Unnatural Histories: The Lonely Mountain, Part 5 - Iron and Tectonics

Okay, the story so far: we've got this "paleo-continent" of Rhovanion with features like the Lonely Mountain and the Iron Hills. Looking at the formation of gold deposits, the most common places on earth include late-stage igneous melts and hydrothermal systems. Something more easily found deep in the bowels of a volcanic system rather than at the surface. Let's take a few moments to look at the Iron Hills - a few leagues to the East. If our volcanic system was once an island arc, could we get iron somewhere from this system too?

Iron ore minerals. Left to right: Taconite pellets, massive hematite, banded iron formation (BIF). Taconite pellets are processed silica-rich iron ore that has been pelletized for easier transport by rail and boat.

Again, let us look to some real-world examples. Most of the iron mined in the world today comes out of "banded iron formations" found in places like Northern Minnesota, Wisconsin, and the Upper Peninsula of Michigan. Other places around the world include the Pilbara region of Western Australia. Many of these deposits are old - like 1-3 billion years old - old. Most of these deposits are associated with deeper marine waters, such as continental shelves or abyssal plains in the open ocean. Conveniently, this deep-water setting is something that could also form in proximity to an island arc. And just for the sake of completeness, an island arc can also be home to carbonate reefs. And carbonate rocks at the surface can yield karst (caves & sinkholes) topography - including caves vast enough to house the elven Woodland Realm palace of King Thranduil. Fun times!

An unexpected tectonic journey: a subducting island arc and associated back-arc basin. The raw materials for our Rhovanion paleo-continent.

Just for fun, let's think about this subducting island a little more. A "Back-Arc Basin" is a feature associated with subduction systems that is counter-intuitive and seems almost magical at first glance. But looking at the dynamics of the situation makes perfect sense. The subducting ocean crust causes the crust on the other side to "roll" over a little bit. Kind of like bending a block of wood creates tension on the top of the bend. This tension can translate into crustal thinning - and produce a small extensional basin adjacent to the compressional subduction zone. Wacky, but then again we're talking about huge slabs of rock that behave differently than we're used to at these immense spatial and temporal scales. And this back-arc basin would have its own associated hydrothermal/igneous system as a result of the decompression melting - hello volcanic-metal-sulfides!

So what do we have? We've got some future Lonely Mountain as an ancient volcanic island arc, rapidly being approached from the rest of the Rhovanion continent acting as the hammer, with the continent of Forodwaith to the north waiting to act as the anvil:

Tolkien-tonics: a hypothesis for the origin of the Lonely Mountain as volcanic island arc. The entire system is collisional in that Rhovanion will eventually collide with Forodwaith, sandwiching the other bits in-between

In the end, we get a bunch of crumply bits along the border of the Forodwaith continent, which may remain as erosional highs (like the Appalachians are today). Then a relatively flat central Rhovanion "craton" and millions of years of erosion exposes the frozen heart of that ancient island arc volcanic system, leaving it as an area of higher, more resistant rock. And chock full of hydrothermal/igneous gold-bearing goodness. Oh, and all those deep marine iron-bearing sediments laid down in that ocean in between Forodwaith and Rhovanion are now pinched up and exposed in the Iron Hills.

Rhovanion/Forodwaith cross-section during the Third Age of Middle Earth

Whew! That is a lot of geologizing. My sixth and last installment will bring us back to the "current" tectonic setting for The Hobbit.

For a complete overview of this series:
Part 1: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain.html
Part 2: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_16.html
Part 3: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_9632.html
Part 4: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_1042.html
Part 5: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_5911.html

Part 6: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_8985.html

Unnatural Histories: The Lonely Mountain, Part 4 - GOLD!

Gold. It's at the heart of nearly every Fantasy trope. It's what any reasonably complex fantasy economy is based on. Aside from the fact that the amount of gold portrayed in "The Desolation of Smaug*" might represent more gold than has ever been mined on Earth, gold is the great shiny, malleable, dense MacGuffin of fantasy.

So where does gold come from? Without going into stellar nucleosynthesis, suffice to say that the bulk composition of Earth's crust contains very little gold. But there are geologic processes that concentrate gold into economically important spots. Places where there is enough gold to dig it out of the rock, or to find it weathering out and washing through streams as little flakes or nuggets. Most of the gold deposits on Earth are the result of two things: 1) really, really hot rocks and 2) really, really hot water.

The popular notion is that our tectonic plates float on an ocean of molten rock. This is not true. Most of the rock that makes up the Earth's crust and mantle are solid - although in the lower crust and the mantle they are hot and under great pressure. Enough heat and pressure to allow them to bend and flow like silly putty instead of crack and shatter like the rocks we're familiar with. But in places where the rocks of the lower crust and upper mantle are affected by other things, these rocks can - and do - melt. Although by "melt" I'm not talking about an orange, syrupy stream - rather it's a mixture of liquids and solid crystals - kind of like a slushy (albeit a two-thousand degree slushy that should come with more than just a "Caution Contents Hot" warning).

Melts are commonly associated with processes that either bring very hot mantle rock near the crust (hot spots), places that lower the confining pressure on the hot lower crust and upper mantle (rifts and spreading zones), or bring water and carbon dioxide to the upper mantle and lower the melting point (flux melting along subduction zones).

As the melt rises (it is less dense than the surrounding solid rock), it begins to cool. If it reaches the surface while molten, a volcano will form. But within the crust, there is far more melt than can reach the surface. And this melt, cooling within the crust, begins to change. Minerals form (or existing minerals might grow larger) within the melt.

Grabbing the materials they need to form stable crystalline molecular structures. Iron, Magnesium, and Calcium are first - bonding with a few silicon and oxygen atoms to form mafic minerals like Pyroxene and Amphibole. As those mafic minerals solidify and the melt cools further, Sodium, Potassium, and Aluminum are grabbed, forming felsic minerals such as Potassium Feldspar and Mica. If these metals aren't available, the Silicon and Oxygen will form Quartz (SiO₂). All the while, what is left in the melt becomes enriched in elements not easily used in these minerals.

A bowl of assorted candies is often reduced to the least popular flavors after being left out in the break room. At first, the proportion of sour-apple-mango is low, but as people pull out the cherry, orange, lime, the remaining candy becomes a super-rich mixture of unwanted taste sensations. Now the snack scavenger must decide how hungry they are and weigh the benefits of that sugar rush with the cost of having one's mouth taste like scented dish soap for the remainder of the afternoon.

The elements left behind in the melt - those that are not easily incorporated into the common rock-forming minerals - are like these unwanted candies. These are the "incompatible" elements. Elements that tend to be shunned - and concentrated - in melts that mostly crystallized the popular elements together.

Within these incompatible elements lies gold. Despite all of its desire by humans, it's relatively unpopular in the mineral world. As the melt finishes solidifying, these incompatible elements start forming really wacky minerals - the gold atoms start hanging out together and form gold crystals - sometimes in solid solution with silver and a few other metals.

Although igneous activity - melts cooling deep within the crust is easy enough to imagine, most gold found on Earth is associated with hydrothermal activity (very high temperature groundwater systems). Water - either forced out of the melt solution as it crystallizes - or that infiltrating from somewhere above can be heated to very high temperatures by the rocks (or nearby melts) deep in the crust. The pressures at this depth are tremendous - and the water does not boil into steam. Instead, these hot fluids interact with the surrounding rocks, leaching some of these minerals and dissolving them. Sulfides, gold, and quartz are easily picked up by these fluids as they pass around and through the gold-bearing rocks. These fluids are under tremendous pressure - and they can force their way into small fractures, or create new fractures for the fluids to move into.

Hydrothermal Gold: Fluids (blue lines) moving through country rock are heated by a melt, collecting incompatible elements like gold (yellow dots), transporting them to a region where they are precipitated in fractures (black zig-zags).

As these hydrothermal fluids get closer to the surface, however, the pressure can drop - either by fractures opening up and making more space, or by getting close enough to the surface for some of the fluid to flash into steam. When the pressure drops, the minerals can precipitate out of solution, sometime very quickly - much like the way steam pipes acquire a coating of lime and scale as the dissolved minerals precipitate when the water changes from liquid to gas. These precipitated minerals form a dense mixture of quartz, sulfides and gold crystals lining the fractures they were forced into. These fractures, now filled with minerals brought in by hydrothermal fluids are called veins. In some instances, the gold crystals within these veins can be very large. In other cases the gold crystals are very tiny an must be dug out in bulk and crushed up, allowing chemical processes to leach the other minerals away.

Most of the gold produced in the US today looks like this. It's then crushed up and the other minerals are leached away to reveal the gold.

So where does that leave us with our paleo-continent of Rhovanion? If we consider the Lonely Mountain as a major gold-bearing feature of this continent, an extinct volcano seems an unlikely place to find the gold. The better spot would be the magma chamber beneath it, encircled by hydrothermal fluids, concentrating the gold into veins and big blobs. That suggests what is described as the Lonely Mountain isn't a volcano at all - at least not the surface expression of one. Rather, the Lonely Mountain is the once beating heart and associated viscera of a long-gone volcanic system. And what better place for a volcanic system than a subduction zone?

Subduction zone volcanic island. Bonus economic geology features include accretionary wedge (blue) and igneous intrusion into carbonate reef (purple) to form skarns

Next up: Iron, tectonics, and exhumed mountains!

*Speaking of which, Rhett Allain over at Dot Physics has a great post about the amount of gold in Smaug's horde.

For a complete overview of this series:
Part 1: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain.html
Part 2: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_16.html
Part 3: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_9632.html
Part 4: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_1042.html
Part 5: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_5911.html

Part 6: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_8985.html

Unnatural Histories: The Lonely Mountain, Part 3

When we left, we had established the possible tectonic system responsible for the two large mountain ranges related to events in The Hobbit. The Misty Mountains to the West, and the Ered Mithrin to the North. Side note: Ered Mithrin means "Grey Mountains," and Gandalf was called "Mithrander" or "Grey Pilgrim" by the elves… Any guesses as to what "Mithril" means? Tolkien was an Oxford Professor of Linguistics after all.

In order to untangle the Misty Mountains and Ered Mithrin, I suggested we take a closer look at earlier tectonic clues. In this case, the clues found within the crust that makes up the region called "Rhovanion." Tolkien describes the area west of the Misty Mountains as "Eriador" and north of Ered Mithrin as "Forodwaith." Since geologists often use regional names as a convenient way of tracking old chunks of crust, we'll look at the paleo-continent of Rhovanion.

Geologically speaking we have a few landscape features that are probably important. First, there are several large rivers - the Anduin (The "Great River" down which Frodo and his eight companions paddled south out of Lorien). The southern-most tributary of the Anduin on this map marks the spot where Isildur was waylaid and killed by Orcs on his march back to Eriador. The One Ring he was wearing slipped off his finger and fell to the bottom of the river, to be picked up years later by a hobbit/hobbit relative named Smeagol whereupon he took it deep into the Misty Mountains, only to lose the ring and have it picked up by Bilbo on his way to the Lonely Mountain… Goodness, it's like Tolkien spent years of his life piecing together all sorts of history for this Children's book he wrote (which he did, of course). Where was I? Oh yes, rivers.

Along with the Anduin is the Forest River and the River Running - both of which flow South and East. Interesting to note the headwaters of both the Anduin and Forest rivers start in the foothills of the Ered Mithrin. While the Anduin continues straight south along the edge of the Misty Mountains, the Forest river veers off to the East towards the Sea of Rhune. Something, then is creating a drainage divide between these rivers - some topographic high in the northwest corner of Mirkwood. The River Running also skirts past and away from other topographic highs such as the Mountains of Mirkwood, the Woodland Realm of Sylvan Elves (from whence Legolas the Dreamy would come to join the Fellowship of the Ring), and the Lonely Mountain. Another river that flows into the Sea of Rhune comes south of the Iron Hills.

Thus the rivers help us locate topographic "lows" and the area between rivers helps define topographic highs. Along with the features important to the Middle Earth narrative, we get a better picture of the paleo-continent of Rhovanion.

But how do we get at it's tectonic history, now that all the easily-identified features have long been stripped away by erosion or editorial expedience? The key lies with economic geology. Namely iron and gold - the topics of my next post.


For a complete overview of this series:
Part 1: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain.html
Part 2: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_16.html
Part 3: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_9632.html
Part 4: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_1042.html
Part 5: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_5911.html

Part 6: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_8985.html

Unnatural Histories: The Lonely Mountain, Part 2

I hope you've had a chance to look at a map of Middle Earth. There is a lot going on - the landscape was formed by the need of Tolkien's narrative. Tolkien was a linguist, not a geologist. But he based his descriptions on the places he knew. So his landscape draws on the same geologic history that formed our own world.

In the map above, we see three main features. On the western edge of the map is the long range of the Misty Mountains (they extend further north and south beyond this sketch). To the North lies the Ered Mithrin, or the Grey Mountains. East of the Misty Mountains lies Mirkwood - a large expanse of oak and beech forest. Incidentally, much of the plot for the Hobbit movies comes from the appendices of The Lord of the Rings and the Silmarillion. Although the plot for the movies doesn't really "follow" these sources as it uses them for general plot-directions for Peter Jackson's story. If you watch Peter Jackson's films with this in mind, you'll be much happier. But I digress...

Physiographically the Misty Mountains and the Ered Mithrin form long, linear topographic highs. These kinds of features are often indicative of tectonic stresses distributed over a large area. On earth, "active" tectonic boundaries tend to have these features. The stresses caused by one plate interacting with another deform the rocks along those boundaries. It's not unreasonable to think that the Misty Mountains indicate such a feature.

We can use the shape and orientation of the mountains to infer plate boundaries and the orientation of the stress. If you hold a phone book and push in with both hands, the book will bend. But the orientation of the bend is perpendicular to the direction you push with your hands - like so:

Deformation either from pushing on the phone book, or that caused by colliding tectonic plates, tends to be oriented away from the direction the stress is coming from. Since stress is coming from the left and the right, the least amount of stress is acting forward and backward (with regards to the above picture). So the deformation occurs in that direction.

Since the Misty Mountains are oriented North-South, we can reasonably assume the stress is coming from East-West. And since we have uplift (mountains), we probably are seeing the crust thickening at this boundary, such as happens when two plates converge on each other like India and Asia. If they were pulling away, we would see thinning of the crust - and perhaps a rift valley or three like we see in East Africa and the Arabian Peninsula.

So, we would probably make the same assumption for the Ered Mithrin. But would we be correct? It's not an unreasonable thought, since these are also a long linear uplift feature. But these are oriented perpendicular to the Misty Mountains, which we've already established as forming because the tectonic stress of two plates is oriented East-West. We need to adjust our thoughts if the Ered Mithrin is also forming due to converging plates. If, perhaps there were several plates - the plate below Mirkwood moving Northwest, into a plate on the other side of the Misty Mountains, and another North of the Ered Mithrin. This isn't easy to do all at once, so perhaps the collision forming Ered Mithrin happened first and the Misty Mountains formed later?

Perhaps - this could work, but maybe we should look earlier in time. It might be that an answer lies not with the most immediate pattern, but one formed earlier. Like the way you have a knotted ball of yarn and you can't undo the second-to-last knot until after you've undone some earlier, deeper knots.

For a complete overview of this series:
Part 1: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain.html
Part 2: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_16.html
Part 3: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_9632.html
Part 4: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_1042.html
Part 5: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_5911.html

Part 6: http://pascals-puppy.blogspot.com/2013/12/unnatural-histories-lonely-mountain_8985.html