Yellowstone magma suspected of accumulation at shallow depths

(ORDO NEWS) — Using seismic tomographic modeling, scientists have refined the phase composition and pattern of the distribution of matter in the magma reservoir under the Yellowstone caldera.

It turned out 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 percent. The study is reported in an article in the journal Science.

Volcanic eruptions, in which the volume of emissions exceeds 1000 cubic kilometers, 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 (such as the Toba eruption 74,000 years ago) or in intraplate magmatism areas such as the Yellowstone hotspot in the northwestern United States.

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.

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, with its help, it was possible to establish that the mantle plume under the Yellowstone hot spot is more complicated than previously thought.

Seismic tomography is based on the inversion (reversal) of seismic wave travel time data into a quantitative description of the physical properties of the subsurface, such as rock density.

Yellowstone magma suspected of accumulation at shallow depths 1
Distribution map of seismic station networks in the Yellowstone Caldera area

Inversion allows you to build a visual model of the boundaries of the source, determine the composition of the substance in it and 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 percent, magma begins to behave like a solid body. This mixture is called “crystal porridge”.

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 percent of the liquid phase.

To refine the model, American geophysicists led by Ross Maguire (Ross Maguire) from the University of Illinois 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 percent.

Yellowstone magma suspected of accumulation at shallow depths 2
Maps of the distribution of shear wave velocity anomalies at various depths in the original numerical model (left column) and in the model optimized by the full inversion method (right column)

According to the simulation results, it turned out that the velocity anomalies of transverse waves are distributed inhomogeneously.

The greatest slowing of the waves (by more than 30 percent, to a value of 2.3 kilometers per second), corresponding to an increase in the proportion of the liquid phase, was found closer to the surface, at depths of three to eight 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 six kilometers.

The zone of gradually decreasing negative velocity anomalies under the caldera extends to a depth of about 35 kilometers.

In addition, the seismic tomogram also contains other low-velocity regions located in the middle (about 20 kilometers deep) and lower (about 40 kilometers) layers of the crust.

Velocity anomalies of transverse seismic waves in the tomographic model constructed by the full inversion method: A) map of the distribution of anomalies at a depth of five kilometers; B, C vertical sections along lines X–X’ and Y–Y’, respectively; inset in (B) is a velocity section in the central part of the caldera.

Shaded in gray is the area of ​​depths corresponding to the data of the petrological analysis of tuffs. Circles mark seismic events with a magnitude greater than 3.0 recorded over the past 20 years.

Yellowstone magma suspected of accumulation at shallow depths 3
Velocity anomalies of transverse seismic waves in the tomographic model constructed by the full inversion method: A) map of the distribution of anomalies at a depth of five kilometers; B, C—vertical sections along lines X–X’ and Y–Y’, respectively; inset in (B) is a velocity section in the central part of the caldera. Shaded in gray is the area of ​​depths corresponding to the data of the petrological analysis of tuffs. The circles mark seismic events with a magnitude greater than 3.0 recorded over the past 20 years
Yellowstone magma suspected of accumulation at shallow depths 4
Vertical slices of a seismic tomographic model at six different latitudes

This means that the system of magma reservoirs of the Yellowstone supervolcano is more complex than scientists have thought so far, and the substance in them is differentiated in terms of phase composition depending on depth.

Maguire and his colleagues estimate the highest content of the molten phase at 16-20 percent – 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 porridge” 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 percent, the magma will become mobile, and after that the explosive devastation of the reservoir will occur relatively quickly, in the next 5000 years.

The total volume of the acid melt, according to researchers, is at least 1600 cubic kilometers.

Yellowstone magma suspected of accumulation at shallow depths 5
Scheme of the structure of the magmatic system under the Yellowstone caldera (section along the southwest–northeast line). The upper reservoir is filled with silicic magma; the inset shows crystalline grains between which the melt is enclosed. In a deep reservoir, presumably basaltic magma

We 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 of the model emphasize that it does not reveal any immediate signs of an approaching eruption, but only helps to detail the picture of the structure and activity cycles of the Yellowstone supervolcano.

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