Postcards from the Field

The first photo shows a desert pavement developed on a terrace surface in the Fort Craig area. There is a blue pen near the center of the photograph, just on the other side of the gully.

El Cerro de Tomé comprises the remnants of a Pliocene volcano that likely was once much larger than the present-day 383-feet-high hill climbed by so many Good Friday pilgrims. Along the trails to the summit, one can view xenoliths of "country rock" (the pre-existing rock through which the volcano's lava was erupted) within the andesite that erupted approximately 3.5 million years ago.

Many gold and silver mines follow quartz veins that were precipitated from hydrothermal groundwater. The first photo [from Death Valley] shows such a quartz vein (just below the hand), which is about 20 cm wide. In the lower-right of the photo is a smooth rock face (covered in dust) that corresponds to a fault plane (dipping to the left). Just above this fault plane is another quartz vein that is 3-4 cm-wide. Many quartz veins form parallel to faults, like what is shown in the first photo.

Geology is truly a multidisciplinary science. On any given day we might work at the intersections of anthropology to zoology. Rock deformation (how rocks break and bend) is one example where these intersections help us to better understand our Earth. Engineering tests conducted to determine the strength of materials have shown us that applying a compressional stress to both sides of a rock will cause it to break in characteristic patterns. One pattern is known as conjugate faults, where failure of the rock results in a pair of faults that form a 60° wedge, of which the maximum compression direction (i.e., the squeezing direction) bisects the middle of the wedge. Armed with this experimental information, detailed field measurements of conjugate faults and other deformation features can tell us the different states of stress that Earth’s crust has enjoyed.

New Mexico is a great place to study extensional tectonics, or what happens to Earth’s crust as it is stretched and thinned. The Reserve graben, a small rift basin in western Catron County, is part of a longer fault system, including the Plains of San Agustin, at the southeastern margin of the Colorado Plateau. It also sits within the massive Mogollon-Datil volcanic field, where some of the largest explosive eruptions in North America’s history have occurred. The San Francisco River and its tributaries have cut down through the sedimentary rock that filled in the basin, exposing many of the basin’s internal faults.

"I have been mapping the Santa Fe Group and post-Santa Fe Group deposits near San Marcial over the past year. In December, I came across this neat exposure of axial sand laid down by the Rio Grande about 2-3 million years ago. The first photo shows very thin-thin beds of volcanic pebbles interbedded in the white, medium-grained river sand. Above the blue pen are a couple of spheroidal clay balls. These balls formed when the river ripped up a pre-existing, hardened clay bed (presumably on a floodplain). The ripped-up clay rolled downstream like a piece of gravel, becoming rounded in the process. If the clay ball rolls across pebbles on the bed of the river (like the pebble beds seen below the pen), then the pebbles can stick to the clay -- like rolling a ball of cookie dough over chocolate chips and having the chips stick to the outside of the ball-of-dough. An example of such an "armored clay ball" is seen in the second photo, with a blue pen for scale. The clay ball in the second photo may actually be comprised of smaller clay balls that have been compressed together to form one larger clay ball, which was later armored."

New Mexico offers a much wider variety of mineral collecting opportunities than my home state of Alabama. This is certainly the case with fluorescent minerals, a major focus of my mineral collection. One mine that offers attractive fluorescent material is the Dictator Mine, located near Winston in Sierra County. The Dictator Mine is a small, former producer of zinc, lead, copper, and silver. According to the 1934 NM Bureau of Mines and Mineral Resources Bulletin 10, which details the ore deposits of Sierra County, the deposit was known to prospectors as early as 1880.

They might seem timeless, but desert badlands are dynamic landscapes that owe their existence to the changes induced by rapid erosion. These badlands in the San Juan Basin of northwest New Mexico are no different. The sediment eroded from steep badlands slopes seen in the background of this photograph is transported downslope in arroyos and in sheetwash floods.

The Jornada volcano, one of the largest shield volcanoes in NM, is located about 35 miles south of Socorro. This volcano was once thought to be ~760,000 years old, but new dating indicates it is only 79,000 years old, making it one of the younger volcanoes in the region!

In the foreground of this photo, at The Box rock climbing area near Socorro, is a basalt lava flow that has a brown varnish. A fresh piece of the rock is shown on the smaller photo below, with a penny for scale. This is the basalt of Broken Tank, which flowed into the area from the south about 8.5 million years ago. This lava has interested Richard Chamberlin, who mapped it out, and myself for many years because one can use it to infer paleo-slopes and paleo-geography at this time period.

But recently I found out another intriguing fact about this basalt: a piece of this same outcrop is now on Mars!