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The NCERT geography chapter on the Earth's interior says the mantle extends "from Moho's discontinuity" downward, and most students learn the phrase without knowing who Moho was. He was Andrija Mohorovičić, a Croatian scientist who found the base of the crust by looking carefully at the arrival times of waves from one earthquake. This post covers his life, the idea behind his discovery, a crossover-distance calculation you can do yourself, and what to learn from his method.
Who was Andrija Mohorovičić?
The Croatian Encyclopedia (Hrvatska enciklopedija), published by the Miroslav Krleža Lexicographic Institute, gives his dates: born in Volosko on 23 January 1857, died in Zagreb on 18 December 1936. He graduated in mathematics and physics in Prague in 1879, taught at schools in Zagreb, Osijek and the nautical school in Bakar, and in 1892 became head of the Meteorological Observatory in Zagreb. His 1893 doctorate was on observing clouds.
An anniversary article by Davorka and Marijan Herak in Seismological Research Letters (2007) notes that his teachers in Prague included Ernst Mach, and that his name is mostly linked with the crust-mantle boundary even though he also contributed to meteorology and other parts of geophysics.
What did the 1909 earthquake show him?
The encyclopedia explains that Mohorovičić started a seismological station in Zagreb in 1901 and acquired two of the most modern seismographs of the time in 1908 and 1909. On 8 October 1909 an earthquake struck the Kupa valley, 39 km south-east of Zagreb.
He compared the Zagreb records with seismograms from many European stations. Two things stood out. Some stations recorded two separate wave arrivals from the same shock. And at stations more than about 200 km away, waves arrived faster than they should have if they had travelled only through the upper crust.
His explanation was that the Earth is layered. One wave travels directly through the upper layer. Another goes down, is refracted along the top of a deeper, faster layer, and comes back up. Beyond a certain distance the deeper route wins, even though it is longer. The encyclopedia records that he called the shallow waves "individual" and the deeper ones "normal"; today they are labelled Pg and Pn. The boundary between the layers became the Mohorovičić discontinuity.
How deep is the Moho?
The NCERT Class 11 chapter Interior of the Earth (2026-27 reprint) gives the crust a mean thickness of 5 km under the oceans and around 30 km under continents, rising to as much as 70 km in the Himalayan region. The Croatian Encyclopedia gives an average Moho depth of 33 km, up to 90 km under some mountain chains and only 5 km under deep oceans.
Worked example: the crossover distance
Here is a two-layer model with sample numbers, chosen for easy arithmetic, not taken from Mohorovičić's data. A crust 30 km thick has P-wave speed v₁ = 6 km/s, and the mantle below has v₂ = 8 km/s. The source is at the surface.
Direct wave: time t₁ = x / v₁.
Refracted wave: it goes down at the critical angle θ, where sin θ = v₁/v₂ = 0.75, travels along the boundary at 8 km/s, then comes up. Its time is t₂ = x / v₂ + 2h cos θ / v₁.
Here cos θ = √(1 − 0.75²) = √0.4375 = 0.661, so 2h cos θ / v₁ = 2 × 30 × 0.661 / 6 = 6.61 s.
The two arrive together when x/6 = x/8 + 6.61. Then x (1/6 − 1/8) = 6.61, so x × 0.04167 = 6.61, and x ≈ 159 km.
Check at 300 km: the direct wave takes 300/6 = 50.0 s; the refracted wave takes 300/8 + 6.61 = 44.1 s. Beyond the crossover the deeper wave arrives about 6 s early, the kind of pattern Mohorovičić saw at his more distant stations.
Where do students go wrong?
Thinking the shortest path is always fastest. Speed matters as much as distance; that is the whole point of the Moho.
Forgetting the critical angle. The refracted wave only travels along the boundary when sin θ = v₁/v₂, the same rule as total internal reflection in optics.
Mixing up the Moho and the core boundary. The Moho separates crust and mantle, tens of kilometres down. The core boundary is about 2,900 km down; our post on Beno Gutenberg and the depth of the core covers that.
When the critical-angle step feels like magic, the Fermi Concept Library has every concept in physics, chemistry, biology, mathematics and computing, so you can revise refraction before returning to this problem.
What can students learn from Mohorovičić's way of working?
Mohorovičić did not throw away the puzzling second arrival as noise. According to the encyclopedia, he only located the earthquake's focus correctly once he dropped the assumption that waves travel at a single uniform speed. Questioning a hidden assumption, rather than forcing the data to fit, is what led to the discovery.
He also worked across fields: clouds, wind, timekeeping and earthquakes. Our post on Luke Howard's cloud names shows the cloud science he started from.
Try it yourself: redo the crossover calculation for a 40 km crust with the same speeds, and see how the distance changes.
