Lecture notes
The Hadean
- Module
- Life On Earth (GS10720)
- Institution
- Aberystwyth University (UWA)
These are notes on the hadean - they are for lecture 1
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The first geological eon that is widely accepted by scientists is the Hadean Eon, this begins with Earth’s formation(4.54bya) and ends with the start of the Archean (4bya). Very little is known about this eon due to the scarcity of
Hadean rocks and zircons. The largest collection of zircons are found in Western Australia, scientists often argue
about the geophysical state of the Earth at this time, although most now agree that plate tectonics probably began
at this time, The Earth would have had a reddish atmosphere due to the presence of Carbon Dioxide, Methane and
the lack of Oxygen present.
The name Hadean was taken from Hades, who was the Greek God of the underworld and sometimes the underworld
itself, and it accurately represents the hellish conditions that would have been present at this time, the name was
first used by American Geologist Dr Preston Cloud. Older literature will sometimes refer to it as the Pre-Archean.
The oldest zircons ever discovered were found inside metamorphised sandstone in the Narryer Gneiss of Terrane in
the Jack Hills of Western Australia, that have been dated to around 4.4bya, although this one is unusual with most
ancient zircons being date to around 4.35bya, which is about 200mya after the Earth formed. There are some
xenocrysts that have been found enclosed in older rocks, suggesting that younger rocks had formed on older rocks.
One example is the Guiana shield.
There would have been some water present on the planet very soon after it’s creating, much of which would have
come from the atoms that were present when it formed. The Molecules would have escape easily when the Earth
was much smaller prior to the collision with Theia, with Hydrogen and Helium escaping constantly (even in the
present era). Much of the water originally present on the Earth would have likely been vaporised during the impact,
leaving just the hot volatiles and creating an atmosphere chocked with heavy CO2, H, and water vapour levels, with
the atmosphere pressure being up to x27 greater than today.
Scientists looking at the zircons that have been found, have discovered the liquid water might have been present as
early as 4.4bya, only 1mya after the Earth first formed.
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