Geoscience Reference
In-Depth Information
zircon crystals from Western Australia, which have been dated as up to 4.4 Ga old.
These crystals are held in a younger (but still early Archaean) meta-sedimentary
rock. A handful of zircon does not make a continent, but this material and the 3.8-
Ga rocks suggest that some sort of continent was in existence at that time. Isotopic
evidence from the zircons suggests that subduction occurred, and deep oceans
existed, even in the Hadean. The cratons which form the cores of the present con-
tinents are, for the most part, rafts of Archaean granitoids and gneisses, formed
in a complex assortment of events from 3.5 to 2.7 Ga. Infolded into the granitoid
gneiss cratons are belts of supracrustal lavas and sediment, including komatiitic
lavas. These are highly magnesian lavas, formed from melts with up to 29%
MgO. Experimental melting has shown that, if dry, such lavas must have erupted
at higher temperatures than did modern basalts. Young (less than 100 Ma old)
komatiite does occur, with MgO content about 20%, but it is very rare. To produce
such hot lavas in abundance, the mantle may have been hotter in the Archaean
than it is today. It is possible that in the Hadean and Archaean some plume-derived
lavas arrived at the surface at temperatures as high as 1580
◦
C, implying temper-
atures of 1800-1900
◦
Cormore at their source.
Various questions can be asked about the Archaean Earth. What was the con-
tinental crust like? Could plate tectonics have operated in the Archaean? Was
there oceanic crust, and, if so, what was it like?
10.5.1 Archaean continental crust
Two tectonic accidents have resulted in exposures of Archaean crust. The Vre-
defort Dome in South Africa is a structure some 50 km in diameter in which a
section of the Archaean crust aged 3.0-3.8 Ga is exposed. The Dome is thought
to have formed at about 2.0 Ga as the result of deformation from within the Earth,
Figure 10.62.
A
geological cross section
across the Vredefort
Dome structure in South
Africa. OGG, outer granite
gneiss; ILG, Inlandsee
Leeucogranofels felsic
rocks. (From Nicolaysen
et al
.(1981).)
