Earth is organized into distinct, chemically differentiated layers. This means the elements that make up our world are not distributed evenly throughout the globe. Heavy and light elements separated long ago, with certain metals concentrating in the deepest regions. Understanding nickel distribution requires examining the planet’s compositional divisions.
Earth’s Compositional Layers
The planet’s interior is defined by three primary compositional layers: the crust, the mantle, and the core. The outermost layer is the relatively thin crust, ranging from 5 kilometers beneath the oceans to 70 kilometers under continental mountain ranges. It is primarily composed of silicate minerals, such as those found in granite and basalt rock.
Immediately beneath the crust is the mantle, which accounts for the vast majority of the Earth’s volume, approximately 83 percent. The mantle consists of denser silicate rocks that are rich in iron and magnesium, such as peridotite. While mostly solid, the intense heat causes this layer to flow slowly over geologic timescales.
The deepest section is the core, which begins about 2,900 kilometers beneath the surface and is composed of metallic material. The core is further divided into a liquid outer core and a solid inner core. This metallic composition results in a significantly higher density, ranging from about 10 to 13 grams per cubic centimeter, compared to the upper layers.
Identifying Earth’s Nickel Reservoir
The layer of the Earth that holds the vast majority of the planet’s nickel is the core. This innermost region is composed predominantly of an iron-nickel alloy. The core is estimated to contain about 5.5 weight percent of nickel, with some models suggesting the inner core could contain up to 10% nickel.
The concentration of this heavy metal within the center of the Earth is immense compared to the overlying layers. The core, despite only accounting for about 16 percent of the Earth’s volume, holds approximately 33 percent of its total mass due to this dense metallic composition. In contrast, the silicate rocks of the mantle and crust contain only trace amounts of nickel.
This dense metallic mixture gives the core its characteristic properties. The liquid iron and nickel of the outer core generate the Earth’s magnetic field through swirling convection currents. Immense pressure compresses the inner core into a solid metallic sphere, which maintains a similar iron-nickel composition.
The Process of Planetary Differentiation
Nickel is heavily concentrated in the core due to a phenomenon called planetary differentiation. This process occurred early in Earth’s history, about 4.5 billion years ago, when the planet was largely molten from accretion and radioactive decay. During this phase, materials separated based on their density and chemical properties.
Nickel is a siderophile element, meaning “iron-loving,” which describes its chemical affinity for iron. In the early, molten Earth, gravitational forces caused the densest materials, primarily molten iron and its partners like nickel, to sink inward. This downward flow of metal formed the dense, metallic core.
The lighter, less dense silicate compounds, rich in elements like silicon and oxygen, rose upward, forming the Earth’s mantle and crust. The separation was efficient, leaving only a small amount of nickel in the upper, silicate-rich layers.
The composition of the core as an iron-nickel alloy is consistent with evidence from space. Iron meteorites, which are fragments from the cores of smaller, differentiated planetary bodies, also show a similar iron-nickel alloy composition. This shared metallic makeup reinforces the understanding of how early planetary heating and gravity drive core formation across the solar system.