There have been many mass extinctions throughout the history of the Earth. Probably the most famous is the extinction that finally saw the end of the dinosaurs reign on the Earth, 65 million years ago. It wasn't just the dinosaurs that died out in this extinction. Whatever caused the death of the dinosaurs also caused the death of around 70% of all of the species on the Earth. There are many theories about why the dinosaurs finally became extinct, some of which are more than a little outlandish.

Self-Destruction

The herbivorous dinosaurs' over-foraging and the carnivorous dinosaurs' over-culling of the herbivorous dinosaurs could have triggered mass starvation.

Global Change

Changes in the Earth's orbit that could have caused climactic cooling might have caused the extinction. In this scenario, the dinosaurs couldn't adapt to the cold, but the furry mammals could. This is consistent with the climate in the late Cretaceous; toward the end of the Cretaceous, there was a drop in sea level, causing land exposure on all continents, more seasonality, and greater extremes between equatorial and polar temperatures. Large amounts of methane changing the Earth's atmosphere (causing a greenhouse effect). The methane source would be from deep-sea algae deposits and/or from plant-eating dinosaurs digestion by-products.

Volcanic Eruptions

Extreme volcanic activity and the accompanying acid raincould have changed the Earth's climate enough to trigger a mass extinction. The late Cretaceous was a time of high tectonic activity and accompanying volcanic activity . The supercontinent Pangea was splitting up and the continents were taking on their modern-day forms. Extreme volcanic activity would spew dust and acidic chemicals (like sulphuric acid) into the atmosphere, causing global cooling, and perhaps, mass extinctions.