The Earth is a complex planetary body, with a structure that has been evolving for over 4.5 billion years. Many of the original elements that the Earth contained immediately after its formation are still a part of its chemical composition today. Early scientists understood that the Earth had different layers, but it wasn't until the 20th century, and the invention of seismology, when scientists began to understand the details and complexity of those layers.
The Internal Structure of the Earth
The Earth is composed of three main layers: the rocky crust, the semi-solid mantle, and the core, which is made up of both solid and liquid layers itself. The physical and petrological properties of these layers are very different, and are the reason that plate tectonics are active on Earth today.
The physical and petrological properties of the crust, mantle and core are:
- Crust - the thickness of the crust varies from ocean floor (less than 1 km) to mountain tops (up to 100 km); the density ranges from 2.2 to 2.9 grams/cubic meter; it is composed mainly of silicic rock, andesite and basalt.
- Upper Mantle - the upper mantle is 720 km thick; the density is 3.4 to 4.4 grams/cubic meter; it is composed mainly of peridotite, eclogite, olivine, spinel, garnet, pyroxene, perovskite and oxides; the upper mantle is cooler and more solid than the lower mantle.
- Lower Mantle - the lower mantle is 2,120 km thick; the density is 4.4 to 5.6 grams/cubic meter; it is composed mainly of magnesium and silicon oxides; the lower mantle is hotter and more fluid than the upper mantle.
- Liquid Outer Core - the outer core is 2,260 km thick; the density is 9.9 to 12.2 grams/cubic meter; it is composed mainly of iron oxides, sulfur and nickel alloy.
- Solid Inner Core - the inner core is 1,220 km thick; the density is 12.8 to 13.1 grams/cubic meter; like the outer core, it is composed mainly of iron oxides, sulfur and nickel alloy.
The thicknesses and densities of these layers have been discovered through studies in seismology. Geologic instruments are used to penetrate the surface of the crust using seismic waves. The waves are reflected back to the crust and these same instruments are able to measure how the physics of the waves have changed after they have been reflected. The physical changes in the wave signals are controlled by the thickness and density of the different layers. An important discovery made using this technique was the discovery of the Mohorovicic discontinuity (aka Moho), which showed a very dramatic change in seismic wave characteristics at the boundary between the crust and the upper mantle.
Plate Tectonics
The density of the layers is what controls their position, either as a top layer or a bottom layer. The layers towards the top, such as the crust (also called the lithosphere), and the upper part of the mantle, are much less dense then the layers deeper down, such as the core. This makes sense because more dense material will naturally want to sink towards the center. Sometimes, heat and pressure melt the upper mantle, creating a thin section known as the asthenosphere, which is the source of magma. This magma rises to the surface, breaking through the crust during volcanic activity. Sometimes, the crust sinks into the asthenosphere and melts, getting recycled back into the Earth's mantle.
It is the density and physical nature of the crust compared to the density and physical nature of the asthenosphere that creates plate tectonics. The crust is very light and brittle, and so it rides on top of a more flexible and dense asthenosphere, breaking apart in many places to form the major tectonic plates.
Sources:
Robertson, Eugene C., "The Interior of the Earth", usgs.gov
Kious, W.J. and Tilling, R. "This Dynamic Earth - Inside the Earth" (online version), usgs.gov
Alexandra Matiella Novak - With a PhD in Geology and expertise in science education, Alexandra is passionate about increasing the public's Earth science literacy.
