(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

On Oct. 8, 1909, an earthquake struck southeast of Zagreb, Croatia. Prior to this event, a number of seismographs had been installed in the region, resulting in a very interesting set of recordings from this earthquake.

These recordings would lead to fundamental discoveries about the internal structure of the Earth.

Dr. Andrija Mohorovicic, a professor of meteorology at the University of Zagreb, had recently become interested in the precise measurement of time for both astronomical and seismic events.

Mohorovicic noticed waves penetrating deeper into the Earth arrived sooner than waves traveling along its surface. He deduced from this observation, that the Earth has a layered structure, an outer layer—the crust—overlying a denser mantle in which earthquake waves could travel more quickly.

The Earth’s crust is aptly named, because it really is only a very thin outer shell on a planet that is almost 8,000 miles in diameter.  If we imagine the Earth as a large, 14 inch pizza, then the Earth’s crust would only be the outer 1/25 inch!

Mohorovicic found when seismic waves strike the boundary between different layers within the Earth, they are reflected and refracted, just as light is when striking a lens.

From the data collected, he estimated the thickness of the upper layer (crust) to be 33 miles near Zagreb.  The boundary between the crust and the mantle has become known as the Mohorovicic discontinuity, usually referred to as the Moho, and its discovery is considered one of the greatest accomplishments of Croatian science.

The Moho is, on average, 4 miles below the ocean floor and 19 to 31 miles beneath continents. The thickest known crust has been found beneath large mountain ranges, such as the Andes (west side of South America) and the Himalayas (south-central Asia), where crustal thicknesses are in excess of 47 miles!

In the spring of 1961, the American Miscellaneous Society, funded by the National Science Foundation, embarked on a mission to drill a hole through the ocean floor to reach the Moho in order to take deep core samples and investigate the chemical nature of the boundary.

During the operation, named Project Mohole, several pilot holes were drilled off the coast of Guadalupe, Mexico.  Due to continuing technical problems and lack of financial support, the project was canceled in 1967.

Simultaneously, the Soviet Union at the Kola Institute drilled a hole that reached a depth of 40,226 feet over a 15-year period, creating the world’s deepest drill hole to date.  This attempt was also abandoned, due to mechanical problems and higher-than-expected temperatures at depth.

Although they did not reach the Moho, drilling programs like these have provided valuable data on the composition of both continental and oceanic crust.

Most of the surface of the Earth is covered by oceanic crust, which is continuously being created at mid-ocean ridges. At these ridges, magma rises into the upper mantle and crust and forms new oceanic crust along the ridge axis.

As it moves away from the ridge, the lithosphere (made up of the crust and the rigid part of the upper-most mantle) becomes cooler and denser, and sediment gradually settles on top of it.

The youngest oceanic lithosphere is at the oceanic ridges and grows progressively older away from the ridges.  The oceanic crust beneath the Hawaiian island chain ranges from about 80 to 120 million years old.  Oceanic crust seldom survives to be more than about 200 million years old before being subducted beneath another tectonic plate, only to be recycled back into the Earth’s mantle.

Mohorovicic’s discovery led to a cascade of discoveries over the next century, including the discovery of the core-mantle boundary, the liquid outer core, and several chemical layering boundaries in the mantle.

And it all started with a few keen observations from an earthquake 100 years ago. So, if you bump into the Earth today, tell her happy birthday!