The Earth’s interior is layered, consisting of the crust, the mantle, and the core. The mantle is the largest of these layers, making up about 84% of the planet’s total volume. This layer is subdivided into the upper mantle and the lower mantle. Determining the precise thickness of the upper mantle is complex because its boundaries are defined by changes in rock composition and physical behavior. This dynamic region drives much of the planet’s geological activity.
Defining the Upper Mantle’s Boundaries and Overall Thickness
The upper mantle begins at the Mohorovičić discontinuity, or Moho. This sharp boundary separates the lighter, silicon-rich crust from the denser, magnesium- and iron-rich mantle rock beneath it. The depth of the Moho is highly variable across the globe, defining the upper limit of the mantle.
Beneath the oceans, the Moho can be found as shallow as 5 to 10 kilometers below the seafloor. Conversely, under continents, the crust is much thicker, especially beneath large mountain ranges, pushing the Moho down to depths that can exceed 90 kilometers. The upper mantle extends downward from this variable starting point to a depth of approximately 670 kilometers.
The lower limit of the upper mantle is the 670-kilometer discontinuity, which marks the boundary with the lower mantle. Therefore, the upper mantle is roughly 600 to 665 kilometers thick, depending on the local depth of the overlying crust. A significant internal marker exists much shallower, at about 410 kilometers depth.
Subdivisions and Physical Behavior
The upper mantle is not a single uniform layer but is distinguished by differences in the mechanical behavior of its rock material. The uppermost part of the mantle is included in the Lithosphere, a rigid, cool outer shell that encompasses the crust and the upper portion of the mantle. The Lithosphere’s thickness varies widely, ranging from about 100 kilometers beneath the oceans to over 200 kilometers under old continental interiors.
Below the rigid Lithosphere lies the Asthenosphere, a zone of the upper mantle where rock material is solid but mechanically weak and ductile. High temperatures and pressure conditions cause the rock here to approach its melting point, allowing it to flow slowly over geological timescales. This plastic behavior enables the rigid tectonic plates of the Lithosphere to slide and move across the Earth’s surface.
A distinct seismic boundary within the upper mantle is the 410-kilometer discontinuity, which is a mineral phase transition rather than a chemical change. At this depth, intense pressure causes the dominant mineral, olivine, to rearrange its crystal structure into a denser form called wadsleyite. This structural change marks the top of the Mantle Transition Zone, a 250-kilometer-thick region extending to the 670-kilometer boundary.
Scientific Methods for Depth Measurement
Scientists cannot directly measure the depth of the upper mantle; instead, they rely on seismology, the study of how earthquake waves travel through the Earth. Earthquakes generate two main types of body waves: compressional P-waves and shear S-waves. The speed of these waves is directly related to the density and rigidity of the material they pass through.
When a seismic wave encounters a boundary, such as the Moho or the 410-kilometer discontinuity, its velocity abruptly changes, causing the wave to reflect or refract. By analyzing the time it takes for these reflected and converted waves to arrive at seismograph stations, geophysicists can precisely map the depth of these internal boundaries. Specialized techniques like seismic receiver functions are used to isolate the faint signals of S-waves converting to P-waves, or vice versa, at deep interfaces.
The abrupt acceleration of P-waves at the Moho, from speeds typical of crustal rock to higher velocities characteristic of mantle rock, was the original observation that first defined the crust-mantle boundary. Similarly, the clear velocity jump recorded at 410 kilometers confirms the depth where olivine undergoes its high-pressure phase change. These seismic measurements provide a detailed, three-dimensional picture of the upper mantle’s structure and its varying thickness beneath continents and oceans.