A Volcano's Crystalline Interior
A close look at the "growth rings" of crystals tell a story about volcanic eruptions
Interested in using work from The Geology Letter for education purposes? You’re free to do so, but I ask that you let me know about it, and that you credit me and link back to the newsletter.
The magma chambers beneath volcanoes are a seething, pressurized cauldron occasionally roiled by injection of more magma from the deep places of the Earth.
Such injections can trigger an eruption, like an inflated balloon that’s close to bursting: A bit more air pumped in could cause it to pop. In a magma chamber, the air pump is upwelling magma from the Earth’s mantle. Rather than bursting everywhere like a balloon, magma may accommodate an overburden of pressure by forcing its way through pre-existing cracks and faults – the plumbing channel leading to the vents and craters of a volcano.
Modern techniques can detect earthquakes from fresh magma entering a volcano’s underground chamber, but does that mean an eruption is coming? If so, how much warning do emergency planners have?
Central Washington University professor Hannah Shamloo says it could be weeks or months, at least for one volcano in the Pacific Northwest. She analyzes crystals that initially form in the deepest part of the magma chamber as the upwelling magma gradually cools. The magma eventually rises high enough to be injected into the main chamber of the volcano, where it encounters a new chemical environment that promotes the rapid growth of an outer layer on the crystal, a bit like a tree ring added during the warm summer months.
Later pulses of magma bring new changes to the chemistry, promoting a new, rapidly adhered outer layer to the crystal, which can acquire many outer layers over time.
After each additional layer, the crystal drifts around in the hot, pressurized magma, and atoms within the crystal slowly diffuse from one layer to the other. What begins as a sharp boundary between two different chemical profiles gradually blends at the margin (see figure above). The more time spent in the thermal soup of the chamber, the blurrier the boundary. The blurring rates for various elements in different minerals, and in different environmental conditions, have been revealed through carefully controlled laboratory experiments.
Sooner or later, the crystal gets ejected during an eruption. Once it cools amidst the lava or tephra, the mobile atoms within the crystal become fixed in place. Researchers then thinly slice the crystal to examine the distribution of atoms across the layers. The known diffusion rates allows researchers to calculate how long the outer layer was exposed to magma. In short, it reveals the time between the most recent change in magma chemistry and the ensuing eruption.
Early results from Mount Baker, located in northwest Washington State, suggest rapid progression. “In some scenarios, injection to eruption time is as short as weeks, but more [often] on the order of months, which is really impressive and much shorter than we thought,” said Dr. Shamloo.
Her team, which includes professors Kristina Walowski and Susan DeBari at Western Washington University, is creating a long-term data bank of the volcano’s activity. “We’ve really just scratched the surface at Mount Baker by looking at four eruptions. Mount Baker [volcanic field] has a 1-million-year-old history,” said Dr. Shamloo.
Inner Workings
Researchers can draw on a wide range of crystals, including olivine, plagioclase, sanidine, ilmenite, pyroxenes, zircons, and apatite. They can track the movement of iron, magnesium, nickel, calcium, manganese, strontium, barium, sodium, potassium, titanium, aluminum, chromium, hydrogen, chlorine, and lithium, depending on the constituents of the crystal and the time frame they are interested in.
When searching for samples, researchers like to find layers of airy tephra, the product of viscous, explosive eruptions. “When it erupts explosively, those tiny particles cool rapidly, so it locks in those pre-eruptive chemical fingerprints, whereas when you have stuff erupting effusively, like [flowing] lava, you can have this secondary thermal history that can interfere with the diffusion time,” said Dr. Shamloo.
Another complication is that a new outer zone in a crystal can be attributable to factors other than magma injection. Changes in pressure, or degassing of magma, may also lead to chemical changes in the magma and a new outer ring on a crystal. “It’s our job to interrogate every part of the chemical system to figure out [the cause of the new layer],” said Dr. Shamloo.
One complementary technique is analysis of melt inclusions, which can be glassy blobs of magma or other constituents trapped within a crystal. Chemical proportions within the inclusion suggest the temperature and pressure in the magma when the inclusion entered the growing crystal, which in turn suggests potential causes for growth of the outer crystal layer.
Kenda Lynn is a research geologist at the US Geological Survey Hawaiian Volcano Observatory who uses diffusion chronometry and other methods to monitor the Big Island’s iconic volcanoes. Her group uses inclusion barometry to examine trapped elements like carbon dioxide that are a fluid because of the high-pressure environment of their formation. “By measuring these tiny fluid inclusions, you can convert that to a pressure, which equates to a depth underneath the surface of the volcano,” said Dr. Lynn.
Old and New
Mount Baker hasn’t been active in historic times. Seismic rumbling and steam emissions raised concerns in 1975, but the magma beneath the volcano stayed underground, and its last major eruption was 6,700 years ago, and as a result Mount Baker is not heavily monitored. “It’s not like Mount Saint Helens, where we have monitoring data and material from a recent eruption. We need diffusion chronometry to understand pre-eruptive timescales for these volcanoes that haven’t erupted in our lifetimes. Otherwise, we have no way of knowing how they behave,” said Dr. Shamloo.
Along with Mount Baker, she and Drs. Walowski and DeBari are investigating Washington State’s Goat Rocks formation, which is the rocky remnant of an ancient volcano that was active between 3 million and 600,000 years ago. Recent glaciation exposed volcanic layers that stretch from the beginning to the end of the volcano’s life, granting geologists an unusual window into the past. “That’s really unique for a Cascade system, because typically the older units are buried or eroded,” said Dr. Shamloo.
She hopes that diffusion chronometry and other methods will shed light on the natural history the Cascade subduction zone volcanoes, including the effect of the basement rock that magma intrudes into and absorbs before it erupts. “I’m most excited about the earliest [history], because we typically don’t get that access. Using diffusion chronometry from the very beginning to the very end to build a true data set of behavior for these eruptions through time at a Cascades [style] eruptive system will be really cool,” said Dr. Shamloo.
Far and Wide
Diffusion chronometry isn’t restricted to sparsely monitored volcanoes like Mount Baker. Its use has exploded in recent years in places like Indonesia and Taiwan, Mount Aetna in Italy, and Mauna Loa in Hawaii.
Dr. Lynn notes that there are decades-long data sets of the seismologic activity and gaseous emissions at some of the world’s most studied volcanoes like Mauna Loa, but the magma chamber remains mysterious. Researchers use seismology, ground deformation, emitted gases, and lava composition to interpret what’s happening underground, but these measurements are indirect. The eruption of Mauna Loa in 2022 presented a unique opportunity to match advanced, real-time monitoring techniques to samples taken directly from the chamber. “Every time we’re able to make this connection between the mineral stories and our monitoring data sets, we can strengthen our confidence in our interpretations the next time we see earthquakes or tilts or other deformation at the surface. We have a better sense of what those data mean in real time, because we’ve done the ground truthing with the diffusion chronometry,” said Dr. Lynn.
Crystal analysis in Hawaiian volcanoes show a varied story, ranging from decades spent in magma chambers to hundreds of years. “These crystals sit around and are kind of recycled through the metaphorical washing machine as magmas come and go. Not every crystal makes it out in an eruption, and many are left behind to then record the next [magma injection] event,” said Dr. Lynn.
To query these different time frames, researchers turn to different atoms that move at strikingly different rates through the crystal. Atoms like hydrogen, which zip quickly through the lattice, reveal the life history of crystals that remain in the chamber for just a few hours or days after the final injection. Atoms like phosphorous move slowly and help researchers date crystals that spent years, or decades, or even centuries in the chamber. “You can leverage all these different elements to piece together the full lifespan of a crystal,” said Dr. Lynn.
The work at Mauna Loa and elsewhere is a good proving ground for diffusion chronometry, says Dr. Shamloo. The agreement between crystal stories and external monitoring in places like Hawaii emboldens her that diffusion chronometry is painting an accurate picture in the Mount Baker and Ghost Rocks volcanoes. “It gives me confidence that applying it to the Cascades, where we don’t have monitoring data, is actually worthwhile,” she said.
References:
Hannah Shamloo interview with Nick Zentner about Goat Rocks: Hannah Shamloo: Goat Rocks
Smudged volcanic crystals offer clues to past eruptions | Science | AAAS