One of the things I did over the past few weeks was travel down to the Washington D.C. area to visit family. We went into the city to visit the Smithsonian Natural History museum and I was very excited to see a sample of the Acasta Gneiss I could actually touch!
Here's the picture with my cell phone (didn't have a good camera on me at the time).
"OK, Steve," I'm sure you're wondering, "why do you have a hard-on for some stupid rock?" BECAUSE IT'S 4 BILLION YEARS OLD! It's the oldest known rock on Earth. I'm sorry, that's fucking cool!
It's a gneiss which means it's a high-grade metamorphic rock. That means it was formed from an older preexisting rock that was altered by very high temperatures and pressures. The gneiss was derived from the metamorphism of an igneous rock called tonalite (similar to granite but lacking K-feldspar). Crystals of the mineral zircon (ZrSiO4) within the Acasta Gneiss record the age of that earlier igneous intrusion as 4.2 billion years ago. Given that the Earth formed around 4.6 billion, that's old!
This was a period of time know as the Hadean Eon - derived from the Greek word hades or Hell. The Earth truly was a hellish place back then - no life, heavy meteorite bombardment, massive tides from a closer Moon, acidic ocean, poisonous (for us) atmosphere, and more volcanic activity. Almost nothing from that time has been preserved due to the Earth's active surface processes (although we have older meteorites and Moon rocks).
Wish I could have a piece of the Acasta Gneiss for my bookshelf but it's found way up in what geologists call the Slave Province about 220 miles north of Yellowknife in the Northwest Territories (click here for a Flash Earth view of this remote area along the Acasta River). That's a long way for a field trip.