Earth's Crust-Mantle Boundary: The Mohorovičić Discontinuity

The Mohorovičić discontinuity, or Moho (the boundary between the crust and mantle), was discovered in 1909 through the observation of an abrupt change in the speed of seismic waves traveling below the surface of the Earth. Its discovery was among the first evidence for the now-famous “onion” model of the layers of the earth, but many of its fundamental properties are still not well understood.

Overview

On October 8, 1909, central Europe was struck by a large earthquake centered in Croatia, near the village of Papuspsko. Andrija Mohorovičić, director of the Meteorological Observatory at the nearby University of Zagreb, collected data on the quake and its aftershocks. In the seismic records from stations at intermediate distances from the epicenter of the quake, he identified two sets of seismic P (longitudinal, or compressional) and S (transverse, or shear) waves, one set arriving at the recording stations sooner than the second. He correctly interpreted the first set of seismic waves to arrive as having traveled deeper and hence faster through material that had a higher density (and possibly different composition) than the crust material through which the shallower and slower second set of waves had passed. Just as light waves moving from water or glass into air bend or refract because of a sudden increase in speed, so also these seismic waves changed speed and refracted at some boundary, then traveled just under the boundary between these layers for some distance before being refracted again back into the lower-density layer and traveling through it to the surface. This boundary, now known to divide the crust from the mantle, was later named after its discoverer, but the somewhat unwieldy title Mohorovičić Discontinuity commonly is shortened to Moho. Similar seismic boundaries have since been discovered between the mantle and outer core, and between the inner and outer cores.

The thickness of the crust, and hence the depth of the Moho, varies from 3 kilometers at mid-ocean ridges to 70 kilometers under young, high mountain ranges created by continental collisions, such as the Himalayas. It is important to note that the Moho does not mark the bottom of the tectonic plates; each plate is made of the crust and the uppermost mantle welded together to form the rigid lithosphere, which moves over the mantle’s plastic layer known as the asthenosphere.

Since its discovery, the Moho has been the subject of intense scrutiny by geophysicists because it is the closest boundary to Earth’s surface and hence the easiest to study. One of the first debates was whether the Moho represented a transition between rock layers of different chemical composition or between rock layers of similar composition but different mineral and crystalline structure (phase changes caused by changes in temperature and pressure). Since the 1960s, the former model has been that favored by most geologists, although some alternate models based on phase transitions still exist. The crust appears to be composed mainly of basaltic rocks in ocean floors and granitic rocks in continents, while the upper mantle is largely made of peridotite. This model is supported by the study and comparison of continental rocks, ophiolite complexes (which are slices of former ocean floor thrust above sea level), and xenoliths (which are pieces of the lower crust and upper mantle brought up to the surface by volcanic activity).

Worldwide studies of seismic data have added considerably to our understanding of the Moho. Seismic refraction data have been gathered not only from earthquakes but also from nuclear weapons testing and other underground explosions. Seismic reflection methods—which generate seismic waves by means of a near-surface explosion or truck-mounted vibrator—have also been widely used. These waves travel down through the crust, reflect off the Moho, and travel back to the surface, like light waves bouncing off a mirror. The out-and-back travel time can be used to determine the depth of the boundary, similar to the way a bat emits sounds that echo off obstacles in its path. Studies at some locations have also detected a change in the electrical conductivity of rocks near the Moho.

While this broad array of experimental data would suggest that the Moho is well defined, the opposite is actually the case. The problem is that the transition depths determined by these different methods often disagree. Therefore, the classic or seismic Moho (defined as the refraction transition, where the velocity of P waves jumps to 7.6 kilometers per second) may differ from the reflection boundary, the electrical conductivity boundary, and most important, from the rock-type or petrologic boundary, which is usually regarded as the true crust-mantle boundary. Compounding this problem is the fact that the seismic Moho is absent in some locations, with the velocity of the P waves only gradually increasing with depth. Taken together, these studies led to a questioning of the early model of the Moho as a thin, sharp, well-defined boundary in favor of a transition layer of definite thickness (perhaps several kilometers), over which the composition of the rocks changes. The Moho also has a complex, multilayered structure in regions of complex tectonic history. These observations have led geologists to view the Moho as a dynamic entity that evolves over time, in terms of both its petrology and its physical structure and substructures.

Knowledge Gained

Although these generalizations about the nature of the Moho are fairly well established, the details of the structure and creation of the Moho are less well understood. Due to the difference in ocean crust and continental crust in terms of composition, thickness, and tectonic interactions, detailed studies of the Moho are usually divided into either oceanic or continental. In addition, the problem of understanding these fundamental processes is so complex that many researchers in this area utilize seismic and petrological data to model the Moho in a single geographic region rather than attempt to make a unified model of the entire Moho.

More is known about the oceanic Moho than its continental counterpart, largely thanks to studies of ophiolite complexes. Distributed worldwide, these samples of former (ancient) oceanic crust and upper mantle allow for direct study of the chemical and mineral composition of the rocks. Coupled with seismic refraction and reflection data, they paint a reasonably clear picture of the oceanic Moho and its history. Its structure is complex, with interlocking layers ranging in composition from mafic (basaltic) at the top to ultramafic at the bottom, finally merging with the peridotite of the mantle. The total thickness of the oceanic Moho layer ranges from 0 to 3 kilometers. Oceanic Moho is created from the same source as oceanic crust, namely the upwelling of Magma at mid-ocean ridges such as the Mid-Atlantic Ridge (which is currently increasing the width of the Atlantic Ocean). It forms rather quickly in geologic terms, within a few thousand years, and after initial creation it is not significantly modified.

The continental Moho is much less well understood. Petrologic information can be gathered from xenoliths and exposed samples of the crust-mantle boundary uplifted by tectonic forces, although the latter must be used with caution, since such rocks have been changed by the same forces that lifted them to the surface. These tectonic forces, such as continental collisions, along with the much longer time frame covered by continental crust samples, lead to a greater complexity in the data. Because of the greater age of the continental rocks, the problem of how the Moho forms in continental regions is central to our understanding of how the early Earth differentiated into layers in the first place.

At least four hypotheses have been suggested for how the continental Moho forms. In the relic Moho model, the continental Moho is the relic of the oceanic Moho, surviving the assembly processes (such as continental collisions and accretion of terranes along the coast) that build the continents. How this might occur without severely disrupting the Moho is unclear, especially given the fact that the crust itself is certainly changed in this process. The magmatic underplating hypothesis suggests that as continents assemble, new Moho material is added beneath them in a process similar to that which creates sills, horizontal intrusions of magma sandwiched between rock layers. In this model, the Moho would be younger than the continental crust, since it is created after the crustal material is set into place.

These first two hypotheses rely on igneous processes, while the final two hypotheses suggest the continental Moho consists of “reworked” rocks. In the metamorphic/metasomatic front model, the Moho is created by additional metamorphism of rocks of the lower crust and/or the upper mantle. Finally, the regional décollement hypothesis posits that the Moho forms as the crust and mantle physically decouple at structurally weak zones, especially under high temperatures.

It should be noted that this list is not suggested as being complete, and all four processes may occur in different geographical areas, depending on the particular geological conditions present. This conclusion has been suggested by researchers working on the Canadian LITHOPROBE program as a result of their study of the subsurface geology of North America. Therefore, it may be that there is no single explanation for the creation and evolution of the continental Moho, and the geologic history of each geographical region must be interpreted to develop a sensible model of its Moho structure and evolution.

Context

All methods described so far to study the Moho have relied on indirect testing, with the exception of samples of ophiolites and xenoliths. However, as previously mentioned, relying on these samples as truly representative of the Moho in general is unwise, since they may have been severely modified by tectonic forces. The most reliable rock sample would obviously be one obtained directly from the current Moho through drilling. However, given the great depths involved and the resulting technological difficulties, no direct Moho sample has yet been obtained, despite a number of programs designed to do so.

The most infamous such project was dubbed Project Mohole, proposed to the National Science Foundation in 1957 and funded until 1966. This three-stage project completed only its first phase, experimental drilling off the coasts of California and Mexico, before it was canceled by Congress (after a series of bureaucratic and financial problems). Despite the failure of the project, geologists remained committed to large-scale drilling projects in the ocean floor, leading to a series of ongoing projects such as the Deep Sea Drilling Project and Ocean Drilling Project. Although individual projects have reached depths of more than a kilometer, identifiable Moho samples have yet to be obtained, in keeping with the problems of identifying the seismic Moho with the petrological crust-mantle boundary and the complex nature of the Moho in general. In December 2015 the Slow Spreading Ridge Moho (SloMo) Project run by the International Ocean Discovery Program (IODP) was launched in the Indian Ocean to drill through Earth's mantle and reach the Moho. If successful, the project will take years and will penetrate 5 to 5.5 kilometers into Earth's mantle. The first of three proposed phases of the project was completed in 2016. The drilling in this phase reached only halfway to its intended goal.

Therefore, nearly a century after the discovery of the Moho, its secrets continue to intrigue geologists. Despite the gaps in our understanding of the terrestrial crust-mantle boundary, planetary geologists are currently applying terrestrial methods and models to its neighbors in space. For example, lunar data suggest that the Moon’s Moho lies about 30 to 70 kilometers beneath its surface, being shallowest beneath the maria. Models of Martian structure suggest that its Moho may lie between 6 and 100 kilometers beneath its surface. Such studies add to our understanding of the geologic evolution of these rocky worlds and how their evolution is similar to, yet differs from, that of Earth.

Bibliography

Amos, Jonathan. "Bid to Drill Deep Inside Earth." BBC, 1 Dec. 2015, https://www.bbc.com/news/science-environment-34967750. Accessed 29 Jan. 2023.

Bascom, Willard. A Hole in the Bottom of the Sea. Garden City: Doubleday, 1961. Print.

Cook, Frederick A. “Fine Structure of the Continental Reflection Moho.” Geological Society of America Bulletin 114. 1 (2002): 64–79. Print.

Eaton, David W. “Multi-genetic Origin of the Continental Moho: Insights from LITHOPROBE.” Terra Nova 18. 1 (2008): 34–43. Print.

Fowler, C. M. R. The Solid Earth: An Introduction to Global Geophysics. 2d ed. New York: Cambridge UP, 2004. Print.

Griffin, W. L., and Suzanne Y. O’Reilly. “Is the Continental Moho the Crust-Mantle Boundary?” Geology 15 (1987): 241–244. Print.

Jarchow, Craig M., and George A. Thompson. “The Nature of the Mohorovičić Discontinuity.” Annual Review of Earth and Planetary Sciences 17 (1989): 475–506. Print.

Perkins, Sid. "A Decades-Long Quest to Drill Into Earth’s Mantle May Soon Hit Pay Dirt." Smithsonian, 25 Jan. 2016, www.smithsonianmag.com/science-nature/decades-long-quest-drill-earths-mantle-may-soon-hit-pay-dirt-180957908/. Accessed 29 Jan. 2023.

Tarbuck, Edward J., Frederick K. Lutgens, Dennis Tasa, and Scott Linneman. Earth: An Introduction to Physical Geology. 13th ed. Pearson, 2019.

Van der Pluijm, Ben, and Stephen Marshak. Earth’s Structure. 2d ed. New York: Norton, 2003. Print.

Vogel, Shawna. Naked Earth: The New Geophysics. New York: Plume, 1996. Print.