Why do mass extinctions occur and why do they often mark divisions of geologic time?

Mass extinctions occur for many reasons. Changes in the environment might make it impossible for the local life to survive, diesease or drought might strike, or there might be a natural disaster. These extinctions make good divisions in geologic time because they are easy to locate over a wide area and mark a definitive change in the Earth's systems.

Answerdistinguish between the mercalli scale and the richter scale;