The variation of magma discharge during basaltic eruptions
Abstract
The rate at which basaltic magma is discharged varies substantially during many eruptions. An individual eruption has an eruption rate ( Qe), the volumetric rate of discharge averaged over the whole or a major part of an eruption, and an effusion rate ( Qf), the volumetric flux rate at any given time. In many examples Qf soon reaches a maximum value after a short period of waxing flow, partly because of magmatic expansion, and then falls more slowly in the later parts of the eruption. The release of elastic strain energy from stored magma and the sub-volcanic reservoir during eruption can produce an exponential form of such waning flow. Comparison of the eruption rates of the historic eruptions of Mauna Loa, Kilauea and Etna shows that for each volcano there is a trend of decreasing Qe with increasing duration of eruption. This relationship is not predicted by a simple elastic model of magma release. Two additional processes are invoked to explain the eruptive histories of these volcanoes: modification of the eruptive conduits, and the continued supply of magma from depth during eruption. Conduits evolving from dikes to plugs by wall-rock erosion or freezing of magma can result in increased early values of Qf and the maintenance of very low values of Qf values for long periods later in the eruption. Discharge variations during three specific eruptions are discussed in detail. Paricutin (1943-1952) had exponentially waning flow, with a time constant of about three years, that is consistent with a deep reservoir. The waning flow of Hekla's 1947-1948 eruption showed some of the characteristics of conduit modification, whilst the 1959 Kilauea Iki eruption is interpreted in terms of a closed system with varying magma rheology.
- Publication:
-
Journal of Volcanology and Geothermal Research
- Pub Date:
- December 1981
- DOI:
- 10.1016/0377-0273(81)90020-2
- Bibcode:
- 1981JVGR...11..139W
- Keywords:
-
- Basalt;
- Magma;
- Volcanology;
- Cones (Volcanoes);
- Effusives;
- Elastic Deformation;
- Flow Characteristics;
- Lava;
- Viscous Flow