Before the Beginning
When early Earth was cooling from its molten state, conditions would have been like hell on Earth – hence the term "hadean" for this earliest segment of the precambrian. Meteors rained down on the hot Earth, ultraviolet radiation was unchecked by an ozone belt, and volcanos belched fumes into a reducing atmosphere, which sparked with lightning. Life, of course, had not yet evolved, but it may have had an earlier start than thought previously (see below).
The term "hadean" was coined to designate the time before the earliest known rocks. However, rocks have been found that are older than the 4550-3800 Ma time-frame of the Hadean.
The oldest rocks so far discovered on Earth are:
- Jack Hills, Western Australia, a 4.4 Ga detrital zircon (sample W74) in the Jack Hills metaconglomerate, Eranondoo Hill. More at Earliest Piece of Earth
- The Acasta Gneisses near Canada’s Great Slave Lake (4.03 Ga)
- The Isua Supracrustal rocks of West Greenland (3.7 to 3.8 Ga)
- Northern Michigan (3.5-3.7 Ga)
- Swaziland (3.4-3.5 Ga)
“No known rocks have survived from the first 500 million years of Earth history, but studies of single zircons suggest that some continental crust formed as early as 4.4 Ga, 160 m.y. after accretion of the Earth, and that surface temperatures were low enough for liquid water. Surface temperatures are inferred from high d18O values of zircons. The range of d18O values is constant throughout the Archean (4.4-2.6 Ga) suggesting uniformity of processes and conditions. The hypothesis of a Cool Early Earth suggests long intervals of relatively temperate surface conditions from 4.4 to 4.0 Ga that were conducive to liquid-water oceans and possibly life. Meteorite impacts during this period may have been less frequent than previously thought.”A Cool Early EarthMore: Introduction to the Hadean / Palaeos Hadean: The Hadean Eon / Geol 2C Hadean lecture / Evolution: Change: Deep Time / Geologic Time: Age of the Earth /