Category Archives: plutonic rocks

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.

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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?

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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?

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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.

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Migmatite in the Santa Fe Range

One of the more unusual kinds of rock, and one of the most striking, can be found near Santa Fe by simply driving up to our ski slopes, Ski Santa Fe, and having a look at the white boulders along the edges of the parking area. Better yet is making the hike up Winsor Trail, which has a well-used trailhead on the northwestern side of the parking area, around to its intersection with the popular Nambe Lake Trail, where you can see intact outcroppings of this rock in a setting of spruce and aspen trees so luminous and peaceful I call it the Zen Forest.

Hiking in the Zen Forest - Winsor Trail near Nambe Creek

The path through the Zen Forest – Winsor Trail near Nambe Creek

And it is a very eye-catching rock. Here is a boulder underfoot in the Zen Forest:

Walking in the Zen Forest

Walking in the Zen Forest

And here is another high up on Raven’s Ridge:

Boulder on Ravens Ridge

Boulder on Ravens Ridge

These rocks are an unusually high grade (you have to visit this link if you like whisky!) of metamorphic gneiss called migmatite. Migmatites are intricately banded interpenetrations of schist and granite which show  evidence of plastic flow. Their name derives from the Greek word migma, or mixture. It’s not the mix of colors you notice so much, however, as the fascinating, almost liquid patterns the dark and light layers make. It almost looks like the rock is in the process of melting:

An outcropping of migmatite on Ravens Ridge

An outcropping of migmatite on Ravens Ridge

Pods of white granite appear to be almost ‘sweated out’ of the original rock, something very likely to have been the case:

An 'eye' of white granite

An ‘eye’ of white granite

and all of the rock seem to be in a state of arrested flow:

Plastic deformation in the migmatite

Plastic deformation in the migmatite

In spite of this appearance the rock itself is very tough and durable, forming sheer crags

Looking down from Ravens Ridge

Looking down from Ravens Ridge

and eroding into a massive bouldery pavement that can be troublesome to hike over:

Rugged ground of weathered migmatite

Rugged ground of weathered migmatite

In fact these rocks hold up some of the highest ground in the Santa Fe Range, underlying the ridge that extends from Aspen Peak, at the bottom of this photograph, all the way up and around to Lake Peak, just above the highest of the ski runs:

Ravens Ridge and Lake Peak. Click on the image to enlarge.

Ravens Ridge to Lake Peak. Click on the image to enlarge.

When the migmatite is blocked out on a map, it seems to form a sort of massive screen or divider between the more abundant granitic plutons around it. Borrowing a term from anatomy, the migmatite forms a septum between the granites, caught up between injections of magma and separating them, as your septum separates your nostrils. This one is called the Aspen Basin septum, named after the drainage basin which cradles Ski Santa Fe.

Migmatites fascinate geologists because they seem to show what granite might look like in the process of formation. They are metamorphic rocks at the extreme limits of metamorphism, caught in the act of passing from metamorphic to igneous. They are a window into conditions deep in the Earth’s crust, where confining pressure and high temperature make even the toughest rock capable of flowing in a plastic state.

This all begs the question, of course, of just what are these rocks doing over two miles above sea level, when they must have formed 15 miles or more below it? But that is a question we’ll have to take up another time.

 

 

 

 

 

An Unusual Granite Near Santa Fe

The mountains that make Santa Fe’s beloved eastern backdrop are part of the southernmost range of the Southern Rockies, the Sangre de Cristo Mountains. Named for their spectacular winter alpenglow,

A winter evening in Santa Fe

A winter evening in Santa Fe

the Sangre de Cristo Mountains in New Mexico are made up of a variety of smaller ranges which link together into a broad chain of forested highlands that extend from the Colorado border south to Glorieta Pass. South of Glorieta the uplift plunges beneath the arid high plains of central New Mexico.

Santa Fe basks in warm southwestern light at the foot of the Santa Fe Range. These mountains are held up by a large block of ancient crystalline crust bounded on the east by large faults in the Pecos Wilderness, and bordered to the west by the picturesque badlands of the Espanola Valley. The rocks that make up the peaks and ridges are of a resistant character, gneisses and schists of various types for the most part, well-laced by granite pegmatites, and sturdy ridges of quartzite in the north. The very highest peaks (not quite reaching 4000 meters in elevation) are frost-shattered and bitten by glaciation

The crest of Santa Fe Baldy, elevation 12,631'

The crest of Santa Fe Baldy, elevation 12,631′

but much of the range has a softly rounded appearance in spite of its underpinnings:

Looking back toward Santa Fe from Tesuque Ridge

Looking back toward Santa Fe from Tesuque Ridge

Even the less vegetated, more arid foothills lack a certain ruggedness:

Western foothills north of Santa Fe

Western foothills north of Santa Fe

Which is why the bumpy-looking ridge at the entrance to Pacheco Canyon has always attracted my attention:

Looking down Pacheco Canyon into the Espanola Valley

Looking down Pacheco Canyon into the Espanola Valley

This ridge shows all the characteristic signs of a massive granite outcropping. When you take the Forest Service road into the mouth of the canyon, the signs become unmistakable:

Bouldery granite peak seen near the entrance to Pacheco Canyon

Bouldery granite peak seen just within the entrance to Pacheco Canyon

This is the typical corestone weathering of moderately jointed granitic rock. Outcrops along the road are much bolder than the usual modest rock exposures in the mountains

Jointed outcrop of granite along Pacheco Canyon Road

Jointed outcrop of granite along Pacheco Canyon Road

and a glance at the rock itself confirms your suspicions. It is a beautiful, very coarse-grained granite rich in biotite and muscovite mica:

Coarse-grained granite along Pacheco Canyon

Coarse-grained granite along Pacheco Canyon

So why is this unusual? Geologic maps of the Santa Fe Range show large regions of ‘granite and related rocks’ – plutonic rocks – of Proterozoic age, interspersed with belts of Proterozoic metamorphic rocks. But nearly all the granites you find in the Santa Fe Range are strongly foliated – distorted by regional metamorphism – showing more or less concordant relationships with the surrounding metamorphics. It would probably be more proper to call them granite gneiss. Excellent examples of these ‘metaplutonic’ granites can be seen in Hyde Memorial State Park near Santa Fe. The ages of these rocks are in the range of 1.6 to 1.65 billion years, right in line with the assembly of juvenile crust that built most of the crystalline basement of the American Southwest.

The granite body in Pacheco Canyon is not foliated and it contains a fair amount of muscovite mica, a ‘wet’ mineral which, in granites, hints at the partial melting of thickened continental crust possibly including metasedimentary rocks. The granite displays signs of emplacement under conditions differing from metamorphosed granites. There are clearly associated pegmatites:

Granite pegmatite with spectacular crystals of feldspar

Granite pegmatite with spectacular crystals of feldspar

and aplite dikes:

Fine-grained aplite dike

Fine-grained aplite dike with typical sugary texture

Features such as these are signs that this granite was intruded under lower confining pressure than a truly deep-seated pluton. Any subsequent metamorphism would have strongly blurred these cross-cutting relationships, as well.

Throughout the crystalline basement of the Southern Rockies the framework metavolcanic and metaplutonic rocks that formed 1.8 to 1.6 billion years ago are intruded by massive and crosscutting granitic rocks emplaced around 1.4 billion years ago. These plutonic rocks  comprise nearly half the exposed crystalline rocks in the mountains, and they very likely played an important role in ‘welding’ the continental crust into the rigid basement upon which all the subsequent geological drama of the American West played out.

Although it isn’t mapped as such an intrusion, I strongly suspect the granite at the mouth of Pacheco Canyon is one of these late stage 1.4 billion year old ‘anorogenic’ granites. The only way to confirm this suspicion would be to date the rock by radiometric techniques, something far beyond my capabilities. Someday, however, some graduate student will take up the challenge, and I hope I’m around to hear the results.