Ordovician-silurian Extinction: 440 million years ago
The shift in the continents and drastic climate change is thought to be the leading cause of this mass extinction. It took place in two waves. The first wave was an ice age that encompassed the entire Earth, lowering sea levels so that many land species could not adapt fast enough to survive the harsh, cold climates. The second wave was when the ice age finally ended. The ocean levels rose too quickly to hold enough oxygen to maintain the species that had survived the first wave.
Devonian Extinction: 365 million years ago
There are several hypotheses for the occurrence of the second mass extinction. The first wave, which dealt a major blow to aquatic life, may have actually been caused by the quick colonization of land—many aquatic plants adapted to live on land, leaving fewer autotrophs to create oxygen for all of the sea life. This led to mass death in the oceans.
The plants’ quick move to land also had a major effect on the carbon dioxide available in the atmosphere. By removing so much of the greenhouse gas so quickly, temperatures plummeted. Land species had trouble adapting to these changes in climate and went extinct as a result.
The second wave of the Devonian mass extinction is more of a mystery. It could have included mass volcanic eruptions and some meteor strikes, but the exact cause is still considered unknown.
Permian-triassic Extinction: 250 million years ago
It is still much of a mystery what set off this greatest of the mass extinction events, and several hypotheses have been thrown around by scientists who study this time span of the Geologic Time Scale. Some believe there may have been a chain of events that led to so many species disappearing; this could have been massive volcanic activity paired with asteroid impacts that sent deadly methane and basalt into the air and across the surface of the Earth. These could have caused a decrease in oxygen that suffocated life and brought about a quick change in the climate. Newer research points to a microbe from the Archaea domain that flourishes when methane is high. These extremophiles may have “taken over” and choked out life in the oceans as well.
Triassic-jurassic Extinction: 210 million years ago
The fourth major mass extinction was actually a combination of many, smaller extinction events that happened over the last 18 million years of the Triassic Period during the Mesozoic Era. Over this long time span, about half of all known species on Earth at the time perished. The causes of these individual small extinctions can, for the most part, be attributed to volcanic activity with basalt flooding. The gases spewed into the atmosphere from the volcanoes also created climate change issues that changed sea levels and possibly even pH levels in the oceans.
Cretaceous-tertiary Extinction: 65 Million Years Ago
It is well-documented that the cause of this mass extinction was a major asteroid impact. The huge space rocks hit Earth and sent debris into the air, effectively producing an “impact winter” that drastically changed the climate across the entire planet. Scientists have studied the large craters left by the asteroids and can date them back to this time.