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Chemistry LibreTexts

12.1: Spaceship Earth- Structure and Composition

  • Page ID
    152210
  • Learning Objective

    • Describe the chemical composition and structure of Earth.

    The layers found inside Earth are divided by composition into core, mantle, and crust or by mechanical properties into lithosphere and asthenosphere. Scientists use information from earthquakes and computer modeling to learn about Earth’s interior.

    THE EARTH’S LAYERS

    The layers scientists recognize are pictured in Figure \(\PageIndex{1}\).

    12AA.png
    Figure \(\PageIndex{1}\) A cross section of Earth showing the following layers: (1) crust (2) mantle (3a) outer core (3b) inner core (4) lithosphere (5) asthenosphere (6) outer core (7) inner core.

    Core, mantle, and crust are divisions based on chemical composition:

    Crust: The Earth's surface is the crust. Generally speaking, the crust is predominately silicon oxide and aluminum oxide. Continental crust is thicker and less dense than oceanic crust. Earth’s crust varies in thickness from less than 5 km (under mid-ocean spreading ridges) to more than 70 km (beneath the highest mountain range).

    Mantle: The next layer down chemically is the mantle. The mantle has an ultramafic composition – it contains more iron, magnesium, less aluminum and somewhat less silicon than the crust. The mantle is roughly 2,900 km thick. In terms of volume, the mantle is the largest of earth’s three chemical layers.

    Core:The final layer is the core, which is mostly iron and nickel. The core is about 3,500 km thick.

    Table \(\PageIndex{1}\) summarizes the chemical layers of the earth.

    Table \(\PageIndex{1}\) Chemical Layers of Earth.
    Crust Mantle Core
    composition: high Si, Al, & O composition: moderate Si, high Mg & Fe composition: Fe & Ni
    thickness: 5 to 70 km thickness: 2,900 km thickness: 3,500 km

    Table \(\PageIndex{2}\) provides the elemental composition of the Earth's crust.

    Table \(\PageIndex{2}\) The Elements of Earth's Crust. Source: Wikipedia
    Most Abundant Elements of Earth's Crust Approximate % by weight
    O 46.6
    Si 27.7
    Al 8.1
    Fe 5.0
    Ca 3.6
    Na 2.8
    K 2.6
    Mg 1.5

    Lithosphere and asthenosphere are divisions based on mechanical properties:

    1. The lithosphere is composed of both the crust and the portion of the upper mantle that behaves as a brittle, rigid solid.
    2. The asthenosphere is partially molten upper mantle material that behaves plastically and can flow.

    Compositional and Mechanical Layers of the Earth

    https://www.khanacademy.org/science/cosmology-and-astronomy/earth-history-topic/plate-techtonics/v/compositional-and-mechanical-layers-of-the-earth

    Video \(\PageIndex{1}\) A comparison of the compositional and mechanical layers of the earth.

    Crust and Lithosphere

    Earth’s outer surface is its crust; a cold, thin, brittle outer shell made of rock. The crust is very thin, relative to the radius of the planet. There are two very different types of crust, each with its own distinctive physical and chemical properties, which are summarized in Table \(\PageIndex{3}\).

    Table \(\PageIndex{3}\) The Physical and Chemical Properties of Oceanic and Continental Crust.
    Crust Thickness Density Composition Rock types
    Oceanic 5-12 km (3-8 mi) 3.0 g/cm3 Mafic Basalt and gabbro
    Continental Avg. 35 km (22 mi) 2.7 g/cm3 Felsic All types

    Oceanic crust is composed of mafic magma that erupts on the seafloor to create basalt lava flows or cools deeper down to create the intrusive igneous rock gabbro (Figure \(\PageIndex{2}\)).

    12AB.png
    Figure \(\PageIndex{2}\) Gabbro

    Sediments, primarily muds and the shells of tiny sea creatures, coat the sea floor. Sediment is thickest near the shore where it comes off the continents in rivers and on wind currents.

    Continental crust is made up of many different types of igneous, metamorphic, and sedimentary rocks. The average composition is granite, which is much less dense than the mafic rocks of the oceanic crust (Figure \(\PageIndex{3}\)). Because it is thick and has relatively low density, continental crust rises higher on the mantle than oceanic crust, which sinks into the mantle to form basins. When filled with water, these basins form the planet’s oceans.

    12AC.png
    Figure \(\PageIndex{3}\) The granite from Missouri is more than 1 billion years old.

    The lithosphere is the outermost mechanical layer, which behaves as a brittle, rigid solid. The lithosphere is about 100 kilometers thick. Look at Figure \(\PageIndex{1}\). Can you find where the crust and the lithosphere are located? How are they different from each other?

    The definition of the lithosphere is based on how earth materials behave, so it includes the crust and the uppermost mantle, which are both brittle. Since it is rigid and brittle, when stresses act on the lithosphere, it breaks. This is what we experience as an earthquake.

    Mantle

    The two most important things about the mantle are: (1) it is made of solid rock, and (2) it is hot. Scientists know that the mantle is made of rock based on evidence from seismic waves, heat flow, and meteorites.

    The properties fit the ultramafic rock peridotite, which is made of the iron- and magnesium-rich silicate minerals (Figure \(\PageIndex{4}\)). Peridotite is rarely found at Earth’s surface.

    Scientists know that the mantle is extremely hot because of the heat flowing outward from it and because of its physical properties.

    12AD.png
    Figure \(\PageIndex{4}\) Peridotite is formed of crystals of olivine (green) and pyroxene (black).

    Core

    At the planet’s center lies a dense metallic core. Scientists know that the core is metal because:

    1. The density of Earth’s surface layers is much less than the overall density of the planet, as calculated from the planet’s rotation. If the surface layers are less dense than average, then the interior must be denser than average. Calculations indicate that the core is about 85% iron metal with nickel metal making up much of the remaining 15%.
    2. Metallic meteorites are thought to be representative of the core. The 85% iron/15% nickel calculation above is also seen in metallic meteorites (Figure \(\PageIndex{5}\)).

    12AE.png
    Figure \(\PageIndex{5}\) An iron meteorite is the closest thing to the Earth’s core that we can hold in our hands.

    If Earth’s core were not metal, the planet would not have a magnetic field. Metals such as iron are magnetic, but rock, which makes up the mantle and crust, is not.

    Summary

    • The core, mantle, and crust are divisions of Earth based on chemical composition.
    • The lithosphere and asthenosphere are divisions of Earth based on mechanical properties.
    • The three most abundant elements of Earth's crust are oxygen (46.6% by weight), silicon (27.7%), and aluminum (8.1%).

    Contributors

    Libretext: Fundamentals of Geology (Schulte)

    Marisa Alviar-Agnew (Sacramento City College)

    Wikipedia

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