Tropical Topics An interpretive newsletter for the tourism industry Wet Tropics geology No. 63 September 2000 The geological connection Notes from the The geological foundation of the Wet Tropics has a profound effect on the Editor distribution and types of plants and animals found there. It is difficult for us to imagine an For a start, the position of the and tree-kangaroos in the Wet Tropics apparently static landscape as a mountains dictates the amount of rain has shown they are definitely more dynamic system, constantly on the which falls. Running roughly parallel abundant on basalt soil. move. Sometimes a volcano erupts, a to the coast, they intercept rain- cyclone causes a river to seek a new bearing clouds moving in from the sea This correlation has been observed in course or our complacency is jolted and, by forcing them upwards, make other animals too. The reason may by an earthquake but, to all intents them drop their moisture. However, the simply be linked to better food and purposes, mountains are a exact orientation of the mountains availability in more diverse forests. For symbol of everything in life which is determines whether a particular slope example, strangler fig trees which fixed and unchanging. It is a challenge gets a lot of rain or not so much. provide food for a wide variety of to our imagination to contemplate most Those slopes which face the birds, fruit bats and other animals, are geological processes — the gradual prevailing south-easterlies are the two to three times more abundant on formation of the landscape by wettest. basalt soils. Even numbers of dung enormous forces working extremely beetles increase due to the greater slowly over a period of time which is Rock types determine soil types. abundance of animal droppings and beyond our comprehension. Rainfall, plant roots, landslides, river researchers’ claims that scrub-itch action and other factors break down mites and leeches are more numerous Nevertheless, an understanding of the rock, particularly where fractures on basalt are probably correct. how our landscape was formed gives make it more vulnerable. Soil builds up us an insight into the origins of the wherever the rock particles come to Unfortunately, clearing for features we find there today and the rest. agriculture has focused on effects the underlying geology has the fertile basalt areas, on the environment in which we live. Soil types, in turn, determine forest resulting in a types. Basalt creates the most fertile widespread loss of soil. Granites and metamorphic rocks those forests which I would like to thank Geoff are more acidic and their soils are less contain the greatest Hodgson for his very valuable fertile. The most complex and diverse diversity of plants assistance with this issue. I rainforest is found where abundant and animals. A would also like to thank Gary rain falls on basalt soils in areas of disproportionate Wilson, Queensland Herbarium, high temperatures*. amount of remaining John Kanowski, Griffith forest in the Wet Tropics University, and Simon Crouch, The soil type also affects the ability of are now found on the Deparment of Mines and Energy. the forest to support animals. John poorer soil types. The Kanowski, a Rainforest CRC distribution of rock APOLOGIES to web users for researcher, has found that leaves types — the result of growing on poorer granite or random geological Lumholtz’s the awkward layout of pages 4 tree- and 5, and the missing southern metamorphic soils are tougher, less processes — therefore kangaroo part of the map. It was too nutritious and contain more toxins prefers to than those growing on the more fertile continues to have feed on difficult to completely divide the basalt soils. This directly affects a profound effect trees original A3 layout for this page growing animals which eat leaves and John’s on the forests of on basalt. and the missing map section is study of the distribution of possums the Wet Tropics. just a computer mystery beyond the comprehension of this editor! *The most ideal conditions are at low altitudes, where temperatures are warmer. However, forest has been cleared from the few places where basalt occurs in the Wet Tropics lowlands. The big picture The geological history of the Wet Tropics region is one of dramatic changes interspersed with long periods of gradual, but steady transformation. The story began about 420 million years ago, at a time when Australia was still part of the ancient continent of Gondwana. The Australian land mass was very different then. The east coast was about 120-150km west of the present coastline, running roughly from Cape Melville to west of Charters Towers. A trip to present day Chillagoe, had we been around, would have entailed a boat ride because it was well under the sea — as were Injection of granite bodies present day Cairns, Atherton and Injection of granite bodies Townsville. The coastal section of the difficult to imagine rocks acting like liquids, but from time to Wet Tropics is therefore a relatively time large pools of molten rock (magma) pushed up into the recent addition to the ancient Australian crust from as far as 50km below the Earth’s surface (see continent. above). Less dense, and therefore more buoyant, than the HODGKINSON rocks above them, this magma squeezed up through them. BASIN As always in geological In many places this material did not make it to the surface, processes, a redistribution of but slowly cooled and solidified to form bodies of granite land was taking place. deep underground. In the southern and western parts of the Gradually, but persistently, Wet Tropics area, some of magma reached the surface, rivers running off the erupting explosively as volcanoes and spreading volcanic continent were carrying ash, dust and debris far and wide. This cooled rapidly to bits of it into the sea in form hard rock known as tuff. the form of gravel, sand and clay. This material For over 100 million years, there were no dramatic geological was being deposited in an undersea basin, known as the changes in the Wet Tropics region. The mountains of the Hodgkinson Basin which lay off the coast possibly between north-east coast continued to rise but were also subjected it and another landmass even further to the east. About to the persistent processes of erosion. Rocks were broken 160km wide, this basin stretched 320km from north to south, down into sediments and carried by rivers to the sea. roughly from Cooktown to Tully. Over a period of about 60 Gradually the granite, once deep below the surface, was million years, sediments accumulated in this basin, creating exposed, as rocks beds some 10km thick. Towards the edges, primitive corals covering it were removed. and other marine organisms were creating mounds of Energy and Mines of Department of permission with limestone. Continental crust Updoming During this period CHILLAGOE FORMATION HODGKINSON FORMATION Mantle Gondwana was breaking Mountain ranges grow apart and Australia was from compression and crumpling of sediments of New convection drifting north. At that Hodgkinson Basin. pattern develops in mantle time the Australian continent extended much Blocks subside further east than it does now and included parts of New Zealand. Sideways movement of convecting mantle Then, about 100-65 place crust under tension million years ago, About 360 million years ago the sediments in the Atherton Coral Sea Basin movements in the molten Hodgkinson Basin was caught in a squeeze (see above). T'land Queensland trough mantle, deep below the Major movements in the earth’s crust put them under Sea level earth’s surface, stretched immense pressure as opposing forces pushed in from east the continental crust District Cairns the of Landscapes and Rocks and west. The accumulated sediments were compressed, above. It was pushed up folded and lifted far above sea level, creating a series of Queensland plateau and, like the crust of a mountain ranges that would rival today’s Andes or cake rising in the oven, it Himalayas in height. Coastal limestone accumulations were eventually cracked under the strain and broke into blocks. pushed up near to the old continent and can be seen today Some of these blocks sank below sea level, one forming the in the limestone formations of Chillagoe. Other sediments — floor to the Coral Sea Basin, and another the Queensland sand, mud and gravel — formed the other mountain ranges, Trough, a deep trench at the edge of the continental shelf. the immense pressure and heat transforming them, in the One block, however, was raised well above sea level. The process, into tougher metamorphic rocks. These are the cliffs at its edge formed a sharp escarpment, at the shoreline, rocks which form running the length of Australia’s east coast at the edge of continent much of the The solid crust what is now the continental shelf. Steep eastern edge Main diagrams reproduced from from reproduced diagrams Main of the earth is just landscape we see Tableland with resistant granite to continent Solid iron in the Wet Tropics hills Sea level a superficial layer, and nickel lying on top of a today. moving molten ocean mass of superheated crust From about 310 to magma which, in turn, surrounds a 260 million years Sediments begin to fill trough solid core of iron and nickel. ago, other events Currents and movements within the deep in the Earth’s crust caused magma affect the hardened crust East further changes in Updoming before breakup of continent ‘floating’ on it. raises plain to form tableland Eastern edge of continent subsides the landscape. It is West along major fractures Continued on page 3 Out and about Wildlife of Tropical Cigarette butts continue to be a During the dry North Queensland problem.