Geologists’ Landscape

When I stand on my back porch and look toward Los Alamos, I see a low ridge before the Jémez composed of horizontal bands of color. Their intensity varies with weather and time of day.

Geologists saw those same layers as strata that revealed the enigmatic past of the earth. They encountered them everywhere - Grand Canyon was perhaps the most captivating.

Within the rocks they found plant and animal fossils. They correlated those between sites, and assigned rough dates to the layers. The diviners grouped these into larger time periods unified by climate: the ferns of the Carboniferous, the dinosaurs of the Jurassic, the glaciers of the Pleistocene.

Their view of the earth was nearly as static as that in Genesis. Some reconciled the third day when the earth was created with the millions of years they saw in rocks by saying both were true if you considered a day to be a metaphor for a complex reality.

When I was student at Michigan State, my geography instructor told us continents weren’t fixed, but were creeping shape shifters. Harm Jan de Blij was from South Africa, land of the great diamond mines. He showed us how South America nested into Africa, and said minerals found on one side of the break were also found on the other. Botanists and paleontologist saw the same continuities across oceans.

Since that time, Alfred Wenger’s theory of continental drift has been refined into plate tectonics. The static has become dynamic. Continents that once sat solidly on a molten interior of the planet, now float above it.

Continents began as much smaller pieces of crust that formed on the molten center described in the post for 28 April 2015. Cratons bumped into each forming supercontinents, and broke apart forming smaller continents and still smaller terranes.

Volcanos formed off the edges of large land masses. When island arcs became large, they were engulfed by their cratons, and new rims of volcanos erupted.

Some of the boundaries between crustal pieces became permanent. Others remained unstable, reemerging eons later.

The crustal mass that matters to us in the valley is the Wyoming Craton. Once it attached itself to the growing continental mass centered on what we once called the Laurentian shield some 1,860 million years ago, volcanic islands began forming along the southern edge. After the Yavapai province attached around 1,700 million years ago, another set of islands formed in the Mazatzal province.

The line of volcanos stretching from the Clayton volcanic field in the northeast, through the Jémez to the White Mountains of Arizona in the southwest may lie on the boundary between the two provinces. If so, we live on the inner edge of Mazatzal. Those ancient Pecos greenstones and zircons from Burned Mountain and Gold Hill survive from those island arcs.

Notes:
Baldridge, W. Scott. Geology of the American Southwest, 2004.

Whitmeyer, Steven J. and Karl E. Karlstrom. "Tectonic Model for the Proterozoic Growth of North America," Geosphere 3:220–259:2007.

Photographs and Diagrams:
1. Badlands in front of Jemez, north of Tchicoma, early morning, 3 March 2015. In Stratigraphy of the Santa Fe Group, New Mexico (1971), Ted Galusha and John C. Blick say Puyé Conglomerate lies below the white line, Chamita Formation above. The low cliffs at the base are Ojo Caliente Sandstone above Chama-El Rito Member, which in turn, is above Pojoaque Member.

2. Terrestrial animal and plant fossils show continuities across oceans. Osvaldo Cangas Padilla, "Snider-Pellegrini Wegener Fossil Map," posted to Wikimedia Commons, 27 August 2010.

3. "North American Cratons and Basement Rocks" today; unidentified USGS map uploaded to Wikimedia Commons by Berkut, 22 April 2005.