The Mohorovičić (Mo‐HOR‐o‐VITCH‐itz) discontinuity is a boundary between the Earth’s crust and the upper mantle. An abrupt increase or jump in seismic wave velocity marks it.
This discontinuity is also known as the Mohorovicic boundary. You can also colloquially call it the Moho or the Moho discontinuity. It is named after Andrija Mohorovičić, who discovered it.
In geology, a discontinuity marks a surface with dramatic seismic wave speed changes. It happens due to changes in density, composition, stress, temperature, fluid saturation, elasticity, etc.
On the other hand, seismic waves are energy waves generated by earthquakes and Earth vibrations. These waves travel within or on the Earth’s surface. We use seismographs to measure seismic waves. The resultant recordings we get are called seismograms.
This article will discuss the Moho in detail. It has where it occurs and insight into its discovery. You will also get its importance and a lot more.
What is Mohorovicic discontinuity?
The Mohorovicic discontinuity is a boundary between the crust and the upper mantle. At this boundary, the P seismic wave velocity abruptly increases by nearly 1 km (0.6 mi) per second, from 6.7–7.2 km/s above it to 7.6–8.6 km/s below it.
The P waves (primary or pressure) are the fastest seismic waves. They travel through fluids and solids and reach seismographs first.
The Moho is usually a 500-meter (some authors put it at 1-2 km) transition between the crust above and the mantle beneath. It occurs both in the oceanic and continental crust and is the accepted lower limit of the crust.
This seismic wave velocity change occurs when waves move through rocks with different densities. Usually, it happens when the waves move from the less dense rocks in the crust to denser rocks in the upper mantle.
The crust has granitic rocks on the upper part. Its lower part has basalts or gabbroic, granulites, and amphibolite, while the mantle has peridotites. Peridotites are denser, olivine-rich ultramafic rocks. Examples are dunite, harzburgite, lherzolite and kimberlite, wehrlite.
Therefore, the Mohorovicic discontinuity represents a change in composition between the crust and mantle. Rocks in these two zones have different geophysical properties.
However, as we will see next, this isn’t always the case. Sometimes, it doesn’t demarcate a change from crust to mantle, i.e., crust-mantle boundary (CMB).
It may not always coincide with the mantle-crust boundary
The Mono doesn’t always coincide with the crust-mantle compositional boundary. A state change can also cause a change in seismic wave velocity. For instance, transforming the less dense gabbro, pyroxene granulite, or amphibolite to eclogite under pressure will change seismic wave velocity. This commonly occurs in collisional orogens.
Eclogite is a denser metamorphic rock with a density (3.3g/cm3) like peridotite. It has mainly garnet and clinopyroxene.
Therefore, eclogite will have seismic velocities comparable to mantle peridotites. However, it doesn’t represent a change in composition or transition to the mantle.
Secondly, serpentinization or hydrothermal alteration of peridotites at the oceanic crust and forearcs will lower its density. This alteration will cause lower seismic velocity and increase the Moho depth. Such will not represent a transition from crust to mantle.
Third, trapping basaltic magma under the Moho or in the crust is another reason. This is known as underplating, and it causes crust thickness. When it occurs beneath the Moho, the base of the crust goes beneath the underplating materials (pyroxenites and garnet gabbros).
Usually, the upper and lower surfaces of the underplated material are high seismic reflectors and have higher wave velocities.
Fortunately, one study notes that their higher Poisson ratios can differentiate underplated material or eclogites from ultramafic (peridotite) rocks in the mantle.
That is not all. Studying xenoliths (rock pieces embedded in magma) brought to the surface by volcanism and seismic reflection shows that transition occurs in basaltic intrusions in areas away from continental cratons. These intrusions can be up to 20 km thick, yet the Moho may lie beneath this transition boundary. Such can affect data interpretation.
Due to these discrepancies, we can define a seismic Moho as a jump in seismic velocities. This differentiates it from petrologic Moho, which represents a change in composition at the base of the crust.
Seismic properties
The Moho boundary has P wave velocities of 6.7–7.2 km/s above it and 7.6–8.6 km/s below. The S or shear wave velocities, on the other hand, will jump from 3.4 and 4.1 km/s to 4.3–4.5 km/s at the Moho.
The lower speeds represent those in basaltic rocks or gabbros in the crust and the higher peridotites in the mantle.
Where is the Moho boundary located?
The Mohorovicic discontinuity occurs entirely in the crust. It separates the crust from the mantle. However, at the mid-ocean ridges (MOR), it separates the crust from the asthenosphere. The asthenosphere is a ductile, weaker part of the upper mantle above which the lithosphere (crust and rigid, solid upper mantle part) moves.
At these ocean-spreading centers, the lithospheric mantle has zero depth. Therefore, the Moho defines the lithosphere-asthenosphere boundary.
The Moho is located at a depth of 20-90 km (10-60 mi) from the surface of continents and 5-10 km (3-6 mi) below the ocean surface. However, the average depth on continental crust is 35 km (22 miles), and oceanic is 7 km (4 mi).
Furthermore, these depths vary from place to place, from thinnest at MOR to thickest at younger, tall collisional mountains.
Who discovered Mohorovicic discontinuity?
Andrija Mohorovičić (1857-1936), a Croatian meteorologist who later became a seismologist, discovered the Mohorovičić discontinuity. This boundary was later named after him.
This discovery occurred following the October 8th, 1909, earthquake in the Pokuplje region, 40 km (25 mi) south of Zagreb. This earthquake had a shallow focus.
At the time, Mohorovičić had some seismographs installed in Zagreb, which gave him seismic data. Also, he requested seismograms from 41 stations in Europe.
Upon analyzing the data, he realized something peculiar. Some of the data had two sets of P-waves and S-waves. One set represented waves that move directly near the surface of the Earth, and the other refracted. These two traveled at different speeds.
He noted that those moving horizontally in the crust arrived first at the seismographs near the focus. On the other hand, the refracted came first in seismographs, far from the focus than those moving horizontally on the crust.
Mohorovičić knew about wave refraction from Snell’s law. Also, he knew seismic velocity is proportional to medium density, and density increased proportionally with depth.
To explain his observation, he noted that the refracted waves were from a zone with a sharp transition in density or physical properties that allowed seismic waves to move fast. This he interpreted to be the crust-mantle boundary (CMB).
Using data from various seismographs, he could compute the depth of this zone or discontinuity, which was about 54km. Later studies confirmed it.
Explaining Mohorovičić observation with a diagram
The diagram below uses Mohorovičić reasoning and seismic waves to explain his observation further.
From the diagram, some seismic waves will move horizontally inside the crust during a shallow earthquake. Others will go to the mantle surface and be refracted.
Those moving in the earth’s crust will have lower velocity. However, they will first reach a seismograph near the focus.
On the other hand, refracted seismic waves go to the crust-mantle surface. At this surface, refraction occurs, and they start to move faster. Some will move inside the mantle, others at the boundary. These refracted seismic waves will generate head waves to the Earth’s surface, which seismographs will pick.
Since they must go to the crust-mantle layer, refracted seismic waves will not arrive first at seismographs near the focus. However, they will arrive first at seismographs further away from the focus. This happens since refracted waves travel faster on the boundary or mantle.
A good analogy is using a beltway to avoid a shorter route through streets because you can travel faster and reach sooner.
At a certain distance from the focus, both the seismic waves moving horizontally on the crust and those refracted arrive simultaneously. This is known as a crossover distance.
The crossover distance can help calculate the depth of the Moho. It tells you how far seismic waves traveled in the crust before reaching the crust-mantle boundary. The further the crossover, the deeper it is, and vice versa.
Why is the Moho important?
It showed that the solid Earth is differentiated chemically and a basis for theorizing the composition of the Earth for the past century. Also, it defines the crust and mantle boundary, the first boundary. Of course, there are other discontinuities like Lehmann’s discontinuity, Rapetti’s, etc.
This observation is one of the earlier seismic studies that laid the foundation for modern seismology. However, it is not the mechanical decoupling base as thought before. Thus, it has little significance in geodynamics. Also, it has no mechanical relevance compared to the lithosphere-asthenosphere boundary.
Frequently Asked Questions (FAQs)
No. Despite the many attempts, including Project Mohole and the 12,260m (40,220 ft) deep Kola Superdeep Borehole and some ongoing, humankind hasn’t reached the Moho. It is an expensive and challenging task due to temperature and pressure conditions. The abandoned Kola Superdeep Borehole by Russian scientists is the deepest dug hole.
No. It doesn’t occur at volcanic island arcs due to delamination, especially at subduction zones. Also, it is absent or poorly developed in the mid-ocean ridges and may be doubled or poorly developed in orogenic belts or hard to detect.
No. Areas undergoing extension, such as the continental rifts, lack a sharp seismic Moho discontinuity. Instead, they show a gradual increase in velocity.
Yes. The Moho is exposed at Gros Morne National Park of Canada, in the northern Oman mountains, and Norway at Leka island. These are part of ancient oceanic crusts.
References
- Tarbuck, E. J., Lutgens, F. K., & Tasa, D. (2017). Earth: An introduction to physical geology (12th ed.). Pearson.
- Gupta, H. K. (2021). Encyclopedia of solid earth geophysics (2nd ed.). Springer.
- Condie, K. C. (2016). Earth as an evolving planetary system (3rd ed.). Academic Press.
- Levin, H. L., & King, D. T. (2017). The Earth Through Time (11th ed.). John Wiley & Sons.
- Giese, P. (2005). Moho Discontinuity. In Selley, R. C., Morrison, C. L. R., & Plimer, I. R. (Eds.). Encyclopedia of geology (Vols. 1-5m, pp.645-659). Elsevier Academic.