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The most recent eruption of Mount Erebus began in 1972 and stopped in 1992. It shares some similarities with both Kilauea and Mount St. Helens but also has some significant differences. Like Kilauea current eruption, the vent was a lava lake that produced some lava flows. Unlike Kilauea, Mount Erebus is a stratovolcano, the same type of volcano as Mount St. Helens. Eruptive activity at Mount Erebus tends to be strombolian , a little more explosive than Kilauea but far less than Mount St. Helens. The composition of the volcanic products at each volcano is different. The silica content of lava from Kilauea and Mount Erebus are about 50 weight percent. However, Mount Erebus rocks have greater amounts of alkali elements (sodium and potassium), about 9-10 weight percent compared to the 3 weight percent for rocks from Kilauea. Ash and lava from Mount St. Helens are called dacite because they contain about 64 weight percent silica, much more than the other two volcanoes.
The tectonics around Erebus are not too clearly understood, but as summarized by Tom Simkin and Lee Seibert in “Volcanoes of the World”, there is a large continental rift that is cutting through the W part of Antarctica (I guess it is this splitting-apart that has formed the Ross Sea). The most famous continental rift is the E. African Rift, and it too is associated with volcanism.
There are chapters on Mt. Erebus in “Volcanoes of the Antarctic Plate and Southern Oceans” by WE LeMasurier and JW Thomson, editors. In the summary chapter on the Mt. Erebus volcanic province, PR Kyle writes “Mt Erebus is an active volcano…and contains a persistent convecting lava lake of anorthoclase phonolite magma. Small Strombolian eruptions occur on a daily basis…often ejecting anorthoclase phonolite bombs onto the crater rim. From September to December 1984, larger Strombolian eruptions occurred more frequently, ejecting bombs up to 2 km from the crater and sending small eruption columns to over 2 km high.,,”
The lava lake in the summit crater counts as an ongoing eruption but it is not a particularly active eruption. The fact that the volcano is composed of layers of lava and ash erupted mainly from the summit means that it does erupt in a bigger way, it is just that no humans have seen it happen.
Source of information:
Simkin, T., and Siebert, L., 1994, Volcanoes of the world: Geoscience Press, Tucson, Arizona, 349 p.