The Hadean Eon spanned the time from Earth’s formation approximately 4.6 billion years ago to the beginning of the Archean Eon around 4 billion years ago. It is a challenging period to study because there is little geological evidence remaining.
Formation of Earth
The Hadean Eon began with the accretion of planetesimals to form Earth. The early Earth was a hot and chaotic place, with frequent impacts from space debris, including everything from dust to asteroids that shared the forming planet’s orbit around the sun.
Intense Volcanic Activity
As the planet took its form as a planet, Earth’s surface was subject to intense volcanic activity. This was the release of heat from the planet’s formation. Lava flows covered vast regions, forming the earliest igneous rocks.
Development of the Early Atmosphere and Oceans
It is still debated where Earth’s water came from. Some was from the material swept up in the formation of the planet, and volcanic outgassing contributed to the formation of the early atmosphere, which likely consisted of water vapor, carbon dioxide, nitrogen, and other gases. Additional water was brought in by comets as they continued to collide with the Earth long after it was first formed. As the Earth cooled, water vapor in the atmosphere condensed to form the primordial oceans.
The Hadean Eon was marked by a period of intense impact bombardment known as the Late Heavy Bombardment (LHB). During this time, large asteroids and comets collided with Earth, leaving behind craters and affecting the planet’s surface, but bringing in water and other materials that would become part of the planet’s crust.
Formation of Earth’s Crust
As the surface continued to cool, the first solid crust formed through volcanic activity and the solidification of molten material. These early crustal rocks have been mostly eroded or transformed over time, making direct study difficult. The cratons around the world, including the Canadian craton, and some rocks in Australia are the closest things to remnants of the early crust.
Earliest Geological Evidence
The lack of preserved rocks from the Hadean Eon makes it challenging to reconstruct this period accurately. The few remaining rock fragments, often found in embedded as clasts in younger rocks, provide insights into the early geological processes.
Origin of Life
While evidence for life during the Hadean is limited, some studies suggest that life could have emerged as early as this period. Some organic molecules might have formed in the primordial oceans, or been brought to Earth in comets and meteorites, setting the stage for the later evolution of life. The Archean, the next Eon, is, however, partly defined as the time when cellular life formed.