[–] Diamond_Deluge 0 points 2 points 2 points (+2|-0) ago
and how do we know its 4b years old?
[–] rwbj 0 points 4 points 4 points (+4|-0) ago
Some minerals are the product of the radioactive decay of other minerals. For instance in really old rocks it's probably going to be uranium turning into lead. In a mineral where you can safely assume that any trace of one element is a product of the radioactive decay of another element you can simply take a sample of it, measure the ratio of the elements and determine its age to a phenomenally high degree of accuracy. For uranium lead dating - the accuracy is higher than 99%. And it doesn't have to be anything like pure uranium. Various minerals contain trace amounts of radioactive elements. Zircon and Uranium for instance. But that trace is far more than enough for dating.
[–] SaneGoatiSwear 0 points 2 points 2 points (+2|-0) ago
no one can fucking break this truth: rocks are fucking cool.
[–] DiscontentedMajority 0 points 12 points 12 points (+12|-0) ago
So for the entire time that life has exited on this planet with entire species rising and dying out, this thing has just sat there unchanging, being a rock.
Kind of cool.
[–] oedipusaurus_rex 0 points 4 points 4 points (+4|-0) ago
It's been changing. Gneiss is the most heavily metamorphosed rock. It's been buried deep enough and heated enough that it started flowing as a solid.
I'm jealous of OP. Gneiss is one of my favorite rocks, and I've been wanting a piece of the Acasta for a while now.