10.4 Mass extinctions

Ongoing sixth mass extinction (ESGCV)

• We are currently living through the 'sixth' mass extinction according to many scientists. The present rate of extinction is thought to be \(\text{140 000}\) species per year. This rapid extinction rate started in approximately \(\text{10 000}\) BC and has continued into the 21st century.
• The extinction includes disappearance of large mammals such as the woolly mammoth.
• The sixth extinction event is thought to be occurring at a rapidly increasing rate.
• The two major causes for the sixth extinction are thought to be human hunting and the rapid rise in human population. You will learn more about the impact of human activity on the environment in Grades 11 and 12.

Causes of mass extinctions (ESGCW)

There is still a lot of debate among scientists as to what caused the mass extinctions. To be a valid theory to explain what caused mass extinctions, the theory must:

• explain all the losses of species at a particular mass extinction event (not just specific losses e.g. dinosaurs).
• explain why some organisms died and others survived.
• be based on natural events and processes that are shown to have occurred around the time of extinction.

Two of the hypotheses put forward are:

• the impact theory of extinction
• massive volcanic activity

Impact event

Scientists believe that an asteroid was responsible for the mass extinction that occurred 65 million years ago. The meteorite that hit Earth was most likely 10 km in diameter, and upon impact it would have released an amount of energy that was 2 million times more powerful than the most powerful man-made device ever created. The massive impact caused earthquakes and volcanic eruptions that produced toxic gases, and ejected dust, soot and debris into the air, blocking out sunlight and preventing photosynthesis. The impact also resulted in the production of sulfur rich acid rain and caused forest fires and mega-tsunamis. The impact of an asteroid smashing into Earth may have caused food chains to collapse both on land and at sea. They believe this impact produced the 180 km wide Chicxulub crater found in the Gulf of Mexico.

Volcanic activity

Some scientists believe that volcanic eruptions may have caused the mass extinction event at the end of the Cretaceous period that led to the extinction of among others, the dinosaurs. The researchers, through examining a trail of dead particles floating in the sea, spanning half a million years, developed a timeline that links mass extinction to large-scale eruptions of the Deccan Traps, an ancient volcanic range in Western India. Evidence suggests that the massive volcanic eruptions pumped out massive quantities of carbon dioxide and sulphur dioxide into the air, thus altering the climate and contributing to the mass extinction event at the end of the Cretaceous period.