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.

How lava flows on Olympus Mons tell us about the volcanic history of Mars

When we think about big mountains on Earth, the first thing that comes to mind is almost always Mt. Everest. Everest, however, is absolutely miniscule in comparison to the solar system’s largest volcano, Olympus Mons.

Standing at 21,229 meters above the Mars global datum (which can be thought of like sea level here on Earth), Olympus Mons is about two and half times as tall as Mount Everest. Unlike Everest, which is the result of two continental plates slamming into each other, Olympus Mons has a volcanic origin. It can be classified as a shield volcano, similar to those that make up the Hawaiian Islands.

Shield volcanoes are characterized by broad, gentle slopes (4-8 degrees) and tend to erupt nonviolently out of fissures on the flanks of the volcano (think the recent lava flows in Hawaii). The tops of shield volcanoes tend to have calderas, which are large collapsed structures found at the top of volcanoes. The magma in the chamber provides pressure that effectively holds up the top of the volcano.The magma chamber is literally inflating the top of the volcano, sustaining its dome-like shape. When the volcano is done erupting and the chamber is partially empty, the top of the volcano collapses and forms a caldera. Things can get a bit more complicated, however, as has been recently discovered at the caldera at the top of Olympus Mons.

Figure 1: The caldera atop Olympus Mons. Colors correspond to elevation, where warmer colors are higher and cooler colors are lower. The blue shapes extending outwards in almost all directions are lava flows. From Mouginis-Mark and Wilson (2019).

The caldera atop Olympus Mons, like the volcano itself, is huge. Its horizontal dimensions are 60 X 80 km, and it has a depth of about 3 km. Lava flows radiate outwards from the rim of the caldera (Fig.1). The strange thing about this caldera, however, is that some of these lava flows appear to flow uphill (Fig. 2). This is not normal, even on Mars. Lava can sometimes travel small distances uphill due to confining pressure or momentum. The lava flow on Olympus Mons, however, didn’t travel a short distance uphill -- it appears to have gone several kilometers!

Figure 2: Zoomed in from figure 1 to show the anomalous lava flow. The left and right images show the same lava flow. The left is CTX image of the flow and the right is an interpreted sketch of the flow with contour lines. The arrow denotes flow direction. Notice how the flow travels uphill. From Mouginis-Mark and Wilson (2019).

Scientists have instead interpreted this to mean that the lava flow was emplaced and flowed downhill normally, cooling as it did so. Later, new magma was brought up to the near surface through cracks in the rock in a sheet-like intrusion, called a dike. This dike caused a localized inflation to occur within the caldera which tilted certain areas. One of those areas was near the lava flow, making it appear to be flowing uphill. But why is this significant? This tilting shows scientists that volcanism didn’t end when previously thought, when the caldera collapse occurred. Instead, there must have been renewed volcanism closer to the present. By understanding the timing of volcanism on Mars, we can better understand Mars’s evolution as a planet, and therefore what past conditions on Mars could have been like. Was Mars more volcanic in its past? Could this volcanism provide a greenhouse effect to allow life to exist? This work takes us one step closer to fully answering these questions.

Original paper:

Mouginis-Mark, P. J., & Wilson, L. (2019). Late-stage intrusive activity at Olympus Mons, Mars: Summit inflation and giant dike formation. Icarus, 319(September 2018), 459–469. https://doi.org/https://doi.org/10.1016/j.icarus.2018.09.038

How Can Molten Rock Transform into Geometric Columns?

Figure 1: (A) Columnar jointing on mars. (NASA, 2007). (B) Tops of columnar joints at Giant’s Causeway, Northern Ireland. (Wikimedia: Code Poet, 2005). (C) Sides of columnar joints at Devil’s Tower, Wyoming. (Wikimedia: Jonathunder, 2017).

Marte Vallis of Mars (Figure 1a), Giant’s Causeway of Northern Ireland (Figure 1b), and Devil’s Tower of Wyoming (Figure 1c) give us just a few examples of the beautiful features formed when lava flows cool and contract to produce geometric columns, a phenomenon called columnar jointing. When columnar joints form, solidified igneous rocks crack in a particular way that causes them to develop columns that resemble  hexagonal  honeycombs from above and a bundle of posts from the side (Figure 2a and 2b). These cracks, called joints, form in lava as it cools and act as pathways for water to flow through the hot rock. Water can flow into the spaces between the columns and react with the hot rocks, even forming valuable mineral deposits such as copper and zinc. This makes them a valuable resource for geothermal and hydrothermal energy, as well as mineral deposits. Columnar joints are common features that can form in any kind of igneous rock, but the conditions under which they develop aren’t perfectly understood.

Figure 2: (A) Close up view of the tops of columnar joints at Giant’s Causeway, Northern Ireland (Wikimedia: Mayer, 2003). (B) Close up view of the sides of columnar joints at Devil’s Tower, Wyoming (Wikimedia: Konstantin, 2003).

Most materials expand when they’re heated up and then contract when they’re cooled down. For a molten lava flow, this means that it takes up less space after it cools. Cracks or joints form in order to compensate for that change in volume. If the contraction happens under specific  conditions, columnar joints will form. But what exactly are these conditions?

Researchers, Lamur et al. 2018, brought rock samples from Iceland’s Eyjafjallajökull volcano into the lab and performed specialized heating and cooling experiments. Their goal was to understand how and why columnar jointing takes place by focusing on the temperature window in which they form. To do this, they heated the field samples to simulate the natural pressure conditions expected in a typical lava flow. They observed how the rocks changed as they cooled to figure out how temperature affects the formation of columns and the size of the gaps between them. Lamur and his team found that columnar jointing happens at 840-890ºC (1544-1634ºF). This means that columnar joints only form when the  molten rock has begun to solidify, but is still hot enough for the rock to be somewhat flexible. They also used their observations to predict how wide the gaps between columns would become under different temperature conditions. The size of those gaps tells us about how well fluids would be able to circulate within the rocks. The larger the gap, the greater its contribution would be to secondary mineral deposition and to geothermal and hydrothermal resources.

References

Code Poet. "Giant's Causeway, Co. Antrim, Northern Ireland." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 28 May 2005. https://commons.wikimedia.org/wiki/File:Causeway-code_poet-4.jpg

Jonathunder. "Devils Tower National Monument at sunset in Wyoming, United States." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 23 August 2017. https://commons.wikimedia.org/wiki/File:SquareDevilsTower.jpg

Konstantin, Phil. "Closeup photo of the columns on Devil's Tower." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 24 May 2003. https://commons.wikimedia.org/wiki/File:DevilsTowerCloseupByPhilKonstantin.jpg

Lamur, Anthony, et al. "Disclosing the temperature of columnar jointing in lavas." Nature communications 9.1 (2018): 1432. https://www.nature.com/articles/s41467-018-03842-4

Mayer, Matthew. "Close up of Giant's Causeway." Wikipedia: The Free Encyclopedia. Wikimedia Foundation, Inc., 25 April 2003. https://commons.wikimedia.org/wiki/File:Giants_causeway_closeup.jpg

NASA. “Marte Vallis, Mars.” Nasa Earth Observatory, 31 October 2007. https://earthobservatory.nasa.gov/IOTD/view.php?id=38904