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The Earth's Crust The earth’s thin rocky outer layer (3–45 mi [5–70 km] thick) is called “the crust.” On the continents it consists of sedimentary rock layers—some containing fossils and some folded and contorted—together with an underlying crystalline rocky basement of granites and metamorphosed sedimentary rocks. In places, the crystalline rocks are exposed at the earth’s surface, usually as a result of erosion. Beneath the crust is what geologists call the mantle, which consists of dense, warm-to-hot (but solid) rock that extends to a depth of 1,800 mi (2,900 km). Below the mantle lies the earth’s core, composed mostly of iron. All but the innermost part of the core is molten. Investigations of the earth’s surface have revealed that it has been divided globally by past geologic processes into what today is a mosaic of rigid blocks called “plates.” Observations indicate that these plates have moved large distances relative to one another in the past and that they are still moving very slowly today. The word “tectonics” has to do with earth movements; so the study of the movements and interactions among these plates is called plate tectonics. Because almost all the plate motions occurred in the past, plate tectonics is, strictly speaking, an interpretation, model, or theoretical description of what geologists envisage happened to these plates through earth’s history. The general principles of plate tectonics theory may be stated as follows: deformation occurs at the edges of the plates by three types of horizontal motion—extension (rifting or moving apart), transform faulting (horizontal slippage along a large fault line), and compression, mostly by subduction (one plate plunging beneath another). Extension occurs where the seafloor is being pulled apart or split along rift zones, such as along the axes of the Mid-Atlantic Ridge and the East Pacific Rise. This is often called “seafloor spreading,” which occurs where two oceanic plates move away from each other horizontally, with new molten material from the mantle beneath rising between them to form new oceanic crust. Similar extensional splitting of a continental crustal plate can also occur, such as along the East African Rift Zone. Transform faulting occurs where one plate is sliding horizontally past another, such as along the well-known San Andreas Fault of California. By Anderw Snelling

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Geology is the study of the nonliving things that the Earth is made of. Geology is the study of soil and rocks. The people that study geology are called geologists. By studying the soil and rocks that the Earth's surface is made of, Geologists can learn more about the history of the Earth. Some of the important events (things that happen) in the Earth's history are floods, volcanic eruptions, earthquakes, orogeny (mountain building), and plate tectonics (movement of continents). Some Geologists study soil and rocks to find minerals, like metals and oil, which are underground. Principles of Geology Geologists use some simple ideas which help them to understand the rocks and soil that they are studying. These ideas are called the "Principles of Geology". 1. Understanding the past: Geologist James Hutton said "The present is the key to the past". He meant that the sort of changes that are happening to the Earth's surface now are the same sorts of things that happened in the past. Geologists can understand things that happened millions of years ago, by looking at the changes which are happening today. 2.Horizontal strata: The layers in a sedimentary rock must have been horizontal (flat) when they were deposited. (laid down). 3.The age of the strata: Layers at the bottom must be older than layers at the top, unless all the rocks have been turned over. 4.In sedimentary rocks that are made of sand or gravel, the sand or gravel must have come from an older rock. 5.The age of faults: If there is a crack or fault in a rock, then the fault is younger than the rock. Rocks are in strata (lots of layers). A geologist can see if the faults go through all the layer, or only some. 6.The age of a rock which cuts through other rocks: If an igneous rock cuts across sedimentary layers, it must be younger than the sedimentary rock. .7The relative age of fossils: A fossil in one rock type must be about the same age as the same type of fossil in the same type of rock in a different place. Likewise, a fossil in a rock layer below must be earlier than one in a higher layer. Uniformitarianism - "The Present is the Key to the Past" Uniformitarianism is a geological doctrine. It states that current geologic processes, occurring at the same rates observed today, in the same manner, account for all of Earth's geological features. Thus, it assumes that geological processes are essentially unchanged today from those of the unobservable past, and that there have been no cataclysmic events in earth's history. As present processes are thought to explain all past events, the Uniformitarian slogan is, "the present is the key to the past." Types of rock Rocks can be found in all sorts of shapes and colors. Some are very hard and some are soft. Some rocks are very common, while others are rare. However, all the different rocks belong to three categories or types, igneous, sedimentary and metamorphic. Igneous rock Igneous rock is rock that has been made by volcanic action. Igneous rock is made when the lava (melted rock on the surface of the Earth) or magma (melted rock below the surface of the Earth) cools down and becomes hard. Igneous rock formed from magma that has become solid inside the earth is called intrusive igneous rock. Igneous rock formed from lava that has become solid on the outside of the Earth is called extrusive igneous rock. This can happen because of the eruption of a volcano. Some extrusive igneous rock types are basalt, andesite, rhyolite, tuff, obsidian, and pumice. Examples of intrusive igneous rock types are gabbro, diorite, and granite. [Sedimentary rock Sedimentary rock is rock that has been made from "sediment". "Sediment" is all solid pieces of stuff that are moved by the wind, water, or glaciers. Sediment can be made from clay, sand, gravel and the bodies and shells of animals. The sediment gets dropped in a layer at the bottom of a river or sea. As the sediment piles up, the lowers layers get squashed together. Slowly they set hard into rock. Some sedimentary rocks are made of just one type of sediment, all about the same size, such as sand. Other sedimentary rocks will have large and small lumps, and pieces made of different types of rock. Well-known sedimentary rocks are sandstone and limestone. Metamorphic rock Metamorphic rock is rock that has been changed. The word "metamorphosis" means "change". Sometimes an igneous, or a sedimentary rock can be heated, or squashed under the ground, so that it changes. Metamorphic rock is often harder than the rock that it was before it changed. One well-known metamorphic rock is marble which is valued for its different colors, and because it can be carved and polished. Slate is a metamorphic rock that is a useful building material. Faults All three kinds of rock can be changed by being heated and squeezed by forces in the earth. When this happens, faults (cracks) may appear in the rock. Geologists can learn a lot about the history of the rock by studying the patterns of the fault lines. Earthquakes are caused when a fault breaks suddenly. The Rock Cycle: The rock cycle changes one type of rock into another. There are three main rock types, igneous, sedimentary, and metamorphic. Igneous rock forms from molten rock. Sedimentary rock forms from eroded pieces of other rocks. Metamorphic rocks are igneous or sedimentary rocks that have been slightly melted. Bits of sand can come off igneous rocks by erosion. These bits can be carried away by water and rivers to seas and oceans where they are deposited. Deposited sediments can harden into sedimentary rock. Igneous and sedimentary rocks can also be changed into metamorphic rock. If molten rock comes near igneous and sedimentary rocks, these rocks can melt slightly to form new metamorphic rocks. Any type of rock that is totally melted becomes magma. Magma that cools and hardens becomes igneous rock.

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