splintering and unsplintering

[Kinetic]

 So on Scadrial, ruin and preservation splintered and created hemalurgy, feruchemy, and alomancy, when they joined up again and unsplintered, how was it that that the magics from their splintering did not cease?

[Spoolofwhool]

Magic, as colloquially known, or manifestation of investiture, as a more literal definition, is the result of a shard investing some of their power into a planet. The interaction between the power and intent of the shard and attributes of the planet, result in the formation of a manifestation of investiture, which appears among the inhabitants of the planet. Doing this does not require a shard to shatter itself, just expend a portion of its power. So Ruin and Preservation invested into Scadrial, which resulted in the creation of the three metallic arts. Afterwards, even as Harmony, the power they invested into Scadrial is still there, so the metallic arts remain.

[Blightsong]

Magic systems aren't created by shards by breaking off a part of themselves, they are a natural outgrowth of a shard investing into a planet.

[Ari]

Splintering is instead what happens when there's no mind attached to the magic of a Shard for a significant period of time, and the magic itself starts to gain sentience, like on Sel or Roshar, with Seons, Skaze, and Spren.
As AonDor has no relation to Seons, nor presumably do the Dakhor monks' abilities relate to the Skaze, you can see that splinters aren't a necessary part of a magic system. (although they are involved on Roshar, but that may be because there were already Spren there when Honour and Cultivation arrived)

[Eki]

20 hours ago, goody153 said:

(not sure if you could consider Atium and the WoA as splinters)

They are not. But there is a very big difference between a shard being splintered, and having splinters. A splintered shard is completely divided into splinters, and no one is controlling the shard as a whole. But on Nalthis, for example, Divine Breaths are splinters of Endowment (which is interesting to think about, since that means the Breath itself should have some kind of consciousness), and Endowment is not considered splintered. I don't know if Preservation or Ruin ever did create splinters of themselves. They did somehow give up a bit of their power to create sentient life, but we don't know the details of that process. I doubt splinters are involved though.

[Spoolofwhool]

20 hours ago, goody153 said:

Preservation and Ruin aren't splintered not in era 2 at least(there's a WoB about this though there's also a WoB about Harmony keeping a chunk of Ruin away to balance himself since his Ruin is bigger than Preservation but it's not splinters). I don't think they were ever splintered in era 1.(not sure if you could consider Atium and the WoA as splinters)

Atium and the Well of Ascension are not splinters since a splinter is a part of a shard's power which has achieved independent sentience. 

[goody153]

Preservation and Ruin aren't splintered not in era 2 at least(there's a WoB about this though there's also a WoB about Harmony keeping a chunk of Ruin away to balance himself since his Ruin is bigger than Preservation but it's not splinters). I don't think they were ever splintered in era 1.(not sure if you could consider Atium and the WoA as splinters)