Re: Confused
"how they can be the earliest?"
I think what was meant was that they are (amongst) the earliest rocks for which we have evidence i.e. they still exist - any older rocks have been entirely reprocessed by erosion or complete remelting deep inside the Earth and no longer exist in any identifiable form.
The Acasta Gneisses are not quite reckoned to be the oldest rocks - at ~4.4 Gy old zircons from the Jack Hills in Australia are older, but they are believed to be the oldest exposed rocks.
What bothers me about the hypotheses is that metamorphic rocks are formed under both temperature and pressure, with pressure seeming to play a greater part - Wikipedia says that the temperatures just need to be greater than 150-200C (the original rock doesn't need to be remelted to be transformed to metamorphic rock) but the pressures need to be greater than 100 megapascals (1,000 bar). Now whilst a meteorite impact will create great pressure, it will be in the form of a brief shock wave, which will have more of a brisant shattering effect than a compressing effect, and indeed, it is these shattering effects, such as 'shatter cones' and 'shocked quartz' that are regarded as proof of an impact.
Another problem is that whilst an impact event could certainly produce temperatures high enough to remelt surface rocks it would also mix them all up in that melting but all the pics of Acasta Gneiss that I've seen show some banding, which suggests that they weren't mixed up - the stratification appears to have been preserved.
Biting the hand that feeds IT
