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Oceans of Ore: How an Undersea Caldera Eruption Created Jerome, Arizona


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“The town and its fluctuating fortunes are a humble reminder that much of human history has been influenced by the vagaries of the geologic processes that shape the land we inhabit, form the minerals from which we construct our civilizations, and produce the riches we covet.”

-Lon Abbott and Terri Cook, Geology Underfoot in Northern Arizona.

While tourists gazed rapt into the billions of years exposed in the layer-rock-cake walls of the Grand Canyon, my mother and I would hop down to Jerome. At Grand Canyon, you have to climb down thousands of feet to touch 2 billion year-old rocks. At Jerome, all you have to do is walk up the street.

Jerome, Arizona circa 1909. Image courtesy Wikimedia Commons

Not that we knew it then. We just knew it was a delightful little destination, filled with beautiful old Victorian houses the artists were gradually restoring. We’d wander amongst shops and storefronts that collided Old West with tourist kitsch, much like subduction zones crush disparate rocks together any-old-how. There were bamboo groves there, a delicious rarity in dry country, and agave sending up their once-in-a-lifetime blooms on rocky hillsides. You could stand on the edge of town and see all the way to the San Francisco Peaks on the Colorado Plateau.

View from Jerome, Arizona. You're perched on the side of the Black Hills (Mingus Mountains, to the locals), looking across the Verde Valley to the Mogollon Rim. The San Francisco Peaks can be seen on the horizon just left of center. There's an amazing amount of geology in this photo. Image credit Cujo359.

But our mecca was Gold King Mine. Mining equipment, rocks, old town buildings filled with the artifacts of a civilization past, more rocks, a working saw mill, and a shop with lots of rocks in it made for a perfect afternoon’s adventure.

View over Verde Valley from Gold King Mine, Jerome, AZ. Note the old trucks, pit wall, and mine tailings. Jerome's raison d'etre was the copper in its rocks. Image credit Cujo359.

Things would have been rather more adventurous here 1,750 million years ago (that’s 1.75 billion, to Americans). The bit that would become Jerome erupted busily near the southern coast of the nascent North American continent. Subduction zones gave rise to at least two volcanic island arcs: Jerome and Prescott. Rhyolitic and basaltic eruptions underwater constructed a submarine dome. The area would have been reminiscent of Indonesia. Fans of Krakatoa know what that means: explosions. And if you were planning to build a time machine and visit the area someday, you’ll want to time your visit very, very carefully, because Krakatoa was a firecracker compared to what was about to happen here.

After an ominous pause in volcanic activity, gas-rich magma filled a chamber. Pressure built, and then boom. I don’t know if you can imagine an undersea caldera eruption. There’s the sea floor, suddenly torn apart, with boiling clouds of ash billowing through the water. Had there been fish back then, they would have been tremendously inconvenienced; also, cooked. Fortunately for them, this was before the Age of Fishes. When the magma chamber emptied, the whole shebang came crashing down. A huge series of fractures propagated in a huge, irregular ring miles in diameter. Sea floor, seamounts, volcanic domes, newly-emplaced Cleopatra tuff and all fell into the chamber magma had so recently vacated.

Cleopatra Hill, Jerome, AZ. It's made of the 1.75 billion year-old Cleopatra Tuff, formed in the caldera eruption. The hill, of course, formed a lot more recently. Cross the road between the two largest buildings in the photo, and you'll have just stepped right over the Verde Fault. Image credit Dana Hunter.

So there’s this enormous depression filled with cracks, fractures and faults, with a sea over it. Seawater percolated through all those crevices, got right down into them and hooked up with the water coming off another magma chamber nearby that hadn’t spilled its guts. These waters were superheated: we are talking temperatures in excess of 600 degrees F (316 degrees C). They were hot enough to dissolve rocks. They chemically altered what they didn’t dissolve. They traveled through the caldera’s ring fracture, carrying dissolved minerals and metals, and emerged from various spots as hydrothermal vents. Yes, just like the modern variety.

An image of a black smoker at the Juan de Fuca Ridge. Image courtesy NOAA.

That black “smoke” is minerals and metals precipitating out as extremely hot water meets cold. This is what the caldera floor would have looked like almost two billion years ago, complete with chimneys formed from mineral deposits. There were “white smokers” where water around 250 degrees F (121 degrees C) did the hot spring thing, too. You can see examples of both in Jerome. They’re eerily like their modern cousins, yet they’re among the oldest found on Earth.

Dissolved metals combined with sulfur and left massive sulfide deposits around the vent fields. Metals lined the fractures and conduits the hot springs flowed through. They filled the chimneys. This is where Jerome’s riches were born: in fire and water, under the sea, before North America had even acquired this profitable bit of real estate. Plate tectonics gradually mashed the Jerome and Prescott volcanic arcs together roughly 50 million years after the big boom, then plastered them both to nascent North America. The Cleopatra tuff didn’t hold, it folded. Its metal-rich deposits folded with it. Once again, fluids coursed through fractures and faults, further concentrating precious minerals.

1,200 million years passed. We don’t really know what happened. Erosion, definitely: there’s no rock record for those years. The area that would become Jerome was probably covered with a thick, cozy sedimentary blanket that was subsequently stripped away: the Grand Canyon preserves some of that record. We do know that 525 million years ago, Jerome was prime beachfront property. Beaches of sand on the shores of an ancient sea, possibly wine-dark, became the Tapeats sandstone. And the continents danced. Jerome dried out, eroded again, went under again 375 million years ago. This time, the limestones of the Martin Formation were laid down in a warm, shallow sea. The Redwall limestone followed 340 million years ago, when Jerome and the Verde Valley area basked near the equator. People joke about oceanfront property in Arizona. There have been plenty of times in its past when that joke has been reality. Take your time machine back to these times, and you could be sipping cocktails on a tropical island right in the middle of what would later become very dry country. And beneath your lounge chair, unimaginable riches awaited. But they weren’t quite ready yet.

Beginning 80 million years ago, North America had an altercation with a bit of ocean crust. The collision began a 40+ million year period of compression. This was the Laramide orogeny, when mountains were raised and the continent crumpled. This episode caused the birth of the Verde fault 70 million years ago. And this fault would eventually become the star of the show.

The mountains raised by the Laramide orogeny did what mountains do, and eroded. Compression gave way to stretching 17 million years ago, and the Basin and Range was born. Basalts poured from the thinned crust, leaving behind the 15 million year-old Hickey formation. If you’re beginning to think central Arizona’s been through interesting times, you’re absolutely right. And it wasn’t nearly over: the Verde fault, dormant for a time, reactivated 10 million years ago. Its handiwork is why Jerome is perched on fabulous wealth on the side of a mountain range.

Looking across the Verde Valley at the Black Hills, locally known as the Mingus Mountains. Jerome is perched halfway up the mountains just left of center. A line of green trees marks the course of the Verde River, and gives the valley its name. This is the view from Tuzigoot National Monument, and it is spectacular. Image courtesy Cujo359.

The Verde fault raised the Black Hills and lowered the Verde Valley. It sliced through the ancient caldera, neatly bisecting the ancient Jerome anticline with its twinned ore bodies. It exposed a portion of one of the ore bodies on the side of Cleopatra hill. Once more, fluids circulated through faults, putting the finishing touches on those ores. Nearly two billion years of geological antics had prepared a bonanza beyond dreams.

Native Americans found it first, collecting beautiful copper-rich minerals from the exposures. Spaniards in the 16th century knew copper and silver were up there, but they were after gold. The first mining claims weren’t staked until 1876. When miners began digging, they found ores averaging over 10% copper. Those ancient hydrothermal deposits, thrice-concentrated, created a boom town. Jerome’s population exploded: it became the second-largest city in Arizona Territory. Exploiting some of the richest copper deposits on the planet, miners extracted over 37 million tons of ore.

A mine car full of copper ore at Gold King Mine. Tons of the stuff were dug out of the mountain. Smelting operations began in Jerome, but the mines eventually opened a large smelter down in the valley in Clarkdale, AZ. Jerome's fortunes were tied up in these blue-tinged rocks, the product of nearly two billion years of geological excitement. Image credit Dana Hunter.

Between 1876, when mining began, and 1953, when the last mine closed, Jerome produced $3 billion modern dollars in copper alone, and a further cool billion in silver, gold, lead and zinc. Things got so posh the city even ended up with an opera house.

After their wooden buildings burned down a few times, the citizens of Jerome began quarrying the Tapeats, Martin and Redwall formations for building stone and limestone to make concrete. Thanks to all those seas, the town ceased to burn – at least above ground.

Mining in Jerome started out as an underground affair. The largest mine was the United Verde, which exploited the ore body revealed by the Verde fault. 88 miles of mine shafts were sunk into the mountains. Rich ores emerged.

Mine entrance at Gold King Mine, complete with ore cars. This mine on the outskirts of Jerome, owned by the Haynes Copper Company, didn't find any copper. But it did strike gold, so that was a little bit of all right, then. Image credit Cujo359.

And United Verde would have gone on happily tunneling if the pyrite hadn’t caught fire in 1905. The flammable fool’s gold burned for a solid twenty years. United Verde, cut off from its rich tunnels by hot rocks, tried blocking off the burning bits. Six miners died in a steam explosion caused by groundwater percolating into the smoldering stones, and by 1917, the fires forced United Verde to excavate a 300 foot deep pit to reach unaffected tunnels. You can still see the pit, and the head frames that lowered miners into the tunnels, which extend down 4,200 feet. The pit also exposes the ancient volcanic rocks.

The United Verde open pit, easily visible from the road. You park on discarded rock from the mines, and gaze at an amazing amount of geology laid bare. The tall metal structures are head frames. Image credit Cujo359.

Incredible, isn’t it? Appreciable portions of the pit are gabbro, basalt that cooled underground without erupting, that is over 1,700 million years old. The rocks have really been through it: sheared by movement on faults, cooked by hot water, scrunched and crunched and squeezed, then excavated by man.

Mine shaft in the wall of the United Verde pit. Image credit Dana Hunter.

United Verde wasn’t the only game in town. A fellow named Rawhide Jimmy Douglas had a brainwave brought on by geology: if the Verde fault had sliced through the United Verde ore body, mightn’t the rest of it have ended up down the hill? He staked a claim to a spot on the lower east slope in 1912. His miners tunneled through the young Hickey basalts downdropped by the fault. They dug into the older rock beneath, bringing up nothing useful, and it began to look like Rawhide Jimmy was wrong. But after two years and 1,000 feet, they struck copper. Fabulously rich copper. Copper ore that was up to 45% copper. If 10% could be considered outstanding, this was beyond bonanza. And yet, Jimmy had been completely wrong about his geology. The Verde fault hadn’t sliced an ore body in half: he’d just found the second ore body from the ring fracture. Not that he knew or cared. He was sitting on the United Verde Extension, his Little Daisy, and he became a very rich man.

Jerome’s wealth came at a cost. Remember that the minerals and metals of the old hydrothermal springs had combined with sulfur. Smelting released that ancient sulfur as sulfur dioxide gas. When it rained in Jerome, it rained sulfuric acid: the trees that hadn’t been cut for fuel and building material died in the acid rain. As miners removed tons of mountain, slopes destabilized, and landslides, combined with concussions as miners dynamited rocks, caused bits of Jerome to go slip-sliding downslope. The slides, fortunately, stopped when mining ceased in 1953. But mine drainage still affects the area, staining some streams vivid shades of blue and orange. And enormous tailings ponds remain in the floodplain of the Verde River. Full of heavy metals, they’ll cause untold environmental damage if containment fails.

Jerome is one of those places where the impact of geology is acutely felt. The Verde fault lifted the mountains that make Jerome’s narrow, steep streets laugh at San Francisco’s; an ancient caldera eruption and the resulting hydrothermal vent fields laid the foundations of its enormous wealth. The bones of mountains young and old shoulder through the thin plant cover on its slopes. The floors of shallow seas and the sands of ancient beaches form its walls. Few towns can draw on two billion years of geological history. Here, it stands above, below and around the city, reminding you at every step that civilization depends on geology, and inviting you to explore the riches it offers. The old mines may be played out, but there’s still a mother lode of geology, just waiting for you to claim it.

Some of Jerome's geologic legacy. Clockwise from top: mica schist from Mingus Mountain, copper ore from the United Verde mine, a bit of pure copper from an Arizona mine, and slag from mining operations at United Verde. Image credit Dana Hunter.

References:

Abbott, Lon and Cook, Terri (2007): Geology Underfoot in Northern Arizona. Missoula, MT: Mountain Press Publishing Company.

Ranney, Wayne (2010): Sedona Through Time. Flagstaff, AZ: Wayne Ranney.

(This post was written for the Accretionary Wedge, a monthly geology blog carnival. It’s an extravaganza of geologic goodness, centered around a theme that can be profound, thought-provoking, or just outright fun. This month, the Wedge is being hosted by Cat of Knowledge Flocs, with the theme “Geology, Life and Civilization.” I figured it was about time to introduce Rosetta Stones readers to the Wedge, and get out of the Pacific Northwest for just a tick. Hence, Jerome, which my intrepid companion Cujo359 and I visited again in 2009. I’ve made myself thoroughly homesick writing this post. I’ll be relieved to return to Mount St. Helens in a few days, but I hope you’ve enjoyed this side trip to one of the best destinations in Arizona for those who like geology, mines, history, old buildings, and kitsch.)

Dana Hunter About the Author: Dana Hunter is a science blogger, SF writer, and geology addict whose home away from SciAm is En Tequila Es Verdad. Follow her on Twitter: @dhunterauthor. Follow on Twitter @dhunterauthor.

The views expressed are those of the author and are not necessarily those of Scientific American.





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  1. 1. BillR 11:16 am 06/4/2012

    Fascinating…. Has anyone traced out the caldera and looked in other places for the minerals that are still there?

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  2. 2. MovableBookLady 11:51 am 06/13/2012

    Very interesting, indeed. I’ve been to Jerome many times (lived in Tucson 14 years). This piece immediately made me think of Kingsolver’s “Animal Dreams” — have you read it? There’s a plot about tailing ponds and their destructive potential. Anyhow, good job!

    Link to this
  3. 3. TheologyGeology 11:52 pm 02/26/2013

    Equally interesting is House Mountain, more or less across the valley from Jerome.

    Link to this

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