The outer surface of the Earth, or crust, is made of a variety of substances formed by a complex series of forces. Understanding the geology of the Earth's surface is important, since it makes up so much of what we see and most of the materials we use in our civilization. Even so, the rocks of the crust make up less than a third of the actual surface area of the Earth, with the rest being water.
Geography
The rocky surface of the Earth is called the lithosphere, with the outer surface being called the crust. However, the visible crust only comprises 29% of the Earth's surface. The remainder is water, either in the form of liquid ocean or frozen ice.
Types
The lithosphere is divided into two categories: the continental lithosphere and the oceanic lithosphere. The former is on the surface and exposed directly to the atmosphere. The latter lies under the Earth's oceans. The crust of the continental lithosphere is thought to be 30- to 50-km thick, while the oceanic crust is much thinner, at an estimated 5- to 10-km thick. Beneath the crust is the mantle, an area of solid matter which is hotter and softer than the crust and thus has a more plastic nature.
Features
The crust is composed of a wide ranging variety of igneous, sedimentary and metamorphic rocks. The continental crust and oceanic crust differ in what types of these rocks are present. Granite, for example, which comprises much of the stone present in the continental crust, is rare in the oceanic crust. Basalt, on the other hand, is common beneath the oceans, but rare on land. Furthermore, most of the rocks that are visible on the surface of the Earth are sedimentary, even though they comprise only a small part of the crust as a whole.
Identification
Igneous rocks are all formed by the cooling of volcanic magma. Granite is a common example of this kind of rock. They make up over 95 percent of the lithosphere, but this fact is hidden from direct human observation. That is because of the presence of virtually all of the sedimentary and metamorphic rock on or near the Earth's surface.
Sedimentary rock is created by the overpressure placed on sediment by the material that has formed above it.--often more sediment. The result of the sustained pressure is the solidification into layers of sedimentary rock. Limestone, sandstone and shale are examples of this type of rock. Much of the visible rock on the surface of the Earth is of this type.
Metamorphic rocks are the result of one of the previous two types of rock being subjected to conditions severe enough to transform them. Sedimentary or igneous rocks that are buried or pressed by tectonic forces are placed under enormous pressure and heat. Likewise, some rocks are altered by direct contact with an intrusion of molten magma from the Earth's core. Examples include slate, marble and quartz.
Considerations
All three types of rocks contain ores--minerals, metals, gems and other substances. Most of the substances we take for granted--gold, iron, copper, uranium--do not exist in usable forms in nature. Instead, they exist in the diluted form of ore. These ores are mined and processed.
Tags: Earth surface, oceanic crust, surface Earth, continental crust, continental lithosphere, outer surface, rocks that
