The Wollumbin / Mount Warning shield volcano and its erosion caldera is a unique volcanic landform which has functioned as a refugium, over the 20 million years since it’s formation, for a core area of Gondwana rainforest flora.
The Caldera is located in a major climatic transition zone, between temperate and tropical floral and faunal species, giving high species diversity under a wide range of habitats, is volcanic origins and very high localised rainfall make this very complex and interesting landform.
The Wollumbin Caldera is part of the Gondwana Rainforests of Australia World Heritage Area, and includes several parks and reserves protected under this UNESCO listing. These are:
- Wollumbin National Park
- Border Ranges National Park
- Nightcap National Park
- Mount Jerusalem National Park
- Mebbin National Park
- Mooball National Park
- Goonengerry National Park
- Limpinwood Nature Reserve
- Numinbah Nature Reserve
- Springbrook National Park (partly in Queensland)
These areas are protected for their rainforest ecosystems and geological significance, as part of the World Heritage listing.
Australia’s rainforest are basically descendants of ancestral Gondwanic flora, as are the rainforest of South America, Africa and India.
Rainforest are basically found on richer soils with high nutrient status.
The flora and fauna of the Caldera extend with the volcanic laval flow soils southwards and westwards.
One of the very few sources of information on the flora and fauna of the caldera at the time of European arrival are the writings of Michael Guilfoyle, father of the eminent botanist and landscaper WR Guilfoyle. The writings of the Guilfoyles are the only scientific descriptions of Caldera vegetation that time.
20 million years ago the continent, bearing its Gondwanic rainforest flora, was moving north, away from Antarctica. The continent had been passing over one of the earths hotspots, and this had given rise to a chain of volcanic activity down the east coast of the continent, including the volcanic eruption and subsequent laval covering of the landscape which was the formation of the Mount Warning / Wollumbin Shield Volcano. The height and spread of the laval mound of the volcano is estimated at 2km high and a spread which for example extended 30km out onto the continental shelf beyond the current sea level.
Over millennia this landform acted as a focus for heavy rains and the consequent massive erosion, especially on the seaboard, carved out the unique erosion Caldera observable today.
Exposed dramatically in the centre of this erosion Caldera is the dominant massif of Mount Warning, 1156 meters high, the ancient magma chamber, composed of erosion resistant plutonic rock which hardened underground.
The Caldera, with it’s steep scarps rising 1150 m in altitude, forms an almost regular horseshoe about 15 km radius around Mount warning and acts as a giant scoop for moisture laden air along the open eastern seaboard. The products of the erosion of the Caldera bowl are deposited on the rich flood plane of the Tweed River estuary.
Mount Warning itself acts as a weather triggering mechanism to the general atmospheric instability above the caldera, especially in spring and autumn. In the rainy season (late January to March) the whole Valley remains developed in a cloud for weeks at a time effectively and closing the humid dump conditions in a greenhouse effect. Along with the protective nature of the Caldera rim, this undoubtedly has been the way the landfall has functioned as a refugium for the many rare and endemic Gondwanic rainforest species it is nurtured on its rich volcanic soils over the millennia.
The changes in land use over the last 150 years have resulted in the removal of 80% of the native vegetation, however the favourable climate and diverse soils have resulted in the survival of many remnant areas of the original vegetation types.
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Please note, the above description of the geological caldera and its flora is extracted from a paper by Jan Hunter, ‘The Erosion Caldera of the Mount Warning Shield Volcano’, a paper which focuses on the flora by which aboriginal people of the caldera sustained.
