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March 14, 1997

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

Why sample the lava?

If you've ever watched videos of the current eruption, you've seen geologists throwing hammer-headed cables into the lava tube to snag a glob of lava, or running to catch molten spatter from lava fountains. Lava sampling can be risky, so why do we bother to do it? Hot lava samples provide important information about what's going on in Kilauea's magma chambers. For example, we can tell the difference between magma that has moved up quickly from deep within the Earth and magma that has been stored for many years in a shallow chamber within the volcano. It's important to get the samples while they're hot and cool them quickly in cold water. A fast `quench' is needed to avoid chemical changes that result from the formation of crystals during slow cooling.

When Kilauea magma forms in the mantle at a depth of ~100 kilometers (60 miles), it has a high concentration of heavy elements such as magnesium and iron. The chemical composition of the magma changes as it rises into the edifice of the volcano, mostly because of crystallization. The first minerals to crystallize are magnesium-rich olivine and small amounts of chromite. As these heavy crystals form, the remaining magma becomes relatively enriched in other major elements, such as silicon, aluminum, iron, calcium and in other minor elements.

We know from laboratory experiments that the more magnesium there is in magma, the hotter it is. Chemical analysis therefore provides the means, not only to determine the crystallization history of lava, but also to establish the temperature at which it was erupted. Lava flows undergo chemical changes that result from cooling during emplacement. Also, our field studies show that lava cools nearly 10°C (18°F) while traveling in lava tubes for 10 km (6 mi) from Pu`u `O`o to the coast. Therefore, samples of vent spatter and from lava tubes nearest the vent provide the best indication of pre-eruptive chemistry.

During periods of continuous eruption, we observe only small chemical changes over time. These changes are due to crystallization and to mixing of magmas with slightly different compositions. The changes take place in the summit magma chamber and in the plumbing system that connects the summit to the east rift zone. From September 1996 through January 1997, lavas from Pu`u `O`o increased steadily in magnesia (MgO) content (6.7% to 7.1%) and temperature (1150°C to 1159°C (2102°F - 2118°F)).

On January 30th of this year, magma moved from both Pu`u `O`o and from the summit magma chamber into an area near Napau Crater, producing brief fissure eruptions (Episode 54).

Episode 54 lavas are chemically different from lavas erupted over the last 12 years. Chemical analyses show that magma was supplied by two distinct magma bodies that have resided for years beneath Napau Crater, where eruptions last occurred in 1983 and in 1968. Lava nearest Pu`u `O`o erupted at 1125°C to 1130°C (2057°F - 2066°F) and contains crystals of olivine, feldspar and pyroxene. Calculations show the source of this lava was intruded at the beginning of the Pu`u O`o eruption in January 1983. The lava erupted from the west wall of Napau tapped a second magma body that was possibly emplaced in August 1968. This magma, which had been cooling and crystallizing for a longer time, erupted with more abundant feldspar and pyroxene crystals and at lower temperatures of 1110° to 1120°C (2030°-2048°F).

Tracing changes in near-vent lava compositions is a way to understand Kilauea's internal plumbing system. These changes mirror those measured by seismic and geodetic methods. The combined use of these varied techniques helps to forecast future eruptive activity at Kilauea and at other volcanoes.

Eruption Update: 12 March 1997

Kilauea's east rift zone eruptive activity is limited to a sustained lava pond deep within the Pu`u `O`o Crater. The pond reappeared on February 24, 25 days after the end of Episode 54. There was an earthquake felt on Thursday, March 13 at 9:55 in the morning by residents of Hawaiian Ocean View Estates subdivision. The magnitude 3.3 temblor was located 16.5 km (10 mi) east of Ka Lae at a depth of 35.7 km (21.4 mi).

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