Weathered rock near Watiyawanu, NT
Our German exchange student, Lea, asked why Australian soil is red. I hope that you won't mind me using this post to explain.
There is a short answer that goes: "Soil starts with rocks, which are ground down and mixed with organic matter that dies, decomposes and becomes part of the soil. Many rocks in Australia are rich in iron which gives them their red colour making Australian soil red."
There is a longer answer, which starts with Australia's geology.
Map showing the continents that made up Gondwana.
A simple geology of Australia
Australia was originally part of the super continent 'Gondwana'. This broke up as the continents drifted apart. The science of 'plate tectonics,' or continental drift, is very new compared to the study of the surface of the earth. It was only accepted in the second half of the twentieth century.
There are seven large plates on the earth's surface which float on the molten rock core. Over time they move slowly causing the continents to move relative to each other. It is very likely that because the continents started out together, they have similar minerals on their surfaces.
As you can see from the image, Africa and Australia were once connected by Antarctica. This led some people to suggest that Antarctica could be mined for similar minerals to those in Africa and Australia. Fortunately, no exploitable deposits have been found because of the huge mass of ice covering 97% of Antarctica.
An early European geological map of Australia
The geology of Australia
Since Europeans came to Australia in the 1770s they have been looking for ways to exploit the land. Some of the earliest mineral uses were stones for building in Sydney and Melbourne and clay for pottery. The map above is an early example of a geological survey, which is coloured to show the different rock types.
There are three classifications of rock: igneous, sedimentary and metamorphic. These are formed in different ways, and all occur in Australia.
This rock is formed when magma from volcanoes cools and solidifies. Igneous rocks are classified by grain size and silica content. Some examples that you may have seen are pumice, which makes the black beaches in Iceland, and obsidian, which is a volcanic glass sometimes used for jewellery.
This is formed from particles that fall in layers and are compressed to form solids which are made up of grains of the same mineral. Examples include sandstone, which forms Uluru, coal, which is a compressed sedimentary layer of organic matter that is mostly carbon, and iron ore.
Metamorphic rocks have changed between being formed and being found. This could be because a sedimentary rock was forced under a tectonic plate, massively heated, and compressed. Quartzite is compressed sandstone and marble is compressed limestone or dolomite. Most famously, diamonds are compressed carbon.
Iron rich rock at sunset near Watiyawanu, NT
Iron ore is a sedimentary rock, which often has a lot of silica associated with it. Silica in its pure form is quartz, and it is the major constituent of sand.
Geologists think that the iron ore came from earth's oceans nearly two billion years ago, when iron dissolved in the salt water reacted with oxygen created by emerging phytoplankton. This created two iron compounds, haematite and magnetite.
Haematite is one of the most common minerals on the earth's surface. It is red in colour, and is not magnetic. It occurs in igneous, sedimentary and metamorphic rock, and can be found around the world. Pure haematite is about 70% iron by weight, but the rocks that it is in contain lots of other minerals too.
Magnetite is also common and occurs in all three types of rocks. It differs from haematite in two important ways:
- it is a black-grey colour
- it is magnetic
Magnetite is also used commercially as an abrasive, in 'emery paper'.
Looking east to Alice Springs along Namatjira Kintore Link, NT
We mentioned earlier that soil is made by grinding down of surface rocks. This can be by glaciers sliding across the surface like in Antarctica or alpine areas; or by water eroding rocks and transporting them to other places. The wind can also erode (sand blast) other rocks when it blows hard enough and temperature changes can shatter rocks.
When the soil is formed it shows characteristics of the rocks and vegetation it is made from. The dominant rocks in central Australia are iron ore bearing and 96% of the iron ore that Australia exports is haematite. In the most productive Western Australian mines, the iron ore contains between 56% and 62% iron.
When there is so little organic matter - either plants or animals - to break down and add to the soil, it keeps its original rock colour.