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.

New research shows hotspots are broad and deeply rooted

Sketch diagram of Hawaii and the hotspot responsible for it. "Hawaii hotspot cross-sectional diagram" by Joel E. Robinson, USGS. Copied from https://commons.wikimedia.org/wiki/File:Hawaii_hotspot_cross-sectional_diagram.jpg#/media/File:Hawaii_hotspot_cross-sectional_diagram.jpg

Several major questions remain about volcanic hotspots: how deeply are they rooted within the mantle – do they originate from partway through the mantle, or do they come from the core-mantle boundary? What shape are they in the mantle? Do they rise vertically or are they affected by mantle currents? These questions have been thoroughly debated, but studies of the subsurface lack the clarity to resolve this problem. New research appears to hold the answer to these questions: mantle plumes rise vertically from the core-mantle boundary, and are broader than previously thought.

Previous studies using seismic tomography weren’t very good at resolving the structure of plumes deep within the mantle. Seismic wave tomography combines signals from earthquakes across the globe, and shows where seismic waves travel more slowly, or more quickly, than normal. Waves travel more rapidly through solid material, and more slowly through more liquid material. Mantle plumes usually show up well because they are warm and fluid, so seismic waves travel more slowly through them than they do the surrounding mantle.

A new study by Drs. French and Romanowicz from UC Berkeley uses data not just from many earthquakes, but also from many different seismometers across the globe. This allowed the researchers to use many different types of seismic waves in their calculations, enabling them to have a significantly better resolution deep within the Earth, and see the deep parts of the mantle plumes with much more clarity.

This cross-section of the earth shows different seismic wave velocities, with blue being fast and red being slow. An area of slow seismic velocities occurs under Hawaii, and appears to connect stretch down to the core-mantle boundary. This suggests that the mantle plume responsible for the Hawaiian volcanic chain is broad and deeply rooted. (Adapted from figure 1 from French and Romanowicz, 2015)

This new technique shows that mantle plumes start at the core-mantle boundary, about 1798 miles beneath our feet. At the base of the plume, there is an area where seismic waves travel more slowly – this indicates that the wave is travelling through a warmer, more liquid area of the mantle. The added resolution of this data set shows that the mantle plume rises vertically from these low-velocity areas through the mantle. Once the plume reaches 621 miles below the Earth’s surface, it is frequently deflected by circulation patterns in the upper mantle. The crust above these plumes begins to melt, and creates massive volcanoes like those in Hawaii.

A three-dimensional model of the large mantle plume, also referred to as the Pacific Superswell, that lies beneath Hawaii and several other nearby islands (indicated by green dots). Separate panels in the figure show cutaways at different depths in the mantle. This shows the complex and broad nature of a mantle plume at different depths. (Figure 2 from French and Romanowicz, 2015)

Unfortunately, the mantle plume for some hotspots is more difficult to image, even using this new technique. Due to small velocity differences between the plume and the surrounding mantle, researchers were still unable to see the mantle plume in some locations – including under Yellowstone.

What does this mean for our knowledge of hotspots? This new research shows that mantle plumes are wider than previously thought. This is more realistic than previous models that predicted a skinny (<200 km) mantle plume; a broad mantle plume (800 – 1,000 km) is more consistent with temperature estimates in the mantle. Additionally, these results show that mantle plumes are sourced at the core-mantle boundary. This result will help us better understand hotspot dynamics, including how hotspots interact with the Earth’s crust and create volcanic systems.