A quantitative analysis of volcanic unrest: Mt. Spurr Alaska, 2002–2006

• We use multiple observations to quantify a volcanic unrest event at Mt. Spurr, Alaska.
• Volatile release at the brittle-ductile transition cannot directly supply the heat needed to form the observed ice cauldron.
• Increased heat output from a two-phase hydrothermal system provides the observed heat energy.

The volcano unrest event at Mt. Spurr, Alaska, which occurred between 2002 and 2006, was characterized by increased seismicity and heat transfer that resulted in ice melting at the summit, increased CO2 transport, and debris flows; but it did not result in a magmatic eruption. We argue that rupturing of the brittle-ductile barrier at a depth of approximately 5 km below sea level, as suggested by the observed seismicity, coupled with the release of magmatic volatiles, and diffusion of the resulting pressure pulse through the edifice likely enhanced heat transfer from the pre-existing hydrothermal system beneath the summit. The estimated heat output of ≈ 32 MW and thermal energy release of ≈ 2.1 × 1015 J that derived from the volume of ice melted over the duration of the unrest event shows that heat transport by CO2 alone was not sufficient to generate the observed thermal effects. Assuming the observed heat output represents a ≈ 10% perturbation on the background heat output, a similar increase in the Rayleigh number in a convecting liquid-dominated reservoir above a background value ~ 100 times critical would produce the observed heat output. This result yields a permeability ranging between 2 × 10− 14 and 10− 13 m2, depending on the thickness and temperature difference assumed. The enhanced heat flux from the liquid reservoir to the overlying vapor-dominated regime, or movement of liquid into the two-phase regime as a result of the pressure drive associated with the rupture of the brittle-ductile barrier, could also yield the observed heat output.