Volcanoes

Mt Schank

Mt Schank

ABOUT VOLCANOES
When you think of a volcano, do you think of Mount St. Helens exploding, or Hawaiian rivers of burning lava flowing to the sea? In essence, a volcano is a point on the earth’s surface where exploding gas and/or molten rock escapes from a confined subterranean space through cracks and vents (‘feed-pipes’). Molten rock is called ‘magma‘ while it is still confined, and ‘lava‘ after it pours out.

This web site will help you to recognize volcanic features and appreciate the natural events that created volcanoes. Some of the patterns of lava in this region resemble those in Hawaii. In both places volcanic activity is attributed to the earth’s crust moving on a tectonic plate that passes over a ‘hot spot‘ where heat rises from the earth’s interior. The whole of the earth’s crust is made up of these tectonic plates. Many of the world’s explosive volcanoes occur at boundaries where tectonic plates collide producing great forces.

LANDSCAPE FEATURES BUILT UP BY VOLCANOES

Conical Hills
From many lookouts on this trail you can see an almost flat horizon broken by isolated conical hills called ‘scoria cones.’ They have smooth sides sloping steeply and tops flattened slightly. At the top of these volcanoes, at the edge of the crater, you can see loose or compacted lumps of frothy lava called ‘scoria’. These lumps are reddish-brown or black.

How were these scoria cones formed?
At the vent, noisy ‘fire fountains’ sprayed out masses of red-hot frothy magma, driven up the feed-pipe by blasts of escaping gases. The lava fell back to earth as scoria. A final quieter phase of runny lava often broke through the scoria-built crater wall, forming a ‘breached cone’. Mounts Sugarloaf, and Schank are scoria cones. Mounts Napier and Eccles are breached scoria cones.

Low, wide craters
Many ground-level crater formations, called maars, are seen on the trail between Colac and Koroit . Maars are wide, flat-bottomed depressions surrounded by a low rim of volcanic ash. In size they vary from 400m to 2km across and are either dry-bedded or filled by a lake, which may be very salty. (Some of these craters also have scoria cones inside their rims.)

How is a maar formed?
When hot magma rising up a feed-pipe meets ground water held in rock, the water becomes super-heated steam. A steam-driven explosion blows up the immediate area, just like dynamite. Powdered rock, called ash, and cooled lava fall back down in layers around the crater left by the explosion. The circle of compacted ash around the depression is called a ‘tuff ring’. Since the crater floor is below ground level it usually fills with sub-surface water to form a lake or swamp.

However, if the underground water has been reduced by the heat, and more magma rises up the original feed-pipe, any more eruptions will form scoria cones inside the crater, which will then be called a ‘nested maar’. Lake Bullen Merri is a clover-leaf shaped lake of three merged craters. Lakes Gnotuk, Leake and Edward are maar lakes, while Tower Hill, Mount Gambier, Mount Leura, Mount Muirhead and Red Rock are nested maars.

In the Mount Napier/Mount Eccles district, molten lava flowed into and filled existing river valleys. The Harman’s Valley area now looks like a ‘river’ of basalt rock varying from 200m to a kilometre wide. No surface streams have yet formed on this young volcanic feature and all drainage occurs through underground channels in the basalt. The last eruption in the region was at Mount Schank (near Mount Gambier), about 5,400 years ago, but whether it was the final eruption, we can’t know. What scientists can say, however, is that at the moment there are few of the tell-tale signs in the region of renewed activity, such as tremors, earthquakes or hot springs.

VOLCANOES and VOLCANIC FEATURES of the
KANAWINKA GEOPARK

E. B. Joyce
9th July 2010

Why was the KGP area selected as a Geopark?
“The young volcanic areas of the Western Plains of Victoria and adjacent South Australia have more than one hundred small scoria cones, maars and lava shields, built up by Strombolian/Hawaiian eruptions over the past 5 million years. Fluid basalt flows have spread laterally around vents, and often for many tens of kilometres down river valleys. These plains are a part of a larger region known as the Newer Volcanic Province of southeastern Australia, which includes a contrasting Uplands volcanic region to the immediate north of the plains, in Central Victoria.”

The volcanoes:
• how many: ~100
• what types: scoria cones & lava shields, and ~40 maar craters
• other features: stony rise lava flows & lava caves, ash deposits downwind from the maar craters

The geology story of the KGP area:
• The Tertiary plain, and the retreating shoreline
• 175 years of study of the area since Major Mitchell in 1836
• What we know about the ages of volcanoes: 5 million years to 5,000 years (Mt Gambier)
• Why they are here: “intraplate volcanism on the trailing edge of a tectonic plate”
• The KGP has monogenetic volcanoes – single eruption, short-lived and small, perhaps due to small magma chambers forming at shallow depth.
• The possibility of further eruptions: 20 volcanoes in the last 40,000 years, so “every 2,000 years”?
• Where? Probably where the most recent volcanoes have erupted i.e. near the SA/Victoria border?
And what sort of volcano?
• Perhaps another maar such as Mt Gambier or Tower Hill?
• Perhaps another lava shield with stony rise flows, like Mt Napier or Mt Porndon?
• Perhaps a scoria cone like Mt Elephant?
• But a new volcano, not an old one being reactivated!

“Individual volcanoes are extinct, but as a whole the Province could be described as dormant, and future eruptions are possible”.

The latest news:
• crater lakes – the climate story
• lunette lakes – more on climate
• Pleistocene marsupial megafauna – evidence of 50,000 to 100,000 year old diprotodontid, macropodid (giant kangaroo) and other animals, both as megafaunal trackways and bone deposits.

For further details of the volcanoes and volcanic features in the Newer Volcanic Province of SE Australia, and that part of that province which is in the Kanawinka Geopark, see: www.geology.au.com


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