Fraser Island 2
Once you start driving around Fraser Island, you realise that it’s really very different from any other place you’ve been to. (But you absolutely need a 4WD to drive in).
It’s the biggest sand island in the world - about 120 km long and 5-25 km wide. It is at least 800,000 years old, and its sand dunes reflect the changes that happened over that time.
To understand Fraser Island, you need to realise for the last 3 million years, the normal state of affairs on our planet has been an Ice Age. The Ice Ages last for about 100,000 years, followed by an Inter Glacial (non-Ice Age) of about 20,000 years, which is then followed by another Ice Age, and so the cycle goes. The periods of the Ice Ages and the Inter Glacials can vary by 30% - and of course, there were warm periods during Ice Ages, and cold periods during Inter Glacials.
During an Ice Age, the ice is about one kilometre thick over New York and Germany. There are glaciers over much of Tasmania and the southern parts of the Great Dividing Range, and the ice sheets reach from Antarctica to within 1,000 km of Tasmania. The water to make all this ice came from the oceans - and so the ocean level was about 100 metres lower than it is today. (Just to keep things in perspective, latest estimates are that by 2100, ocean levels will be 1-7 metres higher than they are today - thanks to Global Warming. If all the ice in Antarctica and Greenland melts, the water levels will be 70 metres higher. So the “maximum” range of ocean level varies from 100 metres lower than today, to 70 metres higher.)
Ice Ages happen because of regular changes in the amount of sunlight falling on the planet. Milutin Milankovitch, a Serbian astronomer of the early 20th century worked out that three separate factors all combined to explain the timing of the Ice Ages. The factors were that the Earth’s orbit changed from circular to oval and back again (100,000 year cycle), that the tilt of the Earth drifted from about 21o to 25o and back again (47,000 year cycle) and that the spin axis (or “tilt”) of the Earth slowly swept out a circle (23,000 years).
Fraser Island was “created” during these massive climate changes. The rocks of the Great Dividing Range (especially the New England Ranges) were eroded into sand, which the rivers delivered to the ocean. Northerly currents carried the sand up past Brisbane. They ran into the obstacle of the hard volcanic rocks of Waddy Point, Middle Head and Indian Head - and were trapped.
Today, Fraser Island gives the geologist the longest and most complete example of overlapping sandunes, running back some 720,000 years.
Fraser Island has dozens of “window” lakes, such as Yankee Jack and Ocean Lake. They are your “standard” lake, where the ground level dips to underneath the local water table.
It also has about 40 “perched dune lakes” - more than half the number known on the planet. “Perched” means that the dune lake is perched above the local water table. The water that ends up in the lake is just rainwater that fell immediately on and around the lake. The water stays there because the bottom of the lake is impermeable, thanks to the long accumulation of mud, peat and sandrock. Some of the sediments have been dated to 300,000 years old. With these lakes, stuff comes in, but doesn’t go out. Perched lakes are usually low in nutrients, and often acid because of decaying plants, so they don’t have much animal life in them. Most of the perched lakes have only a single lifeform - a zooplankton called Calamoecia tasmanica.
Most perched lakes in Fraser Island are between 3-8 metres deep. Lake Boomanjin is the largest perched dune lake on Earth. Its water is tea-coloured, thanks to the tannin chemicals from plants on the lake’s edge. Lake McKenzie is very popular with tourists, because the water is crystal clear and the surrounding trees are magnificent blackbutts. The water near the edge is very clearly broken into distinctly coloured layers, thanks to the different depths of the water. You can really feel the temperature change from the warm shallow water to the deep cold water, if you dive down to the bottom.
The lakes were easy to get to in our Nissan Patrol, but we were slowed in our travel by tourists who had hired 4Wds, and had got bogged. They were pleasantly amazed at how effective low range and low tyre pressure were in getting them out.
I was really amazed that a rainforest could exist on sand. But if you add several hundred thousand years of plant detritus to the top layer of the sand, you get enough nutrients to support a genuine rainforest. The only other thing that you need is good rainfall. That’s why Fraser Island has the only rainforests in the world that grow on sand.