Geology Landscapes of Valencia County: The Rio Grande Rift demonstrates plate tectonics

The geologic landscapes of Valencia County offer visual evidence of plate tectonics. The uprising Manzano Mountains to the east, the Middle Rio Grande Valley and the rising sedimentary mesas to the west illustrate a widening of the earth’s crust.

The volcanos of Tomé, El Cerro de Los Lunas and Isleta were formed by lava rising through vertical cracks caused by the stretching of the crust. These landscapes demonstrate tectonic activity.

Until the 1960s, the Rio Grande Rift was called the Rio Grande trough or depression. The concept of plate tectonics did not come into full acceptance until the 1970s, but the evolution of the theory is interesting to review.

Any elementary school student looking at the map of the world will point out that the shape of South America and Africa appear to fit together if you mentally remove the Atlantic Ocean. In 1912, a German climatologist, Alfred Wegener, proposed the concept of drifting continents. His ideas were categorically dismissed. Geologists, at the time, could comprehend vertical movements of rocks but had no explanation for lateral movements.

Several lines of evidence eventually brought full acceptance to the theory.

(1) When trans-Atlantic communication cables were installed, ocean floors were mapped and found to be much more rugged than previously thought. The mapping showed the presence of ridges in the middle of the Atlantic, providing one of the first clues.

(2) In 1947, the thickness of sediments sitting on the ocean floor was found to be much thinner than expected. If the oceans were old, thick sediments should have accumulated over the past several billions of years.

(3) In the 1950s, the magnetic signature of the lavas spreading away from the mid-Atlantic ridge were systematically mapped, and that work demonstrated that the rising lavas produced in the ridge gradually push away and spread the previous layers.

(4) Studies of earthquakes off Japan showed that the ocean plate was sliding under Japan, providing the evidence that a discrete ocean plate was moving, and making it clear that ocean crust is forming along ridges and the same plate then is subducted at the other end. Mapping of all large known faults then started to provide an earth-wide distribution of large and small plates.

Plate tectonics provides mechanisms for most earthquakes and volcanic events. About seven major plates and a dozen smaller plates are recognized, with some moving at the speed of growing fingernails.

The Rio Grande Rift appears to have started as a split of the crust like the longer, more massive East African Rift that is splitting the eastern part of Africa from the rest of that continent. Multiple GPS stations monitor the movement of the Earth’s surface in the Rio Grande region.

The mechanisms that created the start of the Rio Grande Rift and its recent slowing down are still not fully understood, but all detailed geologic studies confirm the tectonic concept of a rift created by earth crust extension for this geologic feature central to Valencia County.

(Paul Parmentier, a certified professional geologist retired from California and living in Los Lunas, shares the rich geologic features in Valencia County. He earned a Bachelor of Science degree in geology from Belgium and a master’s degree in geochemistry from Japan. The Geology Landscapes of Valencia County are featured monthly.)