IAVCEI 2013 Scientific Assembly - July 20 - 24, Kagoshima, Japan
Forecasting Volcanic Activity - Reading and translating the messages of nature for society

1P1_2H-O12

Room A1

Date/Time: July 21 16:00-16:15

Thermo-rheological feedbacks in silicic lavas and ignimbrites
Alan Whittington1 , Graham Andrews2 , Geoffroy Avard3 , Genevieve Robert1 , Jiyang Ye1
1Department of Geological Sciences, University of Missouri, USA, 2 Department of Geology, California
State University Bakersfield, USA, 3 OVSICORI-UNA, Costa Rica
E-mail: whittingtona@missouri.edu

The rheology of lava is highly dependent on temperature, both directly (via non-Arrhenian temperature dependenceof melt viscosity) and indirectly (via increasing crystal content). Rheology feeds back to temperature, because
rapidly sheared melts can undergo viscous heating (heat production = viscosity × [strain rate]2 ), and rapid
disequilibrium crystallization can cause heating due to latent heat release (∆Hxt ). The heat budget of partiallycrystalline lava offsets these gains with conductive losses controlled by thermal diffusivity (D) and conductivity(k = DρCP , where ρ is density and CP is heat capacity).

We measured the apparent viscosity of several crystalline dacitic lavas from Santiaguito, Guatemala.

At

conditions appropriate to lava flows (shear stress ∼0.1 to 0.4 MPa, strain rate ∼10-8 to 10-5 s-1 ), apparent viscosity
is best modeled as a power-law with no yield strength. Viscosity of the flow core, at ∼850 °C, is estimated∼ 5×1010 Pa.s. There is no evidence for significant crystallization during flow emplacement at Santiaguito, but
viscous heating may be significant ongoing heat source within these flows ∼ 100Wm-3 if most shearing is restricted
to a ∼1m wide zone), enabling highly viscous lava to travel long distances (∼4 km in ∼2 yrs for Santiaguito).

Extremely high-grade, lava-like welded ignimbrites are produced by many large explosive (super-)eruptions.
The lava-like and rheomorphic Grey’s Landing ignimbrite, Idaho, provides abundant field evidence supporting
the upward migration of a transient, 1-2 m thick, sub-horizontal ductile shear zone at the interface between thepyroclastic density current and deposit, through which all of the deposit passed. Using rheological experiments andthermo-mechanical modeling, we demonstrate that syn-depositional welding and ductile flow is achievable withina very restricted field of likely time-temperature-strain space where rapid high-strain deformation (≤1000%) isfavored by higher emplacement temperatures (≥850 °C). The field of ductile deformation is broadened significantly
by accounting for strain-heating, which permits a sustained temperature increase up to 250 °C within the shear
zone, and helps to explain the enormous extents of lava-like lithofacies and intense rheomorphism recorded inextremely high-grade ignimbrites. Recognition of short-lived but very powerful (≥1MWm-3 ) strain-heating withinrheomorphic ignimbrites suggests that large pyroclastic flows may travel over a hot substrate.

We conclude that strain heating, an inevitable result of magma transport that feeds back to rheology andtransport, should be taken into account in thermal modeling of volcanic processes at both high and low strain rates,and both pre- and post-eruption.

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