The geology of Austria consists of Precambrian rocks and minerals together with younger marine sedimentary rocks uplifted by the Alpine orogeny.
Geologic history
Most of Austria's rocks formed in the last 540 million years, during the Phanerozoic explosion of life. Small zircon crystals, eroded out of three billion year old granites are among the few remnants of the Precambrian. Dobra Gneiss, at 1.38 billion years old, is the oldest rock in Austria within the Moldanubian Superunit in the Waldviertel region. Mica schist and phyllite were deposited between 900 and 500 million years ago.
Cambrian
In the Cadomian Event, fragments of continental crust such as the Bohemian Massif and the Alps joined the margin of the supercontinent Gondwana. Igneous activity occurred, in connection with small ocean basins opening. The Maissau granite, dated to 570 million years ago, in the eastern edge of the Bohemian Massif is a remnant of this igneous activity. The Austrian crustal components of Gondwana drifted into the southern hemisphere in the early Cambrian. At the end of the Cambrian, some crustal fragments separated again as microcontinents with the opening of the Rheic Ocean. The Moravian Superunit is one example of this pattern.
Ordovician
Situated near the South Pole at the start of the Ordovician, the region was likely barren and nearly lifeless. The Austroalpine and South Alpine superunits formed a shelf that received clay and sand sediments from the continental interior. The Ordovician also marked a poorly understood period of igneous activity, that resulted in granites and the Blasseneck Porphyroid, a weakly metamorphosed porphyry in the Greywacke Zone, with mixed layers of pyroclastic flow and lava.
Some of the earliest fossils were deposited at the end of the Ordovician on the continental shelf, including orthocerids and cystoids.
Silurian-Devonian
In the Silurian, crustal extension began, opening the Paleotethys Ocean on the northern margin of Gondwana. Basins filled with sand, clay and calcareous sediments while basalt and andesite poured out of cracks, ultimately metamorphosing into amphibolite and greenschist now found in the Austroalpine and South Alpine superunits.
Gondwana drifted northward in the Devonian and large reefs developed on the continental shelf of Austria. Island arc collisions are recorded as amphibolite and gneiss in the Sub-Pennic Superunit of the Hohe Tauern mountain range.
With the closing of the Rheic Ocean and the western Paleotethyic Ocean, the supercontinent Pangaea took shape. Extensive erosion of mountain ranges and basin formations led to coal deposition. These coal deposits were subsequently mined in the Brenner area of Tyrol and Stangalpe in the Nockberge mountain range in Carinthia.
Variscan Orogeny: Carboniferous, Permian and Triassic
A major mountain range formed with the closing of the oceans, spanning thousands of kilometers across Pangaea. Known as the Variscan orogeny in Central Europe, the mountain building event left remnant highlands throughout Europe, northwestern Africa and North America. The Alps and the Bohemian Massif were affected by the orogeny. The Sub-Penninic and Moldanubian superunits were deeply buried within the new range (granulites in the Moldanubian Superunit, for instance, formed at 55 kilometers depth), while the Austroalpine superunit likely formed foothills. A tropical climate dominated the region, supporting 40 meter trees.
Melting occurred, forming granites now found in all the major Austrian superunits. Crustal extension began again in the Permian leading to more basalt melts, granite and pegmatite formation, which are commonly found in the South Alpine and Austroalpine Superunits. Explosive quartz-dominated volcanism took place at the surface, creating the Bozen Quartz Porphyry, which is hundreds of meters thick in South Tyrol. Crustral extension combined with erosion to reduce the height of the Variscan mountains to only a few hundred meters above sea level. Embayments of the Tethys Ocean flooded the edge of the South Alpine and Austroalpine superunits, leaving fossil-rich limestones as well as salt and gypsum deposits. The region experienced a dry climate akin to modern-day Arabia, before the Permian-Triassic mass extinction.