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 1

Like many of you, I saw The Hobbit: The Desolation of Smaug last weekend. Like many of you, I've read "The Hobbit" several times (and had it read to me many times before that). But how many of you have wondered what kind of mountain the Lonely Mountain really was?

Tolkein's description and drawings of the lonely mountain bring to mind an inactive volcano. With its narrow peak and symmetrical slopes. But here's my thought - the Lonely Mountain works much better as a part of a much older and much larger tectonic history of Middle Earth. Let me explain with a little background information, first.

We're told that the Lonely Mountain was a solitary peak perhaps 2,500 - 3,000 meters or so above sea level (since there was some snow on the peak year-round). Assuming the latitude/climate was similar to that of the northern Alps. We also get some backstory on the great Dwarven kingdom of Erebor, which was famous for its deposits of gold and precious gems. Mithril, Tolkien's "unobtanium," was found only in Moria - several hundred miles to the Southwest. To the east lay the Iron Hills. To get to the Lonely Mountain, Bilbo and the Dwarves had to cross the Misty Mountains - a long north-south trending line of imposing peaks.




The photo above is Mt. Baker - A Composite Volcano that I photographed from the (relative) comfort of my economy airplane seat, about 30,000 feet above sea level. The summit reaches a height of about 10,780 feet above sea level. Note the relatively low-lying topography around the summit and the lovely symmetry of the mountain itself.



Here's another grand mountain peak, Grand Teton (summit is 13,775 feet above sea level) from the Garnet Trail, Grand Teton National Park Wyoming.


And here is a wider view of the Teton Range, Wyoming. Relatively young, these mountains are the direct result of extensional tectonic stresses, causing the block of crust I'm standing on (along with the barn) to drop downwards relative to the uplifted mountains in the background. But this tectonic stress is spread out along a north-south trending fault line - made of not one, but many mountain peaks worn sharp by glacial ice and not yet brought down by millions of years of erosion.



This is what gold mining in the western United States used to look like - a long narrow shaft to dig into the gold-rich ore (north-central Nevada). You can see how the mine follows the "vein" of high quality ore along a linear path into the hillside. But there aren't any big mountains nearby...



Longs Peak at sunrise on the Chasm Lake Trail, Rocky Mountain National Park (Colorado). Another lovely mountain peak. By comparison to other areas in Colorado, the rocks in Rocky Mountain National Park are relatively poor in economically valuable minerals like gold and silver. Fortunate for us, because it was easier to set the area aside as a National Park.


Abandoned Iron Mine in Ely, Minnesota. One doesn't need giant mountains to produce economically viable deposits. The iron found in northern Minnesota, Wisconsin, and the Upper Peninsula of Michigan comes from ancient sea-floor deposits over one billion years old.

So we're faced with a few questions. First, if the Lonely Mountain was once a volcano, why is it extinct? What kinds of patterns left by geologic processes here on Earth similar to those described by Tolkien? How do we get gold deposits in such abundance (aside from the "because, magic" clause)? Why did I include a picture of an iron mine?

For this first part, I'm going to leave you with a quick sketch map (drafted in "ArtRage" on my iPad). Look over the topography - where are there areas of likely active tectonics?

Next, I'll go over some of the features and discuss what might lie below the surface as a product of things that happen over what we geologists like to call "deep time."

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