Young rocks

Evy, the skepchick who is now geoblogging at georneys, recently expressed surprise at a paper which labeled rocks with an nine digit age as young. I see nothing wrong with this, however. Depending on how you define 'beginning', the Earth is between 4567 and 4460 million years old. using 4500 as a convenient round number, we can then take half that as a midpoint, and place the old rock / young rock divide at 2250 MA. Alternatively, we could lash out and use 1.5 billion year divisions to call rocks old (> 3.0 Ga), middle-aged (between 3 and 1.5) and young < 1.5 billion.

In Gondwanaland, the 1.2-1.0 Ma Grenville orogeny is much less prevalent than it is in laurentian rocks. It shows up here and there, but is not a major event. The middle proterozoic was in fact fairly quiet in Gondwana, with the Neoproterozoic-to-Cambrian Pan-African / Brasiliano / Ross orogens being the defining tectonic events that assembled the various pieces of Gondwanaland into the supercontinent that we all know and love. So for those of us interested in the pre-assumblage history of Gondwana, it makes perfect sense to describe anything after the midproterozoic as young. I even did so in my thesis, in this sketchmap of the geology of the Brazilian state of Bahia: