Among the oldest intact rocks on Earth at 3.5 billion years, formed when our planet's interior was 400°C hotter. These ancient lavas contain glass beads from asteroid strikes so massive they vaporized rock globally. The layers record at least four mega-impacts.
CSIRO, CC BY 3.0, via Wikimedia Commons
The Archean Eon
The Barberton Greenstone Belt, located in the Makhonjwa Mountains, shows the surface of the early Earth. Its volcanic and sedimentary rocks were formed between 3.6 and 3.25 billion years ago, during the Paleoarchean era. These formations are exceptionally well-preserved, located on the eastern edge of the Kaapvaal Craton, one of the first stable pieces of continental crust to form on the planet.
A feature of the belt is its komatiite lavas. These ultramafic volcanic rocks required immense heat to form, with eruption temperatures reaching or exceeding 1600°C. For comparison, modern basaltic lavas erupt at around 1100 to 1250°C. This temperature difference confirms that Earth's mantle was significantly hotter during the Archean. The komatiite lava was also incredibly fluid, with a viscosity similar to water, allowing it to flow rapidly over vast distances. This rapid cooling is what created its signature spinifex texture, named after a spiky Australian grass.
Record of cataclysm and life
Embedded within the rock layers are thin but distinct seams of tiny glass beads called spherules. They are not volcanic; they are the condensed droplets from clouds of vaporized rock created by enormous asteroid impacts. Scientists have identified at least eight distinct spherule layers, designated S1 through S8, which record at least four, and possibly as many as eight, separate massive impact events between 3.5 and 3.2 billion years ago. The impactors are estimated to have been 20 to 50 kilometers in diameter, far larger than the approximately 10-kilometer-wide asteroid that led to the extinction of the dinosaurs.
Despite this violent environment, the Barberton rocks contain some of the oldest and most widely accepted evidence of life on Earth. Within layers of chert—a dense, microcrystalline quartz—are fossilized microbes dating back 3.42 to 3.5 billion years. Some of these appear to be methanogens that thrived in subsurface hydrothermal systems, meters below the seafloor. Structures interpreted as stromatolites, layered mats built by colonies of photosynthetic bacteria in shallow water, have also been identified and dated to about 3.3 billion years ago.
💡Fun Facts
The region sparked one of South Africa's first gold rushes in the 1880s, and some of the world's oldest gold is still mined here today.
The komatiite lavas were so hot and fluid that they were likely capable of melting the ground they flowed over, carving deep thermal erosion channels.
The area is recognized as a UNESCO World Heritage Site called the Barberton Makhonjwa Mountains.
Some rock layers contain non-terrestrial chromium isotopes and extraterrestrial organic molecules, confirming their origin from meteorite impacts.
The general mountain landscape is accessible 24/7, but it is best and most safely explored during daylight.
Admission
Free to drive through the area. Fees may apply for entry into specific nature reserves or for guided tours.
Accessibility
The terrain is mountainous. A 37-kilometer-long paved road, the Barberton Makhonjwa Geotrail, has 11 marked stops with explanatory displays at roadside pull-offs, making key geological features accessible by vehicle. Hiking trails require appropriate fitness and preparation.
