# These rock hounds collect and analyze the "hot stuff"

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Among the scientists who study volcanoes, the petrologists are the ones who do detailed studies of the "hot stuff" in order to understand the behavior of eruptions. Petrology is literally the "study of rocks," and petrologic monitoring means tracking and interpreting the changes in chemistry and mineralogy of lava during an eruption.

A photograph of a 5 mm portion of a thin section of a basalt with a fine-grained ground mass with plagiocase (long) and clinopyroxene (colored) crystals.

(Public domain.)

When dormant volcanoes awaken, petrologists make quick assessments of the initial ash or lava to classify the type of magma erupted. These analyses tell us about processes that drive volcanic activity beneath the surface. The information is then used to evaluate hazards associated with continuing eruptions.

For ongoing eruptions like those of Kīlauea, which began in 1983, and Mount St. Helens which began in 2004, molten lava samples provide "snapshots" of the magma before it reaches the earth's surface. Over time, variations of temperature, chemical composition, mineralogy, and gas content can be correlated with variations in the style and vigor of the eruptions.

Since lava changes as it cools, degases, and crystallizes at the surface, the nature of pre-eruptive magma is best reflected in samples obtained at, or near, an erupting vent. But different types of eruptions at different volcanoes require unique approaches to lava sampling near vents. When volcanoes on the mainland erupt explosively, ash is scraped from snow or other clean surfaces, well downwind of the vent. Such samples contain sub-millimeter bits of air-quenched magma, scrutinized to decipher what's going on beneath the surface.

At Kīlauea, near-vent sampling is a "catch-as-catch-can" proposition. At times, the only fresh lava samples are from "tear-catcher" boxes set near Puu Oo vents to collect Pele's tears and spatter. During periods of steady eruption, however, samples of molten lava are fished from upper-elevation "skylights," the roof-collapses in active lava tubes. A sledge hammerhead tied to a steel cable is dipped into the fast-moving lava stream and the gob of hot stuff (1,140-1,160 degrees Celsius) is quickly chilled in water to prevent new crystals from forming.

Whether from fountaining fissures, channeled flows, or pahoehoe toes, routine collection of near-vent samples allows us to detect subtle but important changes in the seemingly "same old" basalt erupted by Kīlauea.

In contrast to the rivers of molten lava that are so familiar to Hawaii residents, the 70 million cubic meter (and growing) lava dome in the crater of Mount St. Helens is relatively cold (less than 700 degrees Celsius) and nearly solid when it erupts. Seven successive lava spines have arisen from the vent, all with sheared and fault-gouged outer margins that developed during forceful expulsion of a near-solid dacite plug. This eruption has been punctuated by massive rock falls from these over-steepened megaliths of lava, as they are crushed together during near-continuous extrusion.

Sampling forays on the ground are prohibitive under the hazardous eruption conditions at Mount St. Helens. So, remote sampling is done with a dredging bucket or chain-mail basket slung 100 m (300 feet) below a hovering helicopter. Alert observers stationed on the crater rim and a skilled pilot are essential to this method of sampling. Like a mosquito on a mammoth, the trained pilot buzzes around the sweltering, craggy dome for a place to collect a fresh sample. In addition, and much like the tear-catchers at Puu Oo, a large metal box has been slung to and from positions near the dome's vent to collect lava fragments and airborne ash.

Eruption samples are routinely submitted to the USGS labs in Denver for chemical analysis. Highly polished thin slices of lava are mounted on glass slides are examined with optical microscopes and electron microbeam instruments. Textures and compositions of the minerals are analyzed to determine their origin. Did they grow in place or were they accidentally incorporated by the magma on its way to the surface?

The compositions of the crystals and the glass (lava that did not crystallize before the eruption) give us a picture of the pressure and temperature of the magma before the eruption. Stay tuned for future Volcano Watch articles on the wondrous petrology of the "hot stuff."

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### Volcano Activity Update

This past week, activity levels at the summit of Kīlauea Volcano have remained at background levels. The number of earthquakes located in the summit area is low (usually less than 10 per day). Extension of the summit caldera, indicating inflation, appears to have resumed after pausing earlier in April.

Eruptive activity at Puu Oo continues. On clear nights, glow is visible from several vents within the crater. Lava continues to flow through the PKK lava tube from its source on the flank of Puu Oo to the ocean, with intermittent surface flows breaking out of the tube near the 2,300-ft elevation, and a persistent flow, known as the "March 1 breakout," active on the coastal plain.

Lava continues to enter the ocean at East Laeapuki, in Hawaii Volcanoes National Park. The active lava bench continues to grow following the major collapse of November 28 and is now approximately 1,000 m (3,300 ft) long by 315 m (1,000 ft) wide, with a total area of 17 ha (42 acres).

On April 24, the March 1 breakout reached the ocean about 175 m (575 ft) east of East of Laeapuki. As of April 26, this entry consisted of two narrow streams of lava, only a few meters wide, falling over the sea cliff.

Access to the ocean entries and the surrounding area remains closed, due to significant hazards. If you visit the eruption site, check with the rangers for current updates, and remember to carry lots of water when venturing out onto the flow field.

There were no earthquakes beneath Hawai`i Island reported felt within the past week.

Mauna Loa is not erupting. During the past week, earthquake activity remained low beneath the volcano's summit (five earthquakes were located). Extension of lengths between locations spanning the summit, indicating inflation, continues at slow rates.