Although the Earth is thought to be around 4.5 billion years old, the oldest rocks outcropping in Tasmania are Precambrian quartzites, schists, and other metamorphic rocks. Recent work by Mineral Resources of Tasmania has found zircons in some Precambrian sediments greater than 3 billion years old. The sediments themselves may be up to 1,100 to 1,200 million years old. These rocks, which occur mainly in the western and north western part of the State, originally laid down as sands, silts and muds, were metamorphosed (altered) during an early mountain building episode 750 to 1,000 million years ago. Later Precambrian rocks are much less altered and include dolomites, such as those in the Hastings area in southern Tasmania, and dolerite sills and dykes in the north west.

During the Cambrian period, sedimentary rocks, derived from erosion, were deposited over a long period of time. Many of these rocks were formed in marine environments, a fact indicated by the traces of marine fossils such as trilobites, brachiopods, sponges, and other organisms. Igneous rocks (that is, rocks that have melted, and then solidified by cooling) also formed during the Cambrian. Some of these rocks, such as serpentines, and the Mt Read volcanics, are important hosts for economic minerals (eg: Mt Lyell, Rosebery and Hellyer mines).

Early in the Ordovician, older sediments were squeezed and folded, forming the basis for spectacular mountains such as Mt Murchison, Mt Owen, and Mt Jukes in western Tasmania. These mountains are comprised mostly of Ordovician conglomerate and result mostly from later, Devonian, orogenies. Later, sandstones, mudstones, and then limestones were formed in warm shallow seas.

Sediments continued to be deposited during the Silurian period in the form of sands, silts, and muds over most of Tasmania. Some of the mudstones have since been altered to slate, and may contain phosphate minerals, including turquoise. Among the fossils found in Silurian rocks is one of the earliest land plants.

The middle of the Devonian period saw another phase of mountain building, known as the Tabberabberan Orogeny. Large scale folding took place north-south from Queenstown to Sheffield, and east-west from Sheffield to Deloraine. Well, again this covered practically all of Tasmania (and much of eastern Australia and Antarctica). This was followed in the mid Devonian to early Carboniferous, by the emplacement of granite batholiths with their associated extensive mineral deposits (mostly gold, silver-lead, and tin).

Much of these mountains was subsequently eroded away. Ice covered large areas, not only in Tasmania, but also on mainland Australia from the late Carboniferous. Evidence of this glacial period can be found in the sedimentary rocks laid down in Permian times. Look closely at exposed Permian mudstones in eastern Tasmania, and you will often find pieces of rock or pebbles (perhaps from Antartica!) which were dropped from icebergs into sediments. Many of the rocks are rich in marine fossils, however, fresh water environments also existed, resulting in the formation of coal deposits.

Fossil remains of lungfish, amphibians, and an early reptile, have been found in sedimentary rocks laid down during the Triassic period. Many historic buildings around Tasmania have been constructed using Triassic sandstone. More coal deposits were formed during this period.

The Jurassic period is well known in many parts of the world for its fossils found in sedimentary rocks. This was a time of reptiles, the emergence of birds, and early mammals. However, in Tasmania during the Jurassic, extraordinarily large sills, dykes, and sheets of dolerite were intruded into earlier sedimentary rocks. Faulting and upthrusting of these rocks, coupled with their resistance to erosion, has resulted in spectacular landforms capped by dolerite such as Mt Wellington near Hobart, Cradle Mountain, Ben Lomond and many more. Some probable Jurassic basalts buried prehistoric forests at Lune River. Petrified wood and ferns can still be found here and in other localities.

About 100 million years ago, syenite dykes intruded Jurassic dolerite and Permian sediments near Cygnet in southern Tasmania. Apart from this and some volcanic activity in the north east, the Cretaceous period was mainly once again a time of erosion.

During the Tertiary period, volcanic activity resulted in extensive lava flows, and many river valleys were filled with basalt. Limestones rich in marine fossils, such as Fossil Bluff near Wynyard, were formed, and widespread faulting gave Tasmania much of its present day shape. Tertiary deposits of sands and muds in lakes and rivers often contain tin, gemstones and gold.

This is not the end of the geological history in Tasmania. Recent times have seen rises and falls in sea levels, periods of glaciation and erosion, and new sediments being deposited. There is no current volcanic activity, although minor earthquakes do occur from time to time. Who knows what it will be like in another 1 billion years!