The Earth’s internal structure consists of concentric shells, starting with the thin crust, moving through the massive mantle, and ending at the dense core. The mantle accounts for the largest volume of the planet’s interior. The upper mantle plays a dynamic role in shaping the surface we inhabit. Understanding this region requires examining its physical state and how its slow movements influence the outer shell of the Earth, driving planetary processes fundamental to geology.
Location and Boundaries
The upper mantle is a thick layer situated directly beneath the Earth’s crust. Its uppermost boundary is defined by the Mohorovičić Discontinuity, or Moho. The Moho is a chemical boundary separating the lighter, silica-rich crustal rocks from the denser, ultramafic mantle rocks.
The depth of the Moho varies significantly, ranging from 5 to 10 kilometers beneath the ocean floor to 20 to 90 kilometers under continental landmasses. The lower limit of the upper mantle is marked by the 660-kilometer seismic discontinuity, which separates it from the lower mantle. The region between 410 and 660 kilometers is often referred to as the mantle transition zone.
Physical States: The Lithosphere and Asthenosphere
The upper mantle is divided into two distinct physical layers based on mechanical behavior. The uppermost part of the mantle, combined with the overlying crust, forms the rigid shell known as the lithosphere. This layer behaves as a brittle, solid unit that breaks and fractures when subjected to stress, often resulting in earthquakes.
The lithosphere typically extends to about 100 kilometers and is broken into the moving tectonic plates. Directly beneath this rigid shell lies the asthenosphere, a layer of the upper mantle with far less mechanical strength. Despite being solid rock, extreme heat and pressure cause the asthenosphere to behave plastically, allowing it to deform and flow very slowly over geologic timescales.
This ability to flow is facilitated by temperatures reaching approximately 1300°C in the peridotite rock. This layer of weaker, flowing rock acts as a de-coupling zone, allowing the rigid lithospheric plates to move above it.
Composition and Mineral Makeup
The upper mantle is composed primarily of peridotite, an ultramafic rock rich in magnesium and iron. This rock is significantly denser than the rocks found in the overlying crust. The dominant minerals within peridotite are olivine and pyroxene, with olivine being particularly abundant.
As depth increases, extreme pressure and rising temperature cause these minerals to undergo phase changes in their crystal structure. For example, olivine transforms into denser crystal forms, such as wadsleyite and ringwoodite, at specific depths. These phase changes are responsible for the abrupt increase in seismic wave velocity and density observed at the 410-kilometer and 660-kilometer discontinuities.
The Upper Mantle’s Role in Tectonics
The dynamic function of the upper mantle centers on mantle convection, the primary mechanism for transferring heat from the Earth’s interior to the surface. Heat from the core causes rock material within the asthenosphere to become buoyant and slowly rise. As this material nears the surface, it cools, becomes denser, and sinks again, creating a continuous circulation pattern.
This slow, churning movement in the asthenosphere drives the movement of the rigid lithospheric plates floating above it. Convection currents cause plates to separate at mid-ocean ridges where hot material upwells, and to collide where cooler material sinks through subduction. This interaction generates most of the Earth’s geological activity, including volcanism, mountain building, and the shifting of continents.