From: joe@montebello.soest.hawaii.edu (Joe Dellinger)
Subject: HVO report
Reply-To: joe@montebello.soest.hawaii.edu
Organization: School of Ocean and Earth Science Technology
Date: Fri, 11 Sep 1992 02:44:51 GMT

Date  07 Sep 92 20:31:27 UTC (Mon)
From  wh6dt@wh6af.hi.usa.oc [hilo] (GREG)

[from the Volcano Watch column, Hilo Tribune-Herald, 9/6/92, 
p. 10]

LAVA UNLOCKS SUBTERRANEAN MYSTERIES

To the non-geologist, lava is hot, orange molten rock, or when 
cooled, smooth and gray or black and jagged rock.  However, 
to the geologist or volcanologist, lava contains a set of clues to 
decipher processes occurring in the interior of the Earth and 
the volcano.  These processes, once understood, form the basis 
of eruption forecasting, including eruptive style (passive or 
explosive), magnitude, and timing.  Magma (lava before it 
erupts) consists of molten rock, crystals, dissolved gases, and 
bubbles.  Sometimes there are also fragments of the rocks 
through which the magma moved, called xenoliths.  Many 
lavas have had complex histories that include cooling and 
forming crystals, loss of gases, remelting of rock that 
surrounds the magma body inside the volcano, and mixing of 
different magmas.  In order to understand the initial 
formation of the magma deep inside the Earth, one needs to 
be able to "see through" the effects of all these processes.

Magma that moves up into Kilauea Volcano, from perhaps one 
hundred miles deep, arrives at a temperature of about 1,360! 
Centigrade (about 23,480!F [whoa! should read 2,480]).  
However, lavas with eruption temperatures this high have 
been found only along the submarine part of Kilauea's East Rift 
Zone.  The lavas erupted near the summit and along the 
subaerial part of the rift zones commonly have temperatures of 
about 1,160!C (2,120!F) and range from only about 1,115!C 
(2,040!F) to about 1,200!C (2,190!F).  Clearly, these lavas have 
had to cool by several hundred degrees within the volcano 
before they erupt.  Cooling of magma causes crystals to form.  
In Hawaiian lavas, the first crystal to form as magma cools is 
olivine, the mineral that forms green sand beaches.

As magma is stored inside the volcano in a magma reservoir, it 
also loses some of the gases that are dissolved in the melt.  The 
main gas components in magma are carbon dioxide, water, and 
sulfur gases., Most of the carbon dioxide, and some of the water 
and sulfur dioxide, leaks out of the summit of the volcano as 
the magma is stored in the magma reservoir [about 3 miles] 
below.  During this period of storage, the magma may also melt 
the rocks adjacent to the magma reservoir, thereby changing 
the chemistry of the magma.  Sometimes groundwater or 
hydrothermal fluid is added to the magma with dramatic 
results.  Explosive summit eruptions at Kilauea, such as those in 
1790 and 1924, are caused by addition of water to the magma 
stored inside the volcano [and are rare in the geologic record].

There are numerous separate batches of magma stored inside 
the volcano at any time.  Each has slightly different chemical 
characteristics that can be used to trace mixing of the different 
batches.  Most eruptions at Kilauea appear to involve several of 
three magma batches that have mixed with each other.  
Commonly, the magmas that mix have different temperatures 
and contain distinctive assemblages of crystals.  Upon mixing, 
some of these crystals dissolve back into the melt while other 
crystals grow rapidly.  If the magma is erupted soon after the 
magma batches mix, these crystals are quenched before they 
can dissolve.  However, if mixing predates the eruption by 
more than a few days, the evidence for mixing will have been 
erased.

Lava from the on-going eruption, like nearly all lavas erupted 
at Kilauea, has cooled and formed crystals of olivine before 
transport through the rift system to the eruptive vents.  Most 
of these crystals have apparently settled through the magma, 
because of their greater density, and accumulated inside the 
volcano, as they are not present in the erupted lava.  The 
earliest lavas erupted in 1983 also had abundant evidence of 
magma mixing.  The magma erupting today has also lost some 
of its gas content through the summit and much more through 
the Pu'u 'O'o vent prior to eruption.

The nearly 10-year long eruption on Kilauea's East Rift Zone 
continues to start and stop, as it has for the last six months.  
The eruptive interval that began on August 16 shut down early 
in the morning on August 29.  The pause in activity was brief 
as the episode 51 vents became active once again about 1:00 
a.m. on September 2.  The flows from the August 16-August 29 
interval advanced to about 2,080 foot elevation, just west of 
the flow that cascaded over the pali in late July.  Due to poor 
visibility most of this week, we have little information about 
the extent of the flows since activity restarted on September 2.  
The pond inside Pu'u 'O'o rose about 50 feet to a level about 
120 feet below the rim of the cone between September 1 and 
2, as the eruption began.

In the past two weeks, Hawaii has had four earthquakes with 
magnitudes greater than 3.0.  The first of these was located just 
west of Pahala and occurred at 3:19 p.m. on August 18.  It was 
located about 6 miles deep and had a magnitude of 3.1.  The 
next two were located on the south flank of Kilauea Volcano, 
south of Napau Crater.  The first of these occurred at 2:58 p.m. 
on August 24 and the second at 1:43 a.m. on August 28.  Each 
was magnitude 3.0 and located at a shallow depth (less than 4 
miles deep).  The last earthquake, with a magnitude of 3.5, 
occurred on August 30 at 6:57 p.m. and was located beneath 
the Hamakua coast at a depth of about 8 miles.  These small 
felt earthquakes serve as a constant reminder that Hawaii, 
particularly the Big Island, is prone to earthquakes that can be 
much larger and damaging.

Volcano Watch is a weekly feature provided by the scientists at 
Hawaiian Volcano Observatory.

Aloha+73 de Greg wh6dt@wh6af.hi.usa.oc

     /\    /\    /\/\/\/\/\/\/\.-.-.-.-.......___________
    /  \  /  \  /Hawaii Institute of Geophysics, Honolulu\/\/\.-.-....__
___/    \/    \/Joe Dellinger, Internet: joe@montebello.soest.hawaii.edu\/\.-.__
____________________________________________________
Objectivity is not an unobtainable emptying of mind |
but a willingness to abandon a set of preferences   |----> Stephen J. Gould
when the world seems to work in a contrary way.     |
Newsgroups: sci.geo.geology