Inner Earth Model

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volcanic vent viewed from above. steam and molten lava visible.
Hawaii Volcanoes National Park, Hawaii. The Hawaiian Islands form due to volcanic activity as the Pacific Plate rides over a hotspot rising from deep within the Earth's mantle.

NPS photo by Dale Pate.

Earth's Layers

diagram of earth structure 3 main layers
Earth is differentiated into three main layers (crust, mantle and core) due to different chemical composition. Our planet has a dense iron core, a mantle of silicates rich in iron and magnesium, and a crust with lighter silicates.

Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.

Planet Earth is layered because it consists of different chemical materials. But that’s only part of the story. If you could descend to a great depth within the Earth, you would be incinerated by scorching temperature and crushed by enormous pressure. Those conditions harden and soften materials, and result in an outer shell of tectonic plates (lithosphere) that ride over a softer layer below (asthenosphere). Much of the action on Earth’s surface—earthquakes, volcanic eruptions, and the formation of mountain ranges—occurs along the boundaries of the moving plates, or where a plate moves over a hotspot rising from Earth’s deep interior.

Physical States of Earth's Layers

Earth's main layers (crust, mantle and core) are in different physical states due to increasing temperature and pressure with depth. The outer core is liquid because it’s so hot, but becomes a solid inner core because of the pressure. Earth's mantle (the zone between the thin crust and the heavy core) has hard and soft zones. The crust and outermost mantle are so cold they form the solid lithosphere. But the higher temperature just below that results in a somewhat softer layer, the asthenosphere. Still deeper in the mantle it’s even hotter, but pressure is so great that the lower mantle is a hard solid. These unique properties of Earth’s mantle create a situation where plates of hard lithosphere ride over the softer layer of asthenosphere.

diagram of earth structure 3 main layers with additional layers to show physical states—liquid and solid
Left: Diagram of the different physical states due to increasing temperature and pressure with depth. Right: The structure of hard lithosphere, softer asthenosphere, and hard lower mantle make the outer portions of the Earth similar to an Oreo® cookie.

Modified from “Beauty from the Beast: Plate Tectonics and the Landscapes of the Pacific Northwest,” by Robert J. Lillie, Wells Creek Publishers, 92 pp., 2015, www.amazon.com/dp/1512211893.

Convection Currents

Planet Earth—Viewed as a Giant Heat Engine

Earth was originally molten and remains hot because radioactive elements decay to non-radioactive forms, generating heat in the process. One of the ways the heat escapes upward is through flow of the soft asthenosphere, an extremely slow version of the convection currents in a pot of boiling water or simmering pudding. The rigid plates of lithosphere ride about on the convecting asthenosphere, ripping apart at divergent plate boundaries, crashing together at convergent plate boundaries, and sliding past one another at transform plate boundaries.

Hotspots

Hotspots are plumes of hot material rising from deep within Earth’s mantle. As a hotspot encounters the bottom of a moving plate it melts its way through, forming a line of volcanoes, like the Hawaiian Islands, on the plate’s surface.


Figures Used

Site Index & Credits

Plate Tectonics and Our National Parks (2020)

  • Text and Illustrations by Robert J. Lillie, Emeritus Professor of Geosciences, Oregon State University [E-mail]

  • Produced under a Cooperative Agreement for earth science education between the National Park Service's Geologic Resources Division and the American Geosciences Institute.

Last updated: February 11, 2020

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