3.1: Plates Defined

Figure \(\PageIndex{1}\): Plate boundary map. Arrows at the plate boundaries indicate the direction of movement along that particular boundary. (Public domain; Wikipedia.)

Earth’s outermost rocky surface is broken into about 15 pieces (called plates) that move about relative to each other. The plates are 50-280 km thick and are comprised of the lithosphere which in turn is made up of the solid upper mantle and crust. The crust that sits on top of the solid upper mantle comes in one of two varieties: continental crust or oceanic crust.

Oceanic lithosphere, composed of oceanic crust and the underlying solid upper mantle, is different than continental lithosphere which has a cap of continental crust on the mantle instead. Oceanic lithosphere is a lot thinner (around 10 km thick) and more dense. The continental crust tends to be about 30 km thick, but can get close to 100 km thick under new mountain belts, and is thinner beneath rift zones (weak spots where the crust is stretching). The mantle portion of the lithosphere is the same composition beneath the continental and oceanic crust. It differs from the rest of the mantle not so much in its composition but in the way that it behaves: it rides around with the overlying crust as a more or less stiff unit. The mantle below the lithosphere, called the asthenosphere, is a weak layer in that is mostly solid but perhaps 0.5% molten. This tiny amount of melt dramatically lowers the strength of the asthenosphere. Movement in the asthenosphere contributes to the motion of the lithospheric plates that sit on top of it.

Figure \(\PageIndex{2}\): The layered interior of the Geosphere. Note that Lithosphere includes both the crust and uppermost portion of the solid mantle. This layer comprises Earth’s tectonic plates. (U.S. Geological Survey.)

Beneath the asthenosphere is the lower mantle, and beneath that is the core. The core consists of an inner solid portion and an outer liquid portion. Both are made of an alloy that is mostly iron, plus ~5% nickel and some sulfur. The outer core convects, and in so doing generates a powerful magnetic field that extends through the surrounding shell of rock and out into space.

The ordered layering of crust, mantle, and core is a reflection of a mature planet, that has differentiated into distinct horizons of varying density which increases toward Earth's center:

Continental Crust or Oceanic Crust	2.7 g/cm\(^3\) or 2.9 g/cm\(^3\)
Mantle	3.3 g/cm\(^3\)
Core	11 g/cm\(^3\)

As far as their size goes, the plates can be large or small. The largest plates are the Eurasian Plate and the Pacific Plate. The smallest is harder to define. The Juan de Fuca Plate is only 250,000 km\(^2\), a relatively tiny slab in the eastern Pacific Ocean, but there are even smaller “microplates” of oceanic lithosphere between the Pacific Plate and Nazca Plate, too. The smallest plate consisting primarily of continental lithosphere is the Arabian Plate, at 5,000,000 km\(^2\).

The shape of the plates is kind of like fragments of an eggshell, or pieces of orange peel: they are both “slab like” or tabular in some senses, but also curved, as fragmental pieces of an overall spheroidal shell.

Key Terms

• asthenosphere - the layer beneath the lithosphere, which is in a plastic, semi-solid state
• continental crust - thicker, less dense crust that when paired with the solid upper mantle creates continental landmasses
• lithosphere - the outermost layer of Earth that includes the crust and solid upper mantle; this solid, rock-layer, is broken up into individual pieces called plates
• oceanic crust - thinner, more dense crust that when paired with solid upper mantle creates the ocean floors
• plates - pieces of lithosphere composed of oceanic and/or continental crust and the solid upper mantle
• rift zones - weak spots in Earth's crust where stretching is occurring creating low spots

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