In the lab, Kenna Rubin peers at a core that looks distinctly different from the others we’ve collected – instead of white carbonate, it’s black basalt. As our resident expert in Hawaiian volcanology, Rubin tosses out an array of academic vocabulary: “this alkalic basalt is variably vesicular. This sample has zones that are more scoriaceous and less scoriaceous”.
Translation: this basalt has a relatively high level of alkali (and less silica) compared to other basalts. “Vesicular” and “scoriaceous” refer to the amount and distribution of gas bubbles trapped in the rock.
As Rubin continues describing the various characteristics of this unique sample, it’s clear it came from the lava flow of a nearby volcano. The question is – is it Mauna Loa or Hualalai lava?
Rising 13,679 feet above sea level, Mauna Loa is the planet’s largest active volcano. It last erupted less than a year ago, in November of 2022. And in 1859, it sent a long lava flow directly into the ocean, very close to where we are currently collecting samples.
“Hualālai is a relatively small Hawaiian volcano,” Rubin says. “Lava flows from Mauna Loa go around Hualālai, and some of those flows go directly into the ocean. So, is this basaltic lava flow from Hualālai or Mauna Loa? We don’t know.”
But Rubin is being diplomatic – she has a pretty good hunch which volcano this lava came from – and that hunch is informed by bubbles.
Hawaiian volcanic eruptions are considered effusive (non-explosive), churning out fast-moving rivers of basaltic magma. As these lava flows make their way down the volcano’s slope, they are “de-gassing”.
To explain this process, Rubin compares the lava to soda.
“When you pop a can of soda, it’s super bubbly. But if you let it sit there for a while, all the bubbles will eventually fizzle out. When lava flows from the summit and all the way down into the ocean, the lava cools and degases. If you have two lava flows coming downhill, the one flowing slower will release more gas before it reaches the ocean.”
In the last few eruptions from each of these volcanoes, Hualālai lava flows have been faster moving. The timing of the eruption that formed the sample we’re currently examining is also an open question. But Rubin believes it’s probably much older than anything we could find on the surface of either volcano.
“If you walked all around Mauna Loa, you’d be hard pressed to find a rock that’s more than a few thousand years old,” Rubin says. “It just keeps erupting and ejecting new materials.”
Even on the surface of Hualālai, 80 percent of the rocks are younger than 5,000 years, according to the United States Geological Survey.
But the fossil coral reef where we found this basalt is over 700 meters below sea level and therefore the flow could possibly be hundreds of thousands of years old, but future work will have to be done to confirm this.
“It gives us access to part of the volcano that is orders of magnitude older than we could access on land,” Rubin says.
Having a snippet of a lava from long ago can help us obtain a clearer picture of the historical record, which is critical for understanding how these volcanoes have changed over time and how they might behave in the future.
While this area of the Big Island was sparsely populated in 1801 (when Hualālai last erupted) the base of the volcano now contains billions of dollars’ worth of infrastructure, businesses, and homes.
“We know the recent history of Hualālai includes fast-moving lava flows that don’t allow for a lot of warning,” Rubin says. “There is only one two-lane road to get in and out of Kailua-Kona. The more data we have on long lava flows from either of these two volcanoes, the more we can inform natural hazard mitigation plans.”
“It’s a tiny piece of a bigger puzzle for understanding the hazards of eruptions in Hawaiian communities,” Rubin says.
Blog of IODP Expedition 389: Hawaiian Drowned Reefs 