Image credit: Chris.urs-o via Wikimedia Commons (modified) [CC BY-SA 3.0] (source)

The Musings of a Chunk of Granite

A large chunk of granite sits in the soil, not very deep — only a few feet down — but out of sight of any passers-by. Such chunks of granite are common around here, in this portion of the piedmont of the southern Appalachians. I imagine that chunk of granite resting there, pondering the meaning of life. I imagine it asking the fundamental questions that many of us ask: “Where did I come from? How did I get here? Why am I the way I am? What does the future hold for me?” And I imagine telling that chunk of granite, “Well, I just might be able to answer some of those questions for you!”

1. Where did I come from?

Well, my friend, you and most of the other granite around here are the result of two continents colliding a very long time ago. Five hundred million years ago, North America and Africa were separate continents, as they are today, with an ocean between them. But instead of moving apart, as they do now, the two continents were slowly moving towards each other.

Of course the continents don’t slide around the earth’s surface all by themselves. The entire planet is covered by a mosaic of tectonic plates that form the crust of the earth. The ocean floor is also part of this crust. These plates float on the mantle of the earth, which is hot enough that it can flow — much like a lump of clay will squish and change shape when I press down hard on it. The great heat inside the earth keeps the mantle slowly moving around, which causes the plates — which are more solid than the mantle — to bump and grind at the boundaries where they meet.

Now anywhere that two plates meet, they can do one of three things — they can move slowly apart, or move towards each other, or move sideways, sliding past one another. In places where two plates are moving apart, the gap between is constantly filled with fresh young rock from the mantle, forming new crust at the edge of the plates. But if you have two plates moving towards each other, then there is a problem, because there’s no space for them to get any closer than they already are. The typical result is that the edge of one plate slides underneath the other. We call this subduction, and we call the place where it happens a subduction zone. Most of the deep ocean trenches are subduction zones.

Just as new crust is formed at the place where two plates are spreading apart, old crust is destroyed when the edge of a plate dives beneath another plate. As the diving edge sinks deeper into the hot mantle of the earth, it melts away. But before it does that, all kinds of interesting things can happen. All along the subduction zone, there tend to be lots of earthquakes and volcanoes — a direct result of the subduction process. Furthermore, the edge of the plate that’s on top — the one that isn’t diving beneath the other — tends to get crumbled and lifted up into a range of mountains.

In your case, an important thing to keep in mind is that the subduction in your neighborhood started long before North America and Africa crashed into each other more than 300 million years ago. As soon as the two continents started moving towards each other, this meant that a subduction zone had already begun — right at the boundary where the plate containing North America touched the plate holding Africa.

You were born in this hot subduction zone, deep in the earth where the two plates collided. By the time of your birth, North America and Africa had nearly finished joining together. The collision of the two plates was the principal factor in the creation of the Appalachian Mountains. In fact, at around this time all of continents collided and merged into one huge supercontinent called Pangea — although later on they broke apart again.

2. How did I get here?

If I understand your question correctly, then you want to know how you ended up right here, in this very spot. I can see why that would puzzle you. If you were born deep in a subduction zone, where the edge of a plate is diving and melting into the mantle, then how in the world did you end up here, in this quiet place on the surface of the earth, beneath just a few feet of soil? Yes, that could certainly seem puzzling.

Would you believe me if I said that you floated here? Okay, that does sound unbelievable. But floating really is part of the story. Now you may think that you are far too heavy to float. A floating chunk of granite? I can see why you would be skeptical. But it doesn’t really matter how heavy you are. You just need to be lighter than what you’re floating on. Let me give you an example. In the winter, back when I lived up north, there were times when I tried to lift a great big solid chunk of ice — and man, ice is really heavy! You can hurt your back trying to lift that stuff! And yet, if I throw a big chunk of ice into a pool of water, then the ice floats — because water is heavier. Technically, I should say that water is denser than ice. If you have equal volumes of water and ice — let’s say a gallon of water and a completely frozen gallon chunk of ice — then the ice will weigh slightly less. So when you put the two together, the ice floats to the top of the water.

Now in your case, you happen to be made of minerals that are rich in silicon and aluminum. In comparison, the mantle of the earth is rich in iron and magnesium, which are heavier elements. As a result, you are about 10% lighter, more or less, than a solid rock that came directly from the mantle of the earth. That’s more than enough difference for you to float. Okay, I see that you are still skeptical. So let’s go back to the moment when were born, deep in the subduction zone.

As that segment of crust began its deep dive, it began to melt. Big melted blobs of crustal rock began to form, surrounded by other rock that was also softened by the heat. And some of these giant blobs began to rise up through the softened rock, buoyed upward by their relatively low density. These blobs weren’t able to rise all the way to the surface of the crust, because as they rose higher, the rock around them became cooler and therefore stiffer. (Had they managed to rise all the way up, then they would have produced volcanoes.) But quite a few of these blobs did indeed rise high enough to escape the subduction zone. And you were part of one of those blobs.

Now I should mention that we are not talking about little bitty blobs — we are talking about really big blobs that can be a few miles across. The formal term for one of these blobs is a pluton, although we can also use the term batholith if we to emphasize that we are talking about the big, rounded blobs, and not the skinny blobs that get squirted into the cracks of existing rocks that are fracturing due to stress or other factors.

So, you were part of a pluton, and that pluton floated free of the subduction zone, upward and upward — but then it got stuck partway up. And there you were, still quite deep in the earth. For a long time you just sat there, peacefully minding your own business, gradually cooling from a liquid into a solid mass of granite. So now you must wonder: how did you get the rest of the way up?

An important thing to keep in mind is that the continents are constantly getting worn down. All of the exposed land of the earth is slowly but constantly eroded away by rain, wind, ice, and other factors, until the land becomes low and flat. On the other hand, as the continents float around on their plates, they can experience uplift, which raises the land high enough to start a new round of erosion. Although the most dramatic uplift occurs when two plates collide, other factors can also produce uplift. The Appalachians, along with the nearby piedmont, have experienced more than one period of uplift. You were part of the land that got uplifted, but you were still deep, deep in the earth. However, as soon as you got lifted high enough to be well above sea level, then it was just a matter of time until the land wore down to the point where you would be exposed.

Now admittedly, you are still hiding under a couple of feet of soil, but many of your brother and sister plutons are quite well exposed now, in many parts of the piedmont. Some of the biggest exposed plutons are famous landmarks, such as Stone Mountain near Atlanta.

3. Why am I the way I am?

I’m sure you understand why you are a rock, but you are probably wondering why you are a particular kind of rock — granite. There are lots of kinds of rocks in the world, and some of them are quite different from granite. So I assume that you are asking why you are not something else, such as limestone or sandstone or shale or basalt.

Well, because you formed when liquid magma cooled into a solid, you developed crystals. You are therefore a crystalline rock, which sets you apart from sedimentary rocks like limestone or shale. Sedimentary rocks form when sediment settles in a large basin, often under water. Therefore a sedimentary rock is just a lot of sediment that fused together without melting. In contrast, rocks that formed under intense heat — resulting in melting or partial melting — are composed of a continuous network of crystals. Each crystal is a specific type of mineral, with a particular chemical formula. You are mostly made up of three kinds of mineral crystals — one that is white, one that is mostly clear, and one that is black. The result is an attractive black-and-white speckled appearance.

Your white mineral is called feldspar. There are different kinds of feldspar, although they are all chemically similar. The chemical formula always involves silicon, aluminum, and oxygen, along with an atom of something else, such as potassium. Although feldspar is often white, there are exceptions. For example, pink granite contains a pink type of feldspar. We may not talk about feldspar a lot, but it is an extremely common mineral, found in several kinds of rocks.

Your clear mineral is called quartz. (Small crystals might appear to be light gray.) Along with feldspar, this is the other of the two most common minerals in the continental crust of the earth. Chemically it is very simple, consisting only of silicon and oxygen atoms, in a one-to-two ratio. Anything with this formula can also be called silica — but if it occurs naturally and has crystals, then we call it quartz. Most sand consists primarily of crystals of quartz.

Your black mineral is a type of mica called biotite. The chemical formula has some resemblance to feldspar, except more complicated, with even more different kinds of atoms. In particular, it usually contains a little bit of iron in the formula.

Other crystalline rocks that are NOT granite will have a different combination of minerals. In recent times geologists have gotten more precise about what qualifies as granite — even though the original term was not so precise. For example, at least 20% — but not more than 60% — of the rock should consist of quartz. By this exacting definition, Stone Mountain no longer qualifies as granite, because most of that rock falls below the 20% threshold, consisting primarily of feldspar. But the popular definition for granite remains less strict, and therefore the mountain is usually said to be composed of granite.

You became what you are not only because you formed from the cooling of magma, but also because that magma contained mostly silicon, aluminum, and oxygen, along with some potassium, and very small amounts of other elements, such as iron. If the magma had been similar to the earth’s mantle, then there would have been a lot of iron and magnesium in the mix, and you would not have become granite when you cooled. For example, you might have become peridotite, which consists primarily of crystals of olivine and pyroxene — minerals with lots of magnesium and iron. You are rich in silicon and aluminum because you are composed of recycled bits of the earth’s continental crust. Over time these lighter elements tend to accumulate in the continental crust. But how does this recycling work?

As the continental crust is uplifted and then worn down by erosion, the eroded materials accumulate in low places, usually underwater. Thick layers of sediments can accumulate. But different kinds of sediments accumulate in different places. For instance, right next to the edge of a continent, huge amounts of sand can accumulate. Elsewhere, farther out, small particles of clay can slowly accumulate into deep layers. And in still other places, the sediment might consist mostly of calcium carbonate, the material that sea shells are made of. The sand, if compressed into sedimentary rock, eventually becomes sandstone — and if it becomes hot enough then the grains fuse to become quartzite. Likewise the clay becomes mudstone or shale, or else fused by heat into slate, which is much harder. And the calcium carbonate becomes limestone, or else fused by heat into crystalline marble. All of these rocks — and others too — can be found within a hundred or so miles of here. But right around here, in the piedmont, almost all of the rock is crystalline — so that rules out sedimentary rocks like limestone and shale.

In your particular case, it’s hard to say exactly what you came from, because your previous materials completely melted into magma. But based on the elements that you are made of, we know that you were not pure sand, and you were not pure calcium carbonate. So perhaps your raw materials consisted primarily of clay, with some tiny fragments of quartz sand mixed in.

4. What does the future hold for me?

Your future could go in any of several different directions. Some of the most attractive granite gets quarried and used for construction — the veneer of tall buildings, gravestones, even kitchen counter tops. However, you are not located in a quarry, so becoming construction material is not your likely fate.

A bit higher in the mountains, where the rugged terrain results in rushing rivers, there are places where the streams cut through granite. Over time the exposed granite breaks apart along fractures, and the resulting pieces get scoured and rounded by the water and sand. Therefore the stream gets filled with granite boulders of various sizes. Some of the smaller rocks and pebbles can get washed far downstream during storms, mingling with cobbles and pebbles that came from other types of crystalline rock.

Really large chunks of granite tend to be more resistant than the surrounding rock, so they don’t wear down as fast as what surrounds them. Therefore we see rounded mountains or spires of granite that stick up above the surrounding countryside. But even though these protrusions are relatively resistant to wear, they eventually wear down too.

However you are in a slightly different situation. You are located under a few feet of soil in gently rolling terrain covered by forest. Your bare rock may never see the light of day. Instead, your buried surface gradually turns into soil. All of the soil around here used to be granite — it used to be part of you. Water percolates through the soil, keeping your surface moist. In this warm, wet climate, the minerals in your buried surface undergo chemical changes. The feldspar and the mica both gradually change into clay. In contrast, the crystals of quartz don’t change. The result is that you are gradually converting from a mass of rock into a mass of clay embedded with crystals of quartz sand.

Your dominant material, as you know, is feldspar — and feldspar often changes to a type of clay called kaolin. Like feldspar, kaolin is most made of atoms of silicon, aluminum, and oxygen. However, water introduces hydrogen and additional oxygen into the mix. Pure kaolin is usually quite white. I’ve dug up chunks of decaying granite, called saprolite, while the rock still retains the structure of the original crystals. In the place of each crystal of feldspar, I found soft white clay instead.

Your biotite mica crystals also turn to clay, but biotite contains a bit of iron. As the clay is formed, the iron is released as iron oxide — rust. This rust dyes all of the clay an orange-red color. So you ultimately become a mixture of red clay and sand — which is exactly what I see all around here. Of course, running water easily separates the clay from the sand, so in the streams I see patches of nearly pure sand, and in places where slow-moving water has settled I can find areas of nearly pure clay.

The creeks and rivers gradually wash the clay and sand down towards the coast. The entire coastal plain was built up little-by-little from materials washed down from the mountains and the piedmont. Some of the material gets washed all the way down to the ocean. The sand settles out first, creating sand bars and beaches. The fine particles of clay get carried out farther, forming beds of mud on the ocean floor not very far from the coast. Someday these sediments may get buried deep enough that they turn back into rock — the sand into sandstone, and the mud into mudstone or shale.

But will any of these sediments ever turn back into granite again? Not anytime soon, because America is currently moving away from Africa and Europe. There is currently no subduction zone on the east side of the Americas. Instead, because America is currently heading west, any subduction zones are along the west coast of the Americas — especially along the western coast of South America. The mud and sand washing into the ocean there may be future granite. But eventually the plates could change direction again. Perhaps someday, hundreds of millions of years from now, another subduction zone will develop off the east coast of North America, and material from you and other local rocks could end up there.

The upshot is that someday your parts will probably again coalesce into rock, and just possibly that rock will again be granite. Long ago, before you became a melted blob of magma, you were a collection of materials that had washed into the ocean at the edge of a continent. The cycle slowly continues — rock into soil into rock into soil. So that is your ultimate fate — or at least your most likely fate — to remain within this slow, never-ending cycle, forever a part of the continental crust that floats on top of the denser rock of the earth’s mantle.

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