Sunday, September 11, 2011

Geology and volcanology of Mount Erebus

Mount Erebus is currently the most active volcano in Antarctica and is the current eruptive zone of the Erebus hotspot. The summit contains a persistent convecting phonolitic lava lake, one of five long-lasting lava lakes on Earth. Characteristic eruptive activity consists of Strombolian eruptions from the lava lake or from one of several subsidiary vents, all lying within the volcano's inner crater. The volcano is scientifically remarkable in that its relatively low-level and unusually persistent eruptive activity enables long-term volcanological study of a Strombolian eruptive system very close (hundreds of metres) to the active vents, a characteristic shared with only a few volcanos planetarily, such as Stromboli in Italy. Scientific study of the volcano is also facilitated by their proximity to McMurdo Station (U.S.) and Scott Base (N.Z.), both sited on Ross Island approximately thirty-five kilometres away.

Mount Erebus is classified as a polygenetic stratovolcano. The bottom half of the volcano is a shield and the top half is a stratocone (Mount Etna is like this as well). The composition of the current eruptive products of Erebus is anorthoclase-porphyric tephritic phonolite and phonolite, which constitute the bulk of exposed lava flow on the volcano. The oldest eruptive products consist of relatively undifferentiated and non-viscous basanitic lavas that form the low, broad platform shield of the Erebus edifice. Slightly younger basanite and phonotephrite lavas crop out on Fang Ridge — an eroded remnant of an early Erebus volcano — and at other isolated locations on the flanks of the Erebus edifice.

Lava flows of more viscous phonotephrite, tephriphonolite and trachyte erupted after the basanite. The upper slopes of Mount Erebus are dominated by steeply dipping (−30°) tephritic phonolite lava flows with large scale flow levees. A conspicuous break in slope at approximately 3,200 metres calls attention to a summit plateau representing a caldera less than one hundred millennia old. The summit caldera itself is filled with small volume tephritic phonolite and phonolite lava flows. In the center of the summit caldera is a small, steep-sided cone composed primarily of decomposed lava bombs and a large deposit of anorthoclase crystals known as Erebus Crystals. It is within this summit cone that the active lava lake continuously degasses.

Researchers spent more than three months during the 2007-08 field season installing an array of seismometers around Mount Erebus to listen to waves of energy generated by small, controlled blasts from explosives they buried along its flanks and perimeter. Seismometers measure and record the size and force of underground energy, or seismic, waves. By studying the refracted and reflected seismic waves, the scientists attempted to map the interior of the volcano, much as a CT scan images the inside of an object using X-rays, to understand its deep "plumbing" and how the magma rises to the lava lake.


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