Volcanic features in the Camperdown region

Lake Gnotuk is a maar crater formed as a result of phreatomagmatic explosions during the late Pleistocene/present periods. The date of the last eruption is believed to be 7080-140 years bp. The volcanic features in the Camperdown district were formed within the last 20,000 years making them some of the youngest of their type in Victoria.

While 20,000 years may seem a long time in our terms, it is a relatively short period in geological history. The basement rocks of the district are mostly marine sediments laid down in the later part of the Miocene Period-roughly 15 to 20 million years ago.

Camperdown was established at the foot of Mount Leura and Mount Sugarloaf which are actually part of a much larger volcanic complex known as the Leura Maar.

The Leura Maar is a shallow oval-shaped depression about 2.5 kilometres long, 1.7 kilometres wide and up to 50 metres deep which orginated from a series of major volcanic explosions, possibly 22,000 years ago. Some of the eruptive material was thrown high into the air, but most surged rapidly across the ground surface as a dense cloud of steam, gas and rock fragments. The Leura Maar may have been formed in just a few months.

Inside the Leura Maar, there are many younger eruption points marked by craters and mounds of scoria. These are said to be nested inside the maar and Leura therefore is a nested maar. These younger volcanic features include Mount Leura, Mount Sugarloaf, the deep crater which separates the two mountains, and several smaller craters and mounds in the southern part of the maar.

Although not as explosive as the activity which produces the maar, the growth of Mount Leura and Mount Sugarloaf would have provided a grand spectacle, similar to the present day activity of volcanoes on the island of Hawaii.

They began as vents on the floor of the maar from which, glowing fountains or explosive bursts of lava were ejected. Gases escaping from the lava caused it to break into angular, gravel-sized fragments called scoria which fell back to the ground and heaped up around the volcanic vent forming the high, steep cones. The scoria including larger blocks and smooth-surfaced lava bombs that were hurled into the air and partly cooled before crashing back to the ground to be buried in the scoria.

Mount Sugarloaf is basically a steep conical accumulation of scoria rising as a high point on the same crater rim as Mount Leura. It formed as a result of perhaps several months of lava fountaining from the same point in the crater, continuously building an ever-increasing pile of scoria. Scoria volcanoes can achieve height quickly and Mount Leura could have been built in less that 20 years.

From the summit of Mount Leura, many volcanic and old rock features of Western Victoria are visible. The sand stone ridges of the Grampians form the bulky ranges or the horizon to the north west while more faintly to the north are the older rocks of the Central Highlands near Ballarat. Almost all the other landforms seen from this point are the result of volcanic activity.

The major volcanic features include the single scoria, cones of Mount Noorat, Mount Meningoort, Mount Elephant (the largest scoria cone in Victoria), and Mount Myrtoon, and the multiple eruption complexes forming the Wiridgil Hills and Red Rock. These are all older than Mount Leura, but are still regarded as geologically young features being much less than one million years old. In contrast, the rocks of the Grampians and Central Highlands are from 400 to 500 million years old.

Sources:

Corangamite Shire Council

Lake Gnotuk & Bullenmerri Planning Stratgey Discussion Paper
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