Science & Geological

The basic timeline of a 4.6 billion year old Earth, with approximate dates:

How is the Acasta Gneiss collected?

To collect pieces of this rare rock, a flight must be chartered from the city of Yellowknife to a small outcrop approximately 300km north of the city.  Flights leading to the small, remote island can be complicated by extreme weather conditions such as choppy waters, severe wind, blizzards, and low visibility and chartered flights are not always able to land.  Once arriving at the site, one will first notice a metal silo and once on shore, a closer inspection reveals the title Acasta City Hall, established 4.03 Ga (billion years ago), which is a building used for research expeditions and student field trips.  From this site, Rock of Ages’s owner Mark Brown and others are able to collect less than 800 pounds of rock each trip from the Rock of Ages claim.  Due to expensive nature of collecting, chartered flights to the Rock of Ages occur every other year or sometimes less.

The geological specimen that we get or rocks from contains the mineral zircon, which has been age dated at 3.962 billion years. Each and every piece of this rock came from the site where the world’s oldest known intact crustal rock was discovered, on an island in the Acasta River, Nortwest Territories, Canada.

Discovery

Credit for this find belongs to Dr. Janet King, who is now Assistant Deputy Minister at Northern Affairs Organization in Ottawa was then working with the Geological Survey of Canada. She sent the samples to her research partner Dr. Sam Bowring and a team of geologists from the Department of Earth and Planatary Sciences, Washington University of St. Louis. Missouri, the NWT Geological Division of the Department of Northern Development (DIAND) and the Geological Survey of Canada (GSC). This research was also supported by NASA and the U.S. National Science Foundation. The age dating was done by Bowring, I.S. Williams, and W. Compston at the Research school of Earth Sciences, Australian National University, Canberra Australia. The mineral rights to this area were staked November, 24, 1989 by Yellowknife prospector Walt Humphries and B.E. Weir P.GEOL.

Geology

Without getting into a lot of complex geology and mineralogy, the planet Earth is currently thought to be approximately 4.6 Billion years old. An early crust formed and a few microscopic fragments dating as far back as 4.1 billion years have been found but never an outcropping 4.03 billion years ago, part of the crust was metamorphosed into this Tonalite Gneiss, making it the oldest known rock in the world. Prior to this discovery the oldest known rock was at Isukasia, Greenland, dated at 3.8 billion year, and the oldest known rock in North America, at 3.4 billion years came from the Minnesota River valley in the U.S.A. The discovery of this rock is an important scientific-geological find and from now on most textbooks will refer to the 4.03 billion year old Acasta River Gneiss, Northwest Territories, Canada.

The ca. 4.0 Ga Acasta Gneisses, located on a series of islands and adjacent mainland along the Acasta River on NTS map sheet 86G/04 (approximate coordinates 65 10’N, 115 34’ W), represents the oldest known intact fragment of continental crust on Earth (Bowring et al., 1989a; Bowring and Housh, 1995; Bleeker and Stern, 1997; lizuka et al., 2006, 2007). Attention was first drawn to these rocks when Bowring et al. (1989a) published their landmark paper on the ca. 3.96 Ga U-Pb age of zircons from the site. Since then, a number of research teams have visited and sampled the rocks and refined the radiometric age data. Recently, lizuka et al. (2006) published the first, albeit indirect, evidence for even older, ca 4.2 Ga crust in this region. The Acasta gneiss site represents a unique and finite scientific resource, and also highlights the geology of the NWT, and specifically Wek’eezhii (tlicho Settlement Area) from a tourist and cultural perspective. In addition, public interest in owning a piece of the Earth’s oldest rocks has sustained modest businesses selling pieces for souvenirs (e.g., http://rockofagesnwt.com/)

Relf, C. and Ketchum, J., 2008. Non-renewable Resource Assessment (Phase 1), Acasta Gneiss Candidate Protected Area, Northwest Territories, Canada; Northwest Territories Geoscience Office, NWT Open File 2008-05, 26p.

Gneiss

This is the generic name of a class of rocks geologically defined as: a coarse grained rock in which bands rich in granular mineral alternate with bands in which schistose minerals predominate.

Tonalite

There are many types of gneiss but the Tonaly Pass in Italy was where the first Quartz-Diorite Gneiss of this type was discovered and described, hence the name Tonalite Gneiss.

Mineral Content

The oldest known rock in the world contains the following minerals:

ZIRCONS- ZrSiO4 in this case, less than 1% of the rock is composed of micro zircons and these were formed when the rock originally metamorphosed into a Tonalite Gneiss 4.03 billion year ago. They also grew slightly at latter dates of metamorphism (4.03 billion year ago).

Atoms and molecules have a certain size and shape, and like pieces of a jigsaw puzzle, tend to form crystals of a certain shape. As the crystals grow, atoms of uranium are a close enough fit to become locked inside of the crystal, while lead is completely excluded at this stage. However, uranium slowly deteriorates into thorium slowly deteriorates into thorium and thorium slowly deteriorates into lead. By cleaving the zircon crystal and analyzing the ratio of uranium to lead, the age of the rock can be calculated.

Quartz-Si02 is the greyish-white mineral.

Plagioclase Feldspars - they vary in composition between NaA1Si3O6 and CaA1Si2O6. These are milky white to orange minerals, often seen in seams. A small amount of red potash feldspar is present in some samples.

Biotite – (black mica) K(MgFe)3(A1SiO10)OH2 is the shiny black mineral.

Hornblende – Ca2Na(Mg, Fe”)4(A1,Fe”’,Ti)3S8O22(O,)H)2 is the dark (black) mineral.

Allanite, Sphene, Garnet, Magnetite, Pyrite, Chalcopyrite, Sphalerite, and a whole range of trace minerals may be present. This rock was, after all formed by the planet’s primordial geological stew.