Geol 02C Historical Geology
J Bret Bennington
The Hadean Eon
Currently, the oldest rock ever discovered and dated is metamorphic rock of the Acosta Formation from north-central Canada (3.8-4.0 Ga), although recycled grains of the mineral zircon from rocks of western Australia have been dated to 4.1-4.2 Ga, indicating that the Earths continental crust had begun to form at this time.
However, the age of the solar system, including the Earth, is probably about 4.6 Ga. This date comes from meteorites, the leftover debris from the solar systems formation, as well as some moon rocks that are believed to remain from the formation of the moon (compared to the Earth the Moon is a geologically dead world where new rock has not been created or destroyed for billions of years).
Therefore, there is a period of time, almost a billion years, for which we have little or no direct geological information left on Earth. This interval is sometimes referred to as the Hadean Eon.
The Hadean can be divided into two phases:
1. The initial accretion of the Earth from the solar nebula.
2. The stabilization of the young Earth.
What we think we know about the formation of the solar system comes from two types of studies.
First, using powerful instruments such as the Hubble Space Telescope we can peer out into the galaxy and look for stars like the sun that appear to be in the process of formation. Although we cannot watch an individual star evolve from a cloud of gas (called a nebula) we can study several stars that appear to be at different stages in the process. Recently, the HST has revealed a region of nebula in the belt of the Orion constellation that contains thousands of stars in different stages of formation - what astronomers call a "stellar nursery".
Second, we know a lot about the present composition of the solar system, including the composition, size, mass, and density of the planets. This information comes from physics calculations based on the orbits of the planets and the laws of gravity, from Earth-based telescopic and spectroscopic observations, and from measurements made by robotic space probes sent into the solar system. Any theory or model of solar system formation must be able to explain the present composition, size, and orbital features of the planets. The complex structure of the solar system presents a powerful test of any new theory.
Currently, planetary scientists believe that the initial formation of the solar system took a relatively short amount of time, about 100 million years.
Major Phase II events:
Eventually, the magma ocean would have cooled to form a layer of basaltic crust such as is present beneath the oceans today. Continental crust would form later. It is probable that the Earths initial crust was remelted several times due to impacts with large asteroids.
It is also possible that the Earth has acquired some of its water from comets colliding with the Earth and melting in the upper atmosphere. Recently, some astronomers have argued that as many as 15 million small comets (house-sized and smaller) might be adding water to the atmosphere every year. However, this view is still controversial and concrete evidence for the existence of these comets has not yet been found.
The Earths ancient atmosphere was probably highly enriched in CO2 - perhaps as much as 100 times the present amount. This may have been an important way the early Earth was warmed - astronomers theorize that the young sun was only 80% as bright as it is today, which would cause glacial conditions across the globe under our present atmosphere.
There was likely very little oxygen in the early atmosphere. Atmospheric oxygen appears to be primarily a product of photosynthesis produced by later evolving cyanobacteria and eventually plants.
The oldest evidence for felsic crust is found in grains of the mineral zircon, some of which have been dated to have formed app. 4.2 billion years ago - only 400 million years after the Earth formed.