The largest of the four rocky planets, Earth is the only place known to support life. A dynamic world, unlike any other, it has an abundance of liquid water, and its surface undergoes constant change. Its one companion in space is the lifeless Moon.
Earth was formed about 4.6 billion years ago. since then its material has differentiated into layers. The central core, which is hot and dense, has solidified and consists of iron, with some nickel. Above this is a solid rock mantle, and then a thin crust made up of many different types of rocks and minerals, but predominantly silicate rock. This crust is broken into seven large solid plates and some smaller ones. They float on partly molten under lying mantle. Earth's continents, oceans, and air all support life.
THE PLANET'S iNTERiOR
Movement within the outer core, partly driven by Earth's rotation, produces a magnetic field that deflects solar wind.
solid iron-nickel inner core
mantle of solid silicate rock solid, rocky crust
shaped by water
The Tigre, a tributary of the Amazon, Earth's greatest river, cuts through the Peruvian rainforest. The Amazon carries more water each year than any other river. Water, wind, and life forms have all shaped Earth's surface to varying degrees.
Earth's crust varies in thickness; the thickest parts form the seven major continental landmasses. The rest of the crust, which amounts to over 70 percent of Earth's surface and is generally thinner, is covered in water. Almost all of this is in liquid form and makes Earth's five vast oceans. Just 2 percent is in the form of ice, in the caps around the north and south poles. The plates in Earth's crust move against and away from each other, and at plate boundaries give rise to such features as mountain ranges, deep-sea trenches, and volcanoes.
The Arenal volcano in Costa Rica is one of the many volcanoes along the Ring of Fire. The ring is an arc of volcanic and earthquake activity formed by colliding plates. It stretches around the rim of the Pacific Ocean.
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