When it comes to expensive rocks, one usually thinks only of meteorites, diamonds, platinum, and gold. Nothing could be farther from the truth! Some rocks come from remote locations, where treacherous terrain, bitter cold, and heat are experienced by intrepid geologists searching for geologically specific formations. Such is the story for Acasta gneiss, the Nuvvuagittuq Greenstone Belt rocks, Isua banded iron, and rocks containing one of the earliest biological activity on Earth - Apex chert.
These rocks are sold by the gram, as they did not just impact Earth for easier pickings but have been tortured by primeval atmospheres, blistering climate changes, and tectonic activity. These specimens are from isolated "geologic islands," time capsules we can hold in our hands.
Did you know that the Acasta gneiss, a stunning metamorphic rock found near the Great Slave Lake in northern Canada, holds the title of being the oldest rock on Earth? Its age is only surpassed by a few hundred million years by the Jack Hill rocks, which contain zircon minerals from Australia. These remarkable rocks represent the very beginning of our Earth-based geologic time scale.
The first rocks to provide glimpses of the light of day are the Isua supracrustals found in southwestern Greenland and the Nuvvuagittuq "faux amphibolite" greenstone belt rocks from Canada. These rocks formed about 3.8 billion years ago when the asteroids and comets' early bombardment period finally diminished. The Isua sequence includes sedimentary rocks, records of erosion and deposition by surface water, and even recognizable "pillow" basalts containing signatures of submarine eruptions. These rocks are terrific signposts, guiding our way along a steep path back into geologic time!
These Archean rocks are not the common calcite, geodes, or tourmaline but the absolute prizewinners in geologic time. These specimens are the ones that make any geological collection out-rank any others!
Take the journey back to the Archean Eon, when Earth days were about a mere 18 hours long, the Sun was much younger, tides more aggressive, and the ocean-atmosphere much more dynamic with these dynamic specimens!
Photo courtesy: Wikipedia, NASA By NASA - http://astrobiology.nasa.gov/nai/library-of-resources/annual-reports/2008/cub/projects/sulfur-biogeochemistry-of-the-early-earth/, Public Domain, https://commons.wikimedia.org/w/index.php?curid=11840173