An extinction event (also known as a mass extinction or biotic crisis) is a widespread and rapid decrease in the biodiversity on Earth. Such an event is identified by a sharp change in the diversity and abundance of multicellular organisms.
It occurs when the rate of extinction increases with respect to the rate of speciation. Because most diversity and biomass on Earth is microbial, and thus difficult to measure, recorded extinction events affect the easily observed, biologically complex component of the biosphere rather than the total diversity and abundance of life.
Extinction occurs at an uneven rate. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine animals every million years.
Marine fossils are mostly used to measure extinction rates because of their superior fossil record and stratigraphic range compared to land animals.
The Great Oxygenation Event was probably the first major extinction event. Since the Cambrian explosion five further major mass extinctions have significantly exceeded the background extinction rate. The most recent and arguably best-known, the Cretaceous–Paleogene extinction event, which occurred approximately 66 million years ago (Ma), was a large-scale mass extinction of animal and plant species in a geologically short period of time.
In addition to the five major mass extinctions, there are numerous minor ones as well, and the ongoing mass extinction caused by human activity is sometimes called the sixth extinction. Mass extinctions seem to be a mainly Phanerozoic phenomenon, with extinction rates low before large complex organisms arose.
Estimates of the number of major mass extinctions in the last 540 million years range from as few as five to more than twenty. These differences stem from the threshold chosen for describing an extinction event as “major”, and the data chosen to measure past diversity.
List of extinction events:
| Period | Extinction | Date | Possible causes |
| Quaternary | Holocene extinction | c. 10,000 BCE — Ongoing | Humans |
| Quaternary extinction event | 640,000, 74,000, and 13,000 years ago | Unknown; may include climate changes and human overhunting | |
| Neogene | Pliocene–Pleistocene boundary extinction | 2 Ma | Supernova? Eltanin impact? |
| Middle Miocene disruption | 14.5 Ma | – climate change due to change of ocean circulation patterns and perhaps related to the Milankovitch cycles?. | |
| Paleogene | Eocene–Oligocene extinction event | 33.9 Ma | Popigai impactor? |
| Cretaceous | Cretaceous–Paleogene extinction event | 66 Ma | Chicxulub impactor; Deccan Traps? |
| Cenomanian-Turonian boundary event | 94 Ma | Caribbean large igneous province | |
| Aptian extinction | 117 Ma | ||
| Jurassic | End-Jurassic (Tithonian) extinction | 145 Ma | |
| Toarcian turnover | 183 Ma | Karoo-Ferrar Provinces | |
| Triassic | Triassic–Jurassic extinction event | 201 Ma | Central Atlantic magmatic province; impactor |
| Carnian Pluvial Event | 230 Ma | Wrangellia flood basalts | |
| Permian | Permian–Triassic extinction event | 252 Ma | Siberian Traps; Wilkes Land Crater[38] |
| End-Capitanian extinction event | 260 Ma | Emeishan Traps? | |
| Olson’s Extinction | 270 Ma | ||
| Carboniferous | Carboniferous rainforest collapse | 305 Ma | |
| Devonian | Late Devonian extinction | 375–360 Ma | Viluy Traps |
| Silurian | Lau event | 420 Ma | Changes in sea level and chemistry? |
| Mulde event | 424 Ma | Global drop in sea level? | |
| Ireviken event | 428 Ma | Deep-ocean anoxia; Milankovitch cycles? | |
| Ordovician | Ordovician–Silurian extinction events | 450–440 Ma | Global cooling and sea level drop; Gamma-ray burst? |
| Cambrian | Cambrian–Ordovician extinction event | 488 Ma | |
| Dresbachian extinction event | 502 Ma | ||
| End-Botomian extinction event | 517 Ma | ||
| Precambrian | End-Ediacaran extinction | 542 Ma | |
| Great Oxygenation Event | 2400 Ma | Rising oxygen levels in the atmosphere due to the development of photosynthesis |
Causes of Mass Extinction:
Asteroid impacts, climate change, volcanoes – there have been many theories about the causes of mass extinctions. In some cases, such as the Cretaceous mass extinction event, more than one such factor was involved in the global catastrophe.
Catastrophic methane release: Catastrophic methane release has been suggested as a possible cause of mass extinction. Methane clathrate is an ice-like substance formed from water and methane in the sea bed, arctic lakes and permafrost.
Flood basalt eruptions: Flood basalt eruptions are a type of large-scale volcanic activity, both in terms of extent and duration, that can occur on land or on the ocean floor. A flood basalt may continue to erupt for tens of thousands – possibly millions – of years and the lava can cover hundreds of thousands of kilometres.
Climate change: Earth’s climate is not constant. Over geological time, the Earth’s dominant climate has gone from ice age to tropical heat and from steamy jungles to searing deserts.
Impact events: Impact events, proposed as causes of mass extinction, are when the planet is struck by a comet or meteor large enough to create a huge shockwave felt around the globe. Widespread dust and debris rain down, disrupting the climate and causing extinction on a global, rather than local, scale.
Other Causes:
End-Ordovician:
• Beginning of glacial cycles on Earth, and corresponding changes in sea level
• Changes in atmospheric and oceanic chemistry relating to the rise of the Appalachian mountains
End-Devonian extinction:
• Climate change, possibly linked to the diversification of land plants
• Decrease in oxygen levels in the deep ocean
End-Permian extinction:
• Volcanic activity
• Climate change
• Decrease in oxygen levels in the deep ocean
• Changes in atmospheric chemistry
• Changes oceanic chemistry and circulation
End-Triassic extinction:
• Volcanic activity
End-Cretaceous extinction:
• Asteroid impact
• Volcanic activity
• Climate change
• Changes in atmospheric and oceanic chemistry