The Archean Eon is the oldest unit on the geologic time scale. It began 4.6 billion years ago and ended 2.5 billion years ago. It lasted for 2.1 billion years (2,100,000,000 years).
Earth's oldest rocks are found in Canada. They are about 4.04 billion years old.
But there are even older mineral grains. Sand-sized zircon grains in metamorphosed sedimentary rocks from Australia are 4.4 billion years old.
There are no rocks on Earth that date back to 4.6 billion years ago because the Earth is geologically active, and the oldest rocks have been recycled by plate tectonics or by weathering and erosion as part of the rock cycle.
Much of our knowledge of the Earth's earliest history comes from indirect evidence - meteorites.
The Archean and Proterozoic Eons comprise the Precambrian, which spans 87% of the geologic time scale.
The origin, history, and characteristics of Planet Earth need to be considered in the context of the Universe.
The Universe hosts billions of galaxies. A galaxy is an aggregate of stars, planets, dust, and gases.
Whirlpool galaxy. Image courtesy of NASA and The Hubble Heritage Team (STScI/AURA) Acknowledgement: N. Scoville (Caltech) and T. Rector (NOAO)
Planet Earth orbits the Sun, a dwarf star, that belongs to the Milky Way galaxy.
The Sun and the planets, moons, asteroids, comets and other objects that orbit it, comprise the Solar System.
Schematic view of the Solar System.
The Earth is part of the Solar System, the Solar System is part of the Milky Way galaxy, and the Milky Way galaxy is part of the Universe.
The story of the origin and history of the Earth requires that the origin and history of the Universe and Solar System must be considered.
Evidence to be considered when interpreting the history of the Universe:
The Universe is expanding.
Everything began together at a point.
A big explosion occurred, which astronomers call the Big Bang.
This explosion caused everything in the Universe to begin moving rapidly apart.
In 1914, W.M. Slipher first noted that galaxies displayed the red shift.
Their light is shifted toward the red (or long wavelength) end of the spectrum.
Colors of the spectrum
R O Y   G   B I V
Red = long wavelengths
Blue = short wavelengths
(Microwaves and radio waves are longer than visible
Ultraviolet and X-rays are shorter than visible.)
The spectrum of a star reveals:
Light reaching us from distant receding galaxies has its absorption lines
shifted toward the red end of the spectrum.
This indicates that the galaxy is moving away from the Earth.
The red shift indicates that the universe is expanding.
A similar phenomen, except related to sound, is the Doppler Effect.
Calculations indicate that the Big Bang occurred 18-15 billion years ago.
The Big Bang marked the instantaneous creation of all matter in the Universe.
Variations on the Big Bang theory:
October 25, 2005