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.