SCIENCE

Go Deep: 5 Places on Earth to See Seriously Old Rocks

See geology “in action”—or at least as action-packed as rocks can be—at these five spots

Correspondent

August 10, 2016

"HEY! DID YOU KNOW THAT MACQUARIE ISLAND IS HOME TO SOME OF THE WORLD'S OLDEST ROCKS?" "I DID NOT! WHY ARE WE YELLING?" "I DON'T KNOW, WE'RE SEALS" Hullwarren/Wikimedia Commons

When’s the last time you laid hands on something billions of years old? (That old salsa jar at the back of your fridge doesn’t count.) If you answered “never,” you’re not alone. Most of Earth’s rocks are relative babies: In the grand scheme of things, they just haven’t been around that long. But that’s not the case in a few special places on the planet—places where geology has exposed the oldest and deepest rocks ever discovered.

To understand why really old rocks are special, it helps to understand how the Earth was formed. About 4.5 billion years ago, Earth grew out of a massive disk of gas and dust swirling around the young sun. Planetesimals—boulder-sized bodies made up of microscopic dust and gas—began to accrete. At some point, the one destined to become Earth collapsed under the weight of its own gravity. The heavy stuff made its way to Earth’s core, while the lighter stuff became the mantle and eventually the crust of Earth.

The metals deep inside Earth are much denser than those closer to the surface, but though the core makes up about 15 percent of Earth’s total density, it’s much less accessible and is studied less. The mantle, however, is a different story. Home to about 84 percent of Earth’s total density, it’s the place where Earth’s tectonic plates shift and ride. As a result, volcanoes and earthquakes sometimes expose much older rocks than are normally found on Earth’s surface.

Scientists can use radiometric dating, which measures the number of radioactive isotopes in a rock or mineral, to figure out how old it is. As a result, they’ve located ancient rocks on all seven continents. Here are five places where you can peer deep into Earth’s distant past:

Barberton Mahkonjwa Geotrail (Barberton, South Africa)

(Professor Christoph Heubeck (left) and Tony Ferrar (right), authors of the Barberton Makhonjwa Geotrail guidebook, examine a rock formation. —Kruger Lowveld Tourism)

Also known by the forbidding name “Valley of Death” (legend has it that paranormal activity plagues the area), South Africa’s De Kaap Valley is surrounded by breathtaking mountain passes and old gold mines. But geologists love it because of the Swaziland Supergroup—not a reunited boy band, but rather a cache of volcanic and sedimentary rocks that researchers believe are a full 12 miles thick. Thanks to volcanic activity that turned the chronological layers of the rocks inside out, the exposed rock is 3.5 billion years old. Visitors can check out the rocks by following the so-called Geotrail, a nearly 25-mile-long road studded with interpretive signs that point out everything from volcanic stones to conglomerates of rocks formed by ancient tsunamis. Nearby, you can hike, check out Victorian buildings left over from Swaziland’s gold rush days, or watch local artisans make gold jewelry. The area is so rich in history and natural wonders that it’s being considered for Unesco World Heritage status.

Gros Morne National Park (Newfoundland, Canada)

It’s not that easy to get to Gros Morne—you’ll need to take a ferry from Nova Scotia or catch a flight to the regional airport from Montreal or Toronto. But once you reach Canada’s second-largest national park, it will well make up for the long trip with some seriously old sights. Located on the west coast of Newfoundland, the Unesco-recognized park is home to a fjord that helped geologists figure out plate tectonics. Around 1.2 billion years ago, the plates smushed against one another at the edge of eastern North America, forming a mountain range and exposing the ocean floor. Tectonic activity continued in the area, pushing the Earth’s mantle upward about 500 million years ago. Today, you can hike through the rare slice of mantle—known as the Tableland—and explore the strangely naked rocks usually found beneath the crust. Be sure to check out other nearby attractions like Cape Spear Lighthouse on North America’s easternmost chunk of land, or cruise the freshwater fjord complete with waterfalls.

Qeqertarsuaq (Disko Island, Greenland)

(Some of Disko Island's rocks are 3.8 billion years old. Not bad; you could even say it's quite gneiss. —Mads Pihl / Visit Greenland)

Want to feel really young? Head to Greenland’s west coast during the brief weeks of summer for a glimpse of gneiss—banded, metamorphic rock 3.8 billion years of age. This rock was likely formed when mantle and crust melted into one another as continental plates shifted, thickening the rock into the world’s oldest continental crust. Disko Island is home to three glaciers and tons of flowering Alpine plants, so get your “summer” on as you explore the remote island or ride a dog sled under the midnight sun.

Black Mountains (Inyo County, California)

Here’s a place to see ancient geology that you shouldn’t visit in summer: the Black Mountains above Death Valley. With panoramic views at overlooks like Dante’s View, the mountains are a great place to look over the hot and barren valley below. But they’re also a good place to scope out some of North America’s oldest rocks: these 1.8-billion-year-old rocks are all that remains of an ancient volcanic belt. During the Precambrian era, these rocks were lifted to the surface, then mashed up with other rock types and eroded over time. While you’re there, check out another phenomenon: the Black Mountains’ “turtlebacks,” round, turtle-shell-like formations that are the subject of debate among geologists to this day.

Macquarie Island (Tasmania, Australia)

(In addition to having some of the world's oldest rocks, Macquarie Island is also home to all the world's royal penguins, who nest here in summer. —M. Murphy / Wikimedia commons)

Halfway between Australia and Antarctica lies an island covered in penguins. But Macquarie Island is more than just an adorable refuge for black-and-white birds: It’s a place where you can see geology in action. The Unesco-protected island is the only place on Earth where the mantle is being actively exposed above sea-level (to see the current mantle elsewhere, you’ll have to dive deep into the Atlantic Ocean). Millions of years ago, a new oceanic crust formed and the tectonic plates that surround the area began to compress, squeezing out Macquarie Island. The island is made entirely of mantle rocks. Sure, it’s remote (you’ll have to get there on a cruise to Antarctica), but what it lacks in accessibility it makes up for in geologic wonder.

Learn about this research and more at the Deep Carbon Observatory.