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Upper reaches of Tweed showing Mt.Warning c. 1920.

Tweed Regional Museum Collection, No: pp501.

The scenic Tweed Valley lies in the far north-eastern corner of NSW. Geologically it takes the form of a huge erosion caldera centred on Mount Warning, the sacred mountain that the original Aboriginal inhabitants have known for thousands of years as Wollumbin. During the Miocene epoch between 20 and 24 million years ago, the Indo-Australian tectonic plate upon which the Australian continent sits was moving northwards, and it passed over a hot spot deep below the earth’s crust. This led to a period of volcanic activity during which a chain of volcanoes erupted along the coast of eastern Australia, extending from the Atherton Tableland in North Queensland to Victoria. Some 23 million years ago the Tweed volcano erupted in northern NSW, pouring basalt lavas over the ancient land surface composed of metamorphic and sedimentary rocks, part of the Beenleigh Block and the Clarence Moreton Basin. The Beenleigh Block formed between 360 and 240 million years ago and the Clarence-Moreton Basin between 22 and 130 million years ago.

Over a period of three million years, a vast dome shaped shield volcano was formed, towering over 2 km above sea level and stretching from Lismore in the south to Tamborine in what is now Queensland to the north. Three main series of lava flows occurred during this period at irregular intervals, some the result of violent eruptions, and others more gentle flows. The first group of lava flows produced what are known as the Beechmont basalts to the north and the Lismore basalts in the south; the remnants of these flows can be seen in the lower terraces on the southern side of the Springbrook plateau, and on the caps of the low coastal hills. At Fingal Headland, the flows cooled to form columnar basalt, with distinctive hexagonal columns at right angles to the line of lava flow. This type of landform occurs in many other parts of the world, notably in County Antrim in Ireland and the isle of Staffa off the coast of Scotland, where the original Giant’s Causeway and Fingal’s Cave are found.

The second phase of eruptions was more violent. The volcano ejected rhyolites, volcanic glass, agglomerates and lighter ash, pumice and mudflows. The volcanic ash layers solidified to form tuff. The predominant rocks formed from this phase were rhyolite lavas. As rhyolites are extremely resistant to erosion, deep gorges were formed. They can be seen today on the upper Coomera River in the Lamington National Park and along the cliffs of Springbrook with spectacular waterfalls at their heads plunging from the plateau above. They form cliffs at Sphinx Rock in the Nightcap Range and at Springbrook. Binna Burra to the north was the centre of a large satellite eruption zone, and a similar formation in the south occurs in the Nimbin rhyolite.

The final phase of volcanic activity was a series of relatively quiet basalt flows, called Blue Knob and Hobwee basalts. They form a capping on the high plateaux that ring the caldera: the Nightcap Range to the south, the Tweed and McPherson Ranges to the west and the Lamington and Springbrook plateaux to the north. As the continent slowly moved off the hot spot beneath the earth’s surface, volcanic activity died down and the lava flows became fewer and more sporadic.

The work of wind, water and rain over millions of years eroded the volcanic shield; a myriad of streams flowed in a radial pattern and cut through the Blue Knob basalts, gradually forming gorges and waterfalls in the rhyolite layers.

The Tweed River, the other dominant feature of the Tweed Valley today, cut most quickly into the shield because it drained directly to the sea, received high rainfall and had the steepest gradient. After the period of volcanic activity ceased, vegetation gradually returned. The high rainfall and warm subtropical climate of the Miocene Epoch fostered the development of luxuriant rainforest, and as Australia dried out during the late Miocene and Pliocene, sclerophyll forests, woodlands and coastal heath evolved. The distinctive pinnacle of Mount Warning is the central core of the ancient shield volcano. Encircling the caldera are the Lamington and Springbrook plateaux in the north, the McPherson and Tweed Ranges in the west and north-west, and the Nightcap Range in the south, all remnants of the ancient volcanic shield. Although individually called a range, the Tweed, McPherson and Nightcap Ranges are each in fact a complex network of steep ridges and valleys radiating out from what was once the centre of the Tweed shield volcano. The steep escarpments of the McPherson Range form part of the border between NSW and Queensland. The Pacific Ocean bounds the valley to the east. Other reminders of the Tweed Valley’s volcanic past are secondary vents and dykes exposed by erosion, such as the Pinnacle and Dinsey Rock. The rich volcanic soils created during the complex geological history of the Tweed caldera form the basis of the agricultural industries of the Tweed today, based on sugar cane, bananas, other subtropical fruit crops and cattle rearing.

To explore the geological history of the Tweed in more detail, visit Tweed Regional Museum Murwillumbah's Land | Life | Culture exhibition.

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