massive events such as an asteroid hitting the earth. Even though Hollywood would have us believe that this is the whole story, the reality is that the causes of mass extinctions are much more complicated than that. More than 90% of all organisms that have ever lived on Earth are extinct today. On many occasions new species have adapted better to ever changing ecological niches but at least a handful of times in the last 500 million years, up to 90% of species have disappeared in a geological blink of the eye.
Many still argue that the causes of these mass extinctions are a mystery but there is consensus that volcanic eruptions or asteroids could have caused them. This is because both of these events would create and emit into the atmosphere a lot of debris which would not allow the sun to get through. Without any sunlight, plants and other creatures would slowly die. Both asteroids and volcanoes could also unleash toxic and heat-trapping gases.
Looking at the facts that scientists have been able to piece together, it appears that an asteroid impact would be most closely linked to the Cretaceous extinction event, famed for the death of the dinosaurs. A huge crater off of Mexico’s Yucatán Peninsula is dated to about 65 million years ago, coinciding with the extinction. However, it is often argued that a lot of the other species that disappeared at the same time, such as the ammonites, many flowering plants and the last of the pterosaurs, were already in decline. Some experts therefore suggest that it was also due to flood basalt eruptions – volcanic eruptions that coat large stretches of land or the ocean floor with lava – that affected the world’s climate, in the first place, combined with drastic falls in sea level. The asteroid impact was the last straw that broke the camel’s back.
At the same time, scientists do not seem to be able to explain with any great level of certainty the causes of the Triassic-Jurassic extinction when about 20% of all marine families went extinct, as well as most mammal-like creatures or the Permian-Triassic extinction which took place about 250 million years ago and is considered the deadliest since more than 90% of all species perished. Scientists oscillate between attributing these mass extinction events to an asteroid or comet impact – but there is no crater to support this theory – and flood volcanism from the Siberian Traps, a large igneous province in Russia. A theory that reconciles both is the concept of impact-triggered volcanism, where an asteroid impact triggers a series of volcanic eruptions.
But it is not just volcanoes and asteroids or even a combination of both that can explain all the causes of mass extinctions. For example, 440 million years ago freezing temperatures led to much of the world’s water being trapped as ice, causing the Ordovician-Silurian extinction. Perhaps more interesting is the case of the drawn-out Late Devonian extinction event about 360 million years ago which eliminated about 70% of all marine species from Earth over a period of 20 million years.
So while volcanoes and asteroids are definitely top on the list of what causes mass extinctions, it is also clear that other factors are also at play and that in some cases very different factors are at play. Perhaps what is interesting is the rate at which extinctions unfolded. For example, the effects of a volcano eruption or an asteroid attack are felt immediately but if humans had been living through the Late Devonian extinction, would they have taken note of the demise of species around them?
Perhaps we don’t need much imagination to figure that out. Today scientists are telling us this exact thing: we are on the brink of a sixth mass extinction. There is evidence of the growing and detrimental impact of human activities on species all around the world which is likely to mean that as many as 30 to 50% of all species are possibly heading toward extinction by mid-century³. However, little-felt these impacts are in our daily lives, they are occurring and they are going to impact wider ecosystems until eventually our own survival is directly at risk. We really shouldn’t have to be hit by an asteroid before realising that it is about time we do something for our planet’s species.