Resembling a human profile staring enigmatically up into the sky, this peak in the Grand Teton National Park in Wyoming also testifies to a very old piece of the Earth Story. While the geological history of the region is deep and complex, and far too long for a single post, these rocks are some of the oldest in any US national park, now exposed in some of the country’s youngest mountains.
The dark rocks in the picture are Archaean (around 2.7 billion years old) gneisses, repeatedly squished and baked during the ever rolling rock cycle. They were initially deposited as marine sediments such as limey ooze, sand, silt and clays interbedded with volcanic rocks (geochemistry suggests from a volcanic island arc like modern Japan), maybe around 3 billion years back.
The newly minted limestones, sandstones and shales were then buried by younger rocks up to a depth ranging between 8 and 16km when they were caught up in a mountain building event (called an orogeny) and metamorphosed into completely different minerals, though enough sedimentary structures and original chemistry survived to tease their origin out by a combination of field and laboratory work.
The lighter rocks are only a bit younger, at a mere 2.55 billion years old, and consist of granites, born of a magma that intruded the hard crystalline gneisses. Slabs of dark gneiss can be seen rafted in the lighter granite, or separated by huge veins of it, giving us a visual idea of how things might have been deep in the crust when these events happen.
Granites poke into other rocks in a variety of ways, sometimes uplifting the whole stack above it in one piece, other times following faults and lines of weakness as veins and dykes, and sometimes eating away at the older rocks (referred to as country rocks) from below. The presence of granite above and below huge slabs on gneiss suggest that the last option was at least a part of what happened here, while the veins between the slabs suggest the second also joined in. The issue of how granites make room for themselves as they rise in the crust is still a matter of vigorous debate.
A long series of adventures followed after these rocks were first deposited as marine sediments, metamorphosed and baked again by granitic magma. A gap of maybe 700 millon years occurred before anything that has survived the vagaries of deep time was deposited on top in the Paleozoic aeon. Every subsequent era save the Silurian is represented in the region (usually at the margins of the mountain range), testimony to a diverse history, alternating between terrestrial and marine rocks with occasional volcanic paroxysms.
Around 9 million years ago, the latest part of the great orogeny that is forming the peaks of the American west started to uplift the Teton rocks. At Mt Woodring, all the younger rocks were stripped off by erosion, as ice sheets oscillated back and forth over them during the last couple of million years, exhuming these relics of the time when earth was a low oxygen world, inhabited by nothing more complex than bacterial mats.
Image credit: Acroterion