The Story of Serpentine

An Essay By Hannah D. // 6/7/2017

Trek through the hills of the Northwest’s Sierra Nevada, and you will find red soils peppered with some smooth, waxy rocks called Serpentinite. Attractively streaked with greens and blues, serpentine, as it is locally known, is a type of metamorphic rock with quite the fascinating history. To begin, let’s take a glimpse into a chapter of geology known as Plate Tectonics.

The earth’s crust is fragmented into huge tectonic plates that float above the mantle like toy boats in a bathtub. When it comes to the crust, there are two different types of rocks: felsic and mafic. Felsic rocks are the kind found on continental plates, making it likely that you are seated above felsic rocks at this moment. “Felsic” is just a geological shorthand for rocks rich in feldspar and silicon. Mafic minerals make up the oceanic plates, and are themselves rich in magnesium and iron (the iron’s contribution to the word "mafic" is explained by its Latin name "ferrum").

Back to the toy boats. If I have two boats – a heavy one and a light one – which is going to sink further down into the water? The heavy one. Thus, even if both of the boats are the same size, the heavier one will sink deeper into the water, because of its weight. When comparing mafic and felsic plates, the mafic ones are the heavier boats. They sink deeper into the earth’s mantle, and the light felsic ones bounce up a little higher. The importance of this fact may seem dubious at present, but it’ll come into play again in just a moment.

Now, all these tectonic plates (felsic continental plates, and mafic oceanic plates) are moving around on the earth’s surface, and they can interact together. For example, when two continental plates collide with each other, a mountain range forms. Mountain ranges also form when two oceanic plates collide together.

If oceanic plates collide with continental plates, however, something more complex occurs. After all, these are two plates of different densities. Instead of both piling up on top of one another into a mountain range, the denser plate will sink underneath the lighter one in what is called a subduction zone. A trench forms where the oceanic plate starts sliding underneath the continent, and the heat from the two plates sliding into each other often forms volcanoes. The volcanoes form on the continental plate, above the hot, submerging oceanic plate.

As the oceanic plate slips deeper and deeper down, it gets closer and closer to the mantle. The mantle is the layer of the earth beneath the crust, and its rocks are so hot, they are actually molten. Not only that, but the pressure from the weight of the continental plate on top of it starts to build up too.

Mafic rocks are mostly igneous rocks – they come from volcanoes. Felsic rocks can be igneous too (like granite), but you also get some sedimentary rocks, made from compacted sediments (silicon, after all, is a common component of sand). There is another type of rock, however: metamorphic. To get a metamorphic rock, you need to take an igneous or sedimentary rock and subject it to a lot of heat, or a lot of pressure. In the case of the mafic minerals sliding downwards in subduction zones, you have both! The rocks in the oceanic plates are exposed to some seriously high temperatures and pressures, so they undergo metamorphosis.

Now, oftentimes, these rocks metamorphose as they sink underneath the continental crust, but as they reach the mantle, they just melt down and become a part of the magma again. It is possible, however, for some of those metamorphosed rocks to get caught on the continental plate before slipping further downwards. If that is the case, the metamorphic rocks end up deposited on the continental plates. So mountain ranges on felsic continents can end up with a lot of mafic metamorphic rocks.

What does all this have to do with serpentine? Well, some of the mafic minerals in the ocean’s crust are blue-green gems like olivine and peridotite. Olivine and peridotite are mafic igneous rocks found in some places on the ocean floor. When these rocks undergo metamorphosis, they also go through the chemical reactions that give them their name. When a rock rich in magnesium (like olivine) gets hydrated with water molecules, it is called serpentinization. The serpentinization (and later deserpentinization) actually speeds up, or catalyzes, the reactions involved in their metamorphosis.

Serpentinite is a rock that starts its life on the ocean floor, gets metamorphosed in subduction zones, and then trapped on continental plates. It’s experienced an adventure that brings mafic diversity to felsic environments.

Now, just where do you find subduction zones? On coastlines, of course – where sea meets land, or where oceanic plates meet continental plates. That means that the Sierra Nevada was once a coastline, where tectonic plates collided.


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