Mount Erebus, the second highest volcano in Antarctica with a summit elevation of 3,794 metres, is located on Ross Island, which is also home to three inactive volcanoes. Mount Erebus is part of the Pacific Ring of Fire, which includes over 160 active volcanoes.

Geology and Volcanology
The mineral anorthoclase ((Na,K)AlSi3O8) is a crystalline solid solution in the alkali feldspar series, in which the sodium-aluminium silicate member exists in larger proportion. It typically consists of between 10 and 36 percent of KAlSi3O8 and between 64 and 90 percent of NaAlSi3O8.

Anorthoclase crystal from Mt. Erebus
Anorthoclase occurs in high temperature sodium rich volcanic and hypabyssal (shallow intrusive) rocks. The mineral is typically found as a constituent of the fine grained matrix or as small phenocrysts which may occur as loose crystals in a weathered rock. 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.

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 basanite lavas that form the low, broad platform shield of Erebus.

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 Erebus.

Lava flows of more viscous phonotephrite 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 100 000 years 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.

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