Rock Talk By Larry Larason

The Once and Future Mountains

In 1681 Thomas Burnet wrote a book entitled The Sacred Theory of the Earth. In it he discussed the origin of mountains, and noted: 

"The generality of people have not sence [sic] and curiosity enough to raise a question concerning these things, or concerning the Original of them. You may tell them that Mountains grow out of the Earth like Fuzz-balls, or that there are Monsters under ground that throw up Mountains as Moles do Mole-hills; they will scarce raise one objection against your doctrine…."

If you are reading this article then I assume that you are curious and not among those that Burnet described as "drown'd in stupidity and sensual pleasures, and so little inquisitive into the works of God and Nature." As you may guess from that quote, Burnet was a minister. He truly did try to understand the Earth, but the science of geology would not be invented until more than a century after his death. It is easy to make fun of his idea that the Earth was a hollow sphere filled with water, which rose up to cause the Noachian Flood and jumble the landscape. But his speculations were respectable for his time, and were admired by Sir Isaac Newton, among others. However, had he still been alive, the great Lisbon earthquake and tsunami of 1755, which caused nearly total destruction of that city, might have given him an inkling of the tectonic forces at play on our planet.

We've learned a lot since then. We know that mountains come and go in the geologic record. Tectonic forces lift them up; erosion cuts them down. But in some places, mountains rise more than once.

We have three kinds of mountains in the Four Corners: volcanic, laccolithic and faulted. Laccolithic mountains are created when magma rising from the depths pushes up sedimentary strata into a dome. Examples in the Four Corners include the Henry Mountains, Navajo Mountain, the Carrizos, the Abajos and the La Sals. These are one-time things. But many faulted mountains are recurrent. Let's look at one case: the Zuni Mountains.

It's not clear when the Zunis first rose, but they may have been highlands as early as the Precambrian. The core of these mountains is granite and metamorphic rock more than one billion years old. No younger rock was laid down over the granite until the late Pennsylvanian Period, which ended 290 million years ago. Uplift in the Zunis may have also occurred during the Pennsylvanian, which was a time of mountain building as all the Earth's continents bumped together into the supercontinent of Pangaea. However, the Pennsylvanian was a long period of 33 million years and by late in that time erosion had reduced the mountains enough that shallow sea water covered at least part of the area and left behind thin deposits of fossiliferous limestone and shale.

During the succeeding period, the Permian, the mountains remained low. The Permian sea covered them with more limestone, although the Zuni area was still high enough to be a reef, rather than deep sea. You see some of this limestone, the San Andres Formation, in the road cuts along I-40 as you come down a hill traveling east to the flats west of Milan. When the sea receded, continental shale and mudstone further buried the Precambrian core.

Deposition continued through the three periods of the Mesozoic Era, until the "mountains" were covered by an estimated 20,000 feet of rock. These rocks provide much of the colorful scenery we now enjoy: the Chinle Formation's colored banding and petrified wood, the red sand dunes of the Jurassic, the black shale and coal of the Cretaceous. Then near the end of the Mesozoic the Laramide Orogeny got underway.

"Orogeny" is a fancy word for mountain building. What happened is still debated, but the favored theory, in the simplest form, is that as the North American plate drifted westward it overrode an oceanic plate; this action caused the continental crust to shorten. Compression caused faulting that wrinkled the surface, some areas moving higher, others lower. The Zuni Mountains have been described as the most faulted region on the Colorado Plateau. In addition, they sit on the Jemez Volcanic Lineament, a major fracture zone in western North America. This lineament has been described as "a long leaky flaw in the crust." It features deep faults in the continental basement rocks that are probably the reason it is "leaky." To find it, draw a line between the Raton-Clayton and Springerville [AZ] Volcanic Fields and you will see that many other volcanoes fall on or near that line, including El Malpais and Mt. Taylor. The largest eruptions on the line occurred at the intersection of the lineament and the Rio Grande Rift – a doubly weakened zone in the crust – and built the Jemez Mountains, although that was well after the Zuni Mountains' most recent rise. When compression became great enough during the Laramide Orogeny deep faults along the Jemez Lineament were reactivated and the Zuni Mountains rose as an elongate anticlinal dome.

It didn't happen overnight. And as soon as the mountains began pushing up, erosion began tearing them down. Stripping away 20,000 feet of overburden is impressive, but that's less than one inch per each of the roughly 66 million years that have elapsed since the uplift. In any case, the erosion has left us some wonderful scenery: Pyramid Rock, Church Rock, and the red cliffs along I-40 that are carved out of the northern flank of the Zuni Mountains anticline.

The Defiance Plateau, to our west, has a similar geologic history, except there is no Pennsylvanian rock there. On the plateau Permian strata lie directly on the Precambrian granite.

The Zuni Mountains were abundantly forested when the first settlers arrived. Then logging began for railroad ties. Timbering went big scale beginning in the 1890s with railroad tracks built to the logging camps. The Forest Service road through Sixmile Canyon follows, in part, the old railroad grade. By 1901 millions of board feet were being taken out each year. The biggest trees were gone by the 1920s and the timber companies were leasing cleared land to ranches. Because of clear cutting there was not much left on the land; wildlife and vegetation were mostly gone, and erosion was becoming severe.

The Forest Service created the Zuni Forest in 1909. By 1940 only 5000 acres of virgin forest remained. Local residents sent a petition to the Forest Service asking them to purchase the denuded area and manage it to restore the forest. Responding to the locals' request, the foresters planted seedlings but found that trees from other places, even though of the same species, did not thrive in the Zuni Mountains. Either the soils or the climate were different enough, so that they had to use seeds from trees that were adapted to this environment. By 1970 the forest had had been mostly restored. For history, hikes, and more pick up a copy of Sherry Robinson's book: El Malpais, Mt. Taylor, and the Zuni Mountains; UNM Press, 1994.

If you've read all this you now know more about mountain building than Thomas Burnet did in 1681. I hope you have enjoyed exercising your curiosity, unlike those he railed against who were "drown'd in stupidity and sensual pleasures." And you also know that when North America gets squeezed again millions of years from now the Zuni Mountains and Defiance Plateau will rise again.