Wednesday, September 6, 2017

Comparisons between features in Yellowstone and on Axel Heiberg Island

Hi hi!
Eclipse, 10:22 MST, I took this
through a telescope

I’m writing to you after a phenomenal family vacation I took with my parents, aunt and uncle down to Idaho and Wyoming. The main destination – the totality zone for the eclipse! The eclipse was indeed a very breathtaking, magical experience, but today I will be writing about some of the geology we saw. While passing through Idaho, we stopped at Craters of the Moon National Monument. We briefly visited with Gavin, Kevin, Raymond, and Mike who were a bit knackered after a day of strenuous fieldwork. I won’t write much more about those lava flows, because Gavin has written about them in stylish detail.After Craters of the Moon, we drove into Wyoming and visited what must be a geological, volcanic mecca – Yellowstone National Park. Now, I could write for pages about the geology of Yellowstone, because it is such an exciting, unique place full of epithermal wonders. For this post, I’m going to focus on two features that seemed to show some parallels to what I saw on Axel Heiberg Island. (But you can see all my photos here!)

First are a lot of ignimbrites around the park, whose violent eruption triggered the massively catastrophic caldera collapse 640,000 years ago. Also known as
ash-flow tuffs, ignimbrites are a type of pyroclastic flow deposits characterized by high abundance of ash-sized (<4 mm) particles and pumice.  The scale of eruption required is enough to partially or fully empty magma chambers. The dramatic exodus of material substantially decreases the internal pressure of the chamber, often resulting in caldera collapse if the chamber roof is no longer sufficiently supported. The top of the magma chamber erupts out and is deposited first, so the ash-flow tuff sequence represents the inverted order of the magma chamber’s internal fractionalization.  The layers in ignimbrites can be divided into pyroclastic flow units from multiple eruptive pulses. A typical ignimbrite sequence contains a basal layer with reverse graded pumice and lithic clasts. Above the basal layer, flow units have density separated clasts: pumice is light and floats to the top, showing reverse grading, whereas denser lithic clasts are normally graded. Ash-flow tuffs thin out and have fewer clasts away from their source. If the flow units cool together, they form a compound cooling unit. Sufficiently hot and thick ignimbrites fuse and solidify from partial or complete welding. Welding occurs when hot, pliable pumice clasts flatten under overburden weight and sinter together, giving them a glassy appearance (Francis and Oppenheimer 2004).
Now you are no doubt wondering what on earth a explosive, caldera-building volcanic unit has anything to do with salt diapirs in the Arctic.  Genetically, nothing. But morphologically? Take a look:
The walls of this river-carved valley are jagged and karstic, looking a lot like what we saw at Stolz and Wolf Diapirs.
This looks a lot like that pseudo-karstic topography we saw in the Axel Heiberg Diapirs. The reason is simple. Ash, like salt, is softer than other types of rock. Therefore, it erodes away more readily than the surrounding units and forms jagged peaks. In fact, because ash-flow deposits are soft the Fremont First Nations used faces of ignimbrite for wall carvings in Utah.  I thought this was neat, and worthy of sharing. I imagine that ignimbrite exposed in valley or canyon walls would look very similar to salt diapirs in synthetic aperture radar, which further demonstrates the need to combine different orbital datasets paint a full picture.
Second are the hot spring travertine deposits, like Mammoth Hot Springs.  The mechanism that forms these deposits in hot springs at Yellowstone is like the perennial cold springs in the Arctic. In Mammoth Springs, geothermally heated ground water passes through limestone via a fault, and leaches out calcium. The calcium-rich water upwells to the surface and deposits calcite in the form of travertine as it cools.  

Mammoth Hot Springs has produced the largest known travertine deposit

Seismic activity alters the spring flow paths, enabling expansive terraces to form 

On Axel Heiberg Island, the waters that form Lost Hammer Spring are thought to be passing through subsurface extents of the Wolf Diapir, thereby passing through halite and anhydrite and leaching out sodium and calcium. Subsequently as the spring flows over, the deposits at Lost Hammer are halite, calcite, gypsum, thenardite and mirabilite (Battler et al. 2013). Despite Mammoth springs being a hydrothermal system, and Lost Hammer Spring being a perennial cold spring, I find it interesting that both sites are formed the same way.
Lost Hammer Spring, seen from above

The edge of Lost Hammer Spring, where salt-rich water has overflown and precipitated

In a week and a half, I’m going to the Space Generation Congress and the International Astronautical Congress in Adelaide, Australia! If I find some downtime, I’ll try to provide some short updates here, but I’ll definitely be tweeting about the conferences!
Battler, M.M., Osinski, G.R., and Banerjee, N.R. 2013. Mineralogy of saline perennial cold spring on Axel Heiberg Island, Nunavut, Canada and implications for spring deposits on Mars. Icarus 224: 364–381. doi:10.1016/j.icarus.2012.08.031.
Francis, P. Oppenheimer, C., 2004. Volcanoes Second Edition. Oxford University Press.

1 comment:

  1. Hmm, I'm not sure the ash deposits would look like the salt diapirs in radar. Ash is typically pretty transparent to radar wavelengths, unlike a more solid surface such as salt or rock. Pyroclastic deposits on the Moon have extremely low CPR.

    Also, I love the use of the word 'epithermal'. I've only ever heard it in reference to neutrons before. (A decrease in epithermal neutrons near the poles of the Moon suggest the presence of water ice there.)