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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

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

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