GED Science Practice Test: Structure of Earth

When scientists study or describe the structure of the earth, they typically describe it in terms of its layers or its subsystems.

Layers of the Earth: It was not until the turn of the 20th century that scientists determined that our planet is made up of three main layers: crust, mantle, and core. This layered structure can be compared to that of a boiled egg. The crust, the outermost layer, is rigid and very thin compared with the other two. The crust is found beneath continents and underneath the oceans. Beneath the oceans, the crust varies little in thickness, generally extending only to about 5 km. The thickness of the crust beneath continents is much more variable but averages about 30 km; under large mountain ranges, such as the Alps or the Sierra Nevada, however, the base of the crust can be as deep as 100 km. Like the shell of an egg, the Earth’s crust is brittle and can break.

Below the crust is the mantle, a dense, hot layer of semi-solid rock approximately 2,900 km thick. The mantle is hotter and denser because temperature and pressure inside the Earth increase with depth. As a comparison, the mantle might be thought of as the white of a boiled egg.

At the center of the Earth lies the core, which is nearly twice as dense as the mantle because its composition is metallic (iron-nickel alloy). Unlike the yolk of an egg, however, the Earth’s core is actually made up of two distinct parts: a 2,200 km-thick liquid outer core and a 1,250 km-thick solid inner core. As the Earth rotates, the liquid outer core spins, creating the Earth’s magnetic field.

The diagram below shows a cross-section of earth and its layers. You can see both the position, and the relative thickness of each layer:


Earth’s Subsystems:  Earth’s structure can also be described by its four subsystems:  atmosphere, lithosphere, hydrosphere, and biosphere.  The atmosphere consists of all of the air on earth.  Earth’s atmosphere extends a short distance below the earth’s surface (soil contains air), and outward almost 10,000 km (over 6,000 miles) above the earth’s surface.  The atmosphere includes the ozone layer, which protects us from the sun’s radiation.  The atmosphere will be the focus of the next lesson.

The geosphere contains the crust, mantle, and core of the earth.  Sometimes scientists will refer to the geosphere as the lithosphere.  However, the word lithosphere also refers to just the crust layer of the earth.  The geosphere will be the focus of this lesson.  A number of earth’s natural resources are found in the geosphere, such as coal, salt, metal ores, etc.

The hydrosphere includes all of the water on earth, including frozen, liquid, and gaseous forms of water (see the discussion of the water cycle in the ecology lessons).  Thus, the hydrosphere includes glaciers, icecaps, oceans, rivers, and water vapor in the atmosphere.  Some scientists call the frozen water in the hydrosphere the cryosphere. The liquid water in the hydrosphere also varies in salinity (salt content) and temperature.   Ocean currents, or the movement of water in the oceans is partially due to changes in the salinity and temperature of the ocean water.

The biosphere includes all of the living organisms on earth, including microscopic organisms, plants, and animals.  Some scientists place humans in their own separate subsystem of the biosphere, called the anthrosphere.  This is because humans interact so powerfully with other organisms, and other spheres.

While these four spheres allow us to discuss certain parts of the earth, they do not exist in isolation.   Each sphere is always interacting with other spheres.  Again thinking of the ocean (part of the hydrosphere) and its currents, we said that one of the reasons for the ocean currents was the characteristics of the water itself.  However, winds (part of the atmosphere) also affect ocean currents.

Additionally, the tides, or the rise and fall of the water levels, also affect ocean currents.  Tides can be seen as the interaction of both water (hydrosphere) and land (geosphere).


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