The Last Days of Pompeii
January 17, 2010
One of those must-do day trips on your visit to Santa Fe is a drive to Bandelier National Monument, just south of Los Alamos, a place where you can walk along the shaded Rito de Frijoles under the pines and cottonwoods and have a close look at the Anasazi cliff dwellings that perch along the canyon walls. It’s an old park whose value was recognized very early on, and you are sure to enjoy a visit. But don’t forget to pay attention to some of the other remarkable sights you’ll see on your drive there.
Bandelier National Monument sits on the eastern flank of the Jemez Mountains west of Santa Fe, a rounded range of blue mountains that forms the stage for our spectacular sunsets here. (And you know the quip: Santa Feans take half their salary in sunsets) Steep-walled canyons are cut in the Pajarito Plateau that flank the Jemez, one after the other, dividing the land into narrow pinyon-covered fingers over which Highway 4 winds and dips on its way from the Rio Grande into the great Valles Caldera National Preserve in the heart of the mountains. This plateau was formed in two great acts of creation (disregarding some minor canyon-cutting that came afterwards) when the Jemez supervolcano disgorged its fiery guts not once, but twice, approximately 1.6 and 1.25 million years ago. Incandescent floods of ash formed a vast apron of tuff all around the older volcanic highlands – part of which makes up the Pajarito Plateau – and the bulk of the highlands themselves collapsed into a circular depression, called a caldera, 12 miles across. I could write about this stuff for days (and no doubt will in future) but for now I want to highlight a key outcropping you can’t help but notice as you cruise up from the Rio Grande on Highway 502 from Pojoaque to Los Alamos, just before you make your turn on Highway 4:
Your first thought, upon seeing this cut, is that the Highway Department has sprayed that disgusting, so-called “stabilizing” concrete over the rocks to hold the loose stuff in place. (Can you tell how geologists feel about those tricks?) But you would be wrong. This cliff exposes almost the entire record of those two great explosive eruptions. The lower, greyish-white part of this roadcut is a great fall of pumice that overwhelmed the landscape during the first stage of the cataclysmic eruption 1.6 million years ago. Above the pumice are thick layers of rhyolitic tuff (tuff is consolidated volcanic ash) laid down by massive pyroclastic flows of magnitudes never witnessed by man. The top of the orange tuff, which erupted 1.25 million years ago, marks the top of the Pajarito Plateau.
These events were set up by persistent volcanic activity beneath the Jemez volcanic field, beginning perhaps 16 milion years ago. Eruption after eruption softened the crust beneath the mountains, and a kind of cosmic indigestion developed in the bowels of the field, nurtured by the relentless activity. And then, around 2 to 3 million years ago, renewed pulses of exceedingly hot basaltic magma from the Earth’s mantle began to push queasiness toward catastrophe.
The rocks beneath the mountains began to melt wholesale, forming a vast chamber of viscous and gas-charged magma. The crust that supports continents like North America is lighter than the mantle below it, is richer in silica (most rocks are made of oxygen and silicon – silica – bound together with light metals like aluminum), and is usually richer in ‘wet’ minerals that release water when they melt. Magmas that are oversaturated in silica are ‘sticky’. They are so viscous that it is difficult for them to reach the Earth’s surface. The lavas they produce are lighter in color than basalt (which bleeds up darkly as a partial melting of the mantle beneath the crust), and form rocks called dacite or rhyolite. These magmas are capable of trapping dangerous amounts of superheated steam and other gases. If they do make it to the surface, the violent expansion of those trapped gases has terrible results.
Some of the basaltic magma leaked out around the flanks of the Jemez Mountains before the big events to come. You can see one of these flows at the base of the outcropping:
There’s plenty of space to park and get out of your car at this roadcut, and if you do, you can put your nose on these dense black rocks and see tiny greenish flecks of the Earth’s mantle suspended within. These flecks are unmelted fragments of the mineral olivine, a simple, heavy silicate rich in iron and magnesium. (Larger gem quality crystals of olivine are called peridot, the gemstone for August. Sorry; none of those here. I’ve looked.) What little gas these magmas contain bubbles out peacefully. You can see some of the last of the bubbles trapped in the basalt if you look.
This lava bed sat at the Earth’s surface long enough for a soil to develop on its top. That’s the reddish-orange clay you see above the dark basalt. No doubt it supported a thriving forest on the fatal day when, somewhere to the west, the dangerously swollen magma chamber beneath the mountains could contain itself no longer and blew out.
A massive jet of silica-rich magma fountained skyward with such force that it propelled a boiling cloud of ash into the stratosphere, miles above, within minutes. Clots of stiff rhyolitic magma, bursting with gas and incandescent with heat, swelled into a glassy foam – pumice – light enough to float on water, but not light enough to stay suspended in the blackening atmosphere. The bulk of the magma simply ruptured into a choking ash. The pumice began to fall back to earth in a hellish hail of such magnitude that, here at least, it buried the shaking ground as much as 20 feet deep. (Winds blew ash and smaller bits of pumice over the Santa Fe area and well into Texas) As pictured above, you can put your hand on the very place where the first particles reached the ground.
Worse was yet to come. You can get a hint of this if you examine the top of the pumice layer, where it becomes more stratified. Parts of the eruption column began to collapse and send hurricane-force gusts of pumice and ash across the blanketed landscape in pyroclastic flows hot enough to sterilize everything they touched. This is what happened (on a smaller scale) to Pompeii and its unfortunate citizens. If you want to relive their experience, treat yourself to the climactic scene in Robert Harris’ “Pompeii“. Remarkable.
Fortunately for the history of the Western World, what happened at Pompeii stopped at this point. Things didn’t stop in the Jemez Mountains. The magma chamber was too large, too over-pressured, and too unstable to be ventilated in one central eruption. Multiple vents began to open up in a roughly circular pattern miles across over the crown of the chamber, each one screaming with expanding gas. The crust inside this ring began to collapse into the chamber, like a huge piston, carrying down an entire range of mountains (which were immediately swamped in ash so hot it fused back into liquid rock) So much magma was ejected into the atmosphere that the eruption columns, in a continual state of collapse, spread pyroclastic flows over hundred of square miles, filling valleys, burying mountains up to their necks, and and smoothing the landscape into a sterile plateau of lifeless ash.
Look at the photograph again:
These pyroclastic flows left a deposit which, in this picture, extends from the place where the ash beds turn a brighter white all the way to the base of the orange cliff. It is called the Lower Bandelier Tuff. The reason it is called the Lower Tuff is that, unbelievably, this entire cycle repeated itself 400,000 years later, with even greater force. That eruption left the next massive layer, the Upper Bandelier Tuff, which forms the steep orange cliffs at the top of the pile. This upper tuff is the layer in which Frijoles Canyon, in Bandelier National Monument, is carved, and it is a record of the eruption that formed the Valles Caldera just beyond the monument.
There are two recent publications available in Santa Fe that you must look for if you find these sights as fascinating as I do. The first is Kirt Kempter and Dick Huelster’s “Valles Caldera”, which is a fold out map with associated text and photographs, published by High Desert Field Guides. The second is Fraser Goff’s “Valles Caldera: A Geologic History“. Both of these publications are intended for the lay audience and both of them are well worth having. You can find copies for sale here in Santa Fe at The Travel Bug and Collected Works Bookstore. If you manage to make your day trip without them, you’re sure to find copies at the respective bookstores at the headquarters of Bandelier National Monument and the Valles Caldera National Preserve.
P.S. The matter of Pompeii shouldn’t be taken lightly. There is a partially submerged caldera just west of Naples, called the Campi Flegrei, that is only a little smaller than the Valles Caldera – and it is only the latest scar of some truly catastrophic eruptions on the Italian peninsula.