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.
Firstly, 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:
I 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.
Edited by bonesiii, Sep 25 2012 - 04:40 PM.