu'u O'o vent---of up to 1,000 feet high about once a month. Since the last outbreak on 26 June, the volcano has been swelling in preparation for further activity. The current outbreak (known as `phase 48 of the 3 Jan 1983 eruption') started at 1205 yesterday when a large crack opened up about 0.6 mile to the west of the Pu'u O'o cone, following a swarm of earthquakes starting about 1115. Then, at 1230, a part of the top rim of Pu'u O'o collapsed inward. By mid-afternoon a second crack had opened about a mile northeast of Pu'u O'o. From both fissures the lava was fountaining only about 10 to 15 feet high, but large amounts of smooth, fluid-like pahoehoe flows were produced that flowed off in several directions, typically several hundred feed wide, flowing for about 2 miles. By 7:00pm, the volcan's summit had lost 80% of the swelling it had gained since June, and the level of harmonic tremor---the measure of lava sliding down the rift zone to the erupting fissures---was beginning to ebb. Observation of this latest phase was hampered by federal budget cuts; the Hawaiian Volcano Observatory can no longer air-lift geologists to the site of the eruption via helicopter, they have to hike in (the eruption series has been occuring in a fairly remote area of the big island, more than 12 miles from the nearest road). -- Bob Cunningham cunninghamr%haw.sdscnet@LLL-MFE.ARPA ihnp4!islenet!uhmanoa!bob
eugene@ames.UUCP (Eugene Miya) (07/23/86)
> [for volcano-watchers] > > The character of the Kilaua eruption changed yesterday, as two small > fissures opened up and started producing pahoehoe lava. > message about the some real flaming! > Bob Cunningham > cunninghamr%haw.sdscnet@LLL-MFE.ARPA > ihnp4!islenet!uhmanoa!bob Amazing! An article which has some real content, doesn't flame in the human sense, and is very timely. This is what the net should be about. I hope menlo70 got this. This article made the group worthwhile, I won't unsubscribe, but will "n" everybody else. --eugene miya NASA Ames Research Center com'on do you trust Reply commands with all these different mailers? {hplabs,ihnp4,dual,hao,decwrl,tektronix,allegra}!ames!aurora!eugene eugene@ames-aurora.ARPA
bob@islenet.UUCP (Bob Cunningham) (07/30/86)
[items marked (*) are defined at the end of the message, if you aren't familiar
with the jargon of geology.]
A detailed report (much of this courtesy of the Hawaiian Volcano Observatory,
U.S. Geological Survey)...now that this phase of the eruption finally seems
to be winding down.
Episode 48 of the east-rift (*) eruption of Kilauea (*) began on 18 July with a
swarm of local earthquakes beginning at 1046. The column of underground lava
had reached the top of the conduit (*) and was emitting spatter from a 15m-high
cone at the time of the earthquake swarm. Two fissures, one 1100m long on the
uprift side of the Pu'u O'o cone (*), the other 900m long on the downrift side,
broke out at 1205 or earlier. At about that time the lava column subsided, the
top of the Pu'u O'o spatter cone collapses, and only burning gas has issued
from the small opening since.
Mostly pahoehoe (*) flows were produced from the fissures, the longest moving
rapidly 7.5km to the southeast through a forested part of the national park,
reaching to within 2.5km of the coast and burning approximately 1000 acres of
forest in the process (which endangered several rare species of plants).
After about 20 hours of repose, low fountains returned to the downrift fissure,
first observed at 0828 on 20 July. By 0950 this fissure had shut down and a
new 3rd fissure opened up 1000m downrift eventually extending about 1100m to
the northeast along the rift axis and displaced about 150m northwest of the
original January 1983 fissure---the most recent eruption in that vicinity.
This downrift shift in activity was preceded by a flurry of local earthquakes
commencing at 0430 and incresed tremor (*) amplitude downrift of Pu'u O'o from
0710. A broad pond of pahoehoe lava, covering about 1 square km slowly spread
from both sides of the fissure. Harmonic tremor had reached an intermediate
level in amplitude and remained relatively constant until 0800 on 23 July when
the seismic level decreased to nearly background. Eruptive activity then
decreased to residual bubbling of lava along the northeast fissure.
Deflationary tilt (*) at the summit started at about 1145 and reached a
maximum rate of nearly 2.6 microradians per hour between 1200-1300. The
deflationary tilt at the summit reversed temporarily into an inflationary trend
from about 0500 on 19 July, accompaning a temporary repose in eruptive activity
until about 0000 on 21 July when gradual deflatin resumed and continued
throughout 22 July in response to the renewed eruption. Nearly 21 microradians
of deflation were registered before the resumption of inflation on 23 July.
Current seismicity is characteristic of interphase activity. Harmonic
tremor is continuing at a low level at Pu'u O'o. The number of
micro-earthquakes is below average in the summit region and about average in
the east rift zone.
Meanwhile, the number of microearthquakes recorded on Mauna Loa---which is not
in eruption---is about average in the summit region and northeast rift zone.
..........definitions.........
(*) rift. Eruptions of the Hawaiian "shield" (from a distance, they look like
an old-style Roman shield placed on the ground) volcanoes occurs primarily at
the summit "caldera" (a portion of the top of the mountain, several miles in
diameter, which has collapsed, forming what appears to be a huge crater), and
along several "rift zones" which are ridges radiating from the summit featuring
"rifts" where the earth splits apart---producing gas and/or lava.
(*) Kilauea. The most active of the five shield volcanoes which make up the
"big island" of Hawaii (the 2nd largest volcanic island in the world, after
Iceland, the island of Hawaii being about 5x the area of Rhode Island).
The others are: Kohala (hasn't erupted in historical time, possibly extinct),
Mauna Kea (the biggest, with the astronomical observatories
on top, hasn't erupted in historical time, many geologically recent cinder
cones, evidence of glaciation on top), Hualalai (last eruptions in
1800s), Mauna Loa (rift eruptions about once every two years produce very large
amounts of lava, not currently erupting).
Kilauea tends to erupt at its summit caldera (primarily in the Halemaumau fire
pit), at the edge of the caldera (Kilauea Iki crater), along its southeast rift
zone (along the Chain of Craters road), or along its eastern rift zone (Pu'u
O'o and similar cones). Eruptions---in one place or another---occur often,
usually as a series of "phases" at one particular point every month or so.
Most of the eruptions---except for the recent series along the east rift
zone which are in a fairly isolated area---have been fairly easy to get to,
and (providing you exercise some caution and obey directions of the rangers
at the national park) have provided spectacular viewing.
(*) conduit. Basically a tube where lava flows. In this context, the hole
within the Pu'u O'o cone. Kilauea (and the other Hawaiian volcanoes) contain
extensive sets of large conduits underground through which lava flows to reach
the surface. Near the surface, additional small conduits called "lava tubes"
are created within the central portions of thick lava flows at the exterior of
those flows solidify.
(*) Pu'u O'o. Name for a cone on the east rift of Kilauea for which I don't
have a translation at hand. The "'" signifies a glottal stop in the Hawaiian
language.
(*) pahoehoe. One of the two main types of lava flows, pahoehoe is flows
easily and looks "ropy" on top. As a pahoehoe flow cools and slows
down---providing there is sufficient lava in back to push---it can turn into an
"aa" lava flow, which is less viscous, slower, and appears "clunky". The
Hawaiian words "pahoehoe" and "aa" (really "a'a") have been adopted
internationally.
(*) tremor. Kilauea and Mauna Loa are extensively instrumented. In addition
to being able to detect earthquakes of normal magnitude (they typically produce
at least one Richter magnitude 3.0 or greater quake a week), fine "tremors" can
be detected, indicating deep activity. The term "harmonic tremor" has been
coined to describe the seismic activity that appears to indicate deep-lying
lava in motion.
(*) tilt. Another set of instrumentation, sensitive tiltometers, measure the
tilting---and thus the swelling and deflation---of the main mountain shields.
Increasing tilt means the shield is swelling, decreasing tilt means it is
shrinking. The swelling appears to indicate that lava has rose from deep under
to shield to near the surface, in preparation for erupting at the surface.
--
Bob Cunningham {dual|vortex|ihnp4}!islenet!bob
cunninghamr%haw.sdscnet@LLL-MFE.ARPA
Hawaii Institute of Geophysics, University of Hawaii