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  What goes up, must come down. Or so the saying goes. But in geology, what goes down, must come up, too. This is a key part of what we call the “rock cycle”. Eroded material becomes deposited material. Rocks worn away by wind and water result in new rocks, eventually. Profound? Not really, you might think. But the concept does deserve further analysis.

  It’s the first of these dynamics that I want to consider here: the “erosion” part of geology, not the “deposition”. The creation of rock formations leads to structure, but it is erosion that gives the land character. Like ancient sculptors who turned blocks of marble into Greek goddesses, wind, water, and other agents give the rock formations of the Earth personality.

  Erosion is defined as the “physical breaking down, chemical solution, and movement of brokendown and dissolved rock materials from place to place”. Wind and water are part of it, but it is also facilitated by chemical weathering. Even particular life forms, such as lichens and bacteria, contribute to the decay of rocks by processing the minerals to sustain life.

  Rock formations sometimes contain numerous fractures or cracks along which erosion and weathering can occur. These may be of a large-scale regional nature, or a small-scale local nature, and they can result from destressing, buckling, and with some rock types, cooling of the rock after its formation.

  Many times these fractures form in sets which meet at different angles, known as a joint system. Erosion along such fractures and sets can make for some really spectacular scenery. The sandstone towers of Monument Valley, in the Four Corners area, as well as the granitic spire of Pinnacle Peak locally, were initiated by jointing.

  On a smaller scale, the beautiful granite boulders in and around Carefree also weather out along joint patterns. The dark basaltic lava flows along the Black Canyon Freeway (I17) north of Phoenix lie in blocky and rubbly patterns because the bedrock has disintegrated along intersecting cracks caused by shrinking of the stone when it cooled down.

  Carbon dioxide, which is a naturally occurring gas (as well as human-produced) in our air, reacts with atmospheric water droplets to make “acid rain”, and such precipitation over time dissolves the mineral cement that holds sand and silt particles together in certain hardened rocks. Such rain can actually dissolve some rock, like limestone, completely.

  In Phoenix’s Papago Park, the strangely-weathered, orange rocks of the Papago Buttes are pitted and hollowed by chemical breakdown of the mineral matrix, further enhanced by the physical effects of wind and water. You can see this, too, near Camelback Mountain’s west end.

  And where else better illustrates the power of wind and water than Arizona’s own Grand Canyon? Water especially. In only a few million years, the Colorado River’s flow has carved an immense gorge and its side canyons out of sandstone and limestone that took hundreds of millions of years to accumulate.

 
  The development of a landscape’s look – its geomorphology – is the product of the kinds of rocks there originally, their individual resistances to weathering and erosion, zones of weakness within them, and the types and rates of erosion present.

  So it turns out that the natural beauty of Earth’s rocky landscape is not only a creative process – it is mostly a destructive process. And the nature of how the destruction proceeds amidst the structure gives us our special places.

  In an art gallery one time, while I was viewing a quite mediocre landscape painting, I realized why the artist had failed to get the scene “right”. The rocks in his painting were weathered wrongly.

  Though he had been striving for visual accuracy, he hadn’t broken down the lay of the rock formations correctly, and they just didn’t look believable, even when everything else about the picture did.

  Had the painter looked more closely at how the rocks were placed in nature, he would have seen more clearly how (and why) they had eroded in reality, and how much more character they then would have possessed in his artwork.

  There is an art of placement (for rooms, building sites, gardens, etc.) in Chinese philosophy called Feng Shui, and it’s no coincidence that those words mean “Wind” and “Water” in their language.

  The shape of the landscape, whether big or small, determines the “feel” of the place, and hence, its spirit.

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  Pictured above is the inner north wall of Cerro Colorado. This structure is about one kilometer (3300 feet) in diameter, and about 110 meters (360 feet) deep. You can just barely make out our vehicles in this image. They are the small white dots, across the crater, along its rim. A dark lava flow can be seen in the distance.
 
 

  Just across Arizona’s southern border, on the way to the Gulf of California, lies one of the Sonoran Desert’s most spectacular geologic features – the Pinacate Volcanic Field. Few of the many thousands of tourists that each year visit the party-place we call Rocky Point (Puerto Peñasco to the Mexicans) even know that it is there. That’s a good thing, too, for part of its beauty is its desolation. It is one of the most similar places to the surface of the Moon that you will find anywhere on Earth. Not because of its loneliness, though.

  From the highway to the coast, you cannot see that out there in that barrenness lie a number of impressive craters. Get up in the air, however, and it looks much like what you see through a telescope focused on the lunar landscape. Massive, ring-shaped, and deep, those craters show that the now-quiet countryside was once a pretty violent place.

  Previously, I’ve written about the explosive San Francisco Peak(s) of northern Arizona, towering above Flagstaff, and not hard to miss at all. But the craters of the Sierra Pinacate region of Sonora are not readily apparent until you are right there.

  This area lies within a Mexican National Park – the Parque Natural del Gran Desierto del Pinacate – which also features a sea of sand dunes, lava flows, and a number of volcanic cinder cones. It is not the kind of place you want to venture into light-heartedly, with your passenger car and beach clothes. Take a lot of water – that is some good advice, too.

  To me, the craters are the most interesting things to see, and these are some really good ones. They are different from craters on the Moon, though, because the lunar ones were formed by impact – asteroid, meteoric. Same with Meteor Crater, near Flagstaff.

  The Pinacate craters were created by relatively shallow explosions in the crust of the Earth. They are a type different even from the volcanic craters and cones of northern Arizona, like Sunset Crater. In “geology-speak”, they are called maars, and these happen to be some especially young ones.

  The Gran Desierto (Grand Desert) is a dry, dry place. Yet, deep underneath the sparse desert scrub that does exist there, is groundwater, or very recently was, apparently. That water occupied layers of rock, in turn overlying lower rock units that become hotter with depth.

  Remember, and I’ve written about this in many other articles as well, that this part of North America is very active, geologically. Earth’s crust is and has been breaking up in this zone, and the fractures run deep. Molten rock can move upwards along those fractures, eventually making its way to the surface, hence the cinder cones and lava flows.

  In the past, here in the Pinacate Field, some of that molten rock moved upwards, and encountered groundwater deposits (known as aquifers). When it did, it converted the water instantaneously to steam – massive amounts of it – and the ground literally exploded outward, creating maars.

  Then, at least in some of the cases in the Parque Natural, those exploded chambers collapsed back into themselves, expanding them additionally into structures known as calderas – gaping holes in the ground. There are at least ten of these maar / calderas in the Volcanic Field.

  It is possible that humans witnessed some of the eruptions. Hohokam relics have been found along some of the erosion surfaces in the area. Studies show that the blasts occurred within the last few million years, and some only within the last few thousand years. Very jagged, black, and barren, the basalt lava formations that you drive by between the craters look like they flowed yesterday.

  As you can see, the starkness of the vista adds much to the otherworldly look of this place, so if you can’t make it to the Moon (and most of us won’t have that chance), you can at least get an idea of the lunar scenery by visiting the Pinacates.

  Breathing is easier there, too.

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  It looks more like a dessert, than a desert, in northern Arizona.

  Picture this: you are working for a Hollywood director. The film on which he is working is a science-fiction feature, and your job is to find a filming location that is other-worldly. He wants it to be an extraordinary scene – the countryside of a planet devoid of life, with a forlorn and desolate feel, and with colors like nowhere familiar. Where would you send him and the film crew?

  I know just the site. Every time I drive through it I imagine for a moment that I am there, on that other world, and it wouldn’t surprise me at all to see some Martian-like creatures topping one of the rolling hills, coming to check me out, for good or for worse.

  Where is this spot? You might have guessed already. It is Arizona’s Painted Desert, and it is one of the strangest landscapes anywhere. It’s almost as barren as a place can be, and it looks like it was spray-painted with a variety of hues – colors which have come to symbolize the Southwest. The gently smoothed hills even have a fractal sense to them, as they scale down into smaller, similar forms of themselves. Unless there is something familiar standing next to one, it is difficult to tell a hill’s size from a distance.

  How did our famous scenic desert get that way? The “paint” was volcanic ash; the “canvas” was the coastal plain of earliest North America. And the “brush”? A series of massive eruptions, which blanketed the area with layer after layer of fine-grained dust.

  The Chinle Formation is the name geologists give to the group of these particularly colorful layers of soft, crumbly rock. During Triassic time, around 200 million years ago, vast amounts of ash spewed forth from volcanic ranges far to the west. Carried by the winds, that ash fell in what is now northern Arizona, which was then a lowland area of mud flats and gentle plains.

  Sluggish rivers also carried fine sediments into the basins there, and these mixed with the various ash falls. Varying amounts of organic debris accumulated, and added to the mixture of color. The name of the formation is taken from the nearby town of Chinle, farther east in the Navajo Nation.

  The Triassic Period is responsible for some of the best of the red-rock scenery of the Southwest, as its rocks contain a high proportion of iron compounds. Those minerals have oxidized – basically rusted – giving the land its signature color-scheme. It is the first period of the Mesozoic (“middle life”) Era, the huge span of time that is also commonly called the “Age of Dinosaurs”.

  You would have had mainly reptilian neighbors back then, but may have noticed an occasional little critter scurrying about that looked a bit mammalian. The forests around would have contained stands of cycads, conifers, and ferns. Not many flowers, though, as true flowering plants evolved later.

  And if you had listened intently, you would have noted the missing chirping and singing of birds, as they had yet to make their appearance on the world stage. (You can see what is left of some of the occupants of those prehistoric woods in the eastern end of the Painted Desert, at Petrified Forest National Park.)

  Over time, in that alien panorama, the strata built up, eventually giving way to the great deserts of the Jurassic Period, some 50 million years later. The layers of fine powder that fell from the sky weathered and altered into clay minerals, which are very delicate, and when wet, behave plastically and can even be molded. Geologists call the type of clay found in the Painted Desert bentonite, and the nature of that mineral contributes to the “badlands” found there.

  Bentonite swells when wet, and shrinks when the wet clay dries out. That varying consistency makes it hard for plants to take root, and additionally, since the clay erodes rather quickly when it rains and during windstorms, it makes it difficult for them to stay rooted.

  The impurities within different layers make for the variety of hues and tones in the tinted landscape. Combine those with the easily eroded shapes of the soft rock, and the resulting deposits of slaked-off soil, and you have a first class work of art – one that covers a good deal of northern Arizona. It is perhaps the best place I can think of in which to grasp one of the least appreciated agents of geologic deposition: the air around us.

  Go see for yourself! One of the best viewpoints from which to see a vast expanse of the colorful Chinle Formation is at Painted Desert County Park, just off SR87, about 15 miles north of Winslow, and about 50 miles east-northeast of Flagstaff.

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  Volcanic eruptions, glaciers, landslides, and earthquakes. Those are all topics that elicit questions in the minds of the many tourists (and locals) I talk to every week—not so much in terms of present-day activity (or potential), but in reference to the cause of the many very picturesque and unusual rock formations scattered around the valley.

  Sometimes they are surprised to learn from me that the surrealistic boulder formations around Carefree, for example, were not “pushed there by glaciers,” as had been told to them by a Jeep tour driver. Or that those same massive, rounded blocks of granite were not “blown out of volcanoes and that’s where they fell,” as had been told to them by a hot-air balloon pilot.

  And not that I mind answering their questions and talking about these things. I love it. What better way to help someone discover that geology is about the real world; that it’s not just some boring subject in a dusty textbook.

  Here in Phoenix, we live in an incredibly engaging place to experience geology, and one in which the Earth’s workings and mysteries reveal themselves easily to us. After all, it is desert here, and in the desert you can see for miles and miles, and much of what you do see is bare, naked rock.

  Take Pinnacle Peak, for example (see picture). This strikingly sculptured, towering rock formation stands near the north end of the McDowell Mountains, in north Scottsdale. Those lucky enough to be sipping margaritas on the lovely patio at the Four Seasons Resort see it right there in front of them.

  Most tourists I’ve talked to suppose it to be an ancient volcano. It does kind of have that look, I admit, but it is not a volcano. The rock it is made of, an ancient granite, has simply weathered into the shape you see. And it has taken a long, long time. That rock was once buried deeply in the Earth’s crust. How deep? Probably several miles deep, at least.

  That granite is around 1400 million years old—meaning it cooled down from a molten state almost a billion and a half years ago, when it lay underneath a landscape completely different from that around us now.

  Then, over a vast, almost incomprehensible stretch of time, it got exposed by erosion. Cracks that had formed in it, many of which are nearly vertical, allowed for advanced weathering, and the shape of Pinnacle Peak you see now is just a result of that weathering.

  Look around at all the other granite in the area, and note that it has the same general cracked and blocky appearance. It simply differs from that one nice spire-like shape, you see. (See my Blog / GeoStory “Bones of the Earth” for a similar explanation of the rocks around Carefree.)

  Speaking of Carefree, there are a few other features near that laid-back little town that generate questions for me. They are the gently conical hills a few miles to the southeast of the town center. These are known locally as Big Brownie’s Hill, Little Brownie’s Hill, and an unnamed third cone. Once again, people’s first impression is that they are volcanic, which in this case, is partially true. Just not as you might think, however.

  Most of the rock forming those two hills is volcanic, but the hills are not volcanoes, though once again, they do look like they must be. So what do I mean? The volcanic rock there is mainly of a type called basalt, and when hot and fluid, it flows very readily from its source vent. It forms layers, now mostly tilted and broken-up, which can be seen in many places north of Phoenix.

  In the Brownie’s Hills, already hardened basalt (along with some other rock types) has simply weathered into cone-shaped hills. From a distance, they look like they have to be volcanoes, meaning they look like they should have been major eruptive sources of the rock. A closeup inspection, however, reveals no craters at the summits, and no flows emanating from their slopes.

  By the way, the forces that have shaped Pinnacle Peak and the Brownie’s Hills are still at work—slowly, slowly every day. Those mounds of rock are as ephemeral as the clouds in the sky. They’re just in a different time frame, that’s all. The study of landforms is called geomorphology, and I think it is one of the most fascinating branches of geology. And now that we are mapping and photographing the strange surfaces of other planets and moons, we have even more to figure out about the processes of nature.

  All those shapes around us in the world are not always what they appear to be.

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