Composition and Structure of the Deep Earth

The Earth is differentiated into a metallic core, a silicate mantle, and a thin crust, a layered structure inferred by combining seismic profiles with the physics of how minerals behave at depth.

Definition

The composition and structure of the deep Earth refers to the chemical makeup and layered physical organization of the planet's interior, an iron-rich core, a silicate mantle, and a crust, as inferred from seismic velocity and density models combined with mineral physics and geochemistry.

Scope

This topic covers the chemical composition and physical structure of the Earth's deep interior: the principal layers and the boundaries between them, the iron-dominated core with its liquid and solid parts, the silicate mantle and its phase-transition subdivisions, and the seismically distinctive lowermost mantle. It treats reference Earth models of density and velocity, the Adams-Williamson relation and Birch's law linking velocity to density and composition, and the geochemical and cosmochemical constraints on bulk composition. The emphasis is on what the deep Earth is made of and how it is arranged.

Core questions

• What are the major layers of the Earth and the boundaries between them?
• What is the core made of, and why is part of it solid and part liquid?
• How do phase transitions subdivide the mantle?
• How are seismic velocity and density used to infer composition?

Key concepts

• Crust, mantle, and core differentiation
• Liquid outer core and solid inner core
• Mantle transition zone and phase changes
• Reference Earth models of density and velocity
• Birch's law and the Adams-Williamson relation

Key theories

Reference Earth models

Spherically averaged models such as PREM compile seismic velocity, density, and attenuation as functions of depth, defining the principal layers and serving as the quantitative basis for interpreting the deep Earth's structure.

Birch's law and composition

Birch's empirical relation between seismic velocity and density at fixed mean atomic weight allows the elastic properties measured by seismology to be translated into constraints on the composition and state of the deep interior.

Mechanisms

Early in its history the Earth differentiated, dense iron sinking to form a core while lighter silicates rose to form the mantle and crust; seismic velocities and density jumps mark the boundaries between these layers and the pressure-induced phase transitions within the mantle, and matching these to the elastic properties of candidate minerals and metals, guided by Birch's law and meteorite compositions, fixes the likely composition of each shell.

Clinical relevance

Knowledge of the interior's composition and structure underpins models of mantle convection, the geodynamo, and the Earth's thermal and chemical evolution, and provides the reference framework for locating earthquakes and interpreting seismic tomography.

History

Early-twentieth-century seismology revealed the core and mantle, Lehmann found the inner core in 1936, Bullen subdivided the interior into labeled shells, Birch related velocity to composition in 1952, and the 1981 Preliminary Reference Earth Model synthesized these into the standard radial model still in use.

Key figures

• Inge Lehmann
• Francis Birch
• Adam Dziewonski
• Keith Bullen

Seminal works

• dziewonski1981
• birch1952
• stacey2008

Frequently asked questions

What are the main layers of the Earth?

From the outside in, the Earth has a thin rocky crust, a thick silicate mantle that makes up most of its volume, a liquid iron-rich outer core, and a solid inner core; these are distinguished by their composition and by how seismic waves change speed across the boundaries between them.

Why is the outer core liquid but the inner core solid?

Both are mostly iron, but although temperature rises with depth, pressure rises even faster; deep enough, the very high pressure raises iron's melting point above the local temperature, so the inner core is solid while the somewhat shallower, cooler-relative-to-its-melting-point outer core remains liquid.