What Are They Made From?
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. |
How Were They Formed?
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.
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. 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. 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 6 kilometres. |
How Big/Tall Are They?
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