The Geological History of Mount Warning
and the Tweed Volcano


The geological history of the Tweed Volcano and Mount Warning paints a fascinating picture of nature's awesome power.

Scientific evidence to prove the Continental Drift Theory is now accepted, and supports that millions of years ago the Earth's crust consisted of a huge single landmass - the supercontinent Pangaea.

Then, around 200 million years ago, Pangaea split in two, creating Laurasia in the northern hemisphere and Gondwana (consisting of South America, Antarctica, India, Africa, Australia and New Zealand) in the southern hemisphere. the remnants of the shield Tweed Volcano, Mount Warning, in northern NSW

As the tectonic plates beneath oceans and continents slowly drifted apart, Gondwana began to break up and around 65 millions years ago, Australia separated from the supercontinent and started to drift north at the rate of approximately 5-20 cm per year.

Another several million years passed before the Australian continent began its pass over one of Earth's hotspots, at which stage a number of volcanoes formed along the east coast of Australia.

Hotspots are fixed spots within the Earth's mantle which generate extreme heat and energy - enough to literally melt through Earth's crust and give rise to magma or lava-based volcanoes.

Given Australia was still moving north (albeit only a few centimetres each year), a volcano continued to be active only until that area of land had moved beyond the hotspot.

At this stage, the cycle would begin again with a new volcano being formed over the stationary hotspot - and so on.

Around 23 million years ago, the Gold Coast-Tweed area began its transit over a hot spot giving rise to an enormous shield volcano, the Tweed Volcano, that remained active for some 3 million years.

The central vent of the Tweed Volcano was above the present Mount Warning, and is estimated to have reached a height of around two kilometres at its peak, roughly double the present height of the mountain! Trees, Mount Warning National Park, NSW

Shield volcanoes are built mainly by basalt lava and are so named because of their shape - their shallow, gradual dome being likened to that of a fighter's shield.

The fluid basalt flows easily and quickly along the ground (often through lava tubes) enabling the lava to travel far and wide before cooling - thus shield volcanoes typically cover a large surface area.

The lava flows from the Tweed Volcano covered nearly 7,000 square km, and reached as far as Mount Tamborine in the north, Lismore in the south, and Mount Lindesay in the west!

The lava flows also flowed eastward into the Pacific Ocean where they are still visible today in the form of coastal reefs.

Mount Warning itself is a large plug of magma that was intruded beneath the central vent of the mountain towards the end of its activity.

The huge dimensions of the Tweed Volcano are absolutely mind-boggling. Consider that the Tweed Valley - that's around 40 kilometres wide - has been eroded only from the central part of the mountain.

The attached diagrams - as published in Rocks and Landscapes of the Gold Coast Hinterland, by W.F Willmott, Geological Society of Australia, illustrate the progressive erosion of the Tweed Volcano that was responsible for the magnificent plateaus, jagged cliffs, and deep gorges that dominate the landscape today.

The height of the volcano and its vicinity to the coast resulted in a high level of rainfall which in turn created rivers and streams - and so began the natural erosion process which, over the next 20 million years, created the spectacular landscape that stands today.

Over time, the lava rock softened and steadily wore away; its debris being carried into nearby waterways.

As the lava continued to weaken and decompose, the waterways gradually became deeper and wider, eventually giving birth to the mighty Tweed River. View of Mount Warning from Best Of All Lookout at Springbrook National Park, Queensland

Now a mere shadow of its former self, the Tweed River undoubtedly had the greatest impact in creating the landscape we see today.

Flowing eastward from the top of the Tweed Volcano to the Pacific Ocean, the steep angle combined with gravity and water to create a powerful force that dwarfed the other rivers - some of which were responsible for forming the Currumbin, Tallebudgera, Numinbah, Coomera and Canungra valleys!

The Tweed River quickly eroded the east side of the volcano's cone, creating such a cutting force that the headwaters of all streams leading north, south and west were gradually joined to the huge river.

This sped up the erosion process even further and eventually the cone of the volcano was washed away to expose the plug beneath its central vent - which James Cook was to name Mount Warning a few million years later!

But why wasn't Mount Warning itself eroded away in the process?

For most of its life, the Tweed Volcano erupted basalt - except for a more violent period during which rhyolites spewed forth in a spectacular but less wide-spread fashion.

The plug of Mount Warning was composed of different rocks - gabbro at the base of the walking track, syenite on most of the mountain, and trachyandesite near the top, and these have been more resistant to erosion than the surrounding basalts.

They have indeed withstood the test of time and the force of nature to stand as a spectacular monument to a long gone but fascinating era in Earth's history.


Reference Sources

  • Willmott, W F, Rocks and Landscapes of the Gold Coast Hinterland, published by Geological Society of Australia (Queensland Division), 1986.
  • Blanch, R, and Kean, V, Bushwalking in the Mount Warning Region, second edition published by Kingsclear Books, 1995.
  • NSW National Parks and Wildlife Service, Mount Warning National Park, 1993.
  • URL United States Geological Survey: http://pubs.usgs.gov

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