Thursday, July 24, 2008

Metamorphic rocks of "Middle Earth": boring, or magical?

I love movies with big landscapes, and the New Zealand landscapes of The Lord of the Rings are some of my favorites. So I just had to click on Brian's shared link to a geology.com news post, linking to a Discovery Channel article about 'Middle Earth' Mountains: Steep and Strong.

Executive summary: New Zealand has steep mountains but few landslides. What gives? (Or rather, what doesn't give?)

The answer is: metamorphic rocks. Or rather, the answer is the rapid uplift along the Alpine Fault, which has brought hot young rocks to the surface very rapidly. Many other places with metamorphic rocks (such as New England) have much gentler topography, because they aren't tectonically active. But New Zealand is blessed with the best of both worlds: an active plate boundary, and rapidly exhumed metamorphic rocks.*

Not all metamorphic rocks are strong - many break along cleavage or foliation planes, weakened by flaky sheet silicates (such as biotite, muscovite, and chlorite). But the weakest rocks, those with a lot of sheet silicates, come from metamorphism of aluminum-rich clays. If you metamorphose something that hasn't undergone much weathering - say, a sedimentary rock with a lot of volcanic fragments like a greywacke** - there will be fewer sheet silicates (especially muscovite).

So what are these strong rocks that can hold up spectacular mountains without failing in landslides?

A new survey of the mountain ranges that form the spine of New Zealand confirms the steepest are made almost entirely of tough but otherwise unexciting rocks called greywackes and schists. (Source: Discovery.com)

Ta-da! Greywackes (or maybe meta-greywackes?) and schists. And...

Hold on a minute. What was that modifying phrase?

...tough but otherwise unexciting...

Unexciting? Unexciting?

Those, my friends, are fighting words. (As bad as the gratuitous volcano slander happening on ScienceBlogs.)

And worse, I can't blame a biologist or the faceless media for the slander. It came from an avalanche researcher... a fellow geoscientist:

"They're pretty boring rocks," confirmed avalanche and landslide researcher Oliver Korup of the Swiss Federal Research Institutes in Davos, Switzerland. They are simply petrified deep sea sediments that have been pushed up to form the mountains, he said. "They don't even have fossils."

Ok, then. Let's take a look at what they're calling boring:





(Images are screencaptures from The Fellowship of the Ring. There's obvious CGI on the second one, but the rocks, I suspect, are real.)

If these are boring rocks, I don't think I could handle the adrenaline rush from seeing exciting ones.

It's not just that metamorphic rocks are strong. It's that they remember. They've been through a lot - they still contain traces of their history as sedimentary rocks, sometimes in their textures, sometimes only in their chemical composition. They've been buried and heated and squashed, but they haven't succumbed to melting (or at least, not entirely). Their minerals tell the story of their burial and exhumation; their structures tell of the strains that they have endured. It's hard to tease out their stories, and much of what they experienced has been erased by more recent events. But still - these are rocks worth understanding. These are rocks with sisu.***

So no, it's not elfin magic that makes the mountains of New Zealand strong. It's the magic of things that seem boring and simple on the surface. More hobbit than elf, probably, in Tolkien's world. But wonderful, all the same.

*Chris Rowan did his dissertation on paleomagnetism in New Zealand, and although he is probably less likely to sing the praises of metamorphic rocks than I am (because metamorphic rocks are lousy candidates for paleomag), he knows the tectonics much better than I do.

**Why do many online definitions of "greywacke" describe it as a primarily Pale[a]ozoic rock? Is it because the definitions were written by British geologists, and greywacke is associated with mountain-building, and Britain was shaped by Paleozoic mountain-building? I bet the New Zealand greywackes aren't Paleozoic...

***Thanks to Joe Kopera for introducing me to the word sisu. A Finnish word that can be used equally for metamorphic rocks and some of my favorite literary characters - I love it.

5 comments:

Sabine said...

I think metamorphic rocks are magical and gorgeous, but then I live and work on a passive continental margin, so I may be a wee bit baised.

Kyle(r) said...

I just want to say thanks for a great blog! I stumbled upon your blog when searching for interesting geology-related material and have been reading it for several weeks. This post really helped me to feel much more secure as an undergraduate deciding to study geology and geosciences. Your description of the rock's relationship to history rekindled exactly what originally put me in awe of the geology around us all. Thanks for a wonderful and inspiring post.

Kyle Reeves.

BrianR said...

greywackes boring?! Never!

Besides ... what's a greywacke ... outdated term. Either way, like the metamorphic rocks recording an important history of temperature, pressure, and associated tectonic conditions, the composition of greywackes hold clues to ancient mountains that are long since gone.

Anonymous said...

It's not just that metamorphic rocks are strong. It's that they remember. They've been through a lot - they still contain traces of their history as sedimentary rocks, sometimes in their textures, sometimes only in their chemical composition. They've been buried and heated and squashed, but they haven't succumbed to melting (or at least, not entirely). Their minerals tell the story of their burial and exhumation; their structures tell of the strains that they have endured. It's hard to tease out their stories, and much of what they experienced has been erased by more recent events. But still - these are rocks worth understanding. These are rocks with sisu.

Gorgeous description, of both rocks and hobbits. Why must rocks have fossils to be interesting? Fossils tell the story of life, which is very cool, but you make a great case for these rocks telling the story of the planet. Also very cool. -- Tracy :)

Silver Fox said...

I love metamorphic rocks, and you write about them so well. And I love the tying of these rocks to The Lord of the Rings. Great scenery, and nice to know more about it.