Friday Mountain Blogging: Paleotopography of the Sierra Nevada
No one quite knows how old these hills are. We know how old the rocks are, of course – it’s easy (or at least straightforward) to grab a chunk of granite, crush it up, and throw it in your handy mass spectrometer for a radiometric date. When dinosaurs roamed the earth 100 million years ago, these rocks were coagulating in a magma chamber several miles below. But when did they reach the surface? And how? And why?
The why of the Sierra Nevada is under particular scrutiny these days because they might represent a previously unconsidered kind of tectonic event: a mountain range’s bottom falling off. Large granite bodies like the Sierras are born with a sort of evil twin attached: a body composed of gabbro or eclogite, full of heavy elements like iron and magnesium. This “evil” twin is much heavier than the Earth’s mantle, while granite, the “good” twin, is lighter; joined together, they would sit in the Earth like a boat weighed down with cargo, the top just barely above water. About 3 million years ago, the heavy twin may have detached from the Sierra granite and fallen down into the mantle; if this is true, we would expect that the mountains rose up in response, like a hot air balloon losing ballast.
But an article by Andreas Mulch et al. in this week’s issue of Science places some significant constraints on how fast, and how much, the mountains could have risen in response to this event. Mulch and his co-authors looked at bits of 40-50 million year old clay, formed by weathering granites in the ancient course of the Yuba river. This weathering process takes water from the environment and locks it in to the clay mineral’s crystal structure, so the clay provides a record of the composition of water at various points in the Sierra Nevada of 40-50 million years ago. The water composition, in turn, is correlated with elevation. Their results suggest that the mountains 50 million years ago were about 2 kilometers tall, perhaps half a kilometer shorter than they are today.
Half a kilometer of mountain isn’t anything to sneeze at, but considering that others have proposed a veritable rollercoaster ride – 2 km of uplift, followed by subsidence, followed by more uplift – it’s hard not to be disappointed.