Ta-Koro Geology

[bonesiii]

This is, like, completely irrelevant to anything, but I heart geology, and Bionicle has actually explained some such things (Karda Nui stalactite towers, which this theory gets some inspiration from), so. I just happened to think of it whilst walking my dog the other day, and decided what they hey, I'll make it a theory topic. :PFirstly, there's been a lot of confusion as to what exactly Ta-Koro's arrangement was; there's a common misconception that it's an island in the caldera of the Mangai volcano. No, it's in (or rather was ) a lake that is at some point along a lava river that flows down from the Mangai.Another question is if it is in an open-air lava river and lake, or inside a giant partially-filled lava tube. MNOG originally portrayed it as both; the river is mostly in a tube, but there is a large hole in the ceiling of the tube over the lake. I am taking this as essentially true, and presuming that all other portrayals (which seem to go back and forth on whether the lake is walled highly or lowly) are just approximations of the same basic arrangement. Probably the hole is wider than shown in MNOG.It is known, though, that there are other open-air streams around this area that may come from it, feed into it, etc. I'm mostly not dealing with those; the main question is, how did the Ta-Koro formation itself come to be? At first glance it looks quite unnatural, even though the camouflage system seems to have made most of the rest of the island looking very normal. The easiest explanation would be that Toa Onewa handled that, but I can find no confirmation of this. Re-reading the open of Maze of Shadows where Vakama chooses the location, it's vague, but implies he is looking at a natural ledge as the location, and there's no mention of Onewa getting involved with that.Also important is that MNOG clearly depicts a cave pointing north from the island, somehow getting over the lava (since it's an island) and continuing with no open-air connection at all towards Onu-Wahi. I have pointed out before that this means a hollow, natural rock bridge must have formed between the island and the northern wall of the tube, but how could such a thing form?Also, there are pointed structures on the top of the island to the north, even though to the south its top is flat.Here's the relevant BS01 pages with images:http://biosector01.c...dex.php/Ta-Korohttp://biosector01.c...hp/Lake_of_Firehttp://biosector01.c..._Kolhii_StadiumI propose that among the various proto-stone varieties, there is one that, after melted into lava, cools at a higher temperature than other kinds and will form conical crystalline shapes, and that it tends to be a bit heavier than other rock.I've already theorized elsewhere that the molten protodermis that flows up through pipes from Ta-Metru, around the dome, through the underground (as seen in Onu-Wahi in MNOG) and to the Mangai... then along the lava river -- that it slowly cools as it travels this distance. It would quite naturally give off a lot of heat. This explains why molten protodermis in Metru Nui was portrayed as an intensely hot and deadly substance, yet the same substance on Mata Nui is something even an Av-Matoran can safely surf on, etc.Now, as Mangai originally formed, it would first produce lava in all directions, building up the mountain-caldera shape. (I'm not worrying about the mountains to the northwest of Ko-Wahi and Mt. Ihu here, as I presume these must be just randomized formations from the camouflage system; I can't see how the volcano could generate them.) Soon the flow would lessen somewhat -- I'll refer to this as the first of two major lessening events although likely it was more like a steady decline over time. (Or, the camouflage system might just make false igneous rock to form the whole volcano without the need for a first lessening.)At this point, the lava finds the weakest point of the caldera's ring, which goes east because the land is higher to the west. It follows a natural depression in the land just as a water river would -- perhaps replacing a stream that may have already formed.This is a very thick flow since the second lessening event hasn't happened yet. Surface cooling -- both on top and on the sides which are in contact with the air -- forms a coating of rock. As the lava extends towards the sea, it continually loses heat, breaking out the tip and widening it to extend the tunnel. Since all the substances are still above their melting points, this continues in a rough line the whole way.At various points smaller natural depressions may carry off tributaries from this growing tube, which likeisay I'm not worrying about too much. Suffice to say, the main channel must be big enough and the tributaries small enough that the main river keeps its main amount.However, at one point the land forms a wider basin, where the lava slows down and spreads out sideways. That's key #1 to Ta-Koro's formation. Here a number of branches spread out, while many tributaries may reconnect as well. This forms a lake, but at this point it's still roofed. At the eastmost point it flows over the basin's edge and continues on to the sea.Now, I propose that this distance along this path happens to be just long enough for this special rock type to reach its solidification/crystallization point; key #2.Because the lake is spread more out, the middle-top lava has more opportunity for heat to escape. This works because the walls would naturally be thicker on the sides, and of course the ground is much thicker, but the wide surface of the roof has less rock between it and the air naturally.So the first bead of this crystalline rock forms in the middle of the lake's ceiling.It grows conically downward like a stalactite.As it does, the roof gains weight. The main mix rock that the roof itself is made out of is thus put under more stress.Finally, when the crystal is almost at full size, its weight is too much, and it breaks. Most of it breaks off from the roof main-mix rock, but a part of it, which is more to the northern side, comes off with it.This creates a small hole in the roof, and the cone stabs down into the ground, much like the Karda Nui broken stalactites. Because the lower lava would fade to cooler (heat rises) it would be gel-like and more able to allow this to sink in. It would pile up around it, creating a thicker mass which would rapidly cool, stabilizing it so it doesn't fall over -- but it might tip slightly to the north before this sets, since there's more weight that way.Now the substance continues to crystallize.Some grows upwards from the little chunk of main-mix rock at the top-back of the Ta-Koro island. This forms the little mountain-like shapes towards the back of the Koro. Others radiate off the edge of the top, forming horizontal, diagonal, etc. crystal spikes radiating out in the middle of the lake (more in a moment on those). Others grow up from the new texture on the lake bed, forming smaller islands around the main one (as seen in the MNOGs). Still others continue to form on the ceiling around the new hole.Bit by bit, the extra mass on the ceiling breaks off more and more of it. These rain down into the lake, breaking the early-forming spikes. This happens first and most along the northern side since the hole was that way and thus there was more cracked texture for crystals to seed on.Thus, a large gap over the northern part of the lake opens up first.Now, since nothing more will be raining down there, the Onu-Wahi spike is free to keep growing. It reaches the northern wall and thickens. (It is not naturally hollow; Matoran would have to dig the tunnel through it later.)The rest of the ceiling later collapses, breaking off all the other spikes. Some of the down-pointing spikes on the ceiling would also get quite large and form other flat-topped islands, like the one the Kolhii Stadium is later built on, and smaller spikes may form between them, or begin to form, which bridges between them could be based on. Most of the smallest pieces may re-melt and disappear as they hit the hotter center again, or not be heavy enough to set, so they were carried downriver to the sea.It is about at that time that the second lessening event would occur, and the lava's level would go down to just a fraction of the height of the lava tube. This may simply be the result of more branches opening up at various points, so less lava reaches the lake, and the lava may have started lessening prior to the majority of the ceiling collapsing; that would cause the whole ceiling to go roughly at once as the liquid pressure from below helping to hold it up would suddenly fall away.(I have theorized that there may be a second major river just to the south that formed later, and that this may be used for lava surfing primarily as it is cooler and less dangerous. I took the smaller end of a river south of Ta-Koro in MNOG as the evidence of this, and used the theory in my fanfics. Perhaps the path to that breaking open was what drew away lava from the first river, causing the lessening, rather than any actual built-in flow reduction in the pipes and camouflage system.)The point at which this substance solidifies would move far to the west, so the site of Ta-Koro remains as-is for the most part from then on.This may mean more crystalline structures still form at a much slower pace upriver, and this may also explain some of the other little tributaries breaking out due to the chaos in that part of the river. It is possible these later would form temporary dams that redirected the river naturally, and the Ta-Matoran could take advantage of such a time to burrow into the lake bed and install the stone bridge system, then later use Madu Cabolo to blast the dam open again.They may also have built dams on branches to open up and control overflow when Mangai erupted more than normal; not sure if that's confirmed or not, but if not, most eruptions would not lift the river enough to overflow the village as the top of the broken spike reaches well above the new level.BS01 mentions a small stream flowed through the island. The falling crystal could have cracked, while later crystal growth at the top may have roofed this small path through it. Or it could have been tunneled by Matoran later.The hole in the southern side of the tube wall may have been the original exit point of one of the lake's branches prior to the second lessening, which the lava cannot now reach, or may have been dug.Then all the brick construction seen in the various media atop these rocks, with some other tunneling into it (like the gap in the top of the Kolhii Field island) is added, including the castle walls.Voila. Ta-Koro.

Edited September 25, 2012 by bonesiii

[Taipu1]

I think I followed...some of that. I'm lost on how the tunnel to Onu-Wahi works though. Is it a Stalagtite that falls over sideways, allowing a stone passage to form? That's what I'd ascertain from the rest of what you've written, but it certainly doesn't seem to be what you've written. Some sort of visual guide would be helpful perhaps?

[bonesiii]

I think I followed...some of that. I'm lost on how the tunnel to Onu-Wahi works though. Is it a Stalagtite that falls over sideways, allowing a stone passage to form? That's what I'd ascertain from the rest of what you've written, but it certainly doesn't seem to be what you've written. Some sort of visual guide would be helpful perhaps?

No, a rock crystal that grows sideways while the whole lake part of the tube is still filled to the brim. I'm theorizing that many such sideways & diagonal crystals formed, radiating off the flat top of the original big crystal after it fell, but only the Onu-Wahi one formed under a large early hole in the roof to the north. When the rest of the roof would later collapse, all but that one would be broken off and go downriver or cement into the lakebed.None of this is actual stalactitites or stalagtites as those form from falling water droplets depositing solids (they can only grow up or down); I'm proposing these grow from a crystalline system (fairly common in lava rocks) that would normally try to grow in a perpendicular direction from whatever surface they start on. So depending on the angle of the starting surface they could point in any direction. The dominant direction would be downwards since the ceiling was the coolest part of the interior surface of the lava tube, thus the most likely surface for crystals to begin on. :)I might try to illustrate it if I find time. Edited September 25, 2012 by bonesiii

[Dralcax]

I have a simpler explanation. There is a large rock with a melting point higher than the surrounding stone. When the lava began to flow, the surrounding rock melted, leaving that little island. And there you go

[bonesiii]

I have a simpler explanation. There is a large rock with a melting point higher than the surrounding stone. When the lava began to flow, the surrounding rock melted, leaving that little island. And there you go

I thought of that, but it wouldn't explain the Onu-Wahi tunnel, and it would be difficult to explain the flat tops of some of the islands (Ta-Koro main island & Kolhii Stadium one) while others are conical pointing up. Also, the formation of a heavy mass in the middle of the ceiling helps explain why a hole collapsed over the lake, leaving Ta-Koro itself open to the sky.You could construct a lot of little coincidental theories to explain those away, but that would actually be much more complicated. This simple idea (the substance proposed) causes everything else to naturally flow out of it with standard physics (almost all of it demonstrated consistently to be part of protodermis's physics elsewhere in the Bionicle story) with barely any reliance on coincidence.Of course, likely the story team & MNOG developers didn't think of any of this, or may have just gone with yours while not thinking through the rest, but it's basically just for fun. Edited September 25, 2012 by bonesiii

[Click]

Hmm, very interesting. I never really thought about that. That stalactite theory is cool, and would be very cool to use in fanfics.And just a quick theory I was thinking of when I was reading the first part:Ta-Koro began as a sharp curve in an underground Lava Tube (as evidenced by the two tunnels leaving at about 70-80 degree angles from each other), which, when pressure built up around the curve, the opposite side of the bend was widened outwards into a lake (this also happens in geology, I've seen a lot of evidences of it, particularly in Slot Canyons). Eventually, the widening area becomes too much for the roof, and it collapses into the Lava. Overtime, the smaller fragments melt away, leaving just the few largest fragments, soon to be Ta-Koro, the Kohlii Stadium, and those other two little rocks. The smaller, sharper stalactites formed when they broke out of the ceiling, leaving a cone on top, while the flat ones were directly out of the surface, leaving it more or less flat. The higher area in back of the Koro was previously a small, rough hill on the surface that also contributed to the collapse.And a quick look at MNOLG made it look to me like the tunnel curves downwards. It doesn't get smaller and smaller as it goes, like you would expect from a straight tunnel, but the ground quickly vanishes, leaving me to assume it is down a steep slope. The rest of the images from the tunnel don't show that as well, but it would still be possible. My theory is the Matoran dug their tunnel downwards through the huge hill on the back of the village, going underneath the lava. I'd support this by saying the tunnel also meets the Onu-Koro Lightstone Mine, which should be very deep, but we don't know that much about lightstones.So, just another theory I had, that's a little simpler, but yours has a lot more area for expansion into fanfics and such. Now I wish you could award yourself the Key to Nongu.

[bonesiii]

That's a good alternate theory, Click. I considered the U-bend idea (without crystalline formations); it was basically my running theory for a long time, but I do see some conflicts with MNOG's portrayal.First, if you look down, while looking at Ta-Koro (the scene shown in the image at top of this topic, though it doesn't show this feature), there's a V-shape formation jutting in right at the apex of the hypothetical U-curve. In a non-crystalline, rapid-flow U-bend, that should be smooth. (I theorize it is the remains of one of the crystal formations in a simple lake formation, pointing sideways towards the island, and being on the wall its base may not have been under the parts of the roof that collapsed.)Second, the Onu-Wahi tunnel scenes certainly give no impression of going down steeply. Go in a ways and look back, where you can still see the exit into Ta-Koro -- looks horizontal, doesn't it?Certainly possible anyways, but yeah. Also, opening a path to Onu-Wahi that lava might break into and easily flow down into could be very dangerous. A bridge over it on the other hand would not run as much risk of that. I would think the Onu-Matoran would not bother with such a tunnel if they would have to go under a lava lake. (That could, however, be solved by having the tunnel also go upwards later on before going down again.)Also, it seems to me from MNOG that the lake entrance tunnel is aimed directly at Ta-Koro. If the lava ever came in at a fast pace as you said, I would think the first thing it would do would be to obliterate the island. Even if we pull in Dralcax's theory of a less meltable area, I wouldn't think Ta-Koro would end up in that shape. If you've ever poured hot water into the middle of an ice cube rather than letting the cube melt evenly in still water, I think it would look something like that. Not like a flat-topped cylinder. (And I don't get that same vibe from the other Lake of Fire portrayals; it seems much more like a wide basin than anything like a U-bend, and a basin's entrance and exit may be at any angle with respect to each other.)Finally, if the tunneled down for the Onu-Wahi route, wouldn't it make more sense to build a staircase heading straight down in the middle of the Ta-Koro island, as far away from the lava as possible? Instead the tunnel would have to go straight to the north towards the edge of the island and then angle down at a rate not noticeable to the viewer to be consistent with MNOG. Seems unlikely.Still, possible.

Edited September 26, 2012 by bonesiii

[fishers64]

Crystals? Would the close proximity of the lightstone mines to Ta-Koro, suggesting that those are some type of rare mineral/crystal, suggest that this theory is correct?Otherwise, I'm not so sure this theory is correct, at least the part about the crystals. While they do form in volcanic areas in considerable quantities, I sincerely doubt that they could form a self-supporting structure across a lava lake. The MNOG portrayals seemed to suggest that the back of Ta-Koro is flush with the back of the crater, allowing access to Onu-Koro that way.

[bonesiii]

Crystals? Would the close proximity of the lightstone mines to Ta-Koro, suggesting that those are some type of rare mineral/crystal, suggest that this theory is correct?

Perhaps. Although I don't think those have anything to do with lava, and they keep growing, as I understand it, but I may be wrong.

Otherwise, I'm not so sure this theory is correct, at least the part about the crystals. While they do form in volcanic areas in considerable quantities, I sincerely doubt that they could form a self-supporting structure across a lava lake. The MNOG portrayals seemed to suggest that the back of Ta-Koro is flush with the back of the crater, allowing access to Onu-Koro that way.

Well, I do suspect the MNOG designers may have intended it that way. Or, rather than flush, Click's U-bend idea except without the 'shortcut' ever having eroded (so the tunnel would go through the wall between the two sides of the U).But what I'm seeking to do is reconcile the MNOG portrayal and the others as closely as possible. Canonically we do know it's an island, and we do not actually get a clear view that it isn't in MNOG.

[Taipu1]

No, a rock crystal that grows sideways while the whole lake part of the tube is still filled to the brim. I'm theorizing that many such sideways & diagonal crystals formed, radiating off the flat top of the original big crystal after it fell, but only the Onu-Wahi one formed under a large early hole in the roof to the north. When the rest of the roof would later collapse, all but that one would be broken off and go downriver or cement into the lakebed.

That clears things up. And the smaller bits are crystals/stalagtites that fellin the lava seperately. The depictions in MNOLG and MoL are different, but how d'you suppose they mechanised the entry bridge? MNOLG depiction shows at least they're made from the crystal formation type things, but the MoL appearance appears very manufactured. Eitherway, how did they get any kind of mechanism down there?I vaguely theorised (Based entirely on the behaviour of the activation machinary in MNOLG) that the rocks are in some sort of holes beneath the lava, and when activated, steam forces them up to the surface. But I'm not sure its right to base an entire theory on a tiny piece of animation from semi-canon game.

[bonesiii]

Well are you asking how could they function? Or how could they get them in the lava? I mentioned a theory for the second one in the first post near the end; at some point the main river could be redirected temporarily so it would cool totally, so it'd be safe to install them.As for function, I presume there is a bit that is always beneath the ground that is hooked up to gears. I think all of it is made of a substance with a higher melting point than the lava -- more evidence for this theory -- so they wouldn't be vulnerable to melting.Re: the two designs, there are two basic possible answers, since there's two portrayals that are both in media that is sometimes semi-canon. MNOG's portrayal may be non-canon since contradicted in MOL which is a "normally canon" source. Or, MOL's portrayal may be one of those "artistic license" things.But methinks it doesn't matter; either way they could be made/carved of the same material and even using broken spikes they found intact under the lava. The MOL one would simply require a lot of chiseling to make them boxy and put them closer together.I would personally lean towards the MOL one being the canon one, simply because it is waaaaay safer. One little slip on the MNOG portrayal would be fatal -- and really easy to make.

[dee.3x3]

Sounds like a good theory to me, although I didn't follow it all the way through.But I'm a little confused about where the cable cart to Ko-Wahi is located. Isn't Ta-Koro surrounded by rock? Where would such a thing go? I remember it was depicted being up high on the gates of Ta-Koro, but it doesn't depict the rock wall around the village. Could someone shed some light on that?

[bonesiii]

Sounds like a good theory to me, although I didn't follow it all the way through.But I'm a little confused about where the cable cart to Ko-Wahi is located. Isn't Ta-Koro surrounded by rock? Where would such a thing go? I remember it was depicted being up high on the gates of Ta-Koro, but it doesn't depict the rock wall around the village. Could someone shed some light on that?

Pretty simple -- just go to MNOG's front gate (where the two guards are pacing back and fourth, outside the lava tube). There are stairs at the right of the gate. Following those up leads to a spot on top of the east roof of the lava tube, apparently, which is where the Ta side of the cable ride is anchored. This goes to a high point in Ko-Wahi, presumably on the side of Mt. Ihu. :)This is on the outside of the lava tube. Ta-Koro is on an island on the inside of the tube (the wider part to the west of the cable part). The cable is over where the lava river continues out of the lake to the east.Is that clear at all lol?

[dee.3x3]

Pretty simple -- just go to MNOG's front gate (where the two guards are pacing back and fourth, outside the lava tube). There are stairs at the right of the gate. Following those up leads to a spot on top of the east roof of the lava tube, apparently, which is where the Ta side of the cable ride is anchored. This goes to a high point in Ko-Wahi, presumably on the side of Mt. Ihu. :)This is on the outside of the lava tube. Ta-Koro is on an island on the inside of the tube (the wider part to the west of the cable part). The cable is over where the lava river continues out of the lake to the east.Is that clear at all lol?

Yup, that makes sense. All though I do recall that the cable cart goes across an open lava river to Ko-Wahi. Is that an opening in the tube or just run-off from the main river? Or just artistic license from the game developers?

[bonesiii]

Pretty simple -- just go to MNOG's front gate (where the two guards are pacing back and fourth, outside the lava tube). There are stairs at the right of the gate. Following those up leads to a spot on top of the east roof of the lava tube, apparently, which is where the Ta side of the cable ride is anchored. This goes to a high point in Ko-Wahi, presumably on the side of Mt. Ihu. :)This is on the outside of the lava tube. Ta-Koro is on an island on the inside of the tube (the wider part to the west of the cable part). The cable is over where the lava river continues out of the lake to the east.Is that clear at all lol?

Yup, that makes sense. All though I do recall that the cable cart goes across an open lava river to Ko-Wahi. Is that an opening in the tube or just run-off from the main river? Or just artistic license from the game developers?

There are more like that portrayed in MNOG2 and the flash animations too. I was referring to those in the part of my theory that mentioned many other lava streams, either branching off earlier and rejoining the main river, or branching off from the main river (I suspect there are instances of both).Memory's fuzzy, but I think that one was at a higher elevation than the lava river, so couldn't be coming from the lake but could be next to it or going to it / to the main river downstream.BTW, along with this theory as I posted it, it's interesting that at the Ta-Wahi telescope, almost exactly even with where the lava river seems to be pointing, there is another large upside-down conical shape (with the telescope atop it) much like the one the Kolhii Field is atop, and with a sideways rock bridge that also appears natural (but hewn flat at the top by Matoran presumably).This might be explained if the original full-power river opened up there, and extended farther out to sea. The zone of crystallization need not be only exactly at Ta-Koro; some of the crystalline source could still filter downriver, especially if some of it as theorized earlier re-melts as it hits the hotter center from falling off the roof. A smallish crystal could form near the end of the river where the tube would plunge into the sea, then fall, collapsing the roof there, opening up a hole in the middle.This would probably happen after the main roof over the lake had already broken, so some pieces of that could pile up and form the wall-like shape immediately to the west (upstream slightly) of the fallen crystal. Some of it may have originally been braced up against the crystal's base but later be washed away by wave action, being a less dense material than the crystal (but I'm getting ahead of myself). The sideways bridge could again be part of a series of radiating spike crystals from the big cone after it fell, connecting to the top of the collected rubble.Later, as the flow lessened and wave action hit, the rest of the tube's end could collapse. (There is one angle of viewing the approach to the telescope that even shows, on the left, cracked rock that could be what's left of the southern side of this wall.) The lava flowing downstream could carry most of the falling pieces into the radiating spikes, but both because there is an upriver obstruction lined up with the bridge and because it's under the hole the falling bigger crystal made, that bridge could survive.Also I presume connected crystal bridges would be sturdier anyways -- so in this add-on theory, twice a connected bridge formation survives while merely radiating spikes do not.Meanwhile the collapse effect could spread upriver as there's no more lava pressure to hold it up, and possibly with help from wind and storm action. Lava could form a big mass here filled with stuck cooling stone that obstructs the river and redirects it so that it turns south, forming that long and low hill you first climb over from the beach to Ta-Koro's southern gate. This would be another lava tube, but a much smaller one since this is after the final lessening. There it could rejoin the southern smaller lava river and they both head out to the sea.Most of this I have in fact already theorized long ago, but didn't connect it to the crystalline-resembling formation of the telescope tower.Over the next few hundred years, continued wave, storm, and wind action would tear up the weaker non-crystalline rock around the telescope tower. The tower itself, being denser, would remain intact. So the rock around it would peel away until it and its bridge are exposed, something like revealing a fossil. It in turn shelters part of the wall-like obstruction that the crystal is connected to, leaving most of that intact (and that may contain a conglomeration of smaller dense crystals from the lake's collapsed roof anyways).And all this erosion of rock may partially explain why there is a sandy beach where instead you'd think a rocky peninsula should be, and there is instead a peninsula to the south of igneous rock.Voila, Ta-Wahi beach. :biggrin:Another slightly related note; if you go up to the telescope thing and look towards Ihu, you can see two ridges of rocky mountain to the north and the south. This provides some evidence that the original main lava river follows a somewhat wide natural valley forming a line between the Mangai and the coast.Anywho, more food for thought. Edited September 28, 2012 by bonesiii

[dee.3x3]

Okay, that sounds good. I can finally sleep at night, knowing how Ta-Koro and Ta-Wahi were formed. :PInteresting topic though. I don't really know much about geology but this still makes sense. Thanks for clearing up some of those points.

[The First Speaker]

Well, reading your theory inspired me to make a spinoff of it.My theory is that after the GC , a magma river would flow from the Mangai to the sea. Like you said, the outer surface of the flow would solidificate. Inside the tube, there would be two key processes:1. Assuming that the magma in Mata Nui was made of various minerals, a differentation process would start once the flow slowed down - differentiation. The less dense minerals continue flowing through the top of the "tube", while the more dense ones flow even slowlier through the bottom. If we assume that at one point this river branched off, then the flow would be even slower, allowing only the less dense magma to flow and the more dense one would start to accumulate. This would also make the bottom magma to start to crystalise.2. Magma naturally has gases in it. That's why some kinds of volcanic rock are so lightweight - they are literally rock sponges in which had bubbles in them during solidification. When the river slowed down and the bottom magma started to solidify, it is possible that various bubbles of gas were trapped in the newly formed rock. This could be one possible origin of the Onu-Koro tunnel.After those processes, you would already have some structures formed - the Ta-Koro rock, the pillars that make up the bridge and the Kohlii Stadium collumn. Then the flow of magma disminishes, and the magma starts flowing throught the bottom part. This would mean that structures like the Ta-Koro rock couldn't be formed anymore, as the higher speed current (higher than before the level drop) wouldn't allow crystallisation. Also, since the more dense magma, now solidified, would be made of various minerals, not all the different parts of the rocks would have the same melting point. That means that the magma, which now flew between those rock formations, would melt part of the minerals, giving the formations their current stallactite look. Eventually, the roof of the tube over Ta-Koro would have broken down due to weathering and erosion.