Drilling into Earth’s Mantle

Turns out it was much easier to go to outer space than to inner Earth.

In the 1950s, the U.S. and Russia began competitive spaceflight programs. Twenty-five years later, the U.S. Voyager spacecraft had gone 10 billion miles into space.

Around the same time, both countries also began drilling programs trying to pierce through Earth’s crust to reach the mantle below.

The U.S. project began drilling off the coast of Mexico, because scientists had recognized that oceanic crust is thin, averaging only 4 miles thick, whereas continental crust could be 25 miles thick. But the project was defunded by Congress after only a test bore.

The Russians began drilling on land, above the Arctic Circle, and kept at it for 20 years. In the end, they got 8 miles into the crust without ever reaching mantle.

But last year, an international expedition on the Resolution, the drilling ship we discussed on a prior episode, set out for the Atlantis Massif. Discovered in the seventies, it’s a huge bulge that rises 14,000 feet above the mid-ocean seafloor.

Here, the ocean crust is even thinner, separating into layers as it spreads apart, exposing deep rock that is normally far below Earth’s surface.

By drilling less than a mile into this spreading area, the Resolution finally tapped into mantle and brought core samples to the surface—a geologic first, 66 years in the making!

Background

Synopsis: During the Cold War, intense rivalry existed between America and Russia as they competed in huge engineering challenges. The Space Race prospered and received more publicity, while the race to Earth’s interior proved much more difficult and took a back seat. In 2023, 66 years after the goal was conceived, researchers from the International Ocean Discovery Program (IODP) finally drilled into Earth’s mantle and brought core samples to Earth’s surface.

• Earth is layered like an onion, and the crust is less than 1% of its total thickness.
  • With an average thickness of 25 mi (40 km), Earth’s continental crust ranges up to 45 mi (70 km) thick below places like the Himalayas.
  • Oceanic crust is much thinner, averaging about 4 mi (6 km) thick, but becoming much thinner where it is being created near oceanic spreading centers.
• In 1909, a Croatian seismologist identified a marked change in density at the boundary between the rigid crust and the more ductile mantle. It was named the Mohorovičić Discontinuity to honor the seismologist and is called the “Moho” for short.
  • Below the Moho is the 1800 mi (2900 km) thick mantle, which makes up 40–45% of Earth’s volume. It is the dynamic cushion upon which Earth’s crustal plates ride as they jostle against each other.

    

  • Below the mantle, generating Earth’s protective magnetic field, is the liquid metal outer core. At the planet’s center is its solid metal core.
• Up to now, humans have only been able to directly investigate Earth’s crust, so indirect geophysical evidence had to be used to understand the nature of the mantle.
  • Mantle rocks are occasionally exposed on Earth’s surface as volcanic bombs (peridotite encased in basalt), or where ophiolites (sheets of oceanic crust with a bit of upper mantle still attached) have been pushed up over crustal rocks.
  • The quest to drill into Earth’s mantle has been an unrealized engineering challenge since the time of the Space Race that started in the late 1950s between rival Cold War superpowers, the USA and the USSR.
  • While rockets were directed upward through the atmosphere into space, drill bits were directed downward through the crust with the goal of sampling Earth’s mantle.
• The earthbound mission proved much more difficult; in more than 20 years of drilling, the deepest hole on Earth reached 12 km; whereas in about 26 years, Voyager I escaped the solar system, a trip of 16.5 billion km.
  • In 1957, Americans began planning Project Mohole, to drill through oceanic crust into the mantle. In 1961 a 601-ft (183 m) test hole was drilled near Mexico’s Guadalupe Island. But in 1966 Project Mohole was defunded by Congress because of scientific infighting about program goals and budget mismanagement.
  • In May of 1970 the Soviets began drilling the Kola Superdeep Borehole (ED-216 Kola Superdeep) near the Barents Sea, and they kept drilling for about 20 years! The deepest part of the well got to 40,230 ft (7.62 mi, 12,262 m) vertically below Earth’s surface, deeper than the deepest oceanic trench, ending in 2.6 billion-year-old granites.
  • But finally, in May of 2023, Expedition 399 researchers on the JOIDES Resolution (ED-378 Deep Ocean Drilling) drilled into an uplifted “mountain” along the mid-Atlantic Ridge (ED- 015 The Longest Mountain Range).
  • Drilling below the ocean floor is extremely difficult. The original Expedition 399 plan didn’t pan out, so the researchers began to drill at another location, and the drill bit moved like a hot knife through butter, to their happy amazement.
  • They retrieved core samples of ultramafic peridotites that are thought to be uppermost mantle rock from 4160 ft (1268 m) below the seafloor.
    

    

  • This achieved the goal of Project Mohole 66 years after it was first conceptualized, enabling the first direct scientific observation of mantle rocks.
  • Because many of the peridotites are serpentinized, some scientists think that these rocks could be transitional, from the lowermost crust just above the mantle boundary. Serpentinization decreased progressively in deeper sections of core.
• The Atlantis Massif is a prominent bulge along the mid-Atlantic Ridge that rises about 14,000 ft (4267 m) from the seafloor and is about 10 mi (16 km) across.
  • It is located at approximately 30°8′N latitude 42°8′W longitude.
  • It occurs on the north side of the Atlantis Transform Fault, where it intersects with the mid-Atlantic Ridge, offsetting the ridge eastward by about 37 mi (60 km).
  • It reaches 2,300 ft (700 m) beneath the sea surface at its highest point.
    

    

• The Atlantis Massif is an ultramafic oceanic core complex that formed 1.5-2 million years ago. More than 50 of these complexes have been identified in Earth’s oceans.
  • At most mid-ocean ridges, oceanic crust forms by the continuous process of eruption and intrusion, producing a predictable layered structure of volcanics underlain by sheeted dikes and then gabbro, a coarse-grained version of basalt.
  • But at some locations, like the Atlantis Massif, ultraslow spreading occurs as the ridge makes its big bend, and magmatism can’t keep up.
  • The crust delaminates, splitting into its layers and fanning out like a hand of cards, creating a characteristic corrugated fault system on the seafloor.
    

    

  • This uncovers deeper rocks and allows mantle rocks to bulge upward, producing the elevated oceanic core complex.
  • The bulge provides a tectonic window that exposes rocks that normally only exist far below Earth’s surface.
• Instead of being composed of black basalt like oceanic crust, the Massif exposes dense green serpentinized peridotites at the seafloor, which are characteristic of the mantle.
  • Peridotites are coarse-grained ultramafic mantle rocks mostly made up of green olivine (also known as the gem peridot, ED-224 Gems from Deep Earth or Space) and black pyroxene.

    

  • Serpentinization degrades ultramafic rocks, like peridotites, when they are infiltrated by water that has low quantities of carbon dioxide, producing hydrogen. They typically form at mid-ocean ridges and in the forearc of subduction zones.
  • The core recovered from this expedition will enable researchers to study an actively serpentinizing system. One of the main aims of IODP Expedition 399 is to study the reactions between olivine and seawater that are believed to be actively occurring at depth in the massif today – and may have been important in the origin of life (as we will discuss in the next episode).
  • Another prize for researchers is the number of peridotites that are not yet serpentinized, because they represent mantle rocks that have not yet melted and differentiated into magma. Researchers may be able to learn more about how magma melts, flows and separates, to better understand how volcanoes work.
• Expedition 399 left two boreholes accessible as borehole observatories for future fluid sampling and observation.
  • Meanwhile, scientists continue the long job of detailed documentation of all the core retrieved during the mission.
• Not far from the site of Expedition 399 drilling, more than two decades ago, the DSV Alvin (ED-370 The Longest, Deepest Diver) discovered the remarkable Lost City Hydrothermal Field on December 4, 2000.
  • We’ll talk about the unique physical systems that support extremophiles that flourish in this harsh environment and how they might relate to the origin of life in the next EarthDate episode (ED-401 Life in Hydrothermal Vents).