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Volcanoes & Earthquakes in SE Australia

Volcanoes of the world are generally associated with either subduction zones (eg Pacific rim volcanoes), spreading or rifting zones (eg East Africa, Iceland), or hot spots above mantle plumes ( eg Hawaii). All are associated with some form of seismicity. It has been proposed  that volcanoes of eastern Australia are believed to fall into the latter category. One of the latest such papers is one by  Sutherland (1991) who has suggested Plate migration over rift upwellings over the last 60 million years. The oldest Australian volcanoes are in north Queensland, and the eruptive centres have moved southwards as the Australian plate has drifted northwards over a mantle plume

The region of south-western Victoria and south-eastern South Australia was an active volcanic province in relatively recent times. Several hundred eruptive centres have been identified there, not all named. The most recent activity was at Mount Gambier in South Australia, with the youngest eruption there estimated to have happened approximately 4000 years ago. The youngest Victorian volcanoes may be be Tower Hill ( a scoria cone) and Mt. Eccles which erupted lava. Head et al., have suggested that these volcanoes are older than previously thought, with the Mt. Eccles eruption being 27,000 years BP, and Tower Hill 20,000 yrs BP. A morre recent list of suggested ages is given below. The more recent eruptions are more Alkaline than the older ones.

Many of the volcanoes have preserved their classical volcanic shapes, and the region is considered by some to be volcanically dormant rather than extinct. However, note the conclusion by Cliff Ollier ( in "Landform Studies from Australia & New Guinea, by Jennings & Mabbutt, 1967, p 338) - " Perhaps the most remarkable feature of Victorian vulcanicity is its complete cessation. Older basalts were erupted in Victoria through long periods in the Tertiary: the newer basalts were erupted throughout the Pleistocene and extended into the Recent. The last eruption probably took place only 5000 years ago, and yet there are no fumaroles, no hot springs, no anomalous temperature gradients or seismic effects, in fact nothing to indicate how recently the volcanic activity ceased."

Ollier & Joyce (1964) divided the volcanic landforms of Western Victoria into 3 basic types, with combinations of these types also existing. They were

1) Basalt cones - usually low-angle hills, made up of a number of lava flows, with no apparent scoria. Examples are Mt. Atkinson & Mt. Rebecca.

2) Scoria Cones - ideally a single cone with steep sides and a crater at the top - eg Mt. Elephant & Mt. Noorat. Red Rock is a multiple scoria cone.

3) Maars or Tuff rings. These are low rings or ramparts of pyroclastic material surrounding wide but comparatively shallow craters. Examples are Wangoom Hill, Elingamite & Mt. Ewan.

There are two notable concentrations of volcanoes. One is along the line Geelong-Colac-Portland, which coincides with the axis of a Tertiary trough. The other is the Ballarat-Daylesford area, which is the centre of the volcanics of the Western Highlands Plateau (Ollier, C., in Jennings & Mabbutt, 1967).

Some ages of some SE Australian volcanoes (courtesy B. Joyce)

Mt. Gambier  4,300 BP

East Basin  > 5,200

Red Rock  - between 7,800 & 15,200

Lake Gnotuk   > 9,200

Mt. Eccles  between 19,300 and 27,500

Tower Hill  between 23,000 and 33,000

SEISMICITY OF THE REGION

South Australia's largest earthquake, the magnitude 6.5 Beachport event, occurred approximately 100 km NE of Mount Gambier in 1897.

The accompanying map shows all known earthquakes of magnitude 4.0 or greater in the region 36 to 40 degrees south and 139 to 144 degrees east. Of the 15 events extracted from the Geoscience Australia catalogue, 8 occurred between 1896 and 1907. The most recent event extracted was a magnitude 4.9 event near Nhill in Victoria in 1987.

Remember that there were very few seismographs in Australia before the 1960's, and that location and magnitudes of early events are subject to considerable errors.

With the recent installation of more seismographs in the region, earthquake depths can be determined with more accuracy. Three earthquakes of magnitude approximately 2.5 occurred in the vicinity of Mount Gambier in January and February of 2003, with greater than usual depths ( ie, approximately 40 km deep). Love et. al. (2004) have suggested that they may be generically related to the volcanoes in the area.

Earthquakes of Mag 5.0 or greater

DATE

LAT

LONG

MAG

LOCATION

10 May 1897-37.333139.75

6.5

Beachport SA
02 May 1899-37.2139.6

5.3

Robe, SA
14 Jul 1903-38.43142.53

5.3

Warrnambool, Vic
06 Aug 1948-37.36139.68

5.6

Robe, SA
24 Dec 1960-38.88143.59

5.3

Cape Otway

.

Map of earthquakes and Volcanoes in southwestern Victoria - Southeastern South Australia (36 to 40 degrees South, 139 to 144 degrees East)

Magenta circles - earthquakes of magnitude 4.0 & above, 1890 to present

Blue circles - all earthquakes Jan 1995 - Oct 2004

PHOTOGRAPHS OF SOUTH AUSTRALIAN VOLCANOES  Courtesy Office of Minerals & Enenrgy Resources, South Australia

Below - Mount Gambier - Australia's youngest volcano.

Below - Mount Shank, approximately 10 km south of Mount Gambier

Photographs of some Victorian Volcanoes

1) Red Rock - photo by Colac-Otway shire council

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Mt. Noorat - approximately 50 km east of Tower Hill - photo courtesy State Museum of Victoria

Images from poster by B. Joyce (University of Melbourne), Cities on Volcanoes Conference, Aukland, N.Z. Feb 2001

1) Mt. Elephant scoria cone, Western Victoria

2) Map of Volcanoes, Western Victoria & Eastern South Australia

Tower Hill, Western Victoria

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