physical volcanology of silicic lavas and welded tuffs

I have been investigating the processes by which glassy volcanic rocks deform and lithify at high-temperature - "weld" - since my PhD. Welding turns out to be an important process not just in volcanic deposits like ignimbrites (aka ash-flow tuffs) but in active volcanic conduits where welding influences magma porosity and permeability and therefore is a control on explosivity.

flow dynamics in silicic lavas

I recently received National Science Foundation support for a collaborative grant to study the emplacement and flow dynamics of silicic lavas, working with Alan Whittington (Missouri) and Kenneth Befus (Baylor). We will be working with research students at Obsidian Dome and South Coulee lavas in eastern California. Fieldwork and sampling has already started, and LiDAR should be flown in mid-September 2017 over South Coulee. New WVU PhD candidate Shelby Isom will be working on this project. CSU Bakersfield MS (2017) graduate Abigail Martens worked on Obsidian Dome for her thesis, and presented her results at the GSA Cordilleran Meeting in Honolulu (May 2017) and AGU in New Orleans (Dec. 2017). Abby is now a PhD candidate at the University of South Florida.

rheomorphism and lava-like tuffs

My PhD and much subsequent research has focused on high-grade or extremely high-grade welding in ignimbrites sufficient to return the once porous, fragmental deposit back to a homogeneous, coherent mass of rhyolitic melt capable of flowing ("rheomorphism") like a lava, hence "lava-like". Put another way, these deposits preserve the history of rhyolite magma that was initially a coherent fluid in the volcanic conduit, was fragmented and transported as a dispersed cloud of ash and gas tens of kilometers across the Earth's surface, deposited, and welded back together to form a coherent mass of rhyolite capable of flowing downhill. Because no one has ever had to experience one of these eruptions (thankfully) our evidence and understanding of them comes from their deposits and from experiments.

 Conceptual model of syn-depositional welding and rheomorphism in the Grey's Landing lava-like ignimbrite (Andrews & Branney, 2011).

Conceptual model of syn-depositional welding and rheomorphism in the Grey's Landing lava-like ignimbrite (Andrews & Branney, 2011).

The Miocene Grey's Landing ignimbrite of southern Idaho is an exceptionally well preserved and exposed example of a lava-like tuff. My studies of it allowed us to modify existing conceptual models of how these types of deposits were emplaced and when welding occurred. We were able to show unambiguously that welding and ductile flow were synchronous with deposition of the ignimbrite, and that flow continued after deposition as well (Andrews & Branney, 2011).

 Inclined post-depositional flow folds superimposed upon a syn-depositional sheath fold (the "purple wet pull-over")

Inclined post-depositional flow folds superimposed upon a syn-depositional sheath fold (the "purple wet pull-over")

Subsequent experiments by colleagues in the US, Canada, and Europe have constrained the rates and timescales of welding and ductile flow, and support our interpretation that welding and flow could only occur during and immediately after deposition. Moreover, the mobilization of a flowing ignimbrite mass is sufficient to generate enough heat to sustain the welding and flow for some time (Robert et al., 2013).

quantifying welding parameters

Another long unresolved issue with welding has been understanding how much strain is accumulated during welding and how quickly? Different approaches have been used although most of the most recent quantitative studies rely on experiments on real or analogue tuff rather than studies of actual welded deposits. MS student Roger Ward and I reinvented the wheel by examining and testing different methods of measuring strain in real samples, including 2D shape analysis of thin sections, 3D shape analysis in the field, and helium-pycnometry to measure porosity. Roger showed that the different methods each have their own advantages and limitations, and that no one method reasonably measures the strain distribution through the different rock types in the welded tuff. Instead, an more reasonable appraisal requires that all three techniques be applied as much as possible. Roger's MS thesis defense is captured on video here


Sierra Madre Occidental silicic large igneous province, Mexico

Along with students and colleagues in California, Texas, and Mexico, I am actively engaged in petrologic and stratigraphic studies of the volcanology of the Sierra Madre Occidental (SMO) volcanic province in northwestern Mexico. The SMO is the largest silicic large igneous province in North America and the largest formed during the Cenozoic, it also hosts the world's largest epithermal mineral deposit system. As a bonus, the SMO has stunning scenery and great people.

 Over a kilometer thickness of rhyolite in the Rio Guerachi baranca, Sierra Madre Occidental, Chihuahua

Over a kilometer thickness of rhyolite in the Rio Guerachi baranca, Sierra Madre Occidental, Chihuahua

I am interested in understanding how the SMO developed spatially and temporally, especially in the northern and central portions in Chihuahua and Durango states. To this end my colleagues, students, and I have completed two reconnaissance and sampling transects across the SMO west from Hidalgo de Parral, Chihuahua, from which we are investigating the stratigraphy, petrology, and geochronology. We are particularly interested in developing an accurately dated stratigraphic framework with which to put our geochemical and physical volcanological data into context. New WVU PhD candidate Alyssa Kaess will be studying the sedimentary record of the SMO, especially in dateable sections between large ignimbrites. I work closely with friend and colleague Pablo Davila-Harris from IPICYT in San Luis Potosi, Mexico; together we have advised two CSU Bakersfield MS students: Nick Moreno and Linda Anderson.

I am also interested in the petrogenesis of SMO rhyolite magmas, including their storage and transport through the crust, and I am using zircon U/Pb geochronology and Hf and O isotope studies to investigate this. Large igneous provinces are likely associated with greatly increased thermal energy supplied from the mantle, but how much mantle material (i.e. mafic magma) is added to the continental crust? By investigating the relative contributions of mantle and crustal sources in SMO rhyolites we are attempting to constrain how much crust was added during formation of the SMO, and how quickly. It is likely that such episodic crustal-growth events are among only a limited number of mechanisms by which continental crust has been generated in the Phanerozoic. CSU Bakersfield MS candidate Brenda Pack is completing her thesis on the duration of rhyolite magma batches from formation to eruption using Ar/Ar and U/Pb double-dating, and oxygen isotopes in zircon crystals.

 Ignimbrite stratigraphy along Rio Baleza, Chihuahua

Ignimbrite stratigraphy along Rio Baleza, Chihuahua


IODP Expedition 350 - Izu-Bonin-Marianas rear arc evolution

In April and May 2014 I was part of the scientific team on board the research drilling vessel JOIDES Resolution for IODP Expedition 350 to the rear arc of the Izu-Bonin-Marianas arc in the Western Pacific. I was one of several volcanologists, sedimentologists, and petrologists responsible for describing and sampling core samples. I am currently involved in studies of the U/Pb geochronology of the volcanic stratigraphy and the physical volcanology and sedimentology of specific volcanic deposits with colleagues in California and Germany. The expedition started in Keelung, Taiwan (images below) and finished in Yokohama, Japan, and was the first of three back-to-back expeditions to the Izu-Bonin-Marianas arc region in 2014.

Shipboard research results have now been published, and my colleagues and I are concentrating on specific research projects related to Exp. 350. The Exp. 350 Preliminary and Final Reports, and related post-cruise research papers, are available at http://publications.iodp.org/proceedings/350/350title.html#pgfId-633460

I have separate research themes investigating the geochronology of Hole U1437, the physical volcanology and sedimentology of eruption-fed turbidites in Hole U1437, and the compaction and porosity gradient at Hole U1437.


Neogene volcanism and neotectonics of southern British Columbia

in the works


physical volcanology of kimberlites

Along with Kelly Russell and his students at UBC, I have engaged in volcanological studies of kimberlite deposits at Diavik, NWT, Canada. In a series of studies we identified the first known kimberlite pyroclastic density current deposit as part of a sequence of eruptions within the Diavik group of pipes (Moss et al., 2008; 2009). Subsequent studies of kimberlite have focused on the textures and morphologies of olivine crystals within kimberlite deposits and how they record processes during ascent through the mantle as part of the "kimberlite factory" (image below; Brett et al., 2015).