Speculation about the demise of the dinosaurs continues, but the most popular theory is that a giant meteorite caused their extinction 65 million years ago. The meteorite, that experts believe landed in the Yucatan Peninsula of Mexico, caused dramatic climate change. Why dinosaurs were so specifically affected is uncertain, though their lack of mobility probably did count against them. This dramatic climate change also paved the way for the mammals to take over from the giant reptiles as the dominant force on Earth.
Sudden changes in temperature will always have a drastic effect on many living things, and extreme cold may have resulted in several mass extinctions in Earth's history. Creatures that thrived in warm seas would have soon perished when temperatures plummeted. Ocean water would have ended up on land as glacial ice, which also would have had an impact on land creatures. The glaciation also would have seen sea levels fall, which would have caused more extinctions.
Volcanic ash also has been put forward as a reason for the mass extinction of the dinosaurs millions of years ago. Certainly, if enough volcanoes were spewing out volcanic ash simultaneously it would have a devastating effect on living creatures and plants around the world. A mass extinction also could have been caused by just one enormous volcanic explosion. Apart from the obvious immediate danger to any living thing close to a huge, active volcano, the pollution over a large area also would have had grave implications. Volcanic ash would have blotted out much of the sunlight, and many creatures would have perished because they were unable to breathe.
Mass extinctions are thought of by many as being the result of a dramatic, sudden change. But they can be the result of more gradual changes, including the world's oceans' cycle of expanding and contracting. This has occurred for hundreds of millions of years. During this time, both land and sea creatures would have had to have adapted or face extinction. The changes in the world's oceans could well have resulted in several mass extinctions.