What is the Mantle?

The mantle is the largest structural division of the Earth, extending from the base of the crust (~7–70 km depth) to the top of the outer core at ~2,900 km depth. It accounts for approximately 84% of Earth’s total volume and ~68% of its mass.

Despite this enormous scale, we have never directly sampled more than the very top of the upper mantle. Everything we know about the mantle’s deeper regions comes from seismic wave analysis, high-pressure laboratory experiments, and geochemical modelling.

Key Idea: The mantle is solid rock — but on geological timescales (millions of years), it flows like an extremely viscous fluid. This behaviour, called plastic deformation, enables convection and drives plate tectonics.

Internal Structure of the Mantle

The mantle is not uniform. It is subdivided into distinct layers based on changes in seismic wave velocity:

Upper Mantle (7–660 km)

Composed primarily of peridotite — a coarse-grained rock made of olivine and pyroxene. The upper mantle includes:

  • Lithospheric mantle (7–100 km) — the rigid, brittle upper part that moves with the overlying crust as a single plate
  • Asthenosphere (100–350 km) — a partially molten, weaker zone where seismic waves slow down (the low-velocity zone). This is where the rigid lithospheric plates effectively “float” and slide. The partial melt fraction is only ~1-3%, but it dramatically reduces the rock’s strength.

Transition Zone (410–660 km)

Two major seismic discontinuities mark this region:

  • 410 km discontinuity — olivine converts to a denser mineral structure (wadsleyite)
  • 660 km discontinuity — marks the boundary between upper and lower mantle; ringwoodite transforms to bridgmanite + ferropericlase

Lower Mantle (660–2,900 km)

More rigid than the upper mantle due to the immense pressure. Composed mainly of bridgmanite (formerly called perovskite) and ferropericlase. Despite the high pressure, the lower mantle is thought to participate in whole-mantle convection, driving large-scale mantle circulation.

Mantle Convection

The mantle loses heat from Earth’s interior through convection — the same process as a boiling pot of water, but on a timescale of millions of years:

  1. Hot material at the core-mantle boundary is less dense and rises
  2. As it rises, it cools and eventually spreads laterally beneath the lithosphere
  3. Cool, dense material sinks back down — this is subduction
  4. The cycle continuously transfers heat from core to surface

Key Idea: Mantle convection is the engine of plate tectonics. The heat from Earth’s interior, combined with the latent heat of the still-solidifying inner core, drives the flow that moves plates.

The exact pattern of mantle convection — whether it occurs as two separate layers or as one whole-mantle system — is still debated among geoscientists.

Mantle Plumes

In addition to large-scale convection, some regions of the mantle generate mantle plumes — narrow columns of anomalously hot material that rise from deep in the mantle (possibly from the core-mantle boundary).

When a plume reaches the base of the lithosphere, it melts to form a hot spot:

  • In oceanic settings: hot spot volcanism creates island chains (e.g., Hawaii-Emperor chain)
  • In continental settings: large igneous provinces and flood basalts (e.g., Yellowstone, Deccan Traps)

The Hawaiian islands record the motion of the Pacific plate over a stationary hot spot — each island represents a different phase of volcanic activity, becoming older to the northwest.

Key Terms

  • Mantle — the silicate rock layer between Earth’s crust and core, 7–2,900 km depth
  • Asthenosphere — the weak, partially molten zone of the upper mantle (100–350 km)
  • Lithosphere — the rigid outer layer comprising crust + uppermost mantle
  • Convection — heat transfer through circulation of material; drives plate tectonics
  • Peridotite — the dominant rock type of the upper mantle (olivine + pyroxene)
  • Upper mantle — the mantle from the base of the crust to 660 km depth
  • Lower mantle — the mantle from 660–2,900 km depth; denser and more rigid
  • Transition zone — the 410–660 km interval with major mineralogical phase changes
  • Mantle plume — a column of anomalously hot mantle material rising from depth