Where do you find that a 12 AWG wire is rated for just 30A?
That may be true for standard, PVC-insulated wire used for home electrical systems.
The ampacity of a wire doesn't depend solely on wire dimensions (cross-section) and construction, but also on the characteristics of the insulator and the environment in which it is deployed.
Unless you have a datasheet that says that a specific wire can carry at most 30A, your assumption may be wrong. For example, silicone-insulated copper wire can withstand higher temperatures for the same cross-sectional area, so their ampacity will be higher, all other factors being the same.
Just an example of your ampacity estimation being wrong in general, here is a datasheet of a high-temperature appliance wire, 12AWG (just the first I happened to find):
This is a relevant excerpt from further down in the page:
Note in the header the insulating material il not common PVC, but EPDM (Ethylene Propylene Diene Monomer).
As you can see, it is rated for almost 70A at 30°C. That's not your 100A, but it's getting close, and it is in free air, i.e. with just natural convection acting on it.
As Marla already pointed out in her answer, forced air-flow conditions could well allow higher ampacities.
(To further clarify what ampacity really is, as the OP needed some explanation).
Think of a bare copper wire as a resistor: it disspates power as heat. That heat has to go somewhere and wherever it goes it could cause problems.
For example, it could heat up the wire insulation, other devices nearby, or even other parts of the wire itself if the wire is coiled up or is placed within a closed environment, like a cable conduit.
That's why common household extension cords usually come with a warning stating that their full power-carring rating (or their ampacity) is guaranteed only if the cable is not coiled up, but acutally extended and spread out.
All these factors must be factored in to determine the ampacity.
To cite a more authoritative source, I found this technical report from the US Nuclear Regulatory Commission:
Ampacity Derating and Cable Functionality for Raceway Fire Barriers (Sandia National Laboratories)
Its scope is quite beyond what you asked, but it contains a nice and clear explanation of what is really meant by ampacity.
Excerpt (from page 4 – emphasis mine):
The term ampacity, as used in this report, is
defined as the maximum current carrying
capacity of a given cable conductor applied in
a given installation configuration. A cable's
ampacity is dependent on its routing and
installation configuration. That is, the same
cable will have a variety of individual
ampacity values depending on how and where
it is installed. For example, a cable may have
ampacity values associated with open air,
conduit, and cable tray applications, each of
which will be unique. Furthermore, other
factors beside the raceway type impact
ampacity including environmental ambient
temperature, loading conditions (number of
cables in the raceway), and grouping of
raceways. Hence, ampacity is not a single
valued property of a given cable, but rather, is
a context-driven value that must be
determined (or conservatively bounded) for
each application of interest.
Moreover, to address one of your concerns, actual damage to the conductor itself is fairly rare, since copper melting point is 1083°C.
It is very unlikely that such a temperature will be reached in normal operation, since probably there are other materials in the system that are much less resistant to temperature and that will fail earlier.
OTOH, if something catches fire, like the wire insulating material, for example, then you could have actual copper meltdown, but it's not caused by Joule heating itself.