When speaking of mass extinctions, we evoke a seriousness that transcends anything that we have experienced in human history.

As used in the sciences, it refers to global extinctions of a vast majority of species on Earth. There have been five of these that can be considered major, with more than 75% of species disappeared, and several smaller ones.

These mark the boundaries of geologic eras. Two of these were in the geologic era known as the Paleozoic, or ancient life. Ending the Ordovician, 444 million years ago, 86% of species disappeared. At the end of the Devonian, 375 million years ago, 75% of species disappeared.

At the end of the Permian period, at the Paleozoic/Mesozoic boundary 252 million years ago, 96% of all species disappeared. This is known as “The Great Dying.” In the Mesozoic era at the end of the Triassic period 200 million years ago, 80% of species disappeared.

The most well known among the lay public is the end of the dinosaurs at the Mesozoic/Cenozoic boundary, also called the K-T boundary, 66 million years ago.

The rock record indicates that each extinction was caused by a major and rapid climate change, although it is not clear what specific event or events caused the climate change. Paleontologists think that the extinction events took place over thousands or even millions of years.

These might seem like long time periods to us, but they are merely blips in deep (geologic) time.

There are two favored candidates for the cause of the extinctions. The best known is the strike of an asteroid or a comet.

This seems to be the case for the end of the dinosaurs at the K-T boundary: A huge crater off Mexico’s Yucatan Peninsula is dated to about 65 million years ago, coinciding with the extinction. Global warming fueled by volcanic eruptions at the Deccan Flats in India might also have contributed to the event.

Large volcanic eruptions are the other favored explanation. Over the course of Earth’s history, there have been many basaltic flood eruptions on a scale never seen or imagined in historical times.

Amid continuing speculation about the Great Dying at the end of the Permian period, the eruption of the Siberian Traps stands out as the most likely primary cause.

The Siberian Traps in the northernmost part of Asia began erupting around 300,000 years before the start of the end of the Permian extinction.

The total volume of lava was enough to cover a region the size of the United States in lava more than one-half mile in depth. Most of the eruptions occurred before and during the time of the extinctions, while the last third erupted in the half-million-year period following.

In all of the extinctions, it is not clear how many of the species were killed directly. It is more likely that at least in some cases the majority were a cascading effect caused by ecosystem collapse.

The extinctions share characteristics with an immense power grid failure such as the one that struck the eastern United States in 2003. It extended north into Toronto, west to Michigan and Ohio and covered a huge section of both Canada and the U.S. largely due to a software bug in a control room in Ohio.

This analogy shows that it might not take an earth-shattering event to start a mass extinction. In fact, some people argue that we humans are causing the sixth major extinction by our activities.

Notable is the fact that while formerly all land animals were wildlife, today wildlife accounts for only 3% of Earth’s land animals; human beings, our livestock and our pets take up the remaining 97%.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.