Volcanic ash deposit at base of Uwēkahuna Bluff below Jaggar Museum. Photo by T.A. Jaggar on July 29, 1913. Ash deposit buried by lava flow in 1919. Standing man is circled. From HVO Record Book courtesy of Bishop Museum.

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

For 15 years, we’ve been on a journey of discovery through KÄ«lauea’s volcanic past. Today we review what’s been learned on that journey and how new findings are shaping our thoughts about the future at KÄ«lauea.

Geologists from the Smithsonian, the University of Hawaiʻi, and HVO began their journey in the late 20th century, when it was thought that Kīlauea was almost always effusive (erupting lava flows) and that the opposite kind of eruption, violently explosive, was anomalous and rare. Now, 15 years into the 21st century, we know that explosive eruptions are far more common than previously thought and that the kind of eruptive activity, whether explosive or effusive, alternates over periods of centuries. How did this startling change in thinking take place, and why is it important?

Boots on the ground was the method, coupled with the ability to measure the age (to the nearest few decades) of tiny bits of charcoal found in volcanic deposits. Thousands of field observations of explosive deposits discovered more than 100 sites with charcoal resulting from fires started by eruption. Field work pieced together the sequence in which deposits were laid down—younger deposits overlie older deposits, like papers on your cluttered desk—and determined how far they were dispersed away from the summit of Kīlauea. More sophisticated analysis indicates that some volcanic ash reached upward well into the jet stream, a hazard to air travel were it to happen again.

As our journey progressed, we dated charcoal in the deposits using advanced carbon-14 techniques. The ages confirmed field interpretations that the explosive deposits spanned a considerable time and were not products of rare, solitary eruptions. The most recent explosive period lasted from about 1500 to 1800 CE, and an earlier period from about 200 BCE to 1000 CE. Still older explosive deposits await more study.

We then assembled all previously determined carbon-14 ages for KÄ«lauea lava flows on the volcano and found something remarkable. Most lava flows were erupted BETWEEN explosive periods, not during them.

Wow! Suddenly it dawned on us that KÄ«lauea erupts in cycles. Periods dominated by explosive eruptions alternate with periods dominated by effusive eruptions. KÄ«lauea has mostly erupted lava flows for the past 200 years, and we had been misled into thinking that the volcano was always like that. Now we know better!

We then calculated that the volumes of lava flows surpass the volumes of explosive deposits by nearly 100 times. This was another wow! Apparently the rate at which magma is supplied to the volcano from deep within the earth is almost 100 times faster during effusive periods than during explosive periods. Why this happens is not yet obvious, but it does.

The physical characteristics of the explosive deposits indicate that most eruptions were powered by steam from heated groundwater. This happens when the summit caldera is deep and intersects the water table, which today is about 615 meters (2000 feet) below the rim of the caldera.

Putting all this together, we get a picture of explosive eruptions during periods of low magma supply and a deep caldera, and of effusive eruptions during periods of high magma supply and a shallow or filled caldera, such as today.

The future will probably resemble the past. Eventually the magma supply rate will drop drastically from its currently high level. The caldera will collapse because there is insufficient magma to fill the reservoirs supporting the summit, and explosive eruptions will resume. That would create a hazardous situation at the summit that could last for centuries, until the magma supply rate picks up and effusive eruptions again take command.

What an exhilarating journey of discovery…and it continues.