Steady state volcanism: Evidence from eruption histories of polygenetic volcanoes

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Abstract
Some volcanoes erupt magma at average rates which are constant over periods of many years, even though this magma may appear in a complex series of eruptions. This constancy of output is tested by construction of a curve of cumulative volume of erupted magma, which is linear for steady state volcanism, and whose gradient defines the steady state rate Qss. The assumption is made that Qss is the rate at which magma is supplied to these polygenetic volcanoes. Five general types of eruptive behavior can be distinguished from the cumulative volume curves studied. These types are interpreted in terms of a simple model of batches of magma rising buoyantly through the crust and interacting with a small‐capacity subvolcanic magma reservoir. Recognition of previous steady state behavior at a volcano may enable the cumulative volume curve to be used empirically as a constraint on the timing and volume of the next eruption. The steady state model thus has a limited predictive capability. With the exception of Kilauea (Qss = 4 m3 s−1 ) all the identified steady state volcanoes have values of Qss of a few tenths of one cubic meter per second. These rates are consistent with the minimum flux rates required by theoretical cooling models of batches of magma traversing the crust. The similarity of these Qss values at a variety of volcanoes (producing basalt, andesite, and dacite magmas) in very different tectonic settings suggests that the common factors of crustal buoyancy forces and the geotherm‐controlled cooling rates control the dynamics of magma supply through the crust. Long‐term dormancy at active volcanoes may be a manifestation of the steady state accumulation of magma in large crustal reservoirs, a process that complements the intermittent periods of steady state output at the surface. This possibility has several implications, the most important of which is that it provides a constraint on the supply rate of new magma to the bases of plutons.

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