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Gold Coast Local Geography

Gold Coast Geography and Geology

You don't need a PhD in geology to figure out that the Gold Coast has a lot of beaches, but what you may not know is how they, and other features of the land such as mountains, swamps and plains, came to be formed.

In simple terms, the Gold Coast's geography begins at the ocean, which has beaches to the west, sand dunes behind them, rivers, creeks and swamps behind the dunes, a plain behind the rivers, some foothills, then mountains behind all that.

Want a more technical analysis? Read on...

 

Lamington National Park's Volcanic Influence

Aerial of Hinterland, Hinze Dam

The landscape of Lamington National Park is basically the result of two large, ancient volcanoes. Before the volcanic activity, the rock type was sedimentary rock, which was deposited when the area was under sea level. This sedimentary rock built up to form older hills and valleys.

The two volcanoes, Focal Peak Volcano and Tweed Shield Volcano, erupted, spewing lava over the hills and valley, dramatically changing the landscape into the mountainous areas that it is today. Some scientist believed this to occur around 24 million years ago.

Focal Peak Volcano was situated to the west, near the present Mount Barney. The early eruptions were comprised of basalt, a very dark rock containing many minerals. Due to the fluid nature of basalt, the low valleys towards Beudesert and Kyogle and later as far as Beechmont in the east where first covered. 

Later the lava flows changed composition to rhyolite, being less fluid than basalt was more restricted in its flow. This rock is responsible for the Mount Gillies Volcanics. At the end of the volcanic activity of Focal Peak Volcano, a large mass of rhyolite solidified beneath the volcano. As this rock cooled, it formed granophyre, a rock with larger crystals (but not as large as granite).

Aerial of Hinterland

Pressure from below thrust this granophyre mass upwards, creating circular fractures from which more rhyolite squeezed through. Erosion of this mountain (now Mount Barney) then caused a thin layer of gravel to be spread over its flanks, the Chinghee Conglomerate.

Larger and erupted shortly after the Focal Peak Volcano, the Tweed Shield Volcano was centred over the present Mount Warning (New South Wales). The central summit of this volcano is estimated to have been around 2000m above sea level. 

The lava flows covered much of the lavas from the Focal Peak Volcano in the west as well as many of the older rocks in the east. This volcano built up a broad, gently sloping dome (a shield) of many basalt lavas. These extended over the present border ranges, as far as at least Tamborine Mountain in the north, Lismore in the south and Mount Lindesay in the west. 

Unlike basalt eruptions, which are relatively quiet with the highly fluid lava flowing long distances from the vents, is the activity that formed the Binna Burra Rhyolite. The early eruptions that formed this rhyolite were highly gas charged and explosive. This resulted in the release of fragments and beds of tuff and agglomerate accumulated.

Erosion of the resulting rock from the volcanoes then occured which resulted in the landscape of Lamington National Park.

Continental Drift & Lamington National Park

One explanation as to how the volcanic activity occured in ancient Lamington is related to continental drift.

The volcanic magma is believed to have been generated at a "hot-spot" in the earth's mantles, deep below the crust. As the Australian continent drifted northwards towards New Guinea, basalt lavas were periodically forced up through weaknesses in the crust. These occured as the continent passes over the stationary hot-spot. Presently, the continent is drifting northward at a speed of 65 mm per year. Supposing the rate of continental drift to have been uniform in the past, south-east Queensland would have passed over the hot-spot around 25-23 million years ago.

These articles are re-printed from and www.oreillys.com.au/lamington-national-park and Copyright is attributed to the original authors.