Subduction leads to orogeny
April 19, 2010
That’s a little bit of lame undergraduate geology student humor, occasionally seen on bumper stickers. A sort of an inside joke – but one with more than a grain of truth about it. An orogeny is an episode of mountain-building, not so much in the sense of carving mountains out of the rocks, but more in the way of actually creating the hot, thickened and buoyant rocks themselves, which subsequently rise into a range of mountains. These mountains may be worn right down to the gums, subsequently, but the evidence of the disturbance will remain frozen in the new rocks, once they are uncovered by uplift and erosion.
Subduction, on the other hand, is the pulling-down of dense crust and uppermost mantle into the interior of the planet. Think abduction – pulled away – or induction – pulled in – and you will get the idea. The great unifying theme of geological change – plate tectonics – requires this action, to compensate for new crust and uppermost mantle being born at the vast network of mid-ocean ridges that web our planet. Zones of subduction are naturally places where crust converges, and if conditions are favorable, a sort of piling-up and heating of crust can occur. And there you have it: subduction leads to orogeny. Ha!
You can actually watch this process happening in the microcosm of a soup bowl, next time you visit your favorite Japanese restaurant. Gaze into your bowl of miso soup as it cools – and – if you have an imagination like mine – you can see dark little zones of subduction lacing the surface of your meal:
These patterns drift across the surface of the soup, forming spontaneously, moving, merging, forming again. It’s fascinating to watch.
So what can this possibly have to do with Santa Fe? (Beside the fact that we have some great Japanese restaurants?)
Well, take the wings of geological imagination and have a look at New Mexico from the Space Shuttle, around, say 1.75 billion years ago. We’re right in the center of the picture:
Looks more like Indonesia and Southeast Asia, doesn’t it? Those dark streaks, by the way, mark oceanic trenches that have formed over subduction zones. They also mark plate boundaries. And if we could put the map in motion, those arcs of island would be slowly converging toward the rigid nucleus of early North America – you can just make out the outline of Montana, for reference – destined to crunch up against it and add to the continent.
Island arcs like the ones portrayed above are built by the volcanic activity that occurs above subduction zones as they pull water-soaked oceanic crust down into the hot dry mantle. Basins form near the islands, providing space for volcanic tuffs, lavas, and sediments eroded from the islands to accumulate. Great thicknesses of these materials can pile up, although the bulk of the mass is too “light” to be pulled down into the mantle by subduction. But as the islands and their basins converge and crush together, the bottom of this pile can be pushed as much as 20 miles or more deep into the Earth, where it’s hot enough to begin melting. Formerly horizontal beds and layers are compressed into folds as tight as those of a squeezed accordion as the pile contracts.
Only the very deepest roots of these crustal welts – called orogens – may actually melt. The middle and lower parts, 10 to 15 miles down, approach melting temperatures, however, and are not only under enormous confining pressures, by virtue of their depth, but are subjected to distorting stresses as contraction occurs. The minerals in the buried rocks begin to react with one another and re-crystallize as they adjust to the new conditions of heat and confining pressure. They realign themselves according to the distorting stresses. The sediments and volcanics begin to morph, without melting, into hard, dense, and visibly crystalline rocks of completely different appearance. They are now metamorphic rocks.
As you drive up Hyde Park Road (NM 475) from Santa Fe, on your way to say Ten Thousand Waves for a massage, or for a hike on the Chamisa Trail, or perhaps heading up to Hyde State Park, and begin to leave the soft pinyon-covered hills dotted with expensive homes, you begin to see road cuts like this:
“Such tortured rock!” a friend of mine once exclaimed, upon seeing this. And indeed it is. A closer look reveals a distinctive texture:
You can almost feel this rock flowing like warm taffy. Although it’s counterintuitive, crystalline solids, like rocks, can flow slowly if they are put under a distorting stress, especially if they are close to their melting temperature. The closest actual experience we have to this activity is the flow of ice in glaciers, in which a solid – ice – not all that far from it’s melting temperature, flows slowly downhill under the stress of gravity. And even this activity proceeds with, well, glacial slowness.
The visibly banded and ‘stretched’ texture of this rock, basically made up of the same minerals you find in your granite countertop but profoundly deformed, identifies it as a gneiss (pronounced ‘nice’). It’s a classic metamorphic rock. Based on its mineral content, the rock probably began life as a silica-rich volcanic tuff or a gritty siltstone. It looks nothing like those rocks now.
Quite often you will see features like these:
The “veins” of pink rock really are just like the granite in your countertop, because that’s what they are made of: granite. The pink color comes from the mineral feldspar, the most common mineral in the Earth’s crust: a blocky, moderately hard mineral made up of oxygen and silicon combined with the light metals aluminum and potassium.
A mineral with which you are probably more familiar is also abundant in these granites, and in some places forms nearly pure, milky-white, crystalline masses – quartz:
Quartz is made up of a tight molecular framework of oxygen and silicon with no contaminating metals, and consequently it is a very durable mineral, slow to break down here at the surface. You will frequently find it lying around on the ground while you are hiking in the mountains here, looking like pieces of unmelted ice. (In fact the ancient Greeks thought that quartz was ice that had frozen so firmly it could not melt. Their word for this substance was krystallos – and – I bet you guessed – this is where our word “crystal” comes from.)
You can see from the two photographs above that these veins of granite (technically “dikes” of granite) cut across the stretched and banded grain of the gneiss. That’s because these rocks were injected into the gneiss as melted rock, either as the gneiss was forming, or, in some cases, long after it formed. And since the granite did crystallize from a melt, it is classified as an igneous rock.
Many of the road cuts you’ll see on the lower part of the drive, before you get to Hyde State Park, have a rather motley appearance:
Here the injections of granite have been caught up in the deformation of another kind of gneiss with a somewhat difficult name: amphibolite. A closer look at this dark rock show the same flowing distortions you see in the more traditional gneiss:
Amphiboles are a class of common iron-rich minerals which tend to form elongated and lustrous black crystals. A name for the most common of these minerals is one you might be able to retrieve from your 5th grade memories – hornblende. Based on the abundance of these dark, iron rich minerals, it’s likely that these rocks began life as a silica-poor volcanic tuff, erupted from volcanoes spewing basalt or andesite.
In the sunlight these rocks have a distinctive glitter set up by reflections from all those tiny aligned crystals of hornblende. In fact all of these rocks are characterized by a texture of dense interlocking crystals large enough to glitter and shine, and geologists frequently group all these rocks together as the crystalline rocks.
Remember – these rocks formed at depths of 10 to 15 miles down in the Earth’s crust. Now they are exposed and shivering up over a mile and a half above sea level! It took miles of crustal uplift to bring these rocks up from the depths. It took miles of erosion to strip the overlying rocks out of the way. These are profound movements. Entire mountain ranges had to be eliminated to accomplish this, over 100’s of millions of years. The evidence shows that the ancient orogen was worn as flat as Kansas, and eventually slipped beneath the ocean, only to become the dumping ground for new, “young” sediments, “only” 500 to 300 million years old. You can actually see this very contact along the Hyde Park Road, just as you make the sharp turn east into the canyon:
Normally you’d have to go to the bottom of the Grand Canyon to see this.
By contrast, the relatively few miles of crustal buckling and uplift that brought up the Rocky Mountains, and pushed the crystalline rocks up where we can see them, seems like a minor afterthought.
Geologists used to lump all these rocks into units called “the crystalline basement” or “the Precambrian basement” and plot them up in a single color on their maps, as sort of the foundation upon which the more understandable, fossil-bearing rocks rested. It is hard to untangle their secrets. But with the development of radiometric age dating, and esoteric techniques with names like geobarometry and geothermometry, along with a better understanding of how minerals behave under stressful conditions, we are beginning to gain some insight. Things really started to fall into place with the advent of plate tectonics; now there were globally consistent mechanisms to explain the associations of all these fascinating rocks.
Geologists have long been entranced with orogeny. They just didn’t realized they’d been seduced.
*This representation is taken from “Ancient Landscapes of the Colorado Plateau” by Ron Blakey and Wayne Ranney.