GeologyRocks

About the author

geo_girl
University of Plymouth

What Is A Mass-Extinction?

An extinction event is defined as a sharp decrease in the number of species over a short period of time. A mass extinction is further defined based on four points, pattern, duration, breadth and magnitude, and the event must:

• be confined to a short period of geological time
• Affect a wide variety of clades, that occupy a wide range of Habitats
• And eradicate a high proportion of species.

A high proportion is defined as:

• Greater than 10% of Families

Or

• Greater than 40% of species.

And a species becomes extinct when its total inter-breeding population has been eradicated.

There have been five major extinctions since the Cambrian and these are nick-named the “Big 5”, and are:

1. End Permian
2. Late Devonian
3. End Ordovician
4. End Triassic
5. End Cretaceous

The cause of mass extinctions is unclear and hotly debated, but some favourite ides include Bolides (meteorite impacts), Climate change, Eustacy (Global sea level changes), Volcanics, Anoxia ( no oxygen in marine environments) and disease.
Arens (2008) defines that ;
“A mass extinction results when long-term stress is compounded by short-term shock”
The pattern of an extinction defines whether it occurred at a single horizon and was sudden, or whether it occurred at several horizons and was therefore stepped or Gradual, and it is common for apparent extinctions to precede an extinction event making it appear stepped or gradual, and this is known as the Signor-Lipps effect. There is also another cause for pseudo-extinction, and this happens when a species disappears from the rock record at an extinction horizon and then re-appears at a higher one. This apparent raising from the dead of fossil taxa is known as the Lazarus effect.
The duration of all mass extinctions is less than 10 million years, with most taking place is less than 5 million years and the End-Cretaceous possibly taking place in as little as 10s-100s of thousands of years.