When the eon name “Hadean” was coined in the 1970s, most geologists believed that no rock formations existed from this time and that continental accretion had not yet started. It was believed that the first rocks on Earth formed during the Archean Eon – between 3.9 and 2.5 billion years ago. However, recent discoveries suggest that rock materials exist today that are older than 3.9 billion years and do in fact originate from the Hadean Eon.
Western Australia's Jack Hills
The oldest terrestrial material on Earth can be found in rock formations of the Jack Hills in Western Australia. Within an 80-kilometer band of dark-colored material composed of sedimentary and metamorphic rocks are zircon crystals that have been dated to be about 4.4 billion years old. At this age, the implications are that these zircon crystals mineralized only a few hundred million years after the formation of the Earth. If conditions on the Earth at that time allowed for the mineralization of these crystals, then perhaps other minerals, and therefore whole rock formations, could have been created as well.
Zircon crystals are extremely durable and can therefore survive billions of years’ worth of weathering and erosion, as they have in the Jack Hills. Even after they are recycled through multiple periods of sedimentary and metamorphic rock-forming processes, they are able to tell geologists about the physical and chemical conditions under which they formed. For example, another notable finding that parallels the discovery of these ancient Hadean zircon crystals is that there may have been more water present on the surface of the Earth than previous studies suggested. Geochemical analysis of these crystals implies that mineralization occurred in the presence of great amounts of water, perhaps even an ocean.
Canada's Acasta Gneiss
The Acasta gneiss is a metamorphic rock formation exposed in Northwest Territories, Canada. Radiometric dating of this rock formation gives it an age of just over 4 billion years old. Gneiss is a type of metamorphic rock that forms as a result of granite – an intrusive igneous rock of felsic composition – being deformed and partially re-crystallized under conditions of great heat and pressure. Granite forms as a result of a magma body slowing rising upwards towards the surface and slowly cooling to form continental crust. Additionally, the magma material that forms granite originates from the complete melting of basalt rock, which is only possible in the presence of water since water lowers the melting temperature of rock enough that magma can actually completely melt it. So, the Acasta gneiss tells geologists that both granitic continents and surface water existed during the Hadean Eon.
Canada's Nuvvuagittuq Greenstone Belt
The Nuvvuagittuq greenstone belt is located in northern Quebec, Canada and is also a metamorphic rock. Greenstone forms as a result of metamorphosed volcanic basalts – an extrusive igneous rock of mafic composition. Isotopic analysis of this rock formation gives an age of 4.28 billion years. This tells geologists that as far back as 4.28 billion years ago, well within the Hadean Eon, magma was cooling to shape crust in the form of solidified volcanic rocks. This crust didn’t last long, as it was obviously reburied to form its current metamorphic greenstone state; but the fact remains that solidification of magma to form crust did occur during the Hadean and that remnants of this crust are exposed on the surface of the Earth today.
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Source:
NASA's Earth Observatory Website - Western Australia's Jack Hills
O'Neil, J; Carlson, Rw; Francis, D; Stevenson, Rk (Sep 2008). "Neodymium-142 evidence for Hadean mafic crust". Science (New York, N.Y.) 321 (5897): 1828–31.
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