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October 22, 2009

A weekly feature provided by scientists at the Hawaiian Volcano Observatory.

The layered Earth turns 100 years old

Dr. Andrija Mohorovii (Moh-ho-ro-vee-chich), a professor of meteorology at the University of Zagreb
Dr. Andrija Mohorovii (Moh-ho-ro-vee-chich), a professor of meteorology at the University of Zagreb

On October 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 Mohorovii (Moh-ho-ro-vee-chich), 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. Mohorovii noticed that 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 13,000 km (8000 miles) in diameter. If we imagine the Earth as a large, 14 inch (36 cm) pizza, then the Earth's crust would only be the outer 1 mm (1/25 inch)!

Mohorovii found that 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 54 km (33 miles) near Zagreb. The boundary between the crust and the mantle has become known as the Mohorovii 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, 7 km (4 miles) below the ocean floor and 30 to 50 km (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 75 km (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 12,260 m (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.

Mohorovii'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!

Kīlauea Activity Update

Lava continues to erupt from the TEB vent on Kīlauea's east rift zone and flow through tubes to the ocean at Waikupanaha. Surface flows continue to be active on the lower pali and coastal plain, but have been largely flowing over the existing flow field this past week, creating only very minor expansion of the east margin of the TEB flow field. As of this writing (Thursday, October 22), the active lobe on the coastal plain was 700 yards north of the coast, near the former location of the Kalapana access road.

The lava pond appeared briefly deep within the Halema`uma`u vent cavity on Sunday, October 18, but for the remainder of the week it was out of view, and only degassing holes at the floor of the cavity have been visible since then. Volcanic gas emissions remain elevated, resulting in high concentrations of sulfur dioxide downwind.

One earthquake beneath Hawai`i Island was reported felt this past week. A magnitude-3.1 earthquake occurred at 00:48 a.m., H.s.t., on Tuesday, October 20, 2009, 24 km (15 miles) south of Kalapana at a depth of 38 km (24 miles).

Visit our Web site ( for detailed Kīlauea and Mauna Loa activity updates, recent volcano photos, recent earthquakes, and more; call (808) 967-8862 for a Kīlauea activity summary; email questions to

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Updated: November 29, 2009 (pnf)