Geology

The evolution of Uluru and Kata Tjuta is explained differently by Anangu and the scientific community. According to Anangu, there was nothing on the earth until their ancestors - in the forms of people, plants and animals, travelled widely across the land and formed the world as we know it today. Some of those ancestral beings are still here. Some of the creation stories are public stories, others, according to traditional law, are restricted to certain people. To learn more about Anangu beliefs visit the Anangu culture page.

This page talks about the creation of Uluru and Kata Tjuta from the perspective of a geologist. Geologists use scientific theories to explain the way in which the land has evolved over time.

Composition of Uluru and Kata Tjuta

When looking at the two geological formations, it is visually obvious that they are composed of different rock types. Uluru rock is composed of arkose, a coarse grained sandstone rich in the mineral feldspar. The sandy sediment, which hardened to form this arkose, was eroded from high mountains composed largely of granite. Kata Tjuta rock is a conglomerate - gravel consisting of pebbles, cobbles and boulders cemented by sand and mud. Conglomerate is also a sedimentary rock.

Formation of Uluru and Kata Tjuta

Uluru and Kata Tjuta lie near the southern margin of an area geologists call the Amadeus Basin. This depression in the Earth's crust formed about 900 million years ago, and received layer upon layer of sediment over several hundred million years. This stopped about 300 million years ago.

Read more about the formation...

At times the Amadeus Basin was a shallow sea collecting these sediments. Some of it was blocked off from the sea and the water evaporated leaving crusted salt. A cold period left deposits of glacial rock.

The older sediments in the Amadeus Basin were crumpled and buckled about 550 million years ago, and mountain ranges were uplifted in an event the geologists call the Petermann Ranges Orogeny. At this time there were no trees or grasses covering the landscape. Bacteria and algae were the only life forms and they helped break down the jagged mountain ranges.

These bare mountains eroded easily. Huge amounts of sediment washed away when it rained and formed alluvial fans adjacent to the ranges. It is the remains of at least two of these alluvial fans that are seen today as Uluru and Kata Tjuta.

Conglomerate

Arkose

Folded Proterzoic sedimentary rocks

Igneous and metamorphic rocks

Palaeozoic rocks

Alluvial sediments

By about 500 million years ago a shallow sea again covered the region. The alluvial fans of arkose and conglomerate were at least 2.5km thick and were gradually covered by sand and mud and the remains of sea creatures.

The overlying sediment deposits compressed and cemented the Uluru arkosic sand into arkose and the coarse gravels of Kata Tjuta into conglomerate.

The sea receded between 400 and 300 million years ago and the rocks were folded and fractured. This second major folding is called the Alice Springs Orogeny. It raised the region above sea level. The horizontal layers of the Uluru arkose were folded and turned nearly 90 degrees to their present position. The Kata Tjuta conglomerates were tilted only about 15 to 20 degrees from the horizontal. The sand rocks at the surface eroded rapidly. This erosion still continues now at a slower rate.

A broad valley developed between the two rocks at around 65 million years ago and was partly filled with river sands and swamp deposits, including thin layers of coal. At that time the climate was wet. Only during the past 500,000 years has the climate become drier and a thin blanket of wind-blown sand covered the sediments.

Uluru and Kata Tjuta are therefore the visible tips of huge rock slabs that extend far beneath the ground. It is possible they extend down to 6 kilometres.

 

Caves, patterns and sand dunes

Formation of caves and patterns in the rock

The sculptured shapes are likely to be a combination of mechanical erosion of sand from the rock and other events such as chemical changes caused by moisture.

The major valleys of Kata Tjuta may reflect fractures that formed during the Alice Springs Orogeny. Chemical weathering due to ground water widened the fissures and rainwater run-off gradually formed the canyons and domes we see today.

Read more about caves patterns and sand dunes...

Unlike Kata Tjuta, there are no major joints and fractures visible in Uluru. Water erosion formed the valleys in the arkose on the southern side of Uluru. Rain run-off formed the steep valleys with potholes and plunge pools, often in series. On the north-west side water and weathering has produced parallel raised ridges outlining the sedimentary layers of Uluru.

The flaky surface of Uluru results from the chemical decay of minerals. The characteristic rusty colour of the exposed surface of these flakes is caused by the oxidation or rusting of the iron in the arkose. The fresh arkose is a grey colour.

As the surface rocks are heated and cooled, the release of pressure produces fissures parallel to the rock surfaces. These fissures are visible on Uluru and near the summits of some of the Kata Tjuta domes. Eventually these large "plates" of rock break away and fall down around the base or into high crevices.

Scientists still debate the formation of the caves at Uluru. Some say they can begin with water lying in dimples in the rock. The weathering process can then form a hollow and gradually 'eats' backward and upward into the rock until it becomes a cave. Others say that the 'high' caves on the southern side of Uluru may have begun by water soaking in when the plain surface was at a higher level. Once soaked, the weathering process could again eat upward and backward to form a cave. The spaced high caves on the north-eastern and western faces may have begun by the flaking process and honeycombed out by wind and water over time.

In the middle of the day the sun's rays have to pass through a small thickness of atmosphere so the filtering effect is less pronounced

At sunrise and sunset, the sun's rays pass through a much greater thickness of atmosphere therefore the filtering effect is greatly enhanced

Sand dunes

The landscape the first Aboriginal people saw looked much the same as it did today. The Geologists have found that the dunes have remained in their present position for 30,000 years. However the crests of the dunes are looser sand and shift with the wind.

Colour changes on Uluru and Kata Tjuta

The colour changes on Uluru result from the effects of the Earth's atmosphere on the sun's incoming rays. The ash, dust particles and water vapour present in the earth's atmosphere act as a filter which can remove the bluer light from the incoming rays of the sun, allowing the redder light through at different times of day. At midday when the sun is overhead, the sun's rays only have to pass through a small thickness of atmosphere, minimizing the filtering affect of the atmosphere. In the mornings and evenings when the sun is low in the sky the sun's rays have to travel through a large thickness of atmosphere to reach the earth's surface. The light reaching Uluru and Kata Tjuta near sunrise and sunset is mainly from the red end of the spectrum and its reflection from the rock and clouds in the sky cause the spectacular colours. The reddish-brown colour of the rocks and surrounding sand further enhances these effects.