The Mohorovičić Discontinuity, commonly shortened to the Moho, is a globally present boundary separating the Earth’s crust (the upper shell) from the underlying mantle. It was discovered in 1909 by Croatian seismologist Andrija Mohorovičić. The Moho is a significant marker for geoscientists studying the planet’s layered internal structure, composition, and dynamic processes.
Defining the Moho: Location and Purpose
The Moho is the interface that strictly separates the Earth’s crust from the underlying mantle. This boundary represents a major change in the material makeup of the planet’s interior. Above the discontinuity lies the crust, which is primarily composed of silicate rocks rich in elements like silicon and aluminum.
The rock material immediately below the Moho, which forms the uppermost part of the mantle, is significantly denser. This mantle rock is rich in iron and magnesium and is largely composed of the ultramafic rock type called peridotite. The Moho is a chemical boundary, marking the transition from less dense crustal silicate rocks to the denser, ultramafic silicate rocks of the mantle.
This transition is not necessarily a single sharp line, but can be a narrow zone, sometimes spanning a few kilometers, where the composition changes gradually. This chemical shift establishes the true lower limit of the Earth’s crust, whether continental or oceanic.
Identifying the Boundary: The Role of Seismic Waves
The Moho was not discovered through direct observation, but through the analysis of how energy waves travel through the Earth. Andrija Mohorovičić first identified the boundary in 1909 by studying seismograms from a local earthquake in Croatia. He observed that two distinct sets of seismic waves arrived at seismograph stations at different times.
The first set of waves traveled a direct path through the shallow crust, while the second set arrived sooner than expected for a uniform Earth structure. Mohorovičić correctly theorized that the faster-arriving waves must have been refracted, or bent, at a distinct boundary deep within the Earth. This refraction indicated that the waves traveled a portion of their journey through a deeper layer where their speed increased significantly.
This increase in P-wave velocity is the definitive characteristic used to identify the Moho. Above the boundary, in the lower crust, P-wave velocities are typically 6 to 7 kilometers per second. Once the waves cross the Moho and enter the upper mantle, their velocity abruptly jumps to approximately 7.6 to 8.6 kilometers per second. This acceleration demonstrates a clear transition to a denser material, allowing seismologists to map the Moho’s depth globally.
Depth Variations: Continental Versus Oceanic Crust
The physical location of the Mohorovičić Discontinuity is not constant across the globe, but instead varies significantly depending on the type of crust lying above it. The Moho is much shallower beneath the oceanic crust compared to its depth beneath the continental crust. Under the ocean basins, where the crust is thinner and denser, the Moho is typically found at a depth of about 5 to 10 kilometers below the seafloor. The continental crust is far thicker and less dense than its oceanic counterpart, which pushes the Moho much deeper into the Earth. Beneath continental landmasses, the Moho averages a depth of approximately 30 to 35 kilometers.
This depth can fluctuate greatly, particularly beneath active mountain ranges. In regions like the Himalayas, where continental plates have collided and thickened the crust, the Moho may be found at depths up to 70 kilometers below the surface.