An HVO geologist samples lava from an active lava tube. These samples are analyzed routinely to track changes in lava chemistry. (Photo courtesy of USGS)

(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Petrology is literally the “study of rocks,” and looking carefully at the rocks coming out of a volcano is necessary in order to understand what’s happening on the inside.

Erupting lava, including liquid and crystals, records the journey that magma takes from the mantle into the heart of the volcano and on to the surface.

Based on patterns of earthquakes and swelling and contracting of the summit, scientists have inferred the existence of a 2–3 cubic kilometer (0.5-0.7 cubic mile) reservoir of magma that is around 4 km (2.5 mile) below Halemaumau Crater. Because these patterns at the summit also correlate with events on Kilauea’s rift zones, a magmatic connection must exist between the summit magma reservoir and the rift zones.

What scientists have long speculated about, however, is the residence time of magma in the summit reservoir. Many assumed that the reservoir contains stagnant magma left over from past summit eruptions in 1982, 1974, or perhaps even decades earlier. In March 2008, when a new vent erupted at the summit, speculation about the age of the summit magma reservoir was laid to rest.

Like forensic scientists who identify carefully extracted DNA, petrologists can identify the “genetic signature” of magma from deep within the mantle by determining concentrations of specific elements that are present in trace amounts.

By analyzing these trace elements in older lava, we know that the “mantle DNA” of magma feeding Kilauea has evolved over time, even since the previous summit eruption in 1982.

Our analyses of Kilauea’s new summit lava confirm it to be a genetic match to lava erupting on the east rift zone. This petrologic evidence for summit-to-rift magma continuity is a milestone on our road to understanding the most studied volcano on earth.

It shows us that the active magma pathways from Halemaumau to Puu Oo have been flushed of remnant magma and are now filled by a burgeoning supply of new magma from depth!

So, aside from having the same “mantle DNA,” how does summit lava compare with lava erupting 21 km (13 miles) away on the east rift zone, and what does that comparison tell us about what’s happening in the magma system from the Kilauea’s summit to its east rift zone?

At the summit, lava is erupted at about 1,170 degrees Celsius (2,140 degrees Fahrenheit) and often contains small, amber-green olivine crystals. Out on the rift zone, though, lava is erupting at about 1150 degrees Celsius (2,100 degrees Fahrenheit) and contains a mixed-up assemblage of crystals.

As magma or lava cools under normal circumstances, olivine crystallizes first, followed by pyroxene. “Rockhounds” detect the subtle mixture of amber-green olivine and forest-green pyroxene crystals in the majority of recent rift lava flows.

However, some pyroxene crystals are found with scalloped and embayed edges indicating that they were dissolving in a higher-temperature magma in which olivine crystals are more likely to grow. In fact, some olivines have even crystallized around tiny remnants of dissolved pyroxene crystals!

These microscopic crystal relationships provide clear evidence for mixing between hotter, gas-rich ”new” magma and cooler, degassed, pyroxene-bearing magma mush, before it is erupted on the east rift.

We know that this pyroxene-bearing magma mush is recently derived from the mantle because it, too, has the same genetic signature.

The persistence of this mixed-crystal assemblage in east rift zone lava attests to the presence of magma mush along cooler margins of the active magma pathways, as well as in cavities and cracks within the shallow volcanic edifice. The hotter magma that recharges the edifice could also be mixing with the cooler magma in the summit reservoir, adjacent to the east rift conduit.

Petrology suggests that we have a lot to learn about the complexities of shallow magma storage and transport. The entire Puu Oo eruptive sequence, a prolonged period of magma replenishment, has allowed petrologists a unique view of happenings within and on Kilauea volcano.