Let’s take a moment for Earth Day to approach for a closer look at the Earth’s chemistry; in particular, the chemical elements interspersed in the Earth’s major depths.
With an atmosphere containing 78% nitrogen and 21% oxygen, the Earth is the only planet in the solar system capable of initiating and sustaining life-forms; the various chemical elements that make up the Earth, from the crust, down to the mantle and core, have a little something to do with that.
Defining the Earth’s Boundaries and Elements
As scientists are not able to visit the Earth’s deep interior or place instruments within it, they explore in subtle ways. One approach is to study the Earth with non-material probes, such as seismic waves emitted by earthquakes. As seismic waves pass through the Earth, they undergo sudden changes in direction and velocity at certain depths. These depths mark the major boundaries, also called discontinuities, that divide the Earth into crust, mantle and core.
The Crust. The Earth’s crust is the thin outermost layer of the Earth, with an average depth of 24 km (15 mi). The crust accounts for 1.05% of the Earth’s volume and 0.5% of its mass. The chemical elements oxygen, silicon and aluminum dominate the crustal composition. The major mineral type – the feldspars – are alumino-silicates of the alkali and alkaline-earth metals. Silicon dioxide is the second most common group.
The Mantle. The mantle extends from the base of the crust to the core and is about 2865 km (1780 mi) thick, occupying about 82.5% of the Earth’s volume. The upper mantle is rich in olivine and pyroxenes. The major mineral type in the lower mantle appears to be pyroxenes, especially magnesium silicate. Scientists think that the lowest layer of the mantle called “D layer” is richer in aluminum and calcium than the higher layers of the mantle.
The Core. The core extends from the base of the mantle to the Earth’s center, and is 6964 kn (4327 mi) in diameter – accounting for only 16.3% of the Earth’s volume, but 33.5% of its mass. The core is made up of two distinct parts – a liquid outer core, which is 2260 km (1404 mi) thick, and a solid inner core, which has a radius of 1222 km (759 mi). The core is chemically distinct from the mantle and contains about 89% iron and 6% nickel. The remaining 5% is made of lighter elements, possibly sulfur – but we cannot rule out the presence of oxygen and silicon, in light of a 2013 study published in Nature, which calls them “prime candidates” for the lighter elements in the Earth’s core.
As we celebrate Earth Day, and as in recent times, emphasis has been given to environmental awareness or the value of “green.” This year, let’s pay attention to all the other colors of Earth as well – the colors we see through chemistry.
Lidunka Vočadlo. Earth science: Core composition revealed. (2013). Nature. Accessed April 23, 2013.
Australian National University. Earth Chemistry. Accessed April 22, 2013.
Chambers Book of Facts. (2007). Chambers Harrap Publishers. Edinburgh.
Decoding Science. One article at a time.