The structure of Earth, a layered planet, determines everything from the origin of mountains to the existence of its magnetic field. Scientists use two primary systems to define these subterranean layers: one based on chemical composition and one based on physical properties. The chemical model uses three major divisions—the crust, mantle, and core. The physical model, however, uses differences in rigidity, viscosity, and state of matter to define seven distinct layers. This detailed mechanical structure provides a deeper understanding of dynamic processes like plate tectonics.
Defining the Seven Layers
The seven-layer model separates the Earth based on how materials respond to stress and temperature. These layers, listed from the surface inward, are the Crust, the Lithospheric Mantle, the Asthenosphere, the Mesosphere, the D” Layer, the Outer Core, and the Inner Core. This classification is driven by seismic wave behavior, as the speed and path of these waves change dramatically when crossing boundaries where rock strength or state of matter shifts. The first two layers are physically grouped due to their shared rigid behavior.
The Uppermost Mechanical Layers
The outermost mechanical layer, the Lithosphere, is a rigid shell that forms the planet’s tectonic plates. It is composed of the thin Crust and the uppermost portion of the Mantle, known as the Lithospheric Mantle. The Crust varies significantly in composition and thickness; oceanic crust is thin (5 to 10 km) and rich in mafic minerals like basalt, while continental crust is thicker (up to 70 km) and made of less dense, felsic rocks like granite.
The Lithospheric Mantle is chemically part of the Mantle but remains physically coupled to the Crust, behaving as a single, brittle unit that fractures during earthquakes. This rigid layer floats upon the Asthenosphere, which extends down to approximately 250 to 300 kilometers. The Asthenosphere is solid rock, but its proximity to the melting point causes it to behave plastically. This ductile layer acts as a lubricating medium, enabling the large-scale movement of the overlying tectonic plates.
The Lower Mantle and Transition Zones
Beneath the Asthenosphere lies the Mesosphere, often referred to as the Lower Mantle, extending to a depth of about 2,900 kilometers. This layer is solid, denser, and more rigid than the Asthenosphere, though it still participates in the slow motions of mantle convection that drive surface tectonics. Within the Mesosphere, the Transition Zone exists at depths of 410 and 660 kilometers, where increasing pressure causes the silicate minerals to undergo phase changes, rearranging their atomic structures into denser forms.
The fifth distinct layer is the D” Layer, a complex, seismically defined region about 200 to 300 kilometers thick situated just above the Outer Core. This zone is highly heterogeneous, featuring unique seismic anomalies that suggest compositional variations and distinct mineral phases. The D” Layer is where heat from the core is transferred into the mantle and is a likely source for deep, rising columns of hot rock known as mantle plumes. Its unique properties mark the boundary between the silicate Mantle and the metallic Core.
The Metallic Core
The Core is divided into two metallic layers based on their state of matter. The Outer Core is a layer approximately 2,260 kilometers thick, composed primarily of molten iron and nickel. This layer is liquid because the temperature is high, and the pressure is insufficient to force the metal into a solid state. Convective currents within this highly conductive, turbulent fluid are responsible for generating the Earth’s magnetic field through a process called the geodynamo.
The seventh and innermost layer is the Inner Core, a dense, solid sphere with a radius of about 1,220 kilometers. Although the temperature is estimated to be over 5,700 degrees Celsius, the immense pressure prevents the iron-nickel alloy from melting. Recent research suggests that the Inner Core might exist in a superionic state, where light atoms move fluidly through a solid iron lattice. This unusual behavior helps explain why seismic waves travel through the Inner Core with less rigidity than expected.