All of these are caused by water in some way, however, gully erosion is due to steady and fast-flowing streams; the Grand Canyon is an excellent example of the effect of gully erosion. Splash erosion is the first step in erosion from water, and involves the breakage of soil via the impact between soil and a raindrop. It is often a precursor for other types of erosion since it loosens soil. Sheet erosion involves evenly removing a certain number of layers of material due to regular water patterns. Rill erosion is a result of small, well-defined paths of water known as rills, that flow downhill after rainfall.

Today, 99.9% of all species that have ever existed on Earth are extinct. We are currently in the middle of the 6th mass extinction. The dinosaurs went extinct during the 5th, the Cretaceous, about 65 million years ago.

The Wallace Line, named for Alfred Russel Wallace, is a boundary that separates Asia and Australia. Wallace noticed that the some of the islands in this region contained species common to Australia and some contained species common to Asia, none of which would have crossed the open water in between the continents. Wallace correctly reasoned that this meant that sea levels were once much lower, an artifact of the last glacial maximum.

The correct response is Milankovitch cycles. This theory describes how the orbit, tilt, and axis of the Earth operate on a 100,000 year cycle that cause the Earth to be further from the sun. As the Earth gets farther from the sun, it can trigger an ice age. The theory is aptly named after the man who discovered it. The El Nino and La Nina events occur on a much shorter time scale and have nothing to do with ice ages.