E6 Physical Volcanology

Topic

The physics of a volcanic eruption is fundamentally governed by magma rheology and the way the magma releases its dissolved gases. At depth, gases—primarily water and carbon dioxide—remain dissolved in the molten magma, following a well-established relationship between gas solubility and the pressure exerted on the gas: the lower the pressure, the lower the gas solubility within the magma. Consequently, as the magma rises and pressure drops, the gases can no longer remain dissolved and begin to form bubbles.

The rapid expansion of these bubbles as the magma continues to rise can cause the void fraction within the magma to exceed a critical fragmentation threshold—approximately seventy-five percent of the total volume. Once this threshold is crossed, the magma ceases to behave as a continuous viscous liquid and abruptly fragments into clasts and ash; it is precisely this sudden fragmentation that triggers an explosive eruption.

The height of the eruption column scales with the fourth root of the volcano's material emission rate; thus, significantly increasing the emission rate results in only a moderate rise in column height. Nevertheless, columns exceeding twenty-five kilometers in height reach the stratosphere, injecting sulfate aerosols capable of disrupting the global climate.

When the density of the eruption column exceeds that of the surrounding air, the column collapses under its own weight, generating a pyroclastic flow—a scorching cloud of gas and particles that rushes down the volcano's slopes at speeds of 200 to 300 meters per second and temperatures ranging from 300 to 700 degrees Celsius. Of all volcanic phenomena, the pyroclastic flow is the most lethal. The Volcanic Explosivity Index is the logarithmic scale that classifies the explosivity of an eruption, ranging from a value of zero for a purely effusive eruption—such as those of the Kilauea volcano—to a value of eight for the Toba eruption, which occurred approximately seventy-four thousand years ago and ejected nearly two thousand eight hundred cubic kilometers of volcanic material; this closes the link between the magma degassing mentioned at the beginning of this description and the full spectrum of eruptive violence that such degassing can produce.

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