What is the geosphere?

What is the GEOSPHERE?

The geosphere is considered that portion of the Earth system that includes the Earth's interior, rocks and minerals, landforms and the processes that shape the Earth's surface. The Earth itself (contrary to Christopher Columbus) is not a perfect sphere. It is what is called an oblate spheroid, with a radius of 6,357 kilometers (km) from the Earth's center to the North Pole and 6,378 km from the center to the Equator. Prior to advanced instruments and spacecraft, 17th-century scientist Sir Isaac Newton predicted a similar shape based on the effects of the Earth's daily rotation and his studies of other planets. Geodesy (the study of the Earth's shape) is a very important science, in that it is critical for helping us understand satellite orbits, create maps and navigate on the planet using devices such as the Global Positioning System (GPS).

The Earth's interior includes a thin, 5- to 70 km-thick layer of oceanic and continental crust overlying an additional 6,300 km of rock and metals. The crust varies in thickness and density, with oceanic crust consisting of a thin (around 5 km) layer of dense rock and continental crust consisting of less-dense, lighter-colored rock ranging between 30 and 70 km in thickness. Although the crust is comprised of many types of rocks and hundreds of minerals, these materials are assembled from a very small number of elements. A total of 98.7% of the crust (by weight) consists of just 8 elements, including oxygen (46.6%), silicon (27.72%), aluminum (8.13%), iron (5.00%), calcium (3.63%), sodium (2.83%), potassium (2.70%) and magnesium (2.09%). These elements form the building blocks of most of the inorganic materials we encounter in our daily lives such as glass (SiO2), concrete (CaCO3), and steel. Oceanic crust is dominated by minerals consisting of silicon, oxygen and magnesium and is thus called SIMA; continental crust is made up of SIAL, in which silicon and aluminum dominate.

The Earth's interior is arranged somewhat like a layer cake, consisting of a series of layers that change in density, mineral composition and thickness with depth. Directly below the crust is the mantle. It consists of two parts, an upper layer that is less dense and relatively brittle and a lower (much thicker) layer that is more dense and plastic (it deforms without breaking). The crust and upper mantle combined form the brittle upper layers of the Earth's interior called the lithosphere. The upper mantle is also called the asthenosphere.

The mantle makes up the largest volume of the Earth's interior. The region beneath the mantle is called the core, and consists of two parts, a liquid outer core that is around 2250 km thick and a solid inner core 1220 km thick. The core is primarily made up of iron, with a small amount of nickel. The liquid iron in the outer core is particularly important in that it is the primary source of the Earth's magnetic field. Unlike a common magnet, though, the north and south ends of our "global magnet" are not exactly situated at Earth's poles. Instead, the magnetic north pole is actually situated in northern Canada, and the magnetic south pole resides north of Antarctica and south of Australia. Another interesting feature of the magnetic poles is that their precise location moves over time. Every few million years, even the polarity of the Earth's magnetic field reverses (called a geomagnetic reversal, where magnetic north and south "switch"). While scientists still do not fully understand why geomagnetic reversals occur, the presence of changing magnetic orientations preserved in rocks containing iron was a fundamental clue in unravelling the puzzle of Plate Tectonics. Almost all of our direct knowledge of the Earth's interior is from the upper 10 km. Our knowledge of the remaining 6,300 km is based largely on indirect evidence from seismology, laboratory studies of igneous and metamorphic rocks, computer models and meteorites.

The surface of the Earth is shaped by mountain-building processes, called orogenesis, and the combined effects of wind, water, gravity and chemical weathering. Orogenesis and erosional processes combine to form the rock cycle, in which igneous rocks form directly from molten rock, are changed by heat and pressure into metamorphic rocks and/or eroded, transported and redeposited by wind, water or gravity to form sedimentary rocks. These landforming processes while shaping the land we live on also interact closely with the hydrosphere, biosphere, atmosphere and cryosphere to control the movement of water across the landscape, the distribution of nutrients for plants, formation of soils, the exchange of water and trace gases with the atmosphere and the formation and movement of ice. In turn, local and global climate impacts the rate and type of landforming processes that occur and thus provides a feedback between the geosphere and other spheres. In many ways, it is the linkages between the spheres which interest us most, in that the plants we eat, the soils on which they grow, the water we drink and the air we breathe are found in the interface between the spheres.

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