Martian Volcanic Plumes Shorter than Previously Thought

Martian volcano Olympus Mons, a shield volcano produced by lava flows. (Credit: NASA)

The maximum height of explosive volcanic eruption columns on Mars dictates how far ash could be transported across the planet surface. The higher a volcanic plume rises, the farther ash can be deposited. When ash is being transported in the atmosphere, it is sorted based on size and density; the resulting deposits can look very similar to the sorted material produced by flowing water. Thus, it is vital to constrain which deposits could have been produced by a volcano and which are more likely to have been water-lain when contemplating future Mars Rover landing sites. Previous models have predicted that plumes could rise more than 100 km into the atmosphere, which means that deposits found over 100 km away from the vent could have been caused by a volcano, not water.

Martian plume models are based off terrestrial plume models, developed through laboratory experiments on Earth. These models have several boundary assumptions, including that the rise velocity and expansion rate of the plume are slower than the speed of sound, the expansion rate is less than the rise rate, and the radius of the plume is not larger than the height of the plume. The largest observed terrestrial eruption columns do not violate these conditions, signaling model appropriateness for terrestrial research. To make these models applicable to Mars, scientists changed the Earth specific variables, such as atmospheric conditions and gravity, to reflect those on Mars. However, there is not a compelling reason to believe that the assumptions underlying these models are equally translatable for Martian conditions.

Plume rise velocity vs. altitude. Note that the rise speed exceeds the speed of sound at ~40 km altitude for this example. Different eruption conditions resulted in different heights for the speed of sound violation.  (Figure 5 from Glaze and Baloga, 2002)

A 2002 study assessed maximum Martian plume heights by testing whether the model assumptions were valid. After looking at the model’s physics, which remains the same whether on Earth or on Mars, they found that the source conditions that produced the largest plumes on paleo Mars violated all of these assumptions. They found that the vertical velocity exceeds the speed of sounds at heights 4 times shorter than previously thought. They also found that after 50 km of rise, the radius of the plume is more than 10 times wider than the height! Additionally the radial velocity exceeds the speed of sound at heights 5 times shorter that the maximum plume height. The radial velocity is faster than the rise velocity nearly 7 times faster than previously thought, due to a much lower atmospheric density. When the atmospheric density is lowered, the expansion rate does not change by the same amount. For example, if atmospheric density is lowered by a factor of 2, the expansion rate does not decrease by a factor of 2. The result of all of this violation of model assumptions is that we cannot fully trust the existing plume models to describe eruptions on Mars.

Based on the limitations of the existing models, the authors suggest that the maximum plume height for eruptions on paleo-Mars be considered as 65 km, the maximum height before the model broke down. This decreased maximum plume height similarly decreases the maximum extent expected from associated fall deposits. Further work linking ash transport to plume heights in Martian atmosphere, as well as transport effects from eruption column collapse, can help Earth-bound researchers identify which deposits on the Martian surface are most likely to have been lain by water by ruling out volcanic sources.

Original paper: Glaze, L. S., and S. M. Baloga, Volcanic plume heights on Mars: Limits of validity for convective models, J. Geophys. Res., 107(E10), 5086, doi:10.1029/2001JE001830, 2002.

Frequency of Large Volcanic Eruption in Southeast Asia

Southeast Asia is home to 50% of the world’s active and dormant (not erupting but is expected to erupt again) volcanoes. These volcanoes have produced some of the largest eruptions on record.  The 1991 eruption of Pinatubo was the 3^rd largest eruption of the 20^th century. The eruption of Krakatoa in 1893 resulted in the deaths of over 36,000 people. The eruption of Toba in 1815 released enough volcanic gas to cover the entire earth, leading to massive crop failures and killing more up to 100,000 people globally. Despite large hazard risk, the area has not been rigorously evaluated for eruption frequency due to difficult terrain and dense vegetation. In comparison, these sorts of analyses have existed for Europe and other parts of the world since the 1980’s.

A new study published in the Bulletin of Volcanology uses satellite imagery to classify volcano morphology to determine the decadal chance of different eruption sizes, or VEIs, for the region. The VEI, or Volcanic Explosively Index, ranks the explosively of an eruption based on volume of ejected material. The VEI scale ranges from 1-8, with larger numbers associated with larger eruptions; a VEI 4 or greater will disrupt air traffic. They found that there is a 1% chance of a VEI 4, 0.6% of a VEI 5, 0.15% of a VEI 6, 0.012% of a VEI 7, and a 0.001% of a VEI 8 eruption for the region every decade.

These probabilities were calculated by determining the number of volcanoes in the region possible of producing each VEI and knowing the frequency of an eruption of each VEI. The scientists were able to determine the number of volcanoes of each VEI based on volcano shape using satellite imagery. Each of the 748 volcanoes were sorted into one of five categories: distributed cones and fields, open vent stratocones, semi-plugged stratocones, well plugged stratocones, and large calderas.  To classify these different morphologies, they measured the longest axis of crater in the satellite image and identify any lava flow deposits, pyroclastic deposits, debris avalanche deposits, cones, calderas, collapse scarps, and domes. Areas less than 5 km wide with volcanic centers and lava flows were classified as distributed cones and fields. If the volcano was actively releasing gas, it was classified as an open vent stratocone.

Figure: Map of volcano locations and classifications in Southeast Asia (modified slightly from Whelley et al., 2015)

Each volcano was assigned a maximum VEI, based on these descriptive classification and some simple interpretations: a) more explosive volcanoes produce larger craters; b) small volcanoes and volcanoes with open vents are unable to produce the pressure needed to produce large scale explosions.   Distributed cones and fields were designated VEIs VEI 1-2, open vent stratocones VEIs 3-4, semi-plugged stratocones VEI 5, well plugged stratocones VEI 6, and large calderas VEI 7-8. Smaller eruptions occur more frequency, explaining why there is a much higher chance of a VEI 4 eruption over the next decade in Southeast Asia as opposed to a VEI 8.

Southeast Asia has one of the most rapidly growing airline traffic in the world, with a projected 6.6% annual growth over the next 20 years. There are already increasing concerns about aviation safety in the region, including mechanical safety and weather related incidents. With better long term forecasting, aviation safety boards will be able to better prepare for the next volcanic eruption, both with regard to size and possible location, and will therefore be able to make more timely decisions to ensure passenger safety.

For more information, check out the original article, “The frequency of explosive volcanic eruptions in Southeast Asia,” by Patrick L. Whelley, Christopher G. Newhall, and Kyle E. Bradley, here.

GPS Used To Predict Height of Volcanic Plumes

 Photo of eruption Grímsvötn, May 2011. (SPR AFP/Getty Images) 

Scientists in Iceland have developed an innovative method to predict volcanic plume height using changes in magma pressure.  Using a series of anchored GPS stations, they were able to detect inflation of the Grímsvötn volcano before its eruption in May 2011 and the sudden co-eruptive subsidence.

These GPS stations, originally installed to monitor the movement of tectonic plates over time, continuously recorded position and elevation during the pre-eruptive and eruptive phases of the explosion.  They found that the volcano expanded radially approximately 20 inches in the hour preceding the eruption and sank 10 inches during the initial eruptive phase, resulting in tilting of one hundredth of a degree.  By tracking the change in volume of the volcano, the scientists were able to track the change in pressure of the magma chamber during the eruption.  This process is similar to a tire deflating – as air (or in the case of a volcano, magma) leaks out the tire pressure decreases. 

Based off this change in pressure, the total change in volume of the magma chamber was .027 km3, about 10x smaller than the total erupted volume. Pumice is extremely porous and can contain over 50% void space; this is a result of bubbles being trapped in the solidifying melt as they expand due to decreased pressure at the volcanic vent relative to the magma chamber. The scientists were then able to determine the rate of the eruption using the overall duration and amount of ejected material.  The mass eruption rate is directly correlated with the height of the plume; the faster the eruption rate, the taller the plume. Using this novel method, they estimated that the plume height peaked at 15 km, which compares favorably to satellite imagery.

Column height is an important parameter to predict where an ash cloud will travel.  The heat produced by jet engines cause the ash particles to melt, gunking up the engine and causing the plane to stall.  Currently, the protocol for avoiding this is to completely close airspace.   The eruption of Grímsvötn, which lasted for seven days, caused airspace closures in northern Europe, grounding approximately 900 flights.  Better predictions of column height would result in more accurate hazard maps and fewer plane delays. This new method to estimate plume height uses real time data that does not rely on satellites.  Aviation experts can then better seed their models to create smaller no fly zones due to more accurate predictions of ash movement.

Original Article: Hreinsdóttir, Sigrún, Freysteinn Sigmundsson, Matthew J. Roberts, Halldór Björnsson, Ronni Grapenthin, Pórdur Arason, Thóra Árnadóttir et al. "Volcanic plume height correlated with magma-pressure change at Grimsvotn Volcano, Iceland." Nature Geoscience (2014).