We investigated how thick slow-moving silicic lava flows are emplaced. This type of lava flow has only been observed once in recent history, in 2011-12 at Cordon Caulle in southern Chile, but they have occurred at many places around the world and are potential hazards facing many communities in the western US and elsewhere. Active lava flows are dangerous or impossible to sample, and only reveal their exterior morphology. We studied examples from the Mono-Inyo craters in eastern California, erupted about 650 years ago, and from the Valles caldera in northern New Mexico, erupted about 40,000 years ago, where erosion has exposed the interior of the flows. Both of these examples were sampled in the 1980?s by scientific drilling, and we collected samples in the field and also from drill core. The key questions we asked were:

(i)    Are silicic lavas emplaced as single flows are they multiple stacked flows, one on top of another?
Thick flows can move even when they are more viscous, so multiple flows would require a lower viscosity and faster emplacement, although perhaps over a protracted time interval for multiple flows to be erupted.

(ii)  How does deformation style evolve in space and time during cooling?
Evenly spaced ridges on the surfaces of lava flows have traditionally been interpreted as resulting from folding, meaning that lava near the vent was pushing on lava further away. This requires a lot of stress to be transmitted through the uppermost layer of the flow, which is inconsistent with it being ductile and foldable.  

To answer the first question we developed a new technique for determining the cooling rate of lava, using Differential Scanning Calorimetry. Our new method is faster and more accurate than what was previously used, and will be useful in future studies of cooling lava. We then used the new method on samples from the drill cores, from top to bottom through the lava flows. We found that cooling rates varied by several orders of magnitude through the flows, between 1K/s and 1K per hundred years. They were fastest at the top and bottom, and slowest in the middle, consistent with emplacement as a single flow.  This suggests that these flows have high viscosities and advance slowly in a tank-tread type manner. We found no evidence for inflation by later pulses of lava being injected into the middle of the previously erupted lava, which had been suggested as a mechanism for producing such thick flows.

To answer the second question we combined field observations, high resolution topography derived from remote sensing, and laboratory measurements of lava viscosity at eruption temperature. We found that the ridges on flow surfaces are caused by brittle faulting during extension of the flow. This is completely opposite to the previous interpretation of ductile folding during compression. Our rheological measurements showed that the lava would not be strong enough to support the compressive stresses needed for folding (it would break instead), and there was no mechanism to produce the stresses required for folding ? gravity alone would be insufficient. Gravitational spreading can produce fractures during extension, because the tensile strength of lava is much lower than its compressional strength. We conclude that estimates of lava flow rheology based on the folding interpretation need to be reassessed.       

PI Whittington gave several talks about volcanoes to school groups (in person and online, via the ?Skype a Scientist? program), and to the general public.  This project supported research by three students: one PhD, one MS and one BS, all in Geological Sciences at MU.

Last Modified: 12/20/2020
Modified by: Alan G Whittington