The deep-sea drilling ship Chikyu, which will attempt to drill through the Moho.

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

The Earth’s surface, oceans, and atmosphere are readily accessible to direct exploration. That is, human beings or their instruments can move about, make observations, and collect data. The same is true of space. But the Earth’s interior is less accessible to direct exploration than the surfaces of other planets in our solar system. This, of course, is because it’s much more difficult to penetrate a solid (rock) than it is to move through gas (atmosphere), liquid (water), or the vacuum of space. For example, the Voyager 1 space probe recently left the solar system, yet the deepest hole ever drilled is only 12.3 km (7.6 miles) deep.

Consequently most conventional wisdom about the Earth’s deep structure to its center—6,400 km (4,000 mi) below the surface—is based on indirect measurements, particularly on seismology. By studying the paths and speeds of compressional waves caused by earthquakes, seismologists have concluded that the Earth is layered like an egg, with three main layers. The shell, white, and yolk of an egg are analogous to Earth’s thin crust, mantle, and core.

Andrija Mohorovičić

In 1909 Andrija Mohorovičić, a Croatian seismologist studying a Balkan earthquake, identified an abrupt increase in the speed of compressional waves that marks the boundary between the Earth’s crust and the mantle below. In honor of its discoverer, this seismic discontinuity was named the Mohorovičić discontinuity. Now it’s commonly referred to as the Moho.

The Moho is present under all continents and oceans, but its depth varies—with an average depth of about 35 km (22 miles) under the continents and typically 6 km (3.7 mi) under the oceans. Although the Moho is defined as the boundary between the crust and mantle, the reason for the abrupt increase in compressional wave speed is uncertain. Most scientists think that the wave-speed increase reflects a change in rock type from basalt (like Hawai`i lava) above the Moho to a denser, olivine-rich rock called peridotite below the Moho.

Geoscientists have long wanted to drill through the Moho into the upper mantle to see whether the Moho is caused by a compositional change or by something else. This is a conceptually simple task that’s quite difficult in practice. Drilling where the crust is thin on the sea floor is obviously a more attractive target than drilling on a continent. But drilling from a ship is technologically difficult, and the difficulty increases with the depth of the water.

Earth and atmosphere cutaway illustration 1: Crust, 2: Moho, 3: Upper Mantle, 4: Low-velocity Zone, 5: Lower Mantle, 6: D”- Layer, 7: Outer core, 8: Liquid-solid boundary, 9: Inner core, A: Troposphere, B: Stratosphere, C: Mesophere, D: Thermosphere, E: Exosphere

So a place where the sea floor depth is at a minimum would seem to be the place to drill. The seafloor is relatively shallow near the mid-ocean ridges, where new crust forms from rising magma. So drilling near a ridge seems attractive. However, the young, newly formed crust near ridges is hot, and drilling equipment cannot tolerate the expected temperatures. The trick is to find a place where the Moho is cool enough to drill, yet not too deep to drill.

The first attempt to drill to the Moho was in 1961, off the coast of Mexico near Guadalupe, as part of Project Mohole. The deepest hole from that effort penetrated 183 m (601 ft) into the sea floor, the upper 179 m (557 ft) of which consisted of sediments. In subsequent years, only 4 holes penetrated more than 1 km (0.6 mile) into the oceanic crust; the deepest of these was 2.1 km (1.3 mi) off the coast of Ecuador.

In September 2012, a new state-of-the-art scientific drilling ship—Chikyu—surpassed the old record of 2.1 km. The Chikyu is part of Japan’s contribution to the Integrated Ocean Drilling Program (IODP), an international research effort “dedicated to advancing scientific understanding of the Earth through drilling, coring, and monitoring the subseafloor.” The Chikyu is designed to ultimately drill through the Moho into the mantle.

Planning is currently underway to select a drill site for the Chikyu. Three sites are under consideration: the site off the coast of Mexico that was drilled in 1961, a site off the west coast of Nicaragua that has also been previously drilled, and the North Arch of the Hawaiian Archipelago. The North Arch is about 400 km (250 miles) north and parallel to the Hawaiian Islands.

A location on the North Arch of the Hawaiian Islands, just north of Maui, was seriously considered in the 1960s during Project Mohole but the funding was removed by Congress thus ending the project. We sincerely hope that this important scientific endeavor is more successful this time in pushing back the frontiers of inner space.