Seventy percent of the Earth's surface is covered by water, while the remaining 30 percent consists of the seven continental landmasses. The Earth is unique among the known planets in that is has an abundance of liquid water.
The outer layer of the Earth, known as the crust, was formed 4.5 billion years ago when lava cooled to create a hard surface. The crust is broken into several large continental plates that are in constant movement. These plates drift upon the asthenosphere of Earth's upper mantle, moving slowly in relation to each other. Earthquakes are the result of collision between these plates. Many of the Earth's land features, such as mountain ranges, are a result of their movement.
Earth's crust is made up of several different chemical elements. While iron, oxygen and silicon are the most abundant elements within the crust of the Earth, there are several other elements present in smaller quantities. While the crust is only 3 miles deep at its thinnest point below the oceans, it may be up to 46 miles in depth below the continents. The oceans that cover the vast majority of Earth's surface are thought to have formed by the impact of comets over millions of years.