Category Archives: igneous rocks


One of the joys of teaching is, oddly, learning. Teaching a basic class in physical geology to a wide range of students at the local community college brings me weekly opportunities to hone my communication skills – to try to find concise ways to summarize complex topics in an accurate, yet simple, manner. The struggle to do this teaches me far more, I suspect, than I teach my students.

Volcanism is a case in point. It’s a big topic. Is there a way to capture its essence in a blog-sized post? Here’s a try:

As I’ve mentioned before, New Mexico is the Volcano State. The sheer variety of volcanic features here is unrivaled by any other state in the country, including Alaska and Hawaii. And yet all of this volcanic activity can be encompassed between two poles: effusive eruptions and explosive eruptions.

Effusive eruptions

Effusive behavior refers to relatively quiet outpourings of molten lava from a volcanic vent. Effusive eruptions of fluid basaltic lavas are exceedingly common, forming stacks of thin flows that pile gently into broad shield volcanoes, or spread out across the countryside in lava plateaus.

Valley of Fires, a vast flow of basaltic lava, in southern New Mexico

Valley of Fires, a vast flow of basaltic lava, near Carrizozo, New Mexico. Click to enlarge.

Effusive extrusions of viscous silicic lavas, like dacite or rhyolite, are much less common. These eruptions typically form short, thick, glassy flows, piling into rubble-covered lava domes that usually plug their own vent.

An arc of rhyolitic lava domes in the Valles Caldera

An arc of rhyolitic lava domes in the Valles Caldera, west of Los Alamos, New Mexico. Click to enlarge.

Explosive Eruptions

Explosive eruptions occur when violently expanding gases fragment molten lava into clouds of pumice, scoria and volcanic ash. The first phase of many otherwise effusive eruptions of basalt is a gassy discharge that builds a one-shot, steep-sided cone of scoria, commonly called a cinder cone.

A cinder cone you can walk into, near Grants, New Mexico

A cinder cone you can walk into, near Grants, New Mexico. Click to enlarge.

Eruptions of somewhat more silicic lavas, like andesite, dacite, and trachyte often alternate between explosive and effusive phases, over tens of thousands of years, building “composite” cones of lava and ash that can grow into large – but unstable – mountains.

Mt. Taylor, near Grants, New Mexico, is a classic composite volcano

Mt. Taylor, near Grants, New Mexico, is a classic composite volcano. Click to enlarge.

Although thankfully uncommon, very large eruptions of silicic rhyolite lava can be catastrophically explosive, burying thousands of square miles of countryside under  incandescent blankets of ash flow tuffs up to a thousand feet thick. Events of this magnitude probably alter the global climate. These eruptions leave circular zones of collapsed crust, miles across, called calderas, so large that they are difficult to recognize from the ground.

The Valles Caldera from space

The Valles Caldera from space. Click to enlarge.

What lies below

Magmas trapped and crystallized as shallow intrusives in the feeder systems beneath volcanoes – dikes, sills, and shallow stocks – are often grouped together with the other volcanic rocks.  Where erosion exposes these formerly buried structures, fascinating landforms result, complementing the flows, cones, craters, and calderas of modern New Mexico.

Cabezon Peak, a basaltic volcanic neck, west of San Ysidro, New Mexico

Cabezon Peak, a basaltic volcanic neck, west of San Ysidro, New Mexico. Click to enlarge.


Born of Fire

To the surprise of many, New Mexico is sometimes called the Volcano State. It’s not that we have any erupting volcanoes – at present. But the sheer variety of volcanic features here is unrivaled by any other state in the country, including Alaska and Hawaii. We are definitely an igneous state.

Back in fifth grade you probably heard about the three great groups of rocks on Earth: igneous, sedimentary, and metamorphic. Igneous rocks are rocks that crystallize from melts called magma. Magma is a mix of liquid silica-rich melt, suspended crystals, and dissolved gas like water vapor and carbon dioxide. It’s hot: 1300 to 2400 degrees Fahrenheit, hot enough to glow like fire. When it cools, it freezes in complex ways into igneous rocks – “born of fire”.

Most magma remains trapped in the Earth’s crust. But when it gets out, it forms volcanoes, named after Vulcan, the Roman god of fire, with his smoky forge Vulcano in the Mediterranean Sea. Magma extruded at the Earth’s surface is given an older Italian name, lava, which refers to both the flowing melt and the rock into which it cools. Lava flung into the atmosphere by the explosive expansion of dissolved gas forms a variety of fragmented and glassy materials called pyroclasts - “fire fragments”Lavas, loose pyroclasts, or tephra, and pyroclastic material consolidated into rock, called tuff, collectively form the volcanic rocks.

Since volcanic rocks are quenched at the Earth’s surface, they typically have fine-grained, glassy, or fragmental textures. It is usually easy to recognize a volcanic rock in the field, but assigning them to their specific family – basaltic, andesitic, trachytic, dacitic, or rhyolitic – can be frustratingly difficult. Much depends on finding and identifying small suspended mineral crystals to help out, which is something few of us do on a regular basis. Like algebra. Learning to simply recognize a lava, and to name variations based on texture, like pumice or obsidian, is an easy and rewarding undertaking for natural history buffs.


Magmas trapped deep in the Earth’s crust belong to the plutonic realm, named after a darker god, Pluto, the Roman ruler of the underworld. These magmas are intimately associated with the underworld rocks which they intrude, rocks which have been changed by heat, confining pressure, and shearing stresses into metamorphic rocks.

Plutonic rocks, having cooled slowly deep in the crust, with all their juices sealed in, typically have coarse-grained, visibly crystalline textures – granitic textures. This makes it a little easier to assign them names in the field – granite, granodiorite, tonalite, diorite, gabbro, monzonite, syenite – but since magmatic rocks are mixes, not species, there is always some blurring and overlap. Learning to mentally gauge whether the rock is rich or poor in dark minerals, and rich or poor in visible quartz, helps out here. Light-colored, quartz-rich members of the granite and granodiorite family are much more common than the others.

Simply finding a plutonic or metamorphic rock in the field means something has transported that messenger from the underworld up to the surface. What could it be?


There are any number of rock-identification guides and classification schemes both online and off, but one sweet site you might want to visit has been created by our Australian cousins: Igneous rock types. Go have a look!




What on Earth are you doing with Howard Bannister’s rocks?


The classification of the igneous rocks is a morass from which you would be well advised to steer clear. Even Judy Maxwell said, “I can take your igneous rocks or leave them. I relate primarily to micas, quartz, feldspar. You can keep your pyroxenes, magnetites, and coarse-grained plutonics as far as I’m concerned.”

Personally, I love the igneous rocks. Nevertheless, there is one coarse-grained plutonic up in the mountains above Santa Fe which has given me fits in trying to classify. And it points perfectly to the sort of look-alike confusion which plagues the field identification of these rocks.

Here’s the rock:

The speckled rock along Tesuque Creek

The speckled rock along Tesuque Creek

Ideal countertop material, you might say. I asked a hiking companion what he thought it was and was told “it looks just like the granite back home up in the Sierra” – the Sierra being the Sierra Nevada Mountains in California. And it does look just like those granites, except for the caveat going though my head – the curse of an education – that, as the geologist P.B. King relates, in the Sierra Nevada, “true granites in the technical sense are rather minor, most of them being the somewhat more mafic quartz monzonites, granodiorites, and quartz diorites”.

I thought it might be diorite. Diorite is an interesting construction, a French name built from the Greek root dior izein, ‘to distinguish’. Diorite is a granular igneous rock made up of bright white feldspar and dull black hornblende, with a classic “salt and pepper” appearance that every first year geology student learns to identify on sight.

Unfortunately, diorite is very difficult to distinguish from gabbro, another dark speckled igneous rock, which is what another hiking companion (understandably) always thought it was.

There’s a reason field geologists carry around that little ten-power hand lens, and when you look at this rock up close, you discover that most of the dark minerals are the black mica called biotite, and that there is an awful lot of quartz mixed in with the white feldspar. This throws the ball back into granite’s court, petrologically speaking, and there is a surprising name for the common hybrid between granite and diorite: granodiorite. So that’s where I finally decided to pigeonhole the rock. A very dark granodiorite.

Except that I found out its real name is tonalite.

The point is, the point is… oh god, I’ve forgotten my point.



Cerrillos Hills State Park: get in contact with the Earth

A view of the Sangre de Cristo Mountains from Cerrillos Hills State Park

A view of the Sangre de Cristo Mountains from Cerrillos Hills State Park

One of the most remarkable windows into the natural history of northern New Mexico is accessible to all of us in the Cerrillos Hills State Park, about a 30 minute drive from Santa Fe, and around an hour’s drive from Albuquerque. And with over 1000 years of mining history hidden in these austere cerrillos, the park is a window into the long cultural history of the American Southwest, as well.

There are so many natural features to explore in the park that I’m sure I’ll be coming back many times to touch on them. Although its geology is complex, and still under investigation by State geologists, even a short walk along its dusty trails will reward you with examples of basic physical geology concepts, the sort of things students meet in their very first classes in Earth Science.

Natural history walks in the park begin in the parking area, which you can see in the lower left hand  of this Google Earth image, and wind up and over the Jane Calvin Sanchez trail, which heads off sharply to the right, across the road.

Looking north from the entrance to the park. Grand Central Mountain in the background.

Looking north from the entrance to the park. Grand Central Mountain in the background.

With over 5 miles of trails to explore, there is plenty you can investigate on your own. There are also things you can see from the village of Cerrillos, just south of the park, as well as from the drive down from the village of Madrid, and – if it hasn’t been raining – from the unpaved Waldo Canyon Road which skirts the hills on the south and eventually brings you to Interstate 25 between Santa Fe and Albuquerque.

One of the most basic concepts in physical geology is the contact. A contact is the surface separating a distinctive body of rock from its neighbors. Contacts are what geologists trace on their maps when they are working out the natural history of an area. The contacts between rock bodies form in different ways, and in Cerrillos Hills State Park three important kinds can be seen within a short walk of the parking area.

A few steps down into the dry wash just below the park’s signs brings you to an eroded bank where grey, sharply tilted beds of shale are abruptly overlain by a flat-lying, crudely bedded alluvial gravel, cemented into conglomerate. The shale forms one distinctive body of rock, the conglomerate another, and the surface between them is the contact. Here is a picture of a similar contact, seen from the village of Cerrillos:

Mancos Shale overlain by a thin horizontal layer of Quaternary Alluvium

Mancos Shale overlain by a thin horizontal layer of Quaternary Alluvium (area at the upper left)

This contact is an example of a very significant type of contact called an unconformity. This is an unusual word and it will take a few lines of explanation to unpack its meaning.

Among sedimentary rocks, uniform depositional conditions result in packages of parallel, or conformable, strata. The grey shale, for example, consolidated from mud that accumulated slowly on the floor of an inland sea, and its innumerable thin layers are all more or less parallel to one another.

An outcropping of Mancos Shale

An outcropping of Mancos Shale

Very broadly speaking, accumulations of sediment like this marine mud start and stop depending upon whether the area is above or below water. Once a region rises above sea level erosion dominates and sedimentation stops. The Cerrillos Hills sit far above sea level and erosion is active, but here and there streams have left deposits of alluvial gravels in channels cut into the older shale. These gravels are in unconformable contact with the shale. Like most unconformities, this contact is a buried erosional surface, and like all unconformities, it represents a break in the continuity of the geologic record. Metaphorically speaking, unconformities split the geologic record into chapters. Hence their significance to natural history.

A short walk from the wash up along the Jane Calvin Sanchez Trail brings you to a second kind of contact, the intrusive contact. These contacts reveal the Big Story in the Cerrillos Hills, because, as we will see in other posts, the Cerrillos Hills are a gigantic blister of subvolcanic activity, shot through and through with bodies of formerly molten rock. These magmas burrowed their way through the shale, intruded upon one another, fed a small volcanic complex that is now completely eroded away, and set up the (long dead) hot spring activity that brought the metal ores that brought the generations of miners and gem hunters here.

Just behind the pleasantly smiling ranger you can see a prominent outcropping of rough rock, with a juniper-covered slope resting on its left side. This change marks the contact between a dike of hard igneous rock and the softer shale into which it was intruded.

An intrusive contact just to the right of the juniper bushes

An intrusive contact just to the right of the juniper bushes

The Cerrillos Hills are full of these intrusive contacts. Here is a closer view of the contact between the Mancos Shale and an injection of andesite, one of the first magmas to invade the shale. The molten andesite was hot enough to bake the shale into a hard rind of hornfels, a type of metamorphic rock.

Andesite on left, baked shale on the right

Andesite on left, baked shale on the right

By the way, this kind of metamorphism is called – wait for it – contact metamorphism.

The Cerrillos Hills are pitted all over with prospects dug into promising areas where a miner might find a lode of silver-bearing galena, or lead-zinc sulfides (with a bit of copper thrown in), or crusts of gem-quality turquoise. Several of these holes are visited by the network of trails in the park, all sadly fenced in for your protection. The first one you encounter on the Jane Calvin Sanchez Trail gives us a look at a third kind of contact, the fault contact.

A look into the Christian Lode prospect.

A look into the Christian Lode prospect.

In the photograph above, notice the angled contact, just to the right of the old juniper-pole cribwork, where a body of fractured and bleached andesite (left side) rests against some tilted and distinctly bedded, but not baked shale (right side). This contact represents a shear zone in the rocks along which two different rock types have been dislocated and shifted into contact. In other words, a small fault.

An entire system of northeasterly-trending shear zones transects the Cerrillos Hills. These zones channeled the hydrothermal activity set up by larger, later, and hotter igneous intrusions, which in turn focused the mineralization which has, in its turn, attracted people here for over 1000 years. Below is a Google Earth image looking straight down on the Christian Lode.  Notice the bleached zone angling through the pit and striking off to the northeast, a reflection of the shear zone below:

A raven's eye view of the Christian Lode

A raven’s eye view of the Christian Lode (the black outlined square toward the lower left near the trail)

I think this is a pretty good harvest of examples for such a small investment of walking. And we’ve barely scratched the surface here, so to speak. The Cerrillos Hills are a showplace of igneous activity rarely exposed in such an accessible way, so we will be back to have a look at its other geological treasures in future posts.

Hiking in Cerrillos Hills State Park

Hiking in Cerrillos Hills State Park






Tuff, tephra, tufa, travertine: what’s the difference?

Cliffs of white rhyolite tuff at Kashe-Katuwe Tent Rocks National Monument
Cliffs of white rhyolite tuff at Kashe-Katuwe Tent Rocks National Monument

Our visitors to Northern New Mexico aren’t here very long before they hear the “T” words tossed around. You arrange a visit to Bandelier National Monument and find yourself driving among the most unusual-colored cliffs you’ve ever seen:

On the drive to Bandelier National Monument
On the drive to Bandelier National Monument

When you get to the Monument the ranger tells you you’ve been driving through the Bandelier Tuff. Pretty soon you find yourself inside the stuff:

Exploring the cliff dwellings in Bandelier National Monument
Exploring the cliff dwellings in Bandelier National Monument

Or you make a road trip though the Valle Grande National Preserve, intending to stop at Los Ojos Saloon in Jemez Springs for one of their “Famous Jemez” Burgers, and get sidetracked looking at the Soda Dam right along the road outside of town:

The Soda Dam near Jemez Springs
The Soda Dam near Jemez Springs

This must be tufa, right? We just learned about tuff. Well, no, it’s technically travertine.

And why did they spray all that white concrete all over the rocks way out here?

The Guaje Pumice along the road to Los Alamos
The Guaje Pumice along the road to Los Alamos

Wait! The “High Desert Field Guide” I just bought says that’s a pumice deposit, and geologists call it tephra.

What a confusion of tongues! But as any student of the subject knows, the geological sciences are  afflicted by a kind of logorrhoea when it comes to naming things. The geologist P. D. Krynine called stratigraphy “the complete triumph of terminology over facts and common sense”. And that’s just stratigraphy.

Most people instinctively know that each of these words probably has something to do with volcanoes, somehow, and of course they’re right. Tephra refers to any of the fragmental material a volcano ejects, regardless of the size of the fragments. This includes primary material like ‘bombs’, ‘cinders’, and ‘ash” as well as blocks of rock torn from the volcano’s cone or carried up from crust. As long as this fragmental stuff lies loosely in cones or drifts or layers on the ground, it is still called tephra.

There have been eruptions of pumice – a sort of froth of natural volcanic glass – so recently in the Jemez Mountains that your can scoop up loose pumice out of the roadcuts with your hands:

A roadcut in tephra from the El Cajete crater in the Jemez Mountains
A roadcut in tephra from the El Cajete crater in the Jemez Mountains

Once this fragmental material consolidates into a more or less firm rock, it is called tuff. Some air-fall tephras are turned into rock – ‘lithified’ is the term geologists prefer, which means ‘turned into rock’ – by compaction and by ground water cementing the particles together. But most tuffs are formed almost immediately upon cooling, since the hot glassy fragments that make up the bulk of volcanic ash in pyroclastic flows are soft and hot enough to fuse together. This process is called welding. Depending upon the heat and size of the eruption and the proximity to the vent, a tuff can be welded weakly, or it can be fused together so firmly that you can scarcely distinguish it from lava.

Much of the Bandelier Tuff that visitors see on the way to Bandelier National Monument is only weakly welded, and once the weather-hardened rind is broken through, you can gouge out the tuff with simple tools. As you drive from Bandelier on into the Valles Caldera National Preserve, you might notice the tuff getting firmer and darker, breaking into hard plates of unmistakable rock.

A spectacular example of a welded tuff can be found within San Diego Canyon in the  Jemez Mountains at Battleship Rock:

The grim prow of Battleship Rock
The grim prow of Battleship Rock

The only way to gouge out these rocks is with a jackhammer. This body of tuff was laid down by a very hot pyroclastic flow that surged down a canyon cut in the flank of the Valles Caldera very recently, geologically-speaking. In fact it records a catastrophic phase in the eruption that put down the El Cajete pumice we saw earlier. The flow of ash and pumice was so hot that the fragments of pumice were deformed into squashed and drawn-out tongues of rock, called fiamme (Italian for flames):

Fiamme in the Battleship Rock Tuff
Fiamme in the Battleship Rock Tuff

Firmly welded tuffs deposited from hot pyroclastic flows of ash are sometimes called ignimbrites. You really can’t help loving this word: “glowing cloud stone”.

Travertine is calcium carbonate rock (“limestone”) deposited from a hot spring. Since the rock consolidates from material dissolved in hot water, rather than magma, it is technically a sedimentary rock. You can watch travertine forming right before your eyes at Soda Dam:

Travertine forming around one of the hot springs at Soda Dam
Travertine forming around one of the hot springs at Soda Dam

Heat generated from volcanic activity in the Jemez Mountains has set up a naturally circulating hydrothermal system that dissolves calcium carbonate from beds of limestone  below the mountains, and then redeposits it at the surface where the hot water leaks out. The springs fizz with bubbles of carbon dioxide as the water depressurizes – hence the name Soda Dam – and this abrupt change in chemistry precipitates out the lime. This process is probably assisted by the algae that thrives in the hot water, accounting for the crusty, porous, almost baklava-like texture of the rock:

Older travertine at Soda Dam
Older travertine at Soda Dam

The only difference between tufa and travertine is the minor caveat that travertine is deposited from hot springs and tufa is deposited from springs at ambient temperatures. The only place I’ve seen tufa forming in Northern New Mexico is near Tunnel Springs in the Sandia Mountains near Albuquerque, where mineral-charged water seeping out of limestone beds is forming  deposits of scaly lime, intimately mixed with sticks and leaf litter. Many “petrifying springs” are springs busy laying down tufa. Some of the cooler parts of Soda Dam mimic this, slowly fossilizing grass:

Tufa-like deposits at Soda Dam
Tufa-like deposits at Soda Dam

So there you have it.