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3.6: Layers Of the Earth As Defined By Physical Properties

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    Layers Of the Earth As Defined By Physical Properties

    With increasing depth, Earth’s interior is characterized by gradual increases in temperature, pressure, and density.
    Depending on the temperature and depth, a particular Earth material may behave like a brittle solid, deform in a plastic–like manner, or melt and become liquid. Figure 3.8 illustrates the structure of the Earth highlighting the physical properties of the different layers.

    Structure of the Earth
    Figure 3.8. Structure of the Earth

    Lithosphere (sphere of rock)

    The term lithosphere is used to describe the rigid outer part of the Earth, consisting of the crust and upper mantle. Compared with other layers of the Earth, the lithosphere is a relatively cool, rigid shell and averages about 60 miles (100 km) in thickness, but may be about 155 miles (250 km) or more thick beneath the older portions of the continents. The lithosphere is broken up into moving plates, and the movements of these plates are responsible for all the large-scale features observable on the surface—including ocean basins, continents, and mountain ranges.

    Asthenosphere (weak sphere)

    The term asthenosphere refers to a semi-fluid layer beneath the lithosphere (within the upper mantle), between about 60 to 400 miles (100-650 km) below the outer rigid lithosphere (oceanic and continental crust) forming part of the mantle. The asthenosphere, although solid, is very hot and is thought to be able to flow vertically and horizontally, enabling sections of lithosphere to undergo movements associated with plate tectonics. Geologist use the term plastic to describe how hot solid materials, including rocks, can deform and flow slowly.

    Mesosphere (or Lower Mantle)

    This region is a rigid layer between the depths of about 400 to 1800 miles (650 km and 2900 km), but the rocks at these depths are very hot and capable of gradual flow. Heat from the core drives mantle gravitational convection.

    Earth's Core

    Earth's core is subdivided into to zones based on their geophysical properties: an outer core and an inner core. Outer core: As discussed in section 3.6, the outer core is a liquid layer composed mostly of an iron-nickel alloy (a mixture with similar composition to metallic meteorites). Convective flow within the outer core generates Earth’s magnetic field.

    Inner core: Geophysical studies show that the inner core behaves like a solid, but is very dense, around 16 gm/cc (similar to the physical properties of an iron-nickel meteorite).

    Iron-nickel meteorite
    Figure 3.9. An iron-nickel meteorite is magnetic and has a similar density as the metallic core of our planet.

    How do we know the structure of the Earth?

    By indirect geophysical methods! Geophysical methods use technical applications and equipment to collect information about the earth, oceans, and atmosphere that are not directly observable by our senses. Examples include:

    Geophysical Method

    Instruments Used

    Magnetic measurements

    magnetometers

    Gravity measurements from the surface or from precise measurements from satellites orbiting a planet or moon

    gravitometers and satellites

    Seismic waves from earthquakes or large explosions

    seismographs

    International Space Station
    Figure 3.10. Gravity can be precisely measured from orbiting satellites.

    The sections that follow explore the types of information we can learn from these three methods.