A volcano carved by water
The landscape seen today formed 23 million years of erosion acting on a massive, extinct shield volcano. The original Tweed Volcano was an enormous structure, reaching an estimated height of over 1,900 meters and spanning up to 100 kilometers in diameter. It formed as the Indo-Australian tectonic plate moved northward over a stationary plume of magma in the Earth's mantle, known as the East Australian hotspot. For approximately three million years, successive lava flows built up the colossal shield shape.
The initial eruptions produced fluid basaltic lavas, which are recognized today as the Lismore and Beechmont Basalts. These were followed by more explosive eruptions of viscous rhyolite lava, forming the Nimbin and Binna Burra Rhyolites. A final stage of basaltic flows capped the structure. After volcanic activity ceased, the forces of erosion began to dominate. The Tweed River and its tributaries, flowing steeply to the nearby ocean, aggressively carved into the eastern side of the dormant volcano. This relentless water erosion stripped away the softer outer layers, hollowing out a vast amphitheater-like depression. It is one of the world's best-preserved examples of an erosion caldera.
The caldera and its living laboratory
The dramatic circular escarpment of the caldera visible today—including the McPherson, Tweed, and Nightcap Ranges—forms the rim of that ancient volcano. At the center stands Wollumbin (Mount Warning), the hardened volcanic plug that sealed the volcano's main vent. This central peak, standing 1,159 meters tall, is composed of more resistant intrusive rocks like syenite and microgranite from the volcano's core, which weathered more slowly than the surrounding layers. The caldera is immense, measuring about 40 kilometers in diameter and over 1,000 meters deep.
The volcanic activity created exceptionally fertile volcanic soils on the caldera floor, which now are used for agriculture. The surrounding ranges and steep valleys are part of the Gondwana Rainforests of Australia World Heritage Area, a network of reserves preserving ancient ecosystems. These forests contain plant and animal species with direct lineages to the ancient supercontinent of Gondwana. The area has an extremely high concentration of biodiversity, including over 200 rare or threatened species and stands of Antarctic Beech cool temperate rainforest. The isolated, high-altitude environment on the caldera rim has even driven the evolution of new species, such as distinct types of brooding frogs.