A2 Extreme Volcanic Eruptions

Topic

An extreme volcanic eruption—with a Volcanic Explosivity Index of six or higher—injects vast volumes of pyroclastic material and gases directly into the stratosphere. This index is logarithmic, meaning each additional unit represents roughly ten times the volume of ejected material compared to the preceding unit. The threshold distinguishing an explosive eruption from a quiet one depends on how much dissolved gas the magma can retain before the resulting overpressure exceeds the tensile strength of the rock making up the volcanic edifice itself.

The dynamics of the rising eruption column determine what happens next: if the column remains a stable, convective Plinian-style plume, the pyroclasts within it disperse into the stratosphere; however, if the column fails to remain buoyant, it collapses in on itself, generating devastating dense pyroclastic currents that rush down the volcano's slopes. This transition between a stable column and a collapsing one depends on the balance between the eruption's sustained mass flux and the amount of cold ambient air the column entrains; when the column incorporates too little cold air relative to the mass flux being expelled, it loses buoyancy and collapses.

A dense pyroclastic current resulting from such a collapse travels at speeds of 100 to 300 kilometers per hour and temperatures of 300 to 800 degrees Celsius; at these speeds and temperatures, the current is completely unstoppable by anything in its path.

Massive effusive eruptions that form large igneous provinces—such as those of the Deccan or Siberian Traps—release such vast quantities of carbon dioxide and sulfur dioxide that they alter the global climate for millennia, and these same massive effusive eruptions correlate with major mass extinction events in the geological record. The sulfur dioxide released by these eruptions causes rapid cooling that manifests within just a few years, whereas the carbon dioxide they release produces, by contrast, much slower warming that persists for over ten thousand years.

Supervolcanoes—with a Volcanic Explosivity Index of eight or higher—are capable of triggering volcanic winters that affect the entire planet simultaneously; this is the same type of global climate disruption—albeit taken to its most extreme level—with which this description of extreme volcanic eruptions began.

ID:1365