The Orosirian (Greek: orosira, meaning "mountain range") was the third geologic period in the Paleoproterozoic Era and lasted from 2050 Ma to 1800 Ma (million years ago). Instead of being based on stratigraphy, these dates are defined chronometrically.
Latter half of the period was an episode of intensive orogeny, a process in which a section of the earth's crust is folded and deformed by lateral compression to form a mountain range, on virtually all continents.
Probably during this period Earth's atmosphere changed to oxygen-rich due to photosynthesis of cyanobacteria.
1.9 billion year old Grypania fossils, found in Michigan, were widely seen as the first clusters of organised and communicating cells.
Two largest known impact events on Earth occurred during the Orosirian. At the very beginning of the period, 2023 Ma, a large asteroid collision created the Vredefort impact structure. The event that created the Sudbury Basin structure occurred near the end of the period, 1850 Ma.
The Vredefort impact crater was created around 2.02 billion years ago. The vaporized beads of rock were blasted into the atmosphere and rained down 1,550 miles (2,500 km) away, on a proto-continent that became northwest Russia and Scandinavia. The crater was once 185 miles across. The asteroid or meteor that hit proto-Africa was 6 miles (10 km) wide and excavated a hole 10 times deeper than the Grand Canyon. The impact's tremendous heat melted the Earth's crust, creating a magma lake.
The Sudbury basin formed as an impact from a bolide approximately 10–15 km (6.2–9.3 mi) in diameter that occurred 1.849 billion years ago in the Paleoproterozoic era.
Debris from the impact was scattered over an area of 1,600,000 km2 (620,000 sq mi) and traveled over 800 km (500 mi) away — rock fragments ejected by the impact have been found as far as Minnesota.
Guyana's Highland Range is one of the oldest geological ranges on Earth and date to about 2 billion years ago. Mount Roraima is the best known mountain of this range.