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Layers in the Earth

Nearly a century ago, geophysicists discovered that seismic waves released by earthquakes did funny things when they headed into the mantle. There were layers inside our planet; sometimes seismic waves bounced off those layers and came back to the surface, other times when the seismic waves passed beneath a layer they would speed up. One layer even stopped a certain type of seismic wave. These observations led to our understanding of layers in the planet.

Beneath the crust, the layer we live on, most of the planet is made of about 60% of the mineral olivine, with the rest made of orthopyroxene, clinopyroxene, and garnet. This mineral assemblage, a rock type known as a lherzolite, actually is found at the surface sometimes; volcanoes will even carry these rocks to the surface. Notably, this layer is almost entirely solid – even though the mantle is hot enough that it could melt, it is at high enough pressure that it remains solid.

When minerals are put under enormous pressures, the pressures achieved when hundreds of kilometers of rock sit on top of them, the structures actually change so that the atoms can pack tighter together. About 400 kilometers below your feet right now, the mineral olivine starts changing its structure. It does so twice, turning into 2 different minerals that both have the exact same chemistry as olivine but different structures. These minerals are called Wadsleyite and Ringwoodite, named after two prominent mineralogists/petrologists.

670 or so kilometers below your feet, olivine fundamentally changes its structure. It breaks down into 2 different minerals, one called periclase with the chemistry MgO and another that was named Bridgmanite in 2012 after Nobel Prize winner Percy Bridgman, who worked on the physics of high pressures. This mineral, bridgmanite, exists in the planet for more than 1900 kilometers. Because this volume of the planet is so large and this is the most abundant mineral in that layer, this mineral is the most abundant mineral in the planet Earth.

Below that layer, bridgmanite flips to a different phase. This phase has never been found in nature and analyzed by humans because it only exists at such high pressures – as a consequence it does not have an official mineral name and is just called “Magnesium Silicate Post Perovskite”, which gives both the mineral chemistry and structure. On this plot, that layer is called “D-double-prime” or “D” ‘.

Finally, below those layers we reach the core, where the chemistry changes. The core is an iron-nickel alloy, with at least 1% of a lighter element mixture thought to contain oxygen, silicon, and sulfur. That layer, the core, is much hotter than the mantle, and as a consequence it is molten. Finally, deeper in the planet, there is an inner layer, still iron and nickel minerals, that is at such high pressures that it is fully solid. That layer is the inner core.

-JBB

Image credit: http://science.sciencemag.org/content/346/6213/1057.full