Australia, Geology of. The continent of Australia has rocks that rank among the oldest in the world. Some of them formed over 3 billion years ago. A great variety of rocks of all ages can be found throughout Australia.
Scientists have divided the geological history of Earth into four long stretches of time called eons. Starting with the earliest, the eons are Hadean, Archean, Proterozoic, and Phanerozoic. The first three eons lasted about 4 billion years. Together, they are often called Precambrian time, because they preceded the Cambrian Period. The Cambrian Period marks the beginning of the Phanerozoic Eon, when life became abundant in the fossil record. The Phanerozoic Eon is divided into three eras: the (1) Paleozoic, (2) Mesozoic, and (3) Cenozoic. These eras are further divided into periods.
Precambrian time
During the Precambrian, Earth’s crust formed, and much of Australia’s mineral wealth was created. Major iron ore deposits dating from the Precambrian are mined in the Hamersley Ranges of Western Australia and Iron Monarch, in South Australia. Precambrian strata (rock layers) at Mount Isa, Queensland, provide copper, lead, and zinc. The Kalgoorlie and Pilbara gold fields are in Precambrian shields (stable regions composed of extremely old, hard rocks).
At times in the Precambrian, glaciers were active. At other times, the climate was tropical. Fossils are relatively scarce in Precambrian rocks. But some old fossils called stromatolites suggest forms of bacteria lived in Australia at the time (see Stromatolite). Primitive worms, jellyfish, sponges, and corals were also present.
The Phanerozoic Eon
The Phanerozoic Eon began 539 million years ago and continues to the present day. During this time, many of the large metal deposits found in Australia—including gold deposits—formed. Several other resources, including coal and natural gas, began forming during the Phanerozoic.
The Paleozoic Era
marked the beginning of the Phanerozoic Eon and lasted almost 300 million years. The Paleozoic Era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian periods.
The Cambrian Period.
A shallow sea covered central Australia during the Cambrian Period, which lasted from 539 million years ago to about 485 million years ago. Creatures that lived in this sea included trilobites (small animals with soft shells), brachiopods (lamp shells), graptolites (small compound animals), sponges, sea urchins, worms, and sea lilies.
The Ordovician Period.
Much of the gold mined in Victoria in the 1800’s was found in rocks from the Ordovician Period. This period lasted from about 485 million years ago to about 444 million years ago. A large part of Australia was beneath the sea, and sediments accumulated. Sea life included corals and cephalopods (squidlike creatures). Land creatures did not yet exist.
The Silurian Period
began about 444 million years ago and lasted about 25 million years. Silurian granites and other Silurian rocks have been the source of much of the gold mined in New South Wales.
The Devonian Period
lasted from about 419 million years ago to 359 million years ago. Midway through this period, massive Earth movements in eastern Australia forced the land high above sea level. Fossils indicate that plants first appeared on dry land in Australia during this time. Devonian rocks also contain the fossils of primitive fish. Graptolites died out, and some fish developed the ability to breathe air.
The Carboniferous Period
began 359 million years ago and ended 299 million years ago. Granites from the Carboniferous Period have become the source of gold, copper, lead, zinc, and tungsten in Queensland and tungsten and tin in Tasmania. Fossils show that fish continued to develop during this period. Corals and brachiopods flourished in the seas, but trilobites became extinct.
The Permian Period,
from 299 million to 252 million years ago, provided the valuable coal reserves along the eastern edge of Australia. Permian sediments have also yielded natural gas at Gidgealpa in South Australia and oil in the Perth Basin of Western Australia.
During the early Permian Period, glaciers formed throughout Australia. Later, the climate became warmer and the sea level rose. Four-legged animals with backbones lived on the land. The most common plant was the fernlike Glossopteris.
The Mesozoic Era
is generally thought of as the age of the dinosaurs. The rise and eventual extinction of these great beasts happened during a period when large coal deposits began to form in several areas.
The Triassic Period.
Almost all of Australia was above sea level during the Triassic Period, from 252 million to 201 million years ago. Triassic sediments collected on floodplains, in river deltas, and in lakes. The hard sandstones of the Sydney region and the Blue Mountains collected at this time. Volcanoes erupted in Queensland. Fossils show that ferns, tree ferns, insects, and freshwater fish flourished.
The Jurassic Period
lasted from about 201 million years ago to about 145 million years ago. During this time, thick sediment accumulated in a huge inland depression on the northeastern part of Australia. Cycads (primitive palmlike plants) were the dominant plants. Dinosaurs and other reptiles lived on land.
The Cretaceous Period
lasted from about 145 million years ago to 66 million years ago. Many of the opal fields of South Australia, New South Wales, and Queensland are found in sediments from the Cretaceous Period. However, the opal itself is from a later period. The Burrum and Styx coal fields formed during the Cretaceous in eastern Queensland.
Pines, cycads, and ferns were the most common land plants of the Cretaceous in Australia. Flowering plants first appeared at this time. The modern Australian bush developed from these plants. Mammals and birds were becoming more common.
The Cenozoic Era,
the current era, began around the time the dinosaurs disappeared, 66 million years ago. This period produced deposits of the mineral bauxite, a major source of aluminum. Also during this time, the bedrock on which the Great Barrier Reef sits was formed.
The Paleogene and Neogene periods
together lasted from 66 million years ago to 2.6 million years ago. During this time, low-lying coastal areas were flooded. These floods deposited the limestone of the Nullarbor Plain and the sedimentary rocks on which the coral of the Great Barrier Reef began to form. The brown coal fields of the Gippsland region accumulated at this time. The oil and gas fields of Bass Strait sit in strata from these years. The rich bauxite mines at Weipa, in north Queensland, and Gove, in the Northern Territory, resulted from weathering during this time.
Toward the end of the Neogene Period, tremendous Earth movements raised large areas of eastern Australia, forming the Great Dividing Range. The huge reptiles of the Mesozoic had died out, and only smaller reptiles remained. Birds became common, and mammals flourished.
The Quaternary Period
extends from 2.6 million years ago to the present day. During this period, the world sea level has fallen and risen several times. The rise and fall results from cycles of glacier formation. Growing glaciers hold increasing amounts of water, lowering sea levels. When sea levels fell, the Australian mainland joined with Tasmania and New Guinea, and indirectly to Southeast Asia. Scientists believe that the Aboriginal peoples of Australia arrived on the continent during the last glacial period.
The Quaternary glaciers caused relatively little erosion on the Australian mainland, except in the Snowy Mountains. Animal and plant life in the Quaternary evolved (developed over time) into modern forms.
