List the periods in which there were mass extinctions, and if possible explain why they occurred

"Big Five" extinction events are

1. Ordovician-Silurian - caused by long ice age and gamma ray burst that followed.

2. Late Devonian - speculated to be caused by impact of asteroids. Caused by glaciation.

3. Permian-Triassic;

maybe one or a combination of these events. (plate tectonics, an impact event, a supernova, extreme volcanism, and the release of frozen methane hydrate from the ocean beds to cause a greenhouse effect). 4. Triassic-Jurassic;

- possible causes are gradual climate change, asteroid impact, or massive volcanic eruptions. 5. Cretaceous-Tertiary. - caused by the darkness brought about by the impact based dust. It made photosynthesis impossible and because the animals are herbivores, they starved to death.