You think that this would "flatten/incinerate" an entire continent... I think that this is a bit of an overestimation, but I also think you're dramatically overestimating the amount of antimatter carried at any time by a starship.
Well, just going by canonical references to antimatter being stored aboard. Privately I believe there are other things going on, but then, based on what
I think is happening in a warp engine it would a very different reaction anyway.
FWIW, I have been left with the impression that the bollide impact at Yucatan that wiped out the dinosaurs is estimated to have produced a blast wave that would destroyed everything for several hundred miles and covered much of North America with a blanket of ash and ejecta material. But I admit it's been a couple of years since I last read anything on it so maybe that's out/outdated data?
Oh, I agree with the Yucatan impact's... well, IMPACT on the world (and expect that the blast wave may have had direct impact for a thousand miles more more).
Of course, while we have good geological evidence of this impact, we don't REALLY know what happened... it's always good idea to distinguish between theory (which is what we're talking about) and fact (the dinosaurs might have died for some utterly unrelated reason, as far as we can know for certain, right?)
I believe that this is what happened, but I'm not going to treat it as "fact."
Re: warp drive, again...
I think we'll both have to agree that much of what's seen on-screen in Trek simply doesn't work. That as much does work as we've seen is nothing less than astounding, really, but it's not all one big, coherent structure, and without some "retconning" it can never be.
What I'm talking about is my own perspective on how I envision a "pocket-universe subspace bubble" warp drive system as working. I believe that this largely works in context with Star Trek as aired. But NO "real science" or "hypothetical science" model really fits with everything seen on-screen, does it?
To me, the "warp bubble" is really a separate universe-let, just slightly "offset" from real space-time. Within this little "pocket universe," the ship is essentially stationary. Perhaps momentum is conserved, in a sense, so that the momentum the ship has when entering "warp" is the same momentum is has when re-entering normal space-time, but this has no real relevance to "warp" as I envision it.
It is the "subspace bubble" which is traveling faster-than-light. Not the ship... which is, for all practical purposes, "stationary" inside of the bubble.
Mass which impinges on this bubble "falls into" the bubble. It does not flow at FTL speed within this bubble, though. It basically enters the bubble's volume at the same velocity it was at in real space/time.
The deflector beam pushes these particles, at "sublight velocity" inside of the bubble, back out of the bubble, where they "fall back" into real space-time with a velocity which is the vector sum of their original velocity and whatever new velocity was added to them by the deflector beam.
See? It's not moving FTL particles (relative to the frame of the ship) at all. It's moving sublight particles, outside of the bubble, or it's moving sublight particles, inside of the bubble, but in either case it's still moving particles at sublight.
Now... what happens when at warp, using this model? The very rarified atmosphere of deep space is much more dense inside of the bubble, but is not moving at higher velocity inside of the bubble. YOu're adding a lot more particles per unit time, but these particles are still at their basic "real space" velocity conditions inside of the bubble. So, if you're moving at WF6, the "in-bubble" particle density will be something along the order of 729 times the "warp transition" density. You'd be pushing the big chunks, the gravel, the sand, even the dust aside, but of course, you'd also be permitting free hydrogen (at 729 times warp transition space density) to pass through and to be collected by the bussard system... and this is likely why the ship shows some degree of "weathering" as well.
What is the difference between "real space" density and "warp transition" density? Well, that's a bit rougher... but my own rule-of-thumb is that, since relativity starts coming into play at about .75c, I cut things off there... the ship is in "warp" prior to 1c, and about .75c most likely.
That's my own model of how "warp" works. And I hope you can see how this works when you enter a full-density atmosphere. The relative velocity of the particles entering the "bubble" is not FTL, but the rate of entry is based upon the rate at which the bubble projected front surface is passing through this medium, and the rate of exit is based upon the rate at which the bubble projected aft surface is passing through.
Since density is greater at the leading "edge" than at the trailing "edge," the mass being added will exceed the mass being eliminated. Thus, my statement of "instantaneous hyper-pressure." But you'll still see so much mass passing into this field, even in atmosphere, that the flow rate will result in hyper-velocity flow.
Either, as far as I'm concerned, result in instantaneous destruction of the ship.
Now, going to warp LEAVING the atmosphere would eliminate the "hyper-pressurization" issue entirely... mass flow out will exceed mass flow in. There is no reason that the pressure inside the bubble would rise at all.
And since the particle velocity inside the field is the same as it was outside of the field, only altered by real physics inside the field (such as, for example, the rapid addition of lots of gas atoms and molecules at a very high rate of speed), it's not hard to imagine that as you leave the atmosphere, you'd be subjected to only a very brief period of extreme atmospheric drag, but not the same effect you'd see warping in.
It would still be VERY harsh on your ship, but orders of magnitude less so when exiting the atmosphere than when entering it, due to that pressure-gradient issue.
The only real issue is "how long does the subspace field continue to exist after the ship ceases actively generating it?"
And... as a corollary... is collapse related purely to time, or is it related to energy dissipation (meaning that the more mass is collected in the dissipating field, the faster the field collapses)? I tend to believe it's the latter, rather than the former, but there's no evidence either way.
In either case... warping into a planet will result in the destruction of the ship, either above ground, or at ground level, with the subspace field continuing for some brief period of time after the destruction of the ship.
The antimatter reserves will react with available matter and will result in a massive explosion, but the energy released by this explosion will be what we'd expect from real physics... the only effect of the subspace field would be to tell us WHERE this energy will be released, and how...
That's how I make all this fit together. I see no other way it can fit. But nobody has to buy into my take on warp drive but me, since there is no REAL "warp drive" science at all (and I'm including the hypothetical stuff currently being discussed in real-science circles, not merely fictional circles, by the way... none of it is proven at any level at this point!)