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 Earth’s 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 system’s 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.

Phase I:

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 events

  1. Formation and contraction of the original solar nebula, probably due to shock waves from a nearby supernova (exploding star).
  2. Collapse of the solar nebula into a rotating disk with the majority of mass in the form of hydrogen gas concentrated in the center, forming the protosun.
  3. Formation of dust-sized particles of differing composition. Clumping of particles into larger and larger sizes, forming a range of objects from meteoroids to planetesimals.
  4. Fusion ignition of the Sun followed by 1 my period of violent solar activity. Solar winds sweep lighter materials (H, He, H2O, Ammonia, etc) outward from the Sun, leaving the inner solar system enriched in refractory materials such as silica and iron.
  5. Initial formation of Jupiter near "snow line" at 4 AU. Jupiter’s large mass and high gravity attracted much of the material available from its region of the solar nebula. This left the asteroid belt too depleted in mass to form a planet and resulted in a relatively small mass for the planet Mars. Jupiter’s high mass also makes it a magnet for comets and asteroids, sweeping them up or slinging them out of the solar system. It has been estimated that without Jupiter the frequency of impact between asteroids and comets and the Earth would have been 1000 times greater.
  6. Accretion of remaining planets from planetesimals. Most material swept up by the four inner planets. Major collisions between planets and large planetesimals result in formation of the Earth’s moon (collision with a Mars-sized impactor), loss of much of Mercury’s mantle, and reversal of Venus’ direction of rotation.
  7. Heavy meteoric bombardment - studies of the surface of the moon, mercury, and other planetary bodies reveals that for several hundred million years after the formation of the solar system the planets were continuously bombarded by meteoric debris. Thus the surface of the Earth was probably repeatedly impacted and perhaps remelted by the impacts of large asteroids. This early bombardment continued until about 3.8 billion years ago.

 

Major Phase II events:

  1. Differentiation and cooling of the crust - The earliest Earth was probably very hot due to the release of kinetic energy during accretion, the decay of radioactive elements in its interior, and the collision that formed the moon. Partial melting of the Earth’s interior allowed dense iron to sink to center, forming a core and light, silicate-rich magma to rise to the surface to form a magma ocean. The remaining material between the core and the magma ocean formed the mantle.
  2. 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 Earth’s initial crust was remelted several times due to impacts with large asteroids.

  3. Outgassing of the initial atmosphere and formation of the oceans - partial melting and differentiation of the Earth would have also allowed the release of gaseous compounds formed and trapped in the interior. Modern volcanoes release gases as magma is brought to the surface. These gasses give us an indication of the composition of the Earth’s earliest atmosphere: water vapor, CO2, CO, N2, H2, and hydrogen chloride. Water vapor would have condensed in the atmosphere and rained down as liquid on the surface, covering the Earth with water.
  4. 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 Earth’s 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.

  5. Stabilization of the crust and initiation of plate tectonics - as soon as the crust became cool enough not to remelt plate tectonics must have begun. Initially, because the Earth was much hotter than it was today, more heat would have been flowing up from the mantle. This would have created numerous hot spots and rifts, resulting in many small plates and subduction zones.
  6. Differentiation of the earliest felsic crust (first continents) - Remelting of oceanic crust combined with water along subduction zones would have caused the formation of felsic magmas and the resulting island arcs. Also, remelting of crust over large hot spots might also have created felsic magmas (such felsic magmas are seen erupting from beneath Iceland today). In any case, the first continents were produced as small land masses that eventually accreted together as plates were subducted bringing the protocontinents into collision.

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.