The Bass Strait, as it is known today, is a narrow seaway which connects the Southern Oceans with the Tasman Sea and separates the island of Lutruwita/Tasmania from Victoria in mainland Australia (Figure 1).
The origins of these water bodies began 150 million years ago when the massive supercontinent Gondwanaland began to break apart. Fractures in Earth’s crust developed rifts that filled with seawater, dividing land masses and forming the broad continental outlines of Australia, South America, Africa and Antarctica. Around 135 million years ago more rifting saw India detach and move north, opening the Indian Ocean and defining the Southern Ocean west and south of Lutruwita/Tasmania. At this stage Australia was a large irregular landmass quite different from the Australia of today. The rift that would eventually widen to separate Australia from Antarctica caused parts of the landscape to stretch and sag, developing a series of subsiding depressions that became the Otway, Bass and Gippsland basins. These define the configuration of present-day Bass Strait (Figure 2).
Around 85 million years ago a final major crustal separation broke a huge section off eastern Australia and Antarctica, producing a continent called Zealandia – which today is mostly submerged but for remnant islands such as New Zealand and New Caledonia.
The separation of Zealandia created the Tasman Sea and was accompanied by further subsidence of the Otway, Bass and Gippsland basins. Over time these filled with sediments washed down from ancient rivers and deposited in shallow seas, gradually turning into cemented mudstone, sandstone, and limestone rock. A later phase of tectonic activity uplifted much of the Otway and Gippsland basins forming the high hilly country of the Otway and Strzelecki Ranges in Victoria. The Bass Basin remained as the low point in the terrain, later forming the basic structure of central Bass Strait.
Figure 1. Southeastern Australia defined by oceans. Map by Neville Rosengren. CC BY-NC-ND 4.0.
About the Author
With thanks to
La Trobe University
Today, the Southern Ocean/Indian Ocean and the Tasman Sea, which surround Lutruwita/Tasmania, plunge to deeper than four kilometres. But the Bass Strait is shallow, with an average depth of only 60 metres and a maximum depth of 86 metres in the Bass Basin.
This shallowness is the clue to why the strait has not always been water. Over the past four million years Bass Strait has alternated between being a seaway and a land bridge linking southern mainland Australia with the mountains and high plateaus of Lutruwita/Tasmania. The status at any time is determined by global sea-level, and this in turn is controlled by the Earth’s climate and how much water is trapped in glaciers. Bass Strait has been exposed and submerged many times and is exposed only in the peak coldest periods of glaciation – that is, during an ice age.
On the present-day Bass Strait seafloor, the low-lying Bass Basin is enclosed by two underwater ridges (Figure 2). To the east the granite Bassian Rise traces a semicircle from northeast Lutruwita/Tasmania through the islands of the eastern Bass Strait to meet Victoria east of Yiruk/Wamoon/Wilsons Promontory in Gippsland. West of the Bassian Rise the strait floor slopes gently to the Bass Basin; eastward it drops sharply into the Tasman Sea. King Island High is the western underwater basement ridge. This winds from northwest Lutruwita/Tasmania through King Island then bends eastwards to meet Victoria near Westernport Bay. West of the high the seafloor drops steeply to the deep Southern Ocean/Indian Ocean, passing down in the northwest under the Otway Basin.
115,000 years ago, the world entered its most recent ice age. Evaporated sea water fell as snow which built up as ice, growing into massive ice sheets and glaciers. Less water returned to the oceans. As a result, sea-level began to fall across the globe. Over time, the waters of the Bass Strait began to recede, and more and more land became exposed. By about 43,000 years ago, the sea had fallen low enough in the strait for a land bridge to form. By the time sea-level was at its lowest point of 125-135 metres below current levels, around 28,000 – 15,000 years ago, Bass Strait had become a gently sloping landscape offering uninterrupted connection between Lutruwita/Tasmania and the mainland.
Human occupation spans this period.
Figure 2. The sedimentary basins of Bass Strait. Map by Neville Rosengren. CC BY-NC-ND 4.0.
Figure 3. At the point of lowest sea-level (around 135 metres below present levels) the Bassian Plain was an open plain or group of low rises with areas of exposed rock forming low granitic hills on the eastern side and a brackish central lake. Map by Neville Rosengren. CC BY-NC-ND 4.0.
The Old People would have experienced a wide variety of environments on the Bass Strait landscape. Parts were cold and sparsely vegetated. In some areas sand dunes formed into long sheets and parallel ridges, products of arid cold episodes of the ice age. In other areas shallow intermittent streams meandered across grassy plains. A wide brackish lake filled the deepest part of the Bass Basin, nourished by the Kanamaluka/Tamar River flowing northwards from Tasmania, and possibly by the Toulerm/Tarwin River running south out of Gippsland in Victoria. The large Victorian rivers in Gippsland east of the Toulerm/Tarwin River did not cross the land bridge but flowed southeast across the wide continental shelf into deep-sea canyons. The Yarra River system of modern-day Melbourne also did not meet the land bridge, instead flowing southwest into a bay west of the King Island High off the Otway Ranges (Victoria).
The Old People would have known the Bassian Rise as the highest feature in the landscape, a broad, slightly elevated granite ridge fringing the plains on the eastern side. Although the ridge was only about 20 metres higher than the adjacent plains, steep granite hills and the ridges of what is modern-day Wilsons Promontory and Flinders Island plus isolated pinnacles (now smaller island groups) stood several hundred metres high, their exposed rock and boulder slopes visible from long distances. To the west the King Island High was also prominent, featuring stranded cliffs remnant from periods of higher sea-level fringed by rocky volcanic and sedimentary ridges.
At some point the Old People would have realised their environment was changing. The world began to warm and melt the glaciers, feeding more water back to the sea. About 15,000 years ago rapid sea-level rise began with a meltwater pulse, a rapid influx of water at times rising at 15 to 20 millimetres per year. The lowlands were submerged again by the sea. Sea-level continued to rise until about 8,000 years ago, though the rate was not constant and there were periods of stillstand where the shoreline remained the same for centuries to millennia.
The bridge, which by this time had retracted to a solitary, narrow isthmus along the exposed Bassian Rise, was broken by the sea between about 14,000 and 12,000 years ago when sea-level had risen to around 56 metres below today’s level, high enough to wash over the bridge’s lowest point. Possibly at first the remaining land formed a series of islands and tidal inlets similar to the present highly mobile configuration of Nooramunga and Corner Inlet in Gippsland.
Sea-level reached its present level between 11,700 and 9,500 years ago. There is some evidence that Bass Strait was up to one metre higher from 5,000 years to 2,000 years ago.
As the world enters a new warming phase and oceans rise, the outline of the low-lying lands surrounding Bass Strait will change again in the coming decades to centuries. Just as it did many thousands of years ago, as witnessed by the First Peoples of the land bridge.
Copyright information
Released under a Creative Commons Attribution Non Commercial No Derivatives 4.0 International (CC BY-NC-ND 4.0) license.
This story is subject to disclaimers, copyright restrictions, and cultural clearances. Copyright © Neville Rosengren, 2024.
Further reading
Barrett, N, Monk, J., Nichol, S., Falster, G., Carroll, A., Siwabessy, J., Deane, A., Nanson, R., Picard, K., Dando, N., Hulls, J., and Evans, H. (2021). Beagle Marine Park Post Survey Report: South-east Marine Parks Network. Report to the National Environmental Science Program, Marine Biodiversity Hub. University of Tasmania.
Barrows, T.T., Stone, J.O., Fifield, L.K., Cresswell, R.G., (2002). The timing of the Last Glacial Maximum in Australia. Quaternary Science Reviews 21, 159–173.
Birch W.D. (2003). Geology of Victoria. Geological Society of Australia, Melbourne, Special Publication no. 23, p 842.
Bird, E.C.F. (1993). The coast of Victoria: the shaping of scenery. Melbourne University Press.
Bird, M.A., O’Grady, D., and Ulm, S. (2016). Humans, water, and the colonization of Australia. PNAS | vol. 113 | no. 41 October 11, 11477–11482
Blevin, J.E. | Cathro, D.L. (2008). Australian Southern Margin Synthesis, Project GA707. Geoscience Australia.
Blom, W.M., and Alsop, D.B., (1988). Carbonate mud sedimentation on a temperate shelf: Bass Basin, southeastern Australia. In: C.S. Nelson (Editor), Non-Tropical Shelf Carbonates -Modern and Ancient. Sedimentary Geology, 60, 269-280.
Bowdler, S., (2014). The Bass Strait Islands revisited. Quaternary International 385, 206-218.
Brooke, B.P., Nichol, S.L., Huanga, Z., Beaman, R.J. (2017). Palaeoshorelines on the Australian continental shelf: Morphology, sea-level relationship and applications to environmental management and archaeology. Continental Shelf Research 134 (2017) 26–38.
Cadd, H. et al (2021). A continental perspective on the timing of environmental change during the last glacial stage in Australia. Quaternary Research 102, 5–23.
Costermans, L., & VandenBerg, F. (2022). Stories beneath our feet: Exploring the geology and landscapes of Victoria and surrounds. Costermans Publishing.
Davidson, I., and D. Roberts. 2008. “14, 000 BP – on being alone: the isolation of Tasmania,” in: Turning points in Australian history. Edited by D. A. Roberts and M. Crotty, pp. 18-31. Sydney: UNSW press.
Dickinson, J.A. Wallace, M.W., Holdgate, G.R. Daniels, J. Gallagher S.J. and Thomas L. (2001). Neogene tectonics in SE Australia: Implications for petroleum systems APPEA Journal · January 2001 DOI: 10.1071/AJ00002.
Hamacher, D., Nunn, P., Gantevoort, M., Taylor, R., Lehman, G., Law, K.H.A., Miles, M. (2023). The archaeology of orality: Dating Tasmanian Aboriginal oral traditions to the Late Pleistocene. Journal of Archaeological Science 159, 1-16.
Harris, S., Driessen, M. and Bell, P. (2009). Prime Seal Island Scientific Expedition 2008. Hamish Saunders Memorial Trust, New Zealand and Biodiversity Conservation Branch, DPIPWE, Hobart, Nature Conservation Report Series 09/3.
Jones, D.S. (2023). Planning for Urban Country: Taking First Nations Values into Future Urban Designs. Palgrave macmillan pp.361.
Lavering, I.H. (1994). Marine environments of Southeast Australia (Gippsland Shelf and Bass Strait) and the impact of offshore petroleum exploration and production activity, Marine Georesources & Geotechnology, 12:3, 201-226.
Malikides, M., Harris, P.T., Jenkins, C.J., Keene J.B. (1988). Carbonate sandwaves in Bass Strait. Australian Journal of Earth Science 35:303–311.
O’Sullivan, P.B., Mitchell, M.M., O’Sullivan, A.J., Kohn, B.P., Gleadow, A.J.W. (2000). Thermotectonic history of the Bassian Rise, Australia: implications for the breakup of eastern Gondwana along Australia’s southeastern margins. Earth and Planetary Science Letters 182 31-47.
Sloss, C.R., Murray-Wallace, C.V. and Jones, B.G. (2007). Holocene sea-level change on the southeast coast of Australia: a review. The Holocene 17:7, 999–1014.
Williams A.N., Ulm, S., Sapienza, T. Lewis, S., Turney, S.M. (2018). Sea-level change and demography during the last glacial termination and early Holocene across the Australian continent. Quaternary Science Reviews 182. 144-154.
Williams, A.N., Veth, P., Steffen, W., Ulm, S., Turney, C.S.M., Reeves, J.M., Phipps, S.J., and Smith, M., (2015). A continental narrative: Human settlement patterns and Australian climate change over the last 35,000 years. Quaternary Science Reviews 123, 91–112.
