Holuhraun: Wherefore Art Thou Lava Flow

September 15, 2019

 Let us get into the details on why I went to Holuhraun by first talking about the research motivation. Studies on Martian lava flows have primary involved remote sensing techniques, including radar, high resolution optical imagery, and near infra-red cameras. Using these remote sensing techniques, planetary scientists have studied the surface morphology and roughness of the lava flows, inferred their volcanic origin, and chronologically mapped the surface of planets and moons. The surface morphology and roughness (or surface textures for general simplicity) of Martian lava flows are of great interest for planetary scientists because understanding their origin and formation provides a window into the planets volcanic and magmatic processes. It is difficult however to make inferences about lava flows on Mars because despite having access to a suite of remote sensing datasets we have little ground-truth information (only images and in-situ data collected by Mars landers and rovers). To study Martian lava flows, we require an analogue site on Earth that shares similar morphologies. Martian lava flows exhibit plate-ridged surface textures, analogous to transitional lava flows in Iceland (e.g. mechanically fractured surfaces such as slabby or rubbly pahoehoe) [Keszthelyi et al. 2006]. To exhibit a surface texture similar to Icelandic lava flows, the Martian lava flow would have had to have formed under similar conditions. By studying these lava flows on Earth, we can develop a more refined understanding of how Martian lava flows formed. 

 

     Surface textures such as `a`a clinkers and plate-ridged textures are within the scale of most radar wavelengths (e.g. L-Band (24 cm) and P-Band (70 cm)), creating radar back-scatter effects that are detected by the instrument receiver. Transitional lava flow surfaces are sensitive to decimetre to metre-scale size radar wavelengths and would appear bright under radar imagery and return specific back-scatter signals. However, some lava flow surfaces appear similar under radar, making it difficult to identify the lava flow type. With high resolution imagery, it might be possible to distinguish them. On Mars, imagery at 0.25-1 m/pixel taken by the High Resolution Imaging Science Experiment could identify lava flow types that appear similar under radar. However, in order to confirm our identifications we need quantified results of the flow surface textures. To achieve this, I will be extracting surface roughness statistical values from high resolution topography datasets (UAV DTMs and LiDAR point cloud DEMs) and Unmanned Aerial Vehicle Synthetic Aperture Radar (UAVSAR-L) data (wavelength is 24 cm). The statistical values I will be extracting include circular polarization ratio (CPR), root-mean-square height and slope values and Hurst exponent. They provide information about the centimetre-scale roughness of lava flows, which is hidden from radar data. By comparing the quantified results of high resolution topography data to radar I can improve our interpretations on the origin and emplacement of lava flows on Mars using remote sensing datasets such as HiRISE and Arecibo Observatory radar.

 

Field Work - Holuhraun

 

     Now that we have gone through the motivation for the research, let us dive into the field work. To improve remote sensing interpretations of lava flows I had to study as many different types of lava flows at Holuhraun. My work involved taking detailed field observations, collecting images of the surface textures, selecting locations for LiDAR scans to be conducted, and sample the surface and interior of different lava flow types. I identified five types of lava flows at Holuhraun, all of which have been previously identified in literature: spiny, rubbly, `a`a, pahoehoe, and shelly lava (Figure below). Each of the lava flows formed under different eruption conditions, resulting in their contrasting surface textures. For details on how these types of lava flows form refer to Harris et al., (2017). From each lava flow, I collected a sample of the crust and (if possible) the interior. The surface is what interacts with the remote sensing data, therefore its physical properties (e.g. vesicularity) may explain some of the radar backscatter effects in the UAVSAR-L imagery and the scattering of laser points in the LiDAR point clouds. 

 

     

 

Images above were taken by Gavin Tolometti at Holuhraun in 2019. Spiny lava extruded out of irregular shaped orifices (a) produces grooves on the surface, giving it a rough texture. The spiny lava exhibits wave surface features, showing how the lava flow crust formed. (b) Plate-ridged texture on spiny lava flows. Formed during inflation of the spiny crust. (c) Toes of spiny lava extrude out of the margin similar to pahoehoe toes at Hawaii. (d) Spiny lava with a slabby surface texture. Mechanical disruption of the surface by either change in effusion rate or underlying topography. (e) Spiny lava with a rubbly surface texture. (f)  Rubbly lava with centimetre to decimetre-sized fragments of lava crust. (g) `A`a lava flow breaking out of a channel located at the NE margin of the volcanic vent. (h) Pahoehoe lava with lobes and rope surface textures (d) found in close proximity to the vent. Not found at other locations at Holuhraun. (i) Shelly lava pond with a frail, thin crust.

 

     When weather permitted, I worked with Dr Antero Kukko (a senior scientist from the Finnish Geospatial Institute) to locate and collect LiDAR scans of each lava flow type. His state of the art backpack-mounted, kinematic LiDAR scanning system (backpack LiDAR for short) obtains topographic information at the centimetre-scale (2-5 cm/pixel). The instrument collects point cloud data, which is converted into a digital elevation model for data manipulation. With a resolution this high, we can observe small scale surface textures on the lava flows such as the few centimetre spines on the spiny lava flow. Dr Antero Kukko was able to collect over a dozen LiDAR scans, collecting lava flow surface textures along the lava field margins, lava crust located more than 500 m from the margin, and buried lava flows from older volcanic eruptions adjacent to the margins. The LiDAR scans are currently being processed by Dr Antero Kukko at the Finnish Geospatial Institute. The finished will be point cloud DEMs. Dr Antero Kukko will be sending me the finished products around October when I can begin extracting roughness statistics from the data.

 

Images of Dr Antero Kukko operating his backpack LiDAR system at Holuhraun. Scans were conducted on the lava field surface and along the western and northern margins.

 

     The final part of my field work involved collecting samples (See image of Holuhraun below). The goal for sample collecting was to obtain hand specimens of each type of lava flow at Holuhraun to study their petrography and geochemistry. In the literature, Holuhraun has been discovered to be geochemically homogeneous, so we should expect little to no difference between the samples after they have been processed. However, Holuhraun has a complex eruption and emplacement history, which is why the petrography is of great interest to us. Surface samples were collected from each lava flow that was studied prior to field work using remote sensing data (rubbly, spiny, shelly, pahoehoe and a`a). Interior samples (collected >30 cm below the crust) were collected to study the emplacement history of the particular lava flow type. The crust cooled rapidly in relation to the interior so its petrography only holds information about quenching. The interior however cooled at a slower rate and therefore holds key information about the emplacement history of the lava flow itself. 

 

I am still waiting on the samples to be delivered to my host institution. Once they arrive, I will be preparing them for petrographic and geochemical analysis. A separate blog will be dedicated to this process later on this month.

 

Overview of the Holuhraun lava field. Diverted glacial-fed river channels meander round the margins of Holuhraun, transporting sediment from the base of the glacier and volcanic soil the lava fields margins. The yellow stars mark the GPS locations of sample sites. Majority of sites are located at the vent because shelly, pahoehoe and a`a morphologies are present.

 

     This was a brief summary of what my field work at Holuhraun entailed in July-August 2019. I will be releasing my blog posts about Holuhraun once I receive the samples and LiDAR data. If you are thinking "is there more to know about Holuhraun and the research that has been conducted since it started erupting in 2014?", the answer is YES. On the 18th of October, I will be giving a research forum talk on my research on Holuhraun and the collaboration of research that has occurred since 2014. More information regarding the time and location of the talk will follow very soon. For those of you who are not Western University students or staff that read this blog there will be a link to listen to my forum talk remotely.

 

Until next time, have a wonderful September and start to the fall term!

 

 

References:

  • Harris, A. J., Rowland, S. K., Villeneuve, N., & Thordarson, T. (2017). Pāhoehoe,‘a ‘ā, and block lava: an illustrated history of the nomenclature. Bulletin of Volcanology, 79(1), 7.

  • Keszthelyi, L., Self, S., & Thordarson, T. (2006). Flood lavas on earth, Io and Mars. Journal of the geological society, 163(2), 253-264.

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