5 Places to Find the Oldest Oceanic Crust

Written by Kristen Holder
Published: January 26, 2023
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The earth’s crust is super thin yet it lays the foundation for all life on earth. Oceanic crust currently makes up 60 percent of the planet’s surface but this has fluctuated over the eons. Where are 5 places to find the oldest oceanic crust in existence?

What is an Oceanic Crust?

The very top layer of tectonic plates is their crust. Where the crust exists under the ocean is known as oceanic crust. Tectonic plates are essentially rock slabs that are at least 60 miles thick that fit together like puzzle pieces on the outside of the planet.

A divergent boundary is an underwater volcanic line in the center of ocean basins on tectonic plates. At this line, magma actively comes to the surface through the crust. This creates new seafloor which causes seafloor spreading as more crust is created and pushes the older crust to the side.

Oceans are widening because of this process. It also forms volcanoes which may become islands. The crust that’s moving away from the ridge is older than the crust being created.

Eventually, the oldest oceanic crust meets a continental plate. Continental plates float because they’re made of lighter materials than oceanic crusts. Subduction takes place and the oceanic crust is forced under the lighter continental plate. This returns the oceanic plate to the mantle which recycles it.

Because of this constant creation and recycling process, it’s very rare for any oceanic crust on earth to be over 200 million years old. There are always exceptions to every rule, however.

A sectional view of continental crust and oceanic crust

Because of the constant creation and recycling process, it’s very rare for any oceanic crust on earth to be over 200 million years old.


How They Find the Age of an Oceanic Crust

Geologists have never drilled through the oceanic crust to the mantle. Instead, other measurement techniques are used. Seismic tests are common and there are new novel methods using the earth’s magnetism to chart what’s under the ground.

Different rocks respond to different tests in predictably different ways. This allows scientists to make educated guesses about the composition of the oceanic crust that’s buried miles below the surface substrate.

Every so often, the magnetic fields on earth will change direction. These periods are irregular, but when it happens, it changes some of the properties of crystallizing lava which modern scientists can detect. This loosely means that readable magnetic strips are discernable like tree rings especially when they fan from a mid-ocean ridge or other crust creation spot.

Using the effects of magnetism to judge the age of rocks isn’t new to oceanic crust studies. Because it’s much easier to study the continental crust, this technique is well-documented in terrestrial environments. By applying what’s been learned about earth’s geological timetables from the continental crust, the age of the oceanic crust is easier to estimate.

When lava cools and ages, radioactive elements within it begin to decay at predictable rates. Some of these elements include uranium and potassium.

As these two decay, they sometimes create things like lead. By analyzing the seabed for these elements, scientists can pinpoint with some accuracy when the rock first solidified.

5 Places to Find the Oldest Oceanic Crust

These are 5 of the places to find the oldest oceanic crust:

  1. Dissected Northern Appalachian Orogen
  2. Herodotus Basin
  3. Ionian Sea
  4. Yukon-Tanana Terrane
  5. Mariana Trench

5. Mariana Trench: 180 Million Years Old

The Mariana Trench is a subduction zone in a part of the Pacific Ocean. This means that it has some of the oldest true oceanic crust on the planet. The crust on the Pacific Plate in this region is as much as 180 million years old.

This trench is over 1,500 miles long and over 35,000 feet deep in some parts. Despite its depths, scientists can’t witness subduction in progress at this site because it happens over 400 miles under the surface of the earth.

Mariana Trench

The Mariana Trench is over 1,500 miles long and over 35,000 feet deep in some parts.


4. Yukon-Tanana Terrane: 225 Million Years Old

The Yukon-Tanana Terrane is located in Alaska and it’s 180 to 225 million years old. It’s one of the oldest of a line of landmasses to smack into the region and become part of the landscape over the last few hundred million years. A drifting piece of oceanic crust like this that sticks to a continent is called an accreted terrane. 

The landmass that eventually becomes part of the continent is created by seafloor spreading or volcanic activity somewhere on an oceanic plate. As this oceanic plate undergoes subduction beneath a continental plate, the landmasses drift closer to extant continental land from their origin.

Eventually, these errant landmasses, or terranes, slam into the main continent and become a part of that continent. This process scrapes old oceanic crust onto more static continental crust. The Yukon-Tanana is the oldest of the accreted terranes to slam into the region. 

Yukon-Tanana Terrane in Alaska

Yukon-Tanana Terrane (yellow) in Alaska is 180 to 225 million years old.

©Fama Clamosa, CC BY-SA 4.0 <https://creativecommons.org/licenses/by-sa/4.0>, via Wikimedia Commons – License

3. Ionian Sea: 270 Million Years Old

The Ionian Sea has an oceanic crust that’s been submerged for 270 million years. It’s most likely a remnant of the Tethys Ocean. This area of the ocean, which is technically a bay in the Mediterranean Sea, has a crust that’s more stagnant than other places due to the complicated tectonics in surrounding regions.

The deepest point in the Mediterranean is the Calypso Deep found in the Ionian Sea. It is over 17,000 feet deep and it’s found within the Hellenic Trench.

Ionian Sea map

The Ionian Sea has an oceanic crust that’s been submerged for 270 million years.

©iStock.com/Rainer Lesniewski

2. Herodotus Basin: 340 Million Years Old

The oldest ocean crust that’s still under the water is in the Herodotus basin in the Mediterranean Sea. It is believed to be around 340 million years old.

During the time of Pangaea, the Tethys Ocean existed and the Herodotus Basin may have been a part of it. The current eastern Mediterranean Sea is developing separated basins based on the movement of nearby tectonic plates.

1. Dissected Northern Appalachian Orogen: 400 Million Years Old

There are remnants of ancient terranes from an extinct ocean and a tectonic plate smashed within the rocks of the Appalachian Mountains. These rocks from ancient oceanic crust are over 400 million years old. This ocean was called the Iapetus Ocean.

Any true remnants from the oceanic crust of the Iapetus Ocean had been destroyed by subduction around 450 million years ago. What did survive as terranes were already being stirred up by geological activity before Pangea fully formed. The Iapetus Ocean existed from about 400 to 600 million years ago.

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The photo featured at the top of this post is © 1840489pavan nd, CC BY-SA 4.0 , via Wikimedia Commons – License / Original

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About the Author

Kristen Holder is a writer at A-Z Animals primarily covering topics related to history, travel, pets, and obscure scientific issues. Kristen has been writing professionally for 3 years, and she holds a Bachelor's Degree from the University of California, Riverside, which she obtained in 2009. After living in California, Washington, and Arizona, she is now a permanent resident of Iowa. Kristen loves to dote on her 3 cats, and she spends her free time coming up with adventures that allow her to explore her new home.

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