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There are four main layers of the earth - the crust, mantle, outer core, and inner core. The crust is Earth's outer layer composed of solid, rocky material. If the Earth were compared with an apple, the crust would be as thin as the apple's skin. The mantle is the middle layer made of a soft, solid material (like butter left out on the dinner table). The mantle is very hot and under tremendous pressure. The outer core, which begins more than 1,800 miles beneath the surface, is hot molten liquid. Scientists think this is rich in iron and nickel. The movement of this liquid probably causes Earth's magnetic field. The inner core, also mostly iron and nickel, is squeezed solid due to extreme high pressure.
Scientists believe that the Earth is approximately 4.6 billion years old. Its geology is constantly changing, being restructured and reformed through natural phenomena and also by human impact. The modern theory of plate tectonics (formerly called continental drift) states that the outer crust of the Earth is separated into several "plates," some containing continents, which move slowly, but continually. The constant movement of the mantle layer causes the overlying plates to crack and move. Geologists generally agree that there are 6-8 large plates and a number of smaller ones.
There are three main types of plate boundaries: convergent, divergent, and transform. Tectonic activity, such as earthquakes and volcanoes, often occurs along these boundaries.
A convergent boundary occurs when plates move towards one another. Their collision (convergence) causes folding and uplifting of rocks. Mountain ranges can form through subduction as one plate slides beneath the other or by uplift of the plate boundaries at the collision area. Volcanoes can form when the subducted plate melts deep in the Earth and the molten rock rises as magma to the surface.
A divergent boundary occurs when plates pull apart (diverge) from each other. A rift zone is formed causing the Earth's crust to thin and form a rift valley. If the plates continue to pull apart, magma will rise through the rift forming new crust.
A transform boundary occurs when plates slide laterally past one another. Friction from this movement along plate boundaries can create earthquakes. As the plates move relative to one another, the crust is stretched, compressed, or sheared along the boundaries. A tremendous amount of strain builds up. When the strain finally reaches the breaking point and is suddenly released, the crust breaks, the rocks are displaced, and violent shaking of the earth occurs.
The Earth is undergoing continuous change through the formation, weathering, erosion, and reformation of rock. This process is called the rock cycle. There are three main types of rocks: igneous, sedimentary, and metamorphic.
Rock deep within the Earth encounters temperatures high enough to make it melt. This liquid stage is called magma. Igneous rock is formed when the magma cools and solidifies. Magma forms volcanic rock when it is forced to the surface and cools. Magma forms granitic rock when it cools beneath the Earth's surface.
As rocks are weathered (broken down into smaller pieces) and eroded (moved to new locations), the rock fragments (sediments) build up in layers. The combined weight of the layers along with other pressures within the Earth causes the layers to compact. The tiny spaces between rock fragments fill with natural cementing agents. Mineral grains in the rock may grow and interlock. Thus sedimentary rock has been formed. Sedimentary rock is also formed under water when shells and skeletons of sea creatures accumulate on the ocean floor. Over a long period of time, these sediments compact and harden to form rock. Fossils are most often found in sedimentary rock.
Sedimentary and igneous rocks can be altered by the tremendous pressures and high temperatures associated with the movement and collision of tectonic plates. Metamorphic rock is formed under these extreme conditions. Ultimately, any of the rock types may again return to a hot, molten state deep in the Earth, thus completing the rock cycle.
Studying geology helps people to understand how today's geological formations were created and to predict future changes. Geologists often take a "core sample" by drilling into a rock formation and pulling out a layered specimen of the rocks to determine a timeline of geologic events for that area. The consequences of natural events and human activity can be better analyzed with knowledge of the underlying rock formations. Through this understanding, a student may develop a new sense of respect for our environment and a new commitment for the responsible, caring, and protective behaviors of good citizenship and environmental stewardship.