Category Archives: metamorphic rocks

Strain

One of my absolutely favorite outcroppings in the mountains above Santa Fe is this little stream-polished window into the depths of the Earth’s crust, along Little Tesuque Creek, not far above the Bishop’s Lodge Resort:

Hornblende schist - a kind of amphibolite - along Little Tesuque Creek, Santa Fe, New Mexico

Hornblende schist – a kind of amphibolite – along Little Tesuque Creek, above Santa Fe, New Mexico

The strain displayed by this dark schist is hard to miss. Outcroppings of metamorphic rocks are abundant in the ancient crystalline core of the Santa Fe Range, but few of them exhibit such dramatic stretching as this example. Here’s a close up of a structure known as boudinage:

Boudins of more quartz-rich layers in the schist

Boudins of more quartz-rich layers in the schist

These stony strings of sausages are the result of differing competencies among the mineral components of the schist. The smooth borders attest to the overall ductility of the rock as it was being slowly sheared, deep in the plutonic realm of the middle crust.

These schists are sturdy, and rounded cobbles of this rock are common throughout the foothills and stream beds west of Santa Fe. Most of them glitter in a dark way, as tiny prismatic crystals of hornblende, aligned by shearing strain, send back a little light. Harder to see is their matrix of white plagioclase feldspar, which probably makes up over half the rock’s mineral content.

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.

 

 

 

 

 

A Schist Belt in the Santa Fe Range

One of my favorite hiking trails near Santa Fe is the Chamisa Trail. Its trailhead is just within the border of the Santa Fe National Forest, off of Hyde Park Road, or NM 475, only a few miles from downtown. Because of its relatively low elevation, between 7800′ to 8500′, and the placement of the first mile of the trail along the west-facing slopes of a small open canyon, the trail makes a good walk nearly all year.  The path reaches a saddle after about a mile, and then descends into a shadier drainage for another mile or so, where it intersects with the Winsor Trail along Tesuque Creek.

This is my go-to trail when I need a quick head-clearing walk. It is close by and well maintained. The light is always good. It is particularly luminous in the winter, a time when higher elevation trails require snowshoes. It is practically the poster child for a walk in a mixed conifer forest in the Southern Rockies. If I were designing a diorama for such an ecosystem for a museum, I would copy a few hundred square feet of the forest around the Chamisa Trail.

Ponderosa and White fir along the Chamisa Trail

Ponderosa and White fir along the Chamisa Trail

But for a long time the rocks along this walk puzzled me. The outcrops along the path are not particularly notable; the one in the photograph below is about as good as they get:

A typical outcropping along the Chamisa Trail

A typical outcropping along the Chamisa Trail

Slopes are steep, but rounded, and are covered with a thin soil littered with fragments of rock:

Stony slopes in the forest

Stony slopes in the forest

There is a distinctive mix of colors among the fragments, reflecting the underlying variability of the bedrock:

Rock fragments and pine needles

Rock fragments and pine needles

At a first glance these rocks don’t seem to fit into any straightforward category. They’re clearly metamorphic, with a steeply dipping foliation in outcrop, crystalline, but fine-grained and somewhat blocky close up. Under the hand lens they seem to be made up of either small crystals of quartz and pink feldspar, or small crystals of hornblende and white feldspar. There is very little mica overall. The rocks are moderately platy, but so fractured with finely spaced joints that they weather quickly into angular gravel-sized fragments. (This is not a pleasant trail on which to go barefoot)

Here are some examples:

Chamisa Trail coarse fragments

Chamisa Trail platy fragments

Chamisa Trail variable

Chamisa Trail blocky

 

Metamorphic rocks are classified in the field by their texture. These rocks are certainly not in the coarse-grained gneiss family, nor are they in the directionless-textured fels family. I decided to put them in the schist family by default, in spite of their high feldspar content and low mica content. Schists are typically rich in strongly-aligned micas, causing them to split freely into plates, and giving them a distinctive sheen. Perhaps the protoliths of these rocks – which I suspect are volcaniclastic – were unfavorable for mica to recrystallize. Some of the darker schists break into large angular blocks and are usually called amphibolite in local descriptions of the rocks.

In places along the Chamisa Trail, these metamorphic rocks are intruded by dikes of a distinctive unfoliated pink granite, rich in silvery muscovite mica:

A dike of orange-pink granite intruding the schist

A dike of orange-pink granite intruding the schist

These colorful granites add extra variety to the walk.

There is an entire belt of these schistose rocks in the mountains near Santa Fe, wedged between a mass of monotonous orange gneiss to the south and southwest, and the strongly foliated pink granite, grey tonalite, and high-grade gneiss that make up the core of the Santa Fe Range to the north and east. If you follow the Winsor Trail north from its intersection with the Chamisa Trail at Tesuque Creek, you can walk through the transition from schist to foliated granite within less than a mile.

In this Google Earth view, the Chamisa Trail follows the two end-to-end drainages in the center of the photograph. As far as I can tell, this is just about the center of the schist belt. The higher ridges just to the left of the sharp north turn in the road are made up of foliated granite, underlying Hyde Memorial State Park.

The Chamisa Trail looking north

The Chamisa Trail looking north. Click on photo to enlarge.

Geologic maps of this area show large, but currently inactive, faults cutting through some of the north-south drainages that interrupt the western slopes and streams, including the one the Chamisa Trail follows. These faults were reactivated during past mountain-building activity in the American West, and their movement shattered the brittle schist, gneiss, and granite as they shifted. Shattered rock is more vulnerable to weathering and erosion than neighboring intact rock and as the highlands eroded, the old fault zones helped guide the network of evolving drainages. If you walk down the branch of the trail that follows the dry stream bed back to the Hyde Park Road trailhead, you can see examples of fault breccias:

Fault breccia in amphibolite schist

Fault breccia in amphibolite

and slickensides:

Slickensides ("fault scratches)

Slickensides – the ‘scratched’ pattern

in the outcrops.  Perhaps these strains account for the closely-jointed character of the schist all along the Chamisa Trail.