National Geographic News
Cameras dropped into the Mariana Trench record the presence of giant amoebas as well as other life forms.
The Mariana Trench (pictured in a video still of an illustration) is the deepest place on Earth.

Video still courtesy of NOAA

Richard A. Lovett

for National Geographic News

Published April 5, 2012

James Cameron made headlines last month by successfully diving 6.8 miles (11 kilometers) to the deepest part of the Mariana Trench in a one-person submersible called the DEEPSEA CHALLENGER. (See "Cameron Exclusive: After Record Dive, Why Go Back to Mariana Trench?")

Best known as a Hollywood director and now a National Geographic Society explorer-in-residence, Cameron is one of just three people to dive to Earth's deepest point, and the only one to be able to stay long enough to look around. (Video: Cameron Dive Is an Exploration First.)

Though Cameron and his team are hoping to discover more about the biology of the Mariana depths, geologists already know a lot about how the Mariana Trench formed—and why it's Earth's deepest spot.

(Related: "James Cameron on Earth's Deepest Spot: Desolate, Lunar-Like.")

Ancient Lava Shaped Mariana Trench

The Mariana Trench isn't really the deep, narrow furrow that the word "trench" implies. Rather, the abyss marks the location of a subduction zone.

Subduction zones occur where one part of the seabed—in this case the Pacific plate—dives beneath another, the Philippine plate. Though tectonic forces eventually warp the Pacific plate so that it makes a near-vertical dive into the Earth's interior, at seabed level the plate dips at a relatively gentle angle.

A tectonic plate is a huge hunk of rock, 60 miles (97 kilometers) or more thick, said Robert Stern, a geophysicist at the University of Texas, Dallas. "In order for this to sink back into the earth, it has to bend downward, and these are very gentle bends."

One reason the Mariana Trench is so deep, he added, is because the western Pacific is home to some of the oldest seafloor in the world—about 180 million years old.

Seafloor is formed as lava at mid-ocean ridges. When it's fresh, lava is comparatively warm and buoyant, riding high on the underlying mantle.

But as lava ages and spreads away from its source, it slowly cools and becomes increasingly dense, causing it to settle ever lower—as is the case with the Mariana Trench.

(Read more about our dynamic Earth.)

Mariana Trench Could Trigger Big Quakes?

Two other factors conspire to make the 1,580-mile-long (2,550-kilometer-long) Mariana Trench staggeringly deep.

For one, the trench lies far from any major landmass, which means it's remote from the mouths of muddy rivers.

"Many other deep trenches are more filled with sediment," Chris Goldfinger, director of the Active Tectonics and Seafloor Mapping Laboratory at Oregon State University, said in an email. "This one isn't."

In addition, nearby fault lines cut the Pacific plate into a narrow tongue at the site of the trench, allowing the plate to bend more steeply downward than at other subduction zones.

Because of this orientation, most scientists thought that the Mariana Trench subduction zone wouldn't be a source of major earthquakes, said geophysicist Emile Okal of Northwestern University.

That's because the dense rock of the Pacific Plate at the site of the trench should not press strongly upward against the overriding Philippine plate, creating the friction that would cause earthquakes.

But the 2004 Sumatra earthquake and the 2011 Japan earthquake "killed" that theory by occurring in other, similar places where giant temblors weren't expected, Okal said by email.

So there's no reason to believe the Mariana Trench region couldn't produce an earthquake of perhaps magnitude 8.5, he said.

"There are two events [in the region] known to have generated local tsunamis in 1826 and 1872."

Cameron Dive a "Man on the Moon" Feat

Despite Cameron's record dive, it's impossible to know what really happens in the subduction zone, since most of the action occurs up to 420 miles (700 kilometers) below Earth's surface. (See plate-tectonics pictures.)

(The dive was part of the DEEPSEA CHALLENGE program, a partnership with the National Geographic Society and Rolex. The Society owns National Geographic News.)

"That's the iceberg," the University of Texas's Stern said, in reference to Cameron's movie Titanic. "Cameron wasn't even at the tip of it—11 kilometers out of 700. The trench is the interface between the limits of human experience and the reality humans can't experience."

Even so, with subsequent dives, there's the prospect of retrieving rock samples and looking for life deeper than it's ever been found before.

"Life in extreme environments is fascinating, and I think [that's] a big part of what motivated Cameron," said Oregon State's Goldfinger.

"Hopefully this new vehicle will lead to being able to do science effectively at those depths. I think it's really a mission of discovery at this point."

UT's Stern agreed, calling Cameron's trip "kind of a man-on-the-moon thing."

John Delano
John Delano

The Marianas trench is 180 million years old , and was on the anchient equator.

The ice cap over 20 miles high that formed after the moon forming collision blew the mass that formed our moon 4.6 billion years ago had all the liquid on planet earth collect in the pacific "hollow." The liquid froze over time 4.5 billion years ago and as it froze it rose. The returning upper mantle pushed up the ice cap. The water expanded as it froze , and that made it rise. The heat from radioactivity caused the mass to rise, and by whatever reason the ice cap 10,000 miles wide and rose over 20 miles high at the center. The missing mass from the pacific hollow is the same mass and age (4.6 billion years old) as our moon. Things changed on earth with the changes in mass.

Initially the liquid half of the planet adjusted to the new rotation and change in axis. In time the asymmetrical ice cap that had formed would change the effect it had on the physical geology. In this article we are discussing the Marianas trench, and it was the massive ice cap rotating in the pacific hollow that formed all trenches in the pacific.

The trenches in the Atlantic are created by the circumnavigation of the outflow of ice from the pacific ice cap

John Delano


The Origin Of Mountains

John Delano
John Delano

The Marianas trench is not formed by subduction.

Brett Sutherland
Brett Sutherland

Nice to see  a native Nova Scotian at the forefront of Science and exploration !!


Popular Stories

  • 'Extinct' Bird Rediscovered in Myanmar

    'Extinct' Bird Rediscovered in Myanmar

    The Myanmar Jerdon's babbler was thought to have gone the way of the dodo—until scientists stumbled across it during a 2014 expedition.

  • Lost City Found in Honduras

    Lost City Found in Honduras

    A joint Honduran-American expedition has confirmed the presence of extensive pre-Columbian ruins in Mosquitia in eastern Honduras, a region rumored to contain ruins of a lost "White City" or "City of the Monkey God."

  • Astronomers Find a Galaxy That Shouldn't Exist

    Astronomers Find a Galaxy That Shouldn't Exist

    Small, young galaxies should be free of interstellar dust, but an object called A1689-zD1 is breaking all the rules.

The Future of Food

  • Why Food Matters

    Why Food Matters

    How do we feed nine billion people by 2050, and how do we do so sustainably?

  • Download: Free iPad App

    Download: Free iPad App

    We've made our magazine's best stories about the future of food available in a free iPad app.

See more food news, photos, and videos »