The Edinburgh Geologist
Issue no 41
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The Edinburgh Geologist | | ||||||||
Geological Tables The Atherton Tablelands by Peter Whitehead
About 360 million years ago, the region changed from being an extensional regime to one of compression and the sediments were uplifted to form high mountains of metamorphic rocks such as slate and schist. In the period from 310 to 230 million years ago, episodes of heating led to deep-seated magmatism and the development of granite. In some areas, the acidic magma found its way to the surface, where it produced violent eruptions, producing welded ash flows. After erosion removed the surrounding rocks, the granites remained and formed the large mountain ranges such as Bartle Frere, Queensland's highest mountain, Bellenden Ker, the mountains north of Lake Tinaroo and the coastal ranges to the east. Some of the resulting volcanics now form the ranges on the western edge of the tableland. Not much happened here for the next 100 million years or so, except for erosion gradually lowering the highlands to a plain of low relief and exposing the granites. About 100 million years ago, rising convection currents in the earth's mantle heated the crust, doming it upwards. Plate tectonic movements pulled the crust apart, until, about 65 million years ago, the continent split and the Coral Sea was created. The eastern side subsided beneath the sea, but the western side remained as highlands with a steep eastern escarpment. Since then, the escarpment has retreated west, through continual erosion, forming the present landforms of the coastal plain, escarpment and tablelands, with resistant granite ranges rising above them. About 3 million years ago, basalt volcanoes started erupting lavas over the tablelands. Some of the most voluminous flows poured over the escarpment down a pre-existing valley formed by the Johnson River. These flows form the relatively gentle ramp up the escarpment that is utilised by the Palmerston Highway. The original course of the Johnson River was blocked, and two new valleys, the North Johnson and the South Johnson were formed on either side of the flows. More recently, from about 1 million years onwards, smaller volcanoes have erupted relatively minor basalt flows, but have built small, steep sided, conical cinder cones that can be seen dotted around the Atherton tableland. One notable group is located between Atherton and Yungaburra and contains nine separate vents. Consequently it is called the Seven Sisters. Some of the most recent eruptions
have been the most spectacular, producing maar volcanoes of small height, but
with very wide craters. When the eruptions cease, these craters become crater
lakes, the best known examples being Lake Barrine and Lake Eacham. Another spectacular
volcanic vent can be seen in Hypipamee National Park and is simply known as 'the
Crater'. This volcano has reamed a spectacular vent through the surrounding granite.
The eruption would have involved large amounts of gases, with the extrusion of
only minor amounts of basalt. Volcanic activity has been intermittent on
the tableland for the past 3 million years, with the most recent eruptions being
perhaps as recent as 10,000 years ago or so. Given the age range and the intermittent
nature of the volcanism, it is likely that some volcanism will occur in the future,
although we may still need to wait a few thousand years or so.
Figures
The Atherton Tablelands from the air close to Bartle Frere
The Atherton Tablelands from the air showing Cristmas Falls [Return to Edinburgh Geologist index] |
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