planet

Mohorovičić discontinuity

The boundary between a planet's crust and its mantle, where seismic speeds jump because the rock changes character. Andrija Mohorovičić found it in 1909 by noticing that a quake in the Kupa Valley sent not one set of arrivals but two: a slower set travelling directly through the crust, and a faster set that had dived into a deeper, stiffer layer and come back up. On Earth the Moho lies about 7 km below the ocean floor but 35–70 km beneath continents — and that very difference is the evidence that the planet has two distinct kinds of crust.