Mass extinctions occur when global extinction rates rise significantly above background levels in a geologically short period of time. You can see these spikes in extinction rates in the graph shown at right. This graph shows extinction rates among families of marine animals over the past 600 million years. While background extinction levels hover around five families per million years, during mass extinctions, these rates shoot up.
Mass extinctions can also be observed by looking at diversity levels over time. The graph below shows number of marine genera alive at different points in life’s history. While diversity levels generally increase over time, mass extinctions cause sudden drop-offs in diversity. The largest mass extinctions in Earth’s history are marked on this graph. Here, we will refer to each mass extinction by the name of the geologic period that it ended (e.g., the end-Ordovician extinction marks the end of the Ordovician period around 440 million years ago). During several of these events (notably, the Devonian and Triassic extinctions), low speciation rates also contributed to the loss of diversity. Biologists estimate that each of these mass extinctions correspond to a loss of ¾ of the species on Earth at the time!3
3Jablonski, D. 1994. Extinctions in the fossil record. Philosophical Transactions of the Royal Society B 344:11-17.
Bottom graph adapted from Figure 1 in Sepkoski, J.J. 1997. Biodiversity: past, present, and future. Journal of Paleontology 71:533-539.