Cool carbonate volcano in Africa, Ol Doinyo Lengai… No really, it’s cool!

Figure 1: The Arusha province of northern Tanzania is outlined in pink. White line through center is the border of Tanzania and Kenya. Ol Doinyo Lengai marked with yellow marker. Image courtesy of Google LandSat imagery (2018).

Ol Doinyo Lengai, or “Mountain of God”, is an active stratovolcano located along the East African Rift (EAR) in the Arusha volcanic province of northern Tanzania (Fig. 1 & 2). This volcano is unique in a few different ways: 1) it erupts carbonate material, rather than the usual silica rich material,  2) typical eruptions are extremely effusive, resembling black water more than lava flows; 3) the lavas are very cool compared to other lavas, at ~ 510℃ compared to 1171℃ at Kilauea; and 4) solidified lava weathers rapidly and turns white (Fig. 3).

Figure 2: Ol Doinyo Lengai volcano. Water stream in foreground. (By Clem23 - Own work, CC BY-SA 3.0)

For the past century Ol Doinyo Lengai has been erupting lavas that are uniquely poor in silicate minerals yet enriched in sodium, potassium, and rare earth elements (REE) such as rubidium, strontium, zirconium, niobium, thorium, and uranium. The most common type of lava on Earth today is basalt. Basalts typically contain 45 - 55% silica (SiO2). The carbonatite eruptions of Ol Doinyo Lengai contain <20% SiO2. So where is this cool, silica poor, REE-enriched lava coming from? This is a question scientists have been trying to answer since the discovery of this volcano. Mollex et al. (2018) may have new answers in their paper “Tracing helium isotope compositions from mantle source to fumaroles at Oldoinyo Lengai volcano, Tanzania.”

Figure 3: Satellite imagery of Ol Doinyo Lengai. The white is not snow. It is weathered carbonate material from previous eruptions. (Image courtesy of Google Maps satellite imagery.)

Helium has two stable isotopes: 3He and 4He. 3He was made during the Big Bang, and as the Earth formed it became incorporated into the mantle. 4He is a product of alpha decay of Uranium and Thorium. By studying the relative abundances of 3He and 4He in lavas, scientists can infer where the magma originates. In the Earth’s atmosphere  the ratio of 3He / 4He is 1.384 x 10-6, while crustral ratios are 0.4 to 1 x 10-8. The 3He / 4He ratios are typically reported relative to the atmospheric level as R/RA. In these units crustal rocks typically have 0.01 to 0.1 R/RA, and mantle rocks have 5 - 50 R/RA (Emsley, 2001) (Fig. 4). Mollex et al. (2018) sampled rocks, fumaroles, and cognate xenoliths1 in order to find the 3He/4He ratios of Old Doinyo Lengai.They found that the helium isotope compositions from cognate xenoliths at the summit of the volcano ranged from 5.79 ± 0.82 RA  to 7.24 ± 0.44 RA (Fig. 5). These ratios closely match those of more traditional silicic volcanoes of the surrounding Arusha volcanic province.

Figure 4: 3He/4He ratios found on Earth and in the solar system. Around 10-5 are Ocean Island Basalts (OIB), Mid-Ocean Ridge Basalts (MORB), and Island Arcs (Arcs). Figure 12.1 from White (2015).

Figure 5: Helium isotope ratios from previous studies (left) and the current study of Mollex et al., 2018 (right). RC / RA are helium isotope ratios corrected for atmospheric contamination. SCLM is Subcontinental Lithospheric Mantle (shallow mantle), and MORB is Mid-Ocean Ridge Basalt. The typical ratios of Typical 3He/ 4He ratios for SCLM are shown in orange (Gautheron & Moreira, 2002). And typical 3He/ 4He for MORB are shown in yellow (Graham, 2002). Gray circles denote uncorrected 3He/ 4He ratios. (R / RA). Figure 4 from Mollex et al. (2018).

Based on the helium isotopic ratios found in rocks and summit fumaroles on Ol Doinyo Lengai as well as other volcanoes in the Arusha volcanic province, Mollex et al. (2018) believe that the magma source may be the same for both. They propose that the magma source is old enriched sub-continental lithospheric mantle (SCLM) that has undergone compositional changes from fluids or a mid-ocean ridge basalt (MORB) type mantle magma that mixes with the Ol Doinyo Lengai magma chamber.

The research of Mollex et al. (2018) sheds more light on this enigmatic volcano but more work is required in order to fully understand what is happening in the complex magma system beneath Ol Doinyo Lengai.

References

Emsley, J., 2011, Natures building blocks: an A-Z guide to the elements: Oxford, Oxford University Press.

Mollex, G., Füri, E., Burnard, P., Zimmermann, L., Chazot, G., Kazimoto, E.O., Marty, B., and France, L., 2018, Tracing helium isotope compositions from mantle source to fumaroles at Oldoinyo Lengai volcano, Tanzania: Chemical Geology, v. 480, p. 66–74, doi:10.1016/j.chemgeo.2017.08.015.

White. W.M., 2015, Chapter 12 Noble Gas Isotope Geochemistry, in Isotope Geochemsitry, New York, Wiley-Blackwell, p. 418-452.

13 new acres of prime landslide derived beachfront property available now!

Figure 1: Map showing location of the Mud Creek slide. Before and after photos are present showing the spectacular movement of material. Also present are other similar landslides and instrumentation

Geologic time is slow. Very slow. When geologists talk about mountains rising, such as the Himalayas, they speak on the millimeter per year scale. Every so often, however, drastic movement occurs. On May 17^th2017, the Mud Creek landslide occurred off of Highway 1 in Big Sur (fig.1). The slide was so large, it added 13 new acres to the coastline of California in an instant (fig. 2); amazingly, no one was injured. Since then, scientists have been able to research the slide to figure out how and why the slope failed so dramatically. 

Figure 2: Oblique time lapse view of the slope before failure, after, and the process of rebuilding the road.  

This event was what’s known as a bedrock landslide, which occur at large scales and often move very large, deep masses of earth. Bedrock landslides are highly dependent on groundwater rather than surface water. If the water table rises high enough into the soil that overlies the bedrock, it will effectively cause the overlying rock and soil to float. Water finds its way between individual grains and exerts a pressure on each grain, pushing them away from each other. This force destabilizes the rock, leading to rapid and catastrophic slope failure. 

It takes time for groundwater to move and cause the water table to rise. Groundwater flow isn’t like a raging river of water moving through the subsurface: in reality, groundwater saturates deeper rocks, sort of like a sponge, and only slowly creeps along if there is sufficient water pressure. Because of this, bedrock landslides can occur on seemingly innocuous, sunny days often months after a large rainfall event. In the case of the Mud Creek landslide, the last large rainfall events occurred in February, March, and April (fig. 3), but the slide didn’t occur until mid-May.

Figure 3: Hydrograph showing discharge of the Big Sur River. Discharge can be used as a proxy for rainfall events by looking for spikes in discharge. The landslide occurred in May when there was no rainfall occurring. 

 The Mud Creek landslide is interesting because it was actually moving steadily before the catastrophic slide occurred. Using remote sensing satellites, researchers were able to track the motion of the Mud Creek landslide for the past several years before the big slide occurred. Before total failure, the slope was moving at a stable rate, creeping along at about .24 to .43 m/yr (fig. 4). During this time (2009-2017), California was in a historic drought. Small rainfall events during the winters would cause the velocity of the slide to increase slightly, but it wasn’t until the historically wet winter of 2017 that the slide started to accelerate and deviate from past years’ motion (fig.4).

Researchers attribute the sudden catastrophic failure of the Mud Creek landslide to rapid changes in climate. The fast transition from a drought to very intense rain events caused the Mud Creek landslide to become unstable very quickly. Other similar, slow moving slides in the area remained stable, likely due to their smaller slope angles. 

Figure 4: Top: Downslope velocity of 3 creeping landslides. Notice rapid acceleration and subsequent failure of the Mud Creek slide. Bottom: Solid lines are precipitation values. The dotted line is the calculated pore fluid pressure. Notice how pore fluid pressure rapidly increases before the point of catastrophic failure.  

            California is currently experiencing another historically wet winter after a small drought, and Sierra snowpack currently stands at about 146% of what it was last year. What will this mean for catastrophic slope failure in California? If the Mud Creek landslide is any indication, it will likely mean more landslides during the spring months as snow melts and percolates through the subsurface. Keep a close eye on the coastal ranges of California this coming year and hopefully we’ll see some geology in action! 

Handwerger, A.L., Huang, M., Fielding, E.J., Booth, A.M., and Bürgmann, R., 2019, A shift from drought to extreme rainfall drives a stable landslide to catastrophic failure: Scientific Reports, p. 1–12, doi:10.1038/s41598-018-38300-0.

https://earthobservatory.nasa.gov/images/144552/a-strong-start-to-sierra-snowpack?utm_source=TWITTER&utm_medium=NASA&utm_campaign=NASASocial&linkId=63616203

Buried liquid water on Mars?

The question of whether there is water on Mars has been hotly debated for decades. It has generally been proven that water exists on Mars, though it’s mostly trapped in its ice form. For life to exist as we know it, liquid water is a vital component. Because Mars has such a thin atmosphere, liquid water on the surface isn’t stable and will immediately boil off. This makes it very tricky to even begin to try to observe liquid water. Recently however, using radar instruments, Italian researchers have discovered evidence of a large, liquid water lake beneath the glaciers at the southern pole of Mars.

Aboard the EU’s Mars Express spacecraft is a special instrument called a radar sounder. Radar sounders work by shooting specific frequencies of radar pulses at the surface of a planet and then observing the reflected signal. This can be thought of like clapping in a cave and listening to the echo that comes back to you. Depending on what the echo “sounds” like, scientists can gain a wealth of information about whatever surface they aimed the instrument at. Differing frequencies used by the instrument contain information about the surface rock, ice, or even what lies beneath the surface.

Figure 1: Radargram of the subglacial lake. In this figure, the x-axis is horizontal distance on the planet, and the y-axis is the time is takes the radar signal to bounce back to the spacecraft, which can be interpreted to understand subsurface topography. This figure can be thought of as a cross section. The subglacial lake can be seen at the basal reflection label. The SPLD label shows the South Polar Layered Deposit, which is an area of the south pole where pure water ice is formed in layers. Figure 2A from Orosei et al. (2018).

Using a radar sounder named MARSIS, Italian scientists made several flybys of the south pole region, which is mostly covered in water-ice glacier year round. What they found were bright subsurface radar reflections where, ordinarily, there should be nothing (fig.1). When you shoot a radar wave at a material, the target’s atoms will react by becoming electrically polarized to varying degrees (the atoms will have one side become more positive than the other, or vice versa). This degree of polarization can be unique for different materials, so it’s a great way to identify materials hidden in the subsurface. This quality of materials is known as dielectric permittivity. By analyzing the MARSIS radar reflections, the scientists discovered that the dielectric permittivity of the anomalous reflection matched that of water-bearing materials. By making several flybys of the area, the scientists were able to map out the subglacial lake and found it to be about 20 km wide and approximately 1.5 km beneath the surface (fig.2) 

Figure 2: This image shows the estimated size of the lake. Radar tracks are colored depending on subsurface elevations with warmer colors corresponding to higher elevations and cooler corresponding to lower elevations. The perceived lake is marked by the bold, black line. Figure 3A from Orosei et al. (2018).

Does this confirm that there’s a subglacial lake at Mars south pole? It’s very likely. (SHARAD, another radar sounder orbiting Mars, didn’t see the lake, though this may be to due to the different set of frequencies SHARAD uses). Does this confirm that there’s life on Mars? Not yet. For all we know, the lake could be completely inhospitable, or might not even be there in the first place! What this study does provide is compelling evidence that the secrets to extraterrestrial life in our solar system might be hiding under some ice, just one planet away!

Orosei, R., Lauro, S. E., Pettinelli, E., Cicchetti, A., Coradini, M., Cosciotti, B., … Seu, R. (2018). Radar evidence of subglacial liquid water on Mars. Science, 361(6401), 490 LP-493. Retrieved from http://science.sciencemag.org/content/361/6401/490.abstract

Comparing the Active Volcanism in Hawaii and Guatemala

Figure 1a and 1b. The image on the left is of the basaltic lava flows in Hawai’i, while the image on the right is of the pyroclastic flow in Guatemala. (Image of Hawaiian flow from USGS and image of Fuego pyroclastic flow from Calgary News).

The Hawaiian volcano Kilauea and the Guatemalan volcano Fuego have gotten a lot of media attention recently due to active eruptions that directly impact people. However, their overlapping occurrences are about the only similarities that these volcanoes share. The geology and hazards of these volcanoes are dramatically different. Kilauea is in the news because it is erupting slow moving (~13 feet per hour) basaltic lava, spewing 200-foot tall lava fountains, and periodically producing localized steam explosions (Figure 1a). Fuego is in the news not because of a lava flow, but because explosive volcanic activity triggered an avalanche of hot rocks and gas. This geologic phenomenon is called a pyroclastic flow and in Fuego it traveled more than 50 miles per hour straight toward a heavily populated community (Figure 1b). Due to its speed and destructive force, pyroclastic flows are much deadlier than basaltic lava flows. The latest death toll for Fuego is 114 people, with reports from the Kilauea eruption describing one man having sustained serious injuries on the ground and injuries to another 23 people who were in a boat observing the lava enter the ocean. Pyroclastic flows are not lava flows, but can be described as a landslide of hot cement and toxic gas that is caused by active volcanism.

Other differences in the two volcanoes are that Kilauea is a shield volcano fed by a hot spot of magma rising from the mantle to the surface, while Fuego is a stratovolcano fed by the collision of two tectonic plates (view Figure 2). At Fuego, the tectonic plate under the ocean is actually sliding under the plate that Central America is sitting on. As the lower plate sinks, it heats up and releases water into the rock above, reducing the melting temperature of that rock and creating a chain of volcanoes. The differences in how the magma is created for these two types of volcanoes has a significant impact of the chemistry of the magmas and how they are able to behave. In Figure 2, the clearest distinctions between the two types of volcanoes are the overall shape and the steepness of their slopes. Steeper slopes tend to be more unstable and allow material to travel faster downhill, such as the case in Fuego’s stratovolcano.

Figure 2a and 2b. The image on the left is of a shield volcano, note the gentle slopes. The image of the right is of a stratovolcano, note the steeper slopes. (GHS)

The differences between these volcanoes are linked to their lava compositions. Both volcanoes’ magmas have varying amounts of gas and the compound silica, which affect the overall consistency and therefore behavior of the lava. Lava that is poor in silica and gas, such as the basalts erupting from Kilauea, flow more easily like ketchup forming overall more dome-like shapes. However, the silica- and gas- rich lava of Fuego, tends to build-up and explode instead of flow forming more cone-like shapes.

The increase of media coverage for these volcanoes does not mean that there is any substantial increase of volcanic activity at either location, or that these volcanoes are in some way connected. According to the U.S Geological Survey there are over 1,500 potentially active volcanoes worldwide with a handful of volcanoes erupting daily.