A couple of days to millions of years, this process takes time. In this essay I am going to focus on the rock cycle, how it helped create the Karlu Karlu rock formation (the Devil’s Marbles), and how indigenous Australians use rocks and minerals in their everyday lives.
Rocks are the hard substances that make up the Earth and are classified into three main groups: igneous (granite, pumice, basalt), sedimentary (limestone, chalk, sandstone), and metamorphic (marble and slate). The classification of certain rocks is determined by their properties. Properties that contribute to a rock’s classification are their texture, mineral composition, and general appearance. However, once a rock has been classified, it can still change its classification as a result of external forces acting on it. This common process is referred to as the rock cycle.
Igneous rocks are formed by the process of the cooling and solidifying of magma. This process can be slow and can take thousands of years, or be quick and take a mere couple of days. There are two types of igneous rocks: intrusive and extrusive. Intrusive rocks are formed slowly deep within the Earth with large crystals forming throughout. Extrusive igneous rocks form much quicker on the surface of the Earth, with small or non-existent crystals. Extrusive rocks can also form with small air bubbles, making them vesicular.
Through weathering and erosion, igneous rocks change to sediment, which, in turn, can change to sedimentary rock through compacting and cementing. Sedimentary rock has many layers showing the deposition of sediment from different time periods. There are lots of small grains that are held weakly together, meaning they are often porous, soft, and crumbly. They often have fossils trapped within them. Sedimentary rocks also have two different groupings: clastic, formed by compacted sediments, and organic/crystalline, formed by evaporates, precipitates, and biological matter. Sedimentary rocks can take millions of years to fully develop and form.
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Sedimentary rock can become metamorphic due to heat and pressure. Metamorphic rocks have layer structures of crystals caused by heat and pressure and are usually denser and harder than sedimentary rocks. Sometimes distortions caused by the movements of the rock can create wave or zig-zag patterns in the rock. Again, there are two types of metamorphic rock: contact metamorphism, caused by heat, and regional metamorphism, caused by pressure. Metamorphic rocks can take tens of millions of years to form, the longest formation time of all three rock types.
The Karlu Karlu (the Devil’s Marbles) rock formation is a large cultural landscape spanning over 4,453 acres in the center of the Northern Territory. It is the traditional country for the Wurumungu, Kaytetye, Alyawarra, and Warlpiri people. The ‘marbles’ are scattered throughout the shallow valley of the reserve. Essentially, the Devil’s Marbles are large, rounded boulders balancing on one another, scattered around the Karlu Karlu Conservation Reserve. The name ‘Devil’s Marbles’ came from Scottish-Australian explorer John Ross who was traveling around Australia in 1870 and said: “This is the devil's country; he's even emptied his bag of marbles around the place”.
The formation of the boulders started almost 2 billion years ago when magma cooled within the Earth’s crust, forming the intrusive igneous rock, granite. On top of the granite, a thick sedimentary layer of sandstone formed, compressing the granite under its extreme weight. After millions of years, the sandstone eroded, releasing pressure and resulting in the granite expanding and cracking into the jagged, cube-like blocks we see today. Eventually, the granite weathered, and as it is much harder and stronger, it can withstand any similar weathering to what it had previously experienced. A combination of water and natural acids found in the atmosphere round off the points in the rocks and leave smoother, rounder boulders. Alongside this, the large difference in temperatures in day and night cause the rocks to expand and contract, simultaneously peeling off layers of the marble. The Karlu Kalu is a combination of both sedimentary and igneous rock.
Australian Indigenous people have used rocks and minerals in their everyday life for thousands of years. Powdered minerals are commonly used for things like art. Aboriginals collect minerals as weathered rocks (ochres) and crush them into a powder using a grindstone. Different chemical compositions of minerals cause those minerals to have different colors, for example, hematite is caused by iron oxide (rust), giving it a reddish look. Ochres can be mixed to make different colors. Aboriginal Australians have an extensive knowledge of various rocks and minerals in Australia which is very important in making tools, weapons, and ochres. Rocks are used for different things depending on their hardness, texture, colors, ability to be ground or worn, and ability to flake and form sharp edges. Hard igneous rocks are very commonly seen in indigenous tools like axe heads.
The Karlu Karlu rock formation is a great example of how the rock cycle works, showing two of the three possible rock types, and how they change and transform. Indigenous Australians learned from this formation and used its qualities, like minerals, to their advantage in everyday life, creating art and useful tools.
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