Ancient African bedrock reveals how Earth became a violent, life-bearing planet

The rugged hills of the Makhonjwa Mountains expose some of the oldest intact crust on Earth. These rocks form part of the Barberton Greenstone Belt, a sequence of volcanic and sedimentary layers dated to roughly 3.6–3.2 billion years ago, when the planet was still adjusting to its internal heat and external bombardment.

Within the rocks is direct evidence that vast oceans already covered much of the planet. Pillow-shaped lava flows show where molten rock erupted onto the seafloor, cooling instantly in contact with seawater. Other layers record vigorous hydrothermal systems, where superheated fluids circulated through cracks in the crust and discharged metals back into the ocean.

At that time, Earth’s interior was far hotter than it is today. This heat produced ultramafic magmas that are rare under modern conditions. Repeated eruptions built chains of volcanic islands that periodically rose above sea level, only to be reshaped by further eruptions, erosion, and collapse. Ash and volcanic debris settled across nearby marine basins.

The sedimentary record also shows that these environments were unstable. Large earthquakes repeatedly shook the crust, triggering submarine landslides that swept rock and sediment into deeper water. The resulting chaotic deposits indicate that the early crust was already deforming and recycling, consistent with early forms of plate tectonics or closely related processes.

Life was present in this volatile world. Shallow-water sediments preserve layered structures interpreted as microbial mats, along with textures consistent with biological activity. These early organisms were anaerobic microbes, adapted to an atmosphere without oxygen and dominated by methane and carbon dioxide.

Despite this toxic atmospheric chemistry, surface temperatures remained suitable for liquid water. Greenhouse gases compensated for a fainter young Sun, allowing oceans to persist. Light scattering by these oceans would already have given Earth a blue appearance, long before oxygen accumulated in the atmosphere.

The planet was also repeatedly struck by large asteroids. Distinctive spherule layers in the rock record show that major impacts disrupted Earth’s surface many times during this period. Geological evidence indicates that these events did not sterilize the planet and that early ecosystems survived repeated global disturbances.

Modern volcanic island systems offer a useful point of comparison. In January 2022, the Hunga Tonga-Hunga Haʻapai eruption released energy equivalent to tens of megatons of TNT, generated intense lightning, and produced chemically reactive seawater within a submarine crater. While far younger and better observed, such eruptions illustrate how volcanism, water, and energy interact.

Laboratory experiments show that lightning and hydrothermal chemistry can promote the formation of basic organic molecules. On early Earth, repeated eruptions, earthquakes, and impacts would have created many such chemically active environments, increasing the likelihood that prebiotic chemistry could progress toward living systems.

The geological record preserved in the Makhonjwa Mountains suggests Earth became a stable, ocean-bearing planet within the first tenth of its 4.6 billion-year history. Key factors included its position in the Sun’s habitable zone, sufficient mass to retain an atmosphere, the presence of a magnetic field, and the formation of the Moon, which helped stabilize Earth’s axial tilt.

Today, the region is protected as a UNESCO World Heritage Site. Continued study of these rocks is refining the understanding of how extreme geological activity and biological resilience combined to shape the earliest stages of life on Earth.

References:

¹ Ancient African bedrock reveals the violent beginnings of life on our blue planet – The Conversation – January 6, 2025

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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