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Rosetta Stones


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A Landscape in a Hand Sample: To Transform

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Were you afraid I was Meatloaf? We did two out of the three major rock groups, and then a whole week goes by, and perhaps some of you wondered if I decided two outta three ain’t bad. I assure you this isn’t the case. I just got a bit distracted by other things. I wasn’t going to leave it at igneous and sedimentary and neglect one of my all-time favorite rock groups.

Metamorphic

Marble and orthogneiss, Washington State.

Here we have some quite nice metamporphic rocks from northern Washington State. I collected the marble from Ross Lake, nearly freezing my toes off in the process. Lakes fed by glaciers are bloody cold even in late July. Geologists specifically, and scientists in general, do take risks for science. We’re so absorbed in the awesome we sometimes don’t even think about said risks, at least not until we realize we can’t feel our feet anymore, but marble is a rare type of stone in the northern Cascades, so I figured the reward was worth the risk of a little hypothermia. The lovely little orthogneiss cobble was quite a lot easier: it was washed up on the beach at Carkeek Park, right on Puget Sound, probably brought down from the Cascades by one of the local rivers. These rocks have two very different histories up until the point they were transformed.

Metamorphic comes from the Greek words meaning “after, beyond” and “form, structure.” So this is an after-form, beyond its former structure. The marble began as limestone, laid down in a shallow sea by living creatures. The orthogneiss was igneous, a granitic something-or-other, possibly even a pluton. The two of them ended up shoved cheek-by-jowl by the vagaries of plate tectonics. As the North American and Farallon (later to become the Juan de Fuca) Plates collided, they pushed mountains up. The thing about mountains is, they’re heavy. Really, really heavy. And some of the rocks that ended up on the bottom of the pile found themselves subjected to pretty amazing heat and pressure as they got pressed down. Those conditions caused them to become something else.

Orthogneiss Cobble from Carkeek Park, Seattle, WA.

These two delights are examples of regional metamorphism. As the name implies, that’s metamorphism that happens over a very large area. The next time you admire a mountain chain, keep in mind that beneath it, rocks are being squeezed and strained and heated and becoming something else altogether. Their original minerals are being arranged in new ways. Some of those original minerals are transforming into different minerals. Some of those new minerals can only be formed under conditions of intense heat and pressure. The minerals in a metamorphic rock can tell us a lot about what happened and how deep they were, even long after they’ve been lifted to the surface and their overlying mountains eroded away.

The other major type of metamorphism is contact metamorphism. You’ll see it happen where the country rock (the local stuff) has been invaded by magma underground, or overrun by lava on the surface. There may not be much pressure, but the heat bakes the country rock along the zone of contact into something rather different than what it was.

Those are the basics. Metamorphic rocks get very complicated, but we’re keeping this simple for now. We’ll be revisiting metamorphism plenty of times in posts to come. I’ll introduce you to the joys of foliation, and we’ll no doubt see some pretty exotic minerals here and there, things that can only form deep within the Earth. For now, the main thing is to understand that metamorphic rocks have been changed by heat and/or pressure. The constituent minerals have either recrystallized or transformed into something else entirely. For a taste of that, take a look at a graphite pencil lead (carbon) and a diamond (also carbon). Couldn’t be more different, right? Well, if you could stuff your pencil lead deep down in the Earth and then fetch it up again, you’d have another diamond. All that heat and pressure took ordinary carbon and turned it into something extraordinary by arranging its atoms in a whole different way.

And that isn’t the half of why I find metamorphic rocks so entrancing.

North Cascades, from Ross Lake Boat Ramp, Washington State

The North Cascades are one of those features that make living in a subduction zone worth the risk. Those jagged mountains contain an amazing variety of igneous, sedimentary and metamorphic rocks – you can see all three of your basics in a day. Glacier-fed lakes and rivers fill deep, narrow valleys with eclectic blues. And the peaks, raised by the slow collision of tectonic plates, shaped into jagged points and sway-backed saddles by patient, sculpting glaciers, reach for the stratosphere. If you’re with a geologist, they’ll probably say this place is the schist. It’s pretty gneiss, too. (FYI – there are plenty of fallen branches and/or fist-sized stones with which to menace a geologist who carries the puns too far. You are allowed to wave them at me until I stop. But be vigilant – once a geologist has begun to pun, it’s very hard to get them to stop for long.)

The trails are often steep and strenuous, but mountain streams cascade down in cool, lovely waterfalls quite frequently. In fact, there’s one right along the trail here at Ross Lake. It’s called Happy Creek, and it tumbles down the mountainside with joyful abandon. You’ll discover why it’s called Happy Creek if you’re ever climbing back up the nearly vertical trail from Ross Lake to the parking lot. Believe me: clear, cold water filled with lovely boulders to sit on as it dashes over miniature cataracts on a hot summer’s day is a very happy thing indeed. You can sit there dabbling your toes while your water bottle cools off in the creek beside you, and absorb the fact that beneath you, all sorts of transformations are going on. This place definitely puts the meta in metamorphic. It’s beyond beautiful.

Dana Hunter About the Author: Dana Hunter is a science blogger, SF writer, and geology addict whose home away from SciAm is En Tequila Es Verdad. Follow her on Twitter: @dhunterauthor. Follow on Twitter @dhunterauthor.

The views expressed are those of the author and are not necessarily those of Scientific American.





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