Under the Volcano

Today I’m going to give a little overview of the geology of the Cerrillos Hills. Before I begin, however, I would like to direct the attention of those more serious inquirers and rockhounds to an article by a true expert on this area, Stephen Maynard: “The Geology of the Cerrillos Hills“. My intention is to highlight some of the bold features you might see on a walk in the new State Park on a sunny afternoon, with a few photographs to guide us along. If this piques your interest, don’t hesitate to build some context by having a look at Stephen’s excellent and easy to read summary.

Everyone has seen pictures of volcanoes, but did you ever wonder what it might look like under the smoking mountain? A walk in the Cerrillos Hills will give you that opportunity, with a little guidance from the geologists. All that pent-up magma, liquid and mobile, seeking a new equilibrium in the Earth’s crust above those places where it has been born, exerts a tremendous amount of pressure as it wedges its way up through the rocks. In places it pauses and pools horizontally, splitting the crust and lifting it up, forming a sort of gigantically hot flat pancake in the crust. If the magma freezes there, the body of rock it leaves is called a sill. (Think flat, like a window sill).

While these splits and ruptures can cease at any time, if they do continue upward and breach the surface, the magma gets out. We call the “getting out” a volcanic eruption.

In our part of the American West, it so happens that a very thick body of shale – mud that has accumulated on the floor of an ocean or a lake, and subsequently compacted into rock – stood in the way of the ascending magma and led to some unusual effects.

Laccoliths are sills with bulging roofs, bowing up the rocks above, like a blister. In the Cerrillos area, these blisters actually stacked themselves one above the other, forming – in the fevered imagination of a geologist – something like a stony Christmas tree. The relative weakness of the thick shale encouraged this phenomenon. To put you out of your suspense, the magma did eventually reach the surface, forming a volcano, but persistent erosion dispersed the volcano and etched its way down into the stack of sills and dikes among which we can walk, today.

The pale grey stuff on the left, with the skirts of loose talus, is the shale. (It has a name, the Mancos Shale, about which more in another piece) The craggy orangy-grey cliffs, forming the little peak on the right, is a partially exposed sill of frozen magma. Keep in mind, from this perspective, the magma didn’t so much push up through the shale as out toward you. And it froze in place far underground. The little layer of orange stuff capping the grey shale on the left is a modern blanket of loose rock eroded from the sills and dikes and spread out as a thick rocky soil.

The magma frozen into the sills and dikes in the Cerrillos Hills has a very distinctive texture. Here’s an example:

You can see a thick speckling of white crystals and clots of black crystals suspended in a greenish-gray mass of stone. An igneous rock with this sort of texture – visible crystals floating in a fine grained groundmass – is called a porphyry. To a geologist’s eye, this texture indicates at least two episodes of cooling. And the stony appearance of the groundmass is a clue that the final episode of cooling was fairly rapid and occurred under low confining pressures, a characteristic of volcanic activity.

By the way, that dark mineral you see is rich in iron, so as these rocks weather at the surface, they acquire a patina of rust. That’s why the rocky outcroppings in the hills are more orange than grey.

As if all this blistering wasn’t enough, in a second episode of igneous activity, a big slug of magma of somewhat different composition forced its way through the pile of laccoliths to feed another generation of volcanism. Some of this magma froze into a large, roughly cylindrical plug – called a stock – right in the middle of our stack of sills, and when erosion hacked its way into this mass, it left the stock standing in relief. It’s big. We call it Grand Central, now, and you can see a picture of it at the beginning of this entry.

This second episode of intrusion was sufficiently forceful to dome up and distort the entire package of shale, sills, and dikes. And this mass of melted crust had an additional cargo of elements humans find either useful or attractive – like gold, silver, copper, lead, and zinc. Now we’re talking!

An intrusion of this magnitude takes a long time to crystallize and cool off, and as it does so, all kinds of secondary effects can occur. The heat of the magma sets groundwater into motion. Much of this water is superheated and aggressively corrosive. The crystallizing magma itself rejects volatile elements like hydrogen (i.e. more water) chlorine, and sulfur. It also rejects elements that don’t fit into the atomic framework of the bulk of the minerals that are crystallizing: the heavier metals that we love to use in wedding rings and bullets and car batteries and telephones. This hot brew of chemicals seeks its way toward the surface as best it can, staining everything it touches and leaving behind crusty residues of exotic minerals rich in those valuable metals.

The ancient ones (and not a few modern rockhounds) sought turquoise, which was used for adornment and was traded far and wide. They also used some of the lead minerals to make pottery glazes. The pragmatic Spanish had no use for turquoise, but lead was always useful for bullets, copper for utensils, and silver for money. The turn-of-the-last-century Anglo miners loved that silver too, but also had industrial markets for lead, zinc, and copper. (There’s not much gold in the Cerrillos Hills, but in the rugged mountains you can see just to the south, the Ortiz Mountains, there was – and is – plenty of gold) Turquoise experienced a new vogue in jewelry and small mines were developed to find it. (You always wondered why the box from Tiffany’s had that particular color, didn’t you?)

As usual, the richer and easier to find deposits were exploited to the point of exhaustion. There’s still a faint halo of copper minerals in the rocks, exploitable by modern mining techniques – but it would require the removal of the Cerrillos Hills themselves to get it (not to mention an ocean of unavailable water) and this is unlikely to happen anytime soon. But for now, we can admire the efforts of the early miners, preserved in the park, and we can enjoy a unique natural museum of subterranean activity – under the volcano – laid out for anyone who takes the time to look.