Geoscientist Michael Freeman delivered a fascinating lecture on the story of the bedrock and the overlying regolith of our existence (that is, for those who live in the hills). The Darling Range region is an old, complicated landscape, which in places is not fully understood or agreed upon and the interpretation could be complicated.
Initially, we need to comprehend several basic concepts. Our earth is a huge radioactive ball—the centre of which is an iron alloy sphere that is solid but only because it is under extraordinary pressure. It is also extremely hot. With a geothermal gradient of 25-30°C/km, the base of the earth’s crust is at 800°C and the core is up to 5500°C. When the pressure drops on rocks at these temperatures, they melt, form magma and begin to flow—upwards and outwards.
The magma cools very slowly and becomes granite. The slow cooling enables the atoms within the melt to align into crystals: the slower the cooling, the larger the crystals. Granite can take up to 20 million years (20 Ma) to cool!
Next we look at plate tectonic movements: as one continental plate slides past another it causes huge tensions—and hence fractures—within the plates. Several things can happen: one is the release of pressure deep within the crust that allows molten rock or magma to move to the surface and generate volcanoes. In subduction zones (areas of overriding plates) surface sediments melt; volatiles like steam and carbon dioxide rise to the surface carrying magma upwards.
Granite is the substance of which the Darling Range Batholith (batholith: a large body of igneous rock that crystallised at considerable depth below the earth’s surface) is formed. The Darling Range batholith may have originally been 100,000 cubic kilometres of molten granite and it began cooling around 2700 million years ago. Scientific dating using radioactivity in the rocks shows it possibly cooled over 100 Ma; the youngest date so far is 2610 Ma.
Over the next 1400 Ma, this immense quantity of granite was slowly lifted somewhere in the order of 17km as the overlying rock was eroded off. But where did it go? This is a conundrum for geologists…
If we fast forward to 1200 Ma, continued ructions in the crust create tensional fractures around the edge of the Yilgarn Craton up which flowed basalsitic magma to form ‘dolerite’ dykes. A dyke was explained as a narrow, vertical strip of rock that penetrates surrounding rock like a breadknife. Dolerite is a rock consisting mainly of pyroxenes (a dark mineral) and feldspar, whereas granite consists of feldspar and quartz (both light coloured minerals). Dolerite, coming into contact with the cooler granites, crystallised faster—hence they are significantly more fine-grained.
At the time of Gondwana some 225 Ma, a north-south gulf some 2000km by 200km existed between the Australian plate and the northern Indian plate to the west. Sediment was deposited in the gulf from 400 to 80 Ma. Sediment, both marine and terrestrial, kept the basin filled and the basin floor slowly sank by up to 12km. This sedimentation and sinking process moved the shoreline back and forth (north to south).
Since 80 Ma sand dunes and coastal limestone have covered the basin deposits. Therefore the landscape for those who do not live in the Hills is relatively young, geologically speaking. As the global sea level through much of recent times has been some 70 metres above the present, the shore was 70 metres above the present level, lying at the base of the Darling Scarp. This has caused the coast to have eroded by up to 3km east of the original Darling Fault, between then and now.
Mike continued to explain the distinctive features we see today on the Darling Scarp, particularly the ferricrete. Some of us call it laterite. Laterite is the result of distinctive weathering of rock, in this case of granite. Feldspar in the granite is weathered into kaolin clay, holding the more insoluble metals silicon and aluminium. Alkali metals (sodium, potassium, calcium) are leached away in solution. The remaining iron moves up and down the soil profile (due to successive wet and dry seasons) to eventually form an oxidised iron-cemented layer at the top of the zone—ferricrete.
This ferricrete capping and its associated Jarrah/Marri forest is unique. South-west Australia has the only tall forest in a temperate zone with a Mediterranean climate. Why? Our wet winters but very dry summers stress vegetation. But below the ferricrete layer is a clay-rich (kaolin-rich), water-retaining horizon. The kaolin has a crystal lattice that absorbs water and nutrients making it available to plants, if they can reach it. Jarrah and Marri have evolved roots that penetrate the ferricrete layer to access the clayey zone. Therefore water availability during summer is almost guaranteed.
Diana Papenfus
NB Mike Freeman assisted in the editing of this article. Please understand that the above is a very truncated version of the Darling Range regional geology.