(Wikimedia Commons/Frank Kovalchek https://bit.ly/3H9nv5V)
A team of geologists from University of Illinois used seismic tomographic modeling, and refined the phase composition and pattern of the distribution of matter in the magma reservoir under the Yellowstone caldera.
They found that the substance with a larger proportion of the melt is concentrated 3–8 kilometers from the surface.
The content of the liquid phase in this magma turned out to be higher than previously thought, and amounted to 16%–20%.
Volcanic eruptions that exceed 1000 cubic kilometers of volume are often called mega or super-eruptions.
They not only produce destructive changes over a large area, but also have a global impact on the climate.
They can occur in subduction zones or in intraplate magmatism areas such as the Yellowstone hotspot in the northwestern US.
It forms a continental supervolcano, fed mainly from a reservoir of acidic and viscous rhyolitic magma, which is similar in composition to granitic rocks.
The unloading of such a source, as a rule, occurs in the form of an extremely powerful explosion.
CRYSTAL MUSH
The last catastrophic explosive eruption of the Yellowstone supervolcano occurred about 640,000 years ago and presumably consisted of two events that caused two successive volcanic winters.
After the devastation of the magma chamber, its rhyolite roof sank, forming a huge caldera with an area of 7,500 square kilometers.
Less powerful eruptions of rhyolite lavas also occurred later, from 180 to 70 thousand years ago.
The data of modern observations of gas and hydrothermal emissions, monitoring of ground displacements and earthquakes indicate that the Yellowstone supervolcano remains active to this day.
To more confidently judge the state of the supervolcano, scientists model the structure of the magma chamber and the characteristics of its contents using the seismic tomography method.
So it was possible to establish that the mantle plume under the Yellowstone hotspot is more complicated than previously thought.
Seismic tomography is based on the inversion of seismic wave travel time data into a quantitative description of the physical properties of the subsurface, such as rock density.
The inversion allows:
- to build a visual model of the boundaries of the source,
- to determine the composition of the substance in it and
- to evaluate the phase relationships, that is, the degree of crystallinity of the magma, and, consequently, its ability to move.
It is believed that at an average crystal content of more than 60%, the magma begins to behave like a solid body.
This mixture is called "crystal mush".
The seismic tomographic model built for shear waves showed that there is a vast area under the Yellowstone caldera and far beyond it where they slow down.
It has been interpreted as a large magma body extending to depths of 5 to 15 kilometers, with only about 10% of the liquid phase.
However, due to insufficient resolution, this model suffered from inaccuracy and did not agree with the data of petrological studies of tuff from the deposits of the last powerful explosion.
THE NEW STUDY
To refine the model, a team of researchers led by the geologist Ross Maguire applied the method of complete inversion of the shear seismic wave field.
In this method, a three-dimensional numerical model is repeatedly modified by comparison with field data, including seismic events, which are treated as noise in standard algorithms.
After 10 iterations, the researchers arrived at a model in which inconsistencies with observed data were reduced by 50%.
According to the simulation results, the velocity anomalies of transverse waves are distributed inhomogeneously.
The greatest slowing of the waves (by more than 30%, to a value of 2.3 km/s), corresponding to an increase in the proportion of the liquid phase, was found closer to the surface, at depths of 3 to 8 kilometers.
The peak speed drop (up to 2.15 kilometers per second) was obtained by scientists for a five-kilometer depth in an area slightly shifted east of the center of the caldera.
These values correlate with the petrology of the Lava Creek Formation tuffs formed from the tephra of the last catastrophic eruption.
According to the analysis, the pressure under which the rhyolitic melt that gave rise to them was 80–150 megapascals, and this corresponds to a depth range of three to 6 kilometers.
The zone of gradually decreasing negative velocity anomalies under the caldera extends to a depth of about 35 kilometers.
In addition, the seismic tomography also contains other low-velocity regions located in the middle (about 20 kilometers deep) and lower (about 40 kilometers) layers of the crust.
WHAT IT MEANS
This means that the system of magma reservoirs of the Yellowstone Supervolcano is more complex than scientists have thought so far.
Also, the substance in the magma reservoirs is differentiated in terms of phase composition depending on depth.
The researchers estimate the highest content of the molten phase at 16%-20% higher than was obtained in previous models.
Such a mixture still behaves like a solid, but its differentiation may reflect some dynamics in the development of the magma chamber.
Scientists believe that it is able to exist in a state of "crystalline mush" for at least 100 thousand years, during which phase differentiation should slowly occur.
Upon reaching the threshold content of the melt from 35% to 50%, the magma will become mobile.
After that, the explosive devastation of the reservoir will occur relatively quickly, possibly in the next 5000 years.
The total volume of the acid melt, according to researchers, is at least 1600 cubic kilometers.
Maguire’s team cannot completely rule out the possibility of accelerated unloading of the magma chamber.
Such a scenario is possible in the formation of small "pockets" with a concentrated melt or in the case of slippage in a viscous highly crystalline medium due to deformation.
However, the authors emphasize that it does not reveal any immediate signs of an approaching eruption.
The study helps to detail the picture of the structure and activity cycles of the Yellowstone supervolcano.
Sources:
- Science: https://www.science.org/doi/10.1126/science.ade0347?cookieSet=1
- NewScientists: https://www.newscientist.com/article/2349550-yellowstone-supervolcano-contains-twice-as-much-melted-rock-as-thought/
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One point that I was trying to understand was that this 16-20% compares to the previous estimate of 9%. That's where the "twice as much" in the headline comes from. I found this in the newscientist link.
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