A bit of background:
"Tarmac" is an abbreviation for "tarmacadam," which is itself an abbreviation for "tar-penetration macadam." Macadamization -- named for the Scotsman John Loudon McAdam, who invented the process in the early 19th century -- is a method of road and surface construction that involves breaking stones down into small chunks, then binding them together with a cementing agent -- often, in McAdam's case, a mixture of stone dust and water. The road is then laid atop naked soil. (Before McAdam came along, the conventional wisdom held that roads required deep layers of rock-based foundations to stay level under the weight of people, horses, and carriages.)
The smallness of the stones was the key to the macadamization process. In macademized roads, stones could be no larger than three inches in diameter -- meaning, conveniently, that a worker could check whether a stone was of regulation size by seeing whether it would fit into his mouth. (Technology!)
McAdam's "crusted soil" breakthrough meant that his road construction process was much more economical and efficient than its predecessors. It also meant, however, that roads could become extremely dusty as soil from the lower layers was sucked up to the surface. And though engineers began to stabilize roads with tar, in the early 20th century, as motor vehicles became more common, dust became more common, as well. (Fast-moving cars sucked dust and other debris from the road surface, degrading the roads and making for fairly unpleasant driving conditions.) Edgar Purnell Hooley devised a system that mixed coal tar, cement, resin, pitch, and ironworks slag with the pulverized stones. In 1901, he patented the new macadamized tar mixture -- which would be laid and then flattened by a steamroller -- as "tarmac."
Hooley's invention, with some variation, has been in use since then -- on roads, yes, but also on many other surfaces. And while in the UK, the term "tarmac" lives on to connote any surface that employs the Hooley mixture, here in the U.S. "asphalt" has become the preferred term for the pavement -- the better to clarify the fact that we're using petroleum-based pitch as a binding agent. Except, that is, when we're talking about airport runways -- in which case, "tarmac" is still the standard.
Which brings us back to the case of sinking airplane.
One of asphalt's main selling points is precisely the fact that, because of its pitchy components, it's not quite solid: It's "viscoelastic," which makes it an ideal surface for the airport environment. As a solid, asphalt is sturdy; as a substance that can be made from -- and transitioned back to -- liquid, it's relatively easy to work with. And, crucially, it makes for runway repair work that is relatively efficient.
But those selling points can also be asphalt's Achilles heel. Viscoelasticity means that the asphalt is -- rather frighteningly -- always capable of liquifying. Heat the stuff to around 250 degrees F, and it will become liquid. Cool it back to room temperature, and it will become relatively solid. Again, that's what's great about it -- and, normally, outdoor temperature is close enough to room temperature that asphalt's latent meltiness isn't an issue. Normally, tarmac can function pretty much like concrete. Normally.