Major extinctions happen when a set of causal factors that might not be of serious consequence by themselves become aligned in time. ~ Norman MacLeod
The causes of extinction events are various. A major meteorite impact creates a planetary shock, as it did 66 MYA (the K–Pg event), finishing off the large dinosaurs.
Comet storms and bolides occasion extinction events, as has happened in at least 3 events, with shock waves causing raging wildfires, massive floods, acid rains, and withering winters.
Radiation from supernovae sterilizes and kills surface life, as do solar flares. Geomagnetic reversals forge a flux of cosmic rays to similar effect. Disruptive radiation factored into the extinction events that ended the Ordovician, Devonian, Permian, and Cretaceous periods.
Continental drift has been another facet in mass extinction via global glaciation or warming, increased aridity, and volcanism. The configuration of continents has a pronounced effect on the viability of macroscopic life everywhere.
Volcanoes spew toxicity into the atmosphere, on land, and in the oceans, affecting ecology worldwide, setting up stepwise extinctions. Tectonic plate movements and volcanoes are the 2 sides of the same coin. Volcanism played a role in many mass extinction events, notably the most severe.
Changes in sea level, salinity, and oceanic oxygen levels contributed to several extinction events, as have pattern disruptions in ocean-atmosphere circulation. Rising sea levels from deglaciation can prompt volcanic activity as mantle plumes are put under more pressure.
Finally, life itself creates extinction events: by disease, predation, and other changes into the food web. Most notable is the evolution of new plants, with better protections against herbivory: depriving animals reliant upon the plants of the past.
All mass extinctions stem from a selfsame biotic dynamic: relatively rapid changes in the environment with which life forms are unable to cope quickly enough via adaptation. An extinction event ensues by a cascade of ecological dependencies, or by rapid environmental changes that simultaneously decimate numerous species. Extinction events invariably involve both dynamics.
Mass extinctions are generally non-selective. The species that go extinct or are diminished are not those ill-adapted; just unlucky. The most evolutionarily advanced species may have their candles snuffed by the stiff wind of an extinction event, leaving behind lesser-but-luckier life.
The only selection factor in extinction events is tolerance to adversity. The hardiest organisms tend to be the most archaic: little ones that have persevered through tough times before. That withstanding, microbial species do go extinct.
Most bacterial lineages ever to have inhabited this planet are estimated to be extinct. ~ microbiologist Stilianos Louca et al
The safest place to be during an extinction event is the deep ocean. Benthic life fares better than anywhere else.
Mass extinction is typically followed by high species origination rates within a few million years. The availability of nutrients is a key factor in the revival of life.
Glaciation is a particularly subtle seeding for the next generation of species. Organic remains from the previous period are preserved by the cold, only to be released when the ice melts. At the other extreme, heading to hothouse by global warming provides no such head start.
Environmental change correlates closely with extinction but not with speciation. ~ American geologist Steven Holland