By Charles Bonkowsky
Mantle geologists have it rough.
In its pure form, a sample of the Earth’s red-hot mantle would provide innumerable insights, answering long-standing geologic questions ranging from the precise elemental composition of the planet’s interior to how seismic waves travel. But it’s getting one that’s the tough part. Standing between us and the mantle is several miles of solid rock—that is, the Earth’s crust—and the only way to reach the mantle is straight through.
So far, none of the projects designed to retrieve a sample of the mantle have been able to do so, but not for lack of trying. Expeditions to cut through the crust started during the Cold War—an earthly complement to the moonshot, perhaps—with the US-based Project Mohole. The project took its name from the Mohorovičić discontinuity (usually referred to as the Moho), which marks the boundary between the crust and the mantle. Just above the Moho, seismic waves travel at roughly 7 km/s through the rock—below it, they travel at roughly 8 km/s. The difference in speed means that the crust and the mantle below it must have significantly different compositions, but the specifics of this difference were still left to theory and imaging rather than physical examination when the project was launched in 1959 by a collection of esoteric scientists.
Since the Earth’s crust is much thinner underneath the oceans than under land, the expedition was planned for the deep ocean. But doing so presented its own challenges. Drilling through the seafloor meant the drill would have to be lowered through several kilometers of water before it hit ground, and the ship would have to be kept steady against the influences of wind or waves. Nowadays, oil rigs have become skilled at maintaining their position during drilling, but in the early 1960s, they hadn’t begun seeking oil that deep. A jury-rigged collection of omnidirectional propellers had to be bolted onto their ship, the CUSS I, to complete the first phase of the project, whose auspicious beginning hid the bureaucratic troubles that would eventually doom it.
Though CUSS I managed successfully to drill nearly two hundred meters into the seafloor, the second phase of the project never reached fruition. Questions of funding, feasibility of reaching the mantle, and reshuffling members’ roles led to what one author, observing its demise, called “a lengthy and unattractive trail of bickering, bitterness, and shortsightedness…a classic case of how not to run a big research program.”
Ironically, while Mohole had secured US funding by claiming the Soviets were already beginning research, a formal Russian-run drilling project didn’t begin until 1970, four years after Mohole was formally shut down. The Kola Superdeep Borehole remains the deepest hole in the world, at just over 12 kilometers deep, but still short of reaching the Moho discontinuity. The project’s goal was to reach a depth of 15 kilometers, but machinery problems and heat wrenched it to a halt: the rock they were drilling into was so hot that it acted more like a plastic than a solid, flowing back into the hole they’d just carved out.
Even failing to reach the mantle, their discoveries were groundbreaking (pun intended) in the field of geology. The Moho discontinuity marks an abrupt transition in the speed of seismic waves between the crust and the mantle, but the crust contains similar variations as well, at depths the Kola borehole passes through. Since 1926, scientists had believed that this change in seismic-wave speed occurred where the granite of the crust changed to a deeper ‘basement’ of basalt. The Kola borehole revealed that this transition was instead to a deeper layer of metamorphic rock, thoroughly fractured by the heat and pressure and, more oddly, saturated with water. As seismologist Larry Gedney put it, “free water should not be found at these depths!”
At the incredible pressures 3-6 miles below the surface, water shouldn’t exist naturally, and scientists now believe that the water found is formed from hydrogen and oxygen squeezed out of the rocks themselves. It’s also highly mineralized—if mining companies could drill this deep, they would find highly concentrated ore deposits around this water. Another startling discovery was microscopic fossils four miles beneath the surface, in rocks over two billion years old, preserved in small cavities of limestone even at such depths.
No 21st-century expedition has managed to reach as deep as the Kola borehole, but advances in ocean drilling are giving scientists reason to believe that they might be getting closer to the ‘holy grail’: reaching the upper mantle. Ships like the JOIDES Resolution and the Chikyu have been conducting underwater drilling for ancient samples of the crust, diving deep into the Earth’s last major warming period or seeking the zone where earthquakes are generated. The teams behind both ships are hopeful that new technology will allow them to pierce the mantle for the first time, though both have also faced broken drills and storms on recent missions.
Scientists are now considering three locations: one near Costa Rica, one near Baja California, and one near Hawaii. Each has their tradeoffs between the estimated temperature of the mantle (higher temperatures are harder to deal with), the depth of the ocean (the deeper it is, the harder it is to deal with) and the spreading rates of the tectonic plates beneath (faster spreading rate is better for observing plate tectonics). Drilling here, they hope, will allow them to finally reach the mantle. Understanding the Moho, determining the composition of the upper mantle and the limits of life there can only be done by extracting pure samples—and many of the researchers leading the teams now are the same who were captivated by Mohole or Kola in the 60s and 70s. At the various international organizations, they hope for these ships to begin drilling no later than 2030, while they’re still around to see it.
The deep oceans have been described as the last great frontier. But soon—assuming the drilling goes according to plan—we may reach an even deeper one.