The World's Oldest Rock is in Canada!
The Earth is extremely old (4.5 to 4.6 Billion years old), and there is a healthy rivalry between countries as to who has the oldest rock. In the last two posts, I talked about how "granite" countertops are mainly mined from the cratons of continents. Cratons are the oldest and coldest rocks on the surface of the planet and form the cores of every continent. The rocks that make up cratons are older than 1 billion years, and so there are there lots of contestants in the "Who has the oldest rock?" competition. All countries with large cratons are in the running, but the two leaders are Canada and Australia, and Canada is the winner (with a caveat (see below)).
Earth's age is based on a comparison of rocks from the moon and from meteorites that have a similar chemical composition. These rocks have been dated using zircon, which are tiny little crystals that crystalize as magma (molten rock) cools. Zircon contain uranium (U) but not lead (Pb). Uranium is radioactive and breaks down to form lead, and this reaction releases energy. That is why uranium is used in nuclear reactors and bombs. In Zircon crystals, uranium makes a fantastic "clock" for determining the age at which the zircon (and host rock) formed.
What is the oldest rock on Earth?
The oldest rock on Earth is the Acasta Gneiss, which is situated about 300 km north of Yellowknife. There are numerous tours that will take you there (1). Interestingly, the total outcrop area for the Acasta Gneiss is less than 25 km2. The rock has been metamorphosed, which has altered it's appearance but did not melt it, and so we know the original rock is granite. The zircon in the Acasta Gneiss also preserve the original age of the rock, which is 3.96 to 4.055 (average: 4.03) billion years old depending on the study (2,3). Lots of geologists have worked on the Acasta Gneiss because it is Earth's oldest rock, and it is even thought that the original granite material crystallized from magma that began to form 4.3 billion years ago (3,4)!
If you do visit the Acasta Gneiss, however, don't expected to be overwhelmed. It is a cool looking rock for sure, but it looks like a lot of other granite-turned-gneiss rocks. It would make for a really nice countertop!
Our biggest rival in the oldest rock competition
While the Acasta Gneiss is the undisputed oldest rock on Earth, the oldest zircon are from Australia. In the Jack Hills in Western Australia, zircon have been dated to 4.36 billion years ago, making them the oldest zircon on Earth (5). These zircon are referred to as "detrital zircon". Detrital zircon are found in sedimentary rocks, but they do not record the age of the sedimentary rock. Instead they record the age of the original (igneous) rock that crystallized from magma. The Jack Hills sedimentary rocks are also really old, but not as old as the Acasta Gneiss, so Canada wins!
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2) Mojzsis, S. J., Cates, N. L., Caro, G., Trail, D., Abramov, O., Guitreau, M., Blichert-Toft, J., Hopkins, M. D., and Bleeker, W., 2014, Component geochronology in the polyphase ca. 3920 Ma Acasta Gneiss: Geochimica et Cosmochimica Acta, v. 133, p. 68-96.
3) Orwig, J. (2014), Formation process of Archean rocks in the Acasta Gneiss Complex, Eos, 95, doi:10.1029/2014EO021275.
4) Roth, A. S. G., Bourdon, B., Mojzsis, S. J., Rudge, J. F., Guitreau, M., and Blichert-Toft, J., 2014, Combined147,146Sm-143,142Nd constraints on the longevity and residence time of early terrestrial crust: Geochemistry, Geophysics, Geosystems, v. 15, no. 6, p. 2329-2345.