About Earth Surface & Interiors

Earth, the largest and densest rocky planet, was formed about 4.5 billion years ago. The Earth's interior is divided into four layers which is typical of rocky planets. Each layer has different characteristics and is made of different elements and minerals. There is a gradient in temperature between the cool surface and the hot interior with the center, or core, as hot as 9000 degrees F. The crust is broken into many large plates that move slowly relative to each other. Mountain ranges form when two plates collide and their edges are forced up. In addition, many other surface features are the result of the moving plates. The plates move about one inch per year, so millions of years ago the continents and the oceans were in different positions. About 250 million years ago, most of the land was connected together, and over time has separated into seven continents. The Crust Because the crust is accessible to us, its geology has been extensively studied, and therefore much more information is known about its structure and composition than about the structure and composition of the mantle and core. Within the crust, intricate patterns are created when rocks are redistributed and deposited in layers through the geologic processes of eruption and intrusion of lava, erosion, and consolidation of rock particles, and solidification and recrystallization of porous rock. By the large-scale process of plate tectonics, about twelve plates, which contain combinations of continents and ocean basins, have moved around on the Earth's surface through much of geologic time. The edges of the plates are marked by concentrations of earthquakes and volcanoes. Collisions of plates can produce mountains like the Himalayas, the tallest range in the world. The Structure of the Moon The Moon, our fellow-traveler in space, has a diameter half that of the Earth's core, and it revolves around the Earth, as all the planets revolve around the Sun, under the force of gravity. Moonquakes of very low energy are caused by land tides produced by the pull of Earth's gravity, and, from analysis of moonquake data, scientists believe the Moon has two layers: a crust, from the surface to 65 kilometers depth, and an inner, more dense mantle from the crust to the center at 3,700 kilometers. The crust is presumed to be com- posed primarily of rocks containing feldspar, calcium aluminum silicate, and lesser pyrox- ene, iron and magnesium silicate; the crust also contains basalt in the mares, which con- tains less iron and more titanium than earth basalt. The mantle is thought to be made up of calcic peridotite, containing both pyroxene and feldspar.