Geologic Background:
Heard Island consists of 2 volcanic cones, Big
Ben and Mt. Dixon, joined by a narrow isthmus. Both cones
are young, but only Big Ben has been observed to erupt.
Many young lavas, including 2 that are unvegetated, lie
on the flanks of Mt Dixon. The separation of the 2
volcanoes is evident from the contrasting petrographic,
geochemical, and isotopic characteristics of their
respective eruptive products.
Big Ben is a large, glacier-covered, composite cone
20-25 km in diameter at sea-level, consisting mainly of
basaltic lavas and lesser ash and scoria. Its summit
region consists of a SW-facing semi-circular ridge 5-6 km
in diameter, 2200-2400 m above sea-level. The ridge
appears to have formed from breaching of the SW flank of
Big Ben, psssibly by landsliding caused by seismicity or
a laterally directed blast. The E, N, and W flankes of
Big Ben have been deeply scoured by glacial erosion,
forming high-standing radial ribs to 7-8 km long.
Eruptions have built a new regularly shaped cone,
Mawson Peak, withing the breached region of the summit.
Mawson Peak is snow- and ice-covered on all sides, its
summit lies 2745 m above sea-level, and its SW flank
slopes smoothly to the coast. All of Heard Island's
historic volcanism has apparently originated at the
summit of Mawson Peak.
Mt. Dixon, much smaller than Big Ben, appears to be
the latest manifestation of volcanic activity that has
created a peninsula 9 km long and up to 5 km wide
extending from the NW side of Big Ben. Mt. Dixon, at the
end of the peninsula, is a glacier-covered rounded cone
706 m tall. More than 20 separate relatively young
basaltic lava flows have been identified on its flanks,
including 2 that are largely vegetation-free and may have
been erupted within the last few hundred years.
Historic Activity:
- Eruptions in 1881, 1910, 1950-1954.
Recent Activity:
- January 14-15, 1985, eruption of small lava flow.
Reports (from ships and space shuttle) in October 1985 of
volcanic activity. A deep, well-formed crater at the top
of Mawson Peak was discovered on helicopter overflights
in Dec. 1986.
- Satellite images and observations from an Australian
base revealed eruptive activity in 1992. Satellite
inmages revealed activity on Jan. 17, 1992. Activity was
observed from the Australian base on the island from
March thru June and included and orange summit glow and
gas emissions.
- A new lava flow was observed on the southwest flank
in mid-January 1993, start of eruption is known but
earthquakes were felt on the island by a team of
biologists on Dec. 19, 1992.
- On Jan. 5, 1997, a pilot on an Antarctic sightseeing
tour near Heard Island saw what appeared to be a volcanic
plume.
- Heavy fumarolic activity was observed from Mawson
Peak in October 2000.
- Increased fumarolic activity in Feb and March 2001.
Photos taken in Feb at night show two locations of
activity high on Big Ben.
Data Sources:
- Smithsonian's SEAN Bulletin (V. 10, No. 2 and V. 10,
No. 10)
- Global Volcanism Network (V. 17, No. 5 to V. 25, No.
9).
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