GEOL 102 Historical Geology

Spring Semester 2018
TIntroduction to the Strange Aeons of the Precambrian: The Hadean Eon

Earth duing the Late Heavy Bombardment, by NASA Goddard Space Flight Center

That is not dead which can eternal lie
And with strange æons even death may die
-- "The Call of Cthulhu" (1928), H.P. Lovecraft

Precambrian: general term for all time before the Phanerozoic Eon.
Other names for Precambrian: Azoic (lifeless) or Cryptozoic (hidden life).

Special terminology for Precambrian rock exposures and distributions:

Standard international stratigraphic nomenclature does not recognize a formal "Precambrian". Instead, uses three divisions: the Hadean (of Hades), Archean (ancient), and Proterozoic (earliest life) Eons.

Before The Hadean Eon: >4.56 Ga
Formation of Proto-Earth (& the Rest of the Solar System): Solar system coalesces out of remains of dust clouds produced by earlier generations of supernovae. Particles clump together to form planetessimals, which are comprised of various combinations of:

As these lump together, they become more massive and exert more gravitational pull, attracting more material, becoming more massive, exerting more gravitational pull, etc. Soon the vast majority of this material is clumped into the various large bodies of the Solar System, with a tiny fraction left over to form today's asteroids, comets, and Kuiper Belt Objects. The Proto-Earth (called "Tellus" by some) is one of the largest bodies in the interior part of the Solar System.

A Star Is Born: However, the VAST majority of the Solar System's mass is in the Sun. Gravitational forces of this mass starts fusing hydrogen into helium, and the Sun ignites. Originally it was dimmer than today, but still it shone. Solar wind and photon pressure began to sweep the Solar System clear of much debris.

Hadean Eon:

The Iron Catastrophe: Although the Earth is not a star, it (or at least the Proto-Earth) too went through a profound change in its earliest history. Originally the ices, organics, silicates, and metals would have been randomly mixed together, but a combination of gravitational heating, heating from impacts of planetessimals, and radioactive heating from some of the isotopes found in the silicates and metals caused the material of Proto-Earth to melt. The heaviest materials (mostly metals, especially iron and nickel) sank to the interior to form the core; the silicates "floated" on top to form the mantle. Ices melted into gasses and liquids, and they and organics got mixed in with other material. With a metallic core, Earth now had a magnetosphere.

The Primordial Atmosphere: Proto-Earth's original atmosphere would have mostly been hydrogen and helium, like the gas giants. However, it lost much of this atmosphere to a number of factors:

Bad Moon Rising: About 4.510 Ga, a Mars-sized planet (called "Theia" by some) collided into the Proto-Earth ("Tellus"). Theia and a substantial fraction of the Earth's mantle was vaporized. The matter that remained in orbit around the Earth coalesced to form the Moon; the rest rained back down to the surface of the Earth. (So the matter of Proto-Earth + Theia became redistributed to form today's Earth + Moon). (It is worth noting that recent studies suggest that instead of a single "Big Thwack", multiple large planetary bodies may have smashed into Tellus in a geologically short period of time.)

As a result of the birth of the moon:

Here's a video with an overview of the rest of the course, entitled "A Brief History of Geologic Time":

To Lecture Notes.

Last modified: 7 March 2018

Detail from "Formation of Earth's Continents" (1952) by Chesley Bonestall, from Life Magazine's "The World We Live In" series