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