Volcanic Crystals Record Quick Trip from Mantle to Surface

An analysis of crystals in volcanic rocks suggests that during Iceland’s Borgarhraun eruption, magma ascended rapidly from the base of the crust, reaching the surface in just days to weeks.

Everyone living on or around a volcano would appreciate an early warning system, but thus far, volcanic eruptions have been notoriously difficult to predict. One possible warning sign is earthquakes, which indicate when melted rock, called magma, moves into the crust. Another potential signal is a spike in gas emissions, when carbon dioxide comes out of the ascending magma. Knowing how long it takes for the magma to navigate a volcano’s plumbing system and erupt also would be useful for volcano eruption forecasting. Scientists, however, have struggled to make these measurements.

Now for the first time, DCO Reservoirs and Fluxes Community members Euan Mutch, John Maclennan, and Marie Edmonds (all at University of Cambridge, UK) with colleagues have estimated how long it takes for magma to move from the base of the crust to the surface before a volcanic eruption. In a new paper in Nature Geoscience, they estimate that magma in the Borgarhraun eruption that occurred in northern Iceland about 8000 years ago ascended 24 kilometers in about 10 days [1]. They made this estimate by examining volcanic rocks from the eruption and seeing how crystals in the magma changed during the trip to the surface. Similar estimates at other volcanoes could help us understand the movement and storage of carbon in the subsurface and may help refine eruption forecasting in concert with monitoring of earthquakes and gas emissions.

“[This approach] is certainly quite useful for volcanic hazard management and also allows us to understand how things like [carbon and other] volatiles are transferred through the crust, and heat, which can be quite useful for geothermal power plants,” said Mutch.

Borg Nodule
This microscopic nodule that erupted from the Borgarhraun volcano is made up of several olivine crystals (green). Some surfaces have grown rims of the same mineral, (light green), but with a slightly different composition, with more iron and less magnesium. Over time, elements from the rim diffused into the olivine, which is an indicator of how long the nodule stayed in the magma chamber before erupting. Credit: Euan Mutch

The researchers used a technique called diffusion chronometry, which uses crystals in magma like tiny stopwatches to record how long they spent inside the magma chamber. The consistency of magma deep in a volcano is much like the ice and sugar syrup inside a “slushie,” a sweet, frozen drink usually drunk with a straw. Inside the Borgarhraun volcano, the ice represents crystals of minerals like olivine, the sugar syrup is the liquid part of the magma, (called melt), and the straw is the path to the surface. As olivine crystals swirl around in the melt, they grow a coating with a slightly different composition, with more iron and less magnesium. Over time, elements from the coating gradually diffuse into the olivine crystal and vice versa. Based on how far those elements travel, the researchers estimated how long the crystals had spent inside the magma before erupting. 

In this study, the researchers looked at 20 olivine crystals from volcanic rocks that originated as magma from the boundary between the mantle and crust beneath the Borgarhraun volcano. Measurements from each crystal indicated a slightly different amount of time that it took the magma to erupt, ranging from five to 30 days. This is not surprising, however. To go back to the slushie analogy, crystals can spend different amounts of time swirling around in the cup before going up the straw. 

The magma moved quickly, reaching speeds of up to 0.1 meters per second (or about one third of a kilometer per hour). The rates estimated in this study are similar to ones made by geophysicists who studied earthquakes and ground deformation caused by magma moving in the crust. When monitoring stations detect magma movement in the lower crust, scientists may be able to estimate how long it will take for the magma to reach the surface, using estimates from diffusion chronometry.

Once the researchers calculated how fast the magma moves, they could estimate when Borgarhraun would have emitted a tell-tale carbon dioxide spike, which often occurs in volcanoes prior to eruption. Previous studies determined that Borgarhraun’s magma had a fairly low carbon content and so carbon dioxide likely didn’t bubble out of the magma until it was close to the surface. Thus the spike in carbon dioxide emissions likely occurred only a day or two before the eruption 8000 years ago, which if it happened today, would give people little time to evacuate.

Considering how rapidly magma can ascend, magma speeds are one factor to consider for volcano hazard management. “You have to use a combination of techniques – seismicity, petrology, and gas – to take a holistic approach to understand what volcanoes are doing, particularly in magmas that are ascending quickly from the base of the crust,” said Mutch. 

The amount of carbon and other volatile compounds in the magma also affects its behavior. In magmas with a higher carbon content, carbon dioxide would likely bubble out at lower depths, announcing the eruption sooner. However, volatile-rich magmas also move faster.  

Of course, magma may not always travel straight from the mantle to the surface. Sometimes it pools in the shallow crust before an eruption, and so Mutch and his colleagues also are using a similar approach with these samples to understand how long the magma spent in storage. These estimates will be helpful for estimating how quickly magma moves between the mantle and crustal and surface reservoirs. “If we know how fast magma can move through there, we ultimately know how fast carbon can move,” said Mutch.

Main image: In 2014, Mutch witnessed an eruption at Iceland’s Holuhraun lava field, the largest lava field to form in Iceland since the 1780s. Credit: Euan Mutch

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