Oregon State University

College of Earth, Ocean, and Atmospheric Sciences

rock samples

Coring the deep blue sea: Frank Tepley digs deep to understand underwater volcano

I am Frankie T
Coring on the deep blue sea
Brothers, come to me

Frank Tepley is an igneous petrologist – he studies the magma-sourced rocks that form the bulk of the Earth’s continental and oceanic crusts. But he might pass as a poet. He wrote the above haiku while on a research cruise aboard the research vessel JOIDES Resolution through the International Ocean Discovery Program (IODP Expedition 376). In 17 syllables, Tepley captured the essence of how he spent two months at sea: drilling down into the active submarine Brothers volcano, part of the massive Kermadec Arc of volcanoes in the Pacific Ocean near New Zealand.

No one had before attempted to plunge instruments several hundred meters into the guts of an active volcano – maybe for good reason. The water is extremely acidic and in places metal-toxic. But the 30 scientists on board were keen to investigate how this volcanic system evolved and how various metals were transported and deposited within the volcanic edifice and seafloor.

Researchers on the expedition were also looking at hydrothermal vents within the arc. First discovered in 1977 by a team that included CEOAS scientists, hydrothermal vents are typically found along mid-ocean ridges, where two tectonic plates diverge and allow for seawater to seep through fissures in the ocean crust. But hydrothermal vents, including black smokers, are also being discovered at a different kind of tectonic environment: along submarine arcs such as Kermadec. Because hydrothermal vents are a source of mineral deposits and home to unique microbial life, expedition scientists wanted to understand differences between mid-ocean ridge systems and those within volcanic arcs.

Tepley’s particular contribution to the cruise was to collect and examine rocks from the volcano to understand how Brothers formed and erupted in the past. Crystals inside rock samples – plagioclase crystals to be exact – provide telling clues about the volcano’s history. You can think of these crystals like tree rings, recording their environment as they grow.

“How they grow is a reflection of the thermal, barimetric and compositional environment the crystals are experiencing at that time,” Tepley says. “With this in mind, by examining the growth and compositional patterns, we can determine the thermal, pressure and liquid compositional regime to which these crystals were subjected at various times during their ‘life.’”

Frank Tepley and Dominique Tanner
Frank Tepley and Dominique Tanner (University of Wollongong, Australia) aboard the JOIDES Resolution during his two-month International Ocean Discovery Program research cruise (photo courtesy of Bill Crawford, IODP)

Not surprisingly, drilling into a submarine volcano to retrieve unaltered rock samples proved challenging. The recovery rate was poor, because the drill was going down into loose material. Sometimes a 15-meter drill would only recover a single cobble. Additionally, the hot and acidic environment put stress on drilling operations.

Once the few, hard-won samples came on board, Tepley and five other igneous petrologists worked to identify and categorize them. They spent 12-hour shifts describing rocks, writing reports and trying to set up a classification scheme for the samples.

The research vessel had several analytical instruments so Tepley could measure a handful of elements during the cruise. But in the near future he plans to analyze key samples in his electron microprobe lab. There he will “interrogate the crystals” by pummeling them with a focused electron beam. The beam, only a micron thick (smaller than one of your red blood cells), excites the atoms in the sample to produce an X-ray unique to a particular element.

“The more X-rays there are, the more atoms there are in this material,” Tepley says. Together these X-rays can tell him the elemental composition of the crystals.

What could such elemental compositions tell him? While his analysis is still ongoing, his crystal interrogation could put key events into the geologic chronology – in other words, he can create a timeline of what happened when. His analysis could tell him whether samples on the outside and older portion of the Brothers caldera are different from the more active cone, thereby providing clues to the volcano’s evolution. He can glean insights on magma storage conditions and how long the volcano took to erupt.

“If we see a big transition in concentrations of elements, then I’m going to apply a diffusion model to say, it took x amount of time for these crystals to grow, mix in, and then get erupted on the surface,” he says.

The impact of Tepley’s research during the IODP cruise is both basic and applied. “Part of this effort was simply to understand the way the world works. How is it that these volcanic systems do what they do? But if we can understand volcanic mechanisms a bit more thoroughly, then maybe we can understand other systems that threaten population centers,” he says.

Posted June 6, 2019

By Abby Metzger


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