Looking at the oldest rocks of any region requires that we free ourselves of the present-day geography. In the case of the Colorado Plateau, the landscape that existed 1.7 billion years ago was very far removed from the relatively stable arrangement of horizontal sedimentary rocks and gently faulted and folded monoclines and upwarps that exist today. It was not at the same latitude and longitude, possibly not even in the same hemisphere. The land we now know as the Plateau was connected to a large landmass that today is Australia, Antarctica or Siberia. How could everything be so completely different? Amazingly enough, movements of 2-3 inches a year, added up over millions and billions of years, are more than enough to account for the huge changes we see in the rocks. Entire continents move laterally, collide, split up, and reconfigure the world's geography on a constant basis.The oldest rocks of the Colorado Plateau are much changed from their time of origin. The original sandstone, shale and volcanic rocks have been contorted by heat and pressure brought on by burial deep in the crust, as much as five miles or more. They may date from hundreds of millions of years before the time they became metamorphic, perhaps 2.0-2.2 billion years before the present time. Very little of the original textures can be detected in these metamorphic rocks. Still, we can make some reasonably accurate conjectures about how these rocks formed.
As a rule, the conditions that form schist or gneiss require a major mountain-building event, such as the collision of major continents (like India and Asia today), or by the collision of a terrane with the edge of a continent (such events have happened in Alaska and Indonesia). The best evidence, illustrated very nicely in the Blakey reconstructions linked below, shows a regional continental arrangement somewhat similar to the situation that exists today in southeast Asia. North America was a smaller continent, and only a small corner of California existed as dry land. Two island masses, the Yavapai and Mazatzal islands were moving north towards the continent. They made contact, one after the other, about 1,700 million years ago, causing an extensive mountain range to rise. The range extended through New Mexico, Arizona, California, and an unknown distance west. We can't say for sure how tall the mountains were, perhaps 10-20 thousand feet, but we can say that they were absolutely barren of life: only rock, gravel and maybe snow. Even in the seas, nothing above the level of single-celled life existed.
The mountains rose to the sky, the mountains eroded. And eroded. Over several hundred million years, rock was removed until nothing remained but a nearly flat plain near sea level. Whatever else happened in this interval is lost to us forever, for erosion, by definition, removes information. But the roots of the mountains still exist for us to study, in the Inner Gorge of the Grand Canyon, at Frenchman's Mountain outside Las Vegas, in the Black Mountains of Death Valley, on the Uncompahgre Uplift in Colorado (and Unaweep Canyon), at Colorado National Monument and Black Canyon of the Gunnison National Park, and at other sites scattered in the deepest canyons on the Plateau and around the Basin and Range, especially in southern Arizona.
The rocks, in my humble opinion, are some of the most beautiful one can find. The biotite and muscovite mica shines like glitter, the black and white layers form incredible folds and crinkles (crinkles, there's a geological term for you), and the occasionally there are the bright greens of epidote and the red-browns of garnet. It is humbling to hold rocks from a different time and place far changed from the world we know today.
Paleogeographic maps from Ronald Blakey at Northern Arizona University
No comments:
Post a Comment