“Hans!” Arthur calls when the meal is over and the bowls and pots have been scraped clean. “Get over here, Hans. Have a seat. There you go. Sit down.” Marie is passing around her flask of brandy. “All right, Hans, tell me something. Why is this cave here? And why, for that matter, is this escarpment here? Why is Olympus Mons the only volcano anywhere to have this encircling cliff?”
Frances says, “It's not the only volcano to have such a feature.”
“Now, Frances,” Hans says. “You know it's the only big shield volcano with a surrounding escarpment. The analogies from Iceland that you're referring to are just little vents of larger volcanoes.”
Frances nods. “That's true. But the analogy may still hold.”
“Perhaps.” Hans explains to Arthur, “You see, there is still not a perfect agreement as to the cause of the scarp. But I think I can say that my theory is generally accepted—wouldn't you agree, Frances?”
“Yes. . . .”
Hans smiles genially and looks around at the group. “You see, Frances is one of those who believe that the volcano originally grew up through a glacial cap, and that the glacier made in effect a retaining wall, holding in the lava and creating this drop-off after the glacial cap disappeared.”
“There are good analogies in Iceland for this particular shape for a volcano,” Frances says. “And it's eruption under and through ice that explains it.”
“Be that as it may,” Hans says, “I am among those who feel that the weight of Olympus Mons is the cause of the scarp.”
“You said that once before,” Arthur says, “but I don't understand how that would work.”
Stephan voices his agreement with this, and Hans sips from the flask with a happy look. He says, “The volcano is extremely old, you understand. Three billion years or so, on this same site, or close to it—very little tectonic drift, unlike on Earth. So magma upwells, lava spills out, over and over and over, and it is deposited over softer material—probably the gardened regolith that resulted from the intensive meteor bombardments of the planet's earliest years. A tremendous weight is deposited on the surface of the planet, you see, and this weight increases as the volcano grows. As we all know now, it is a very, very big volcano. And eventually the weight is so great that it squishes out the softer material beneath it. We find this material to the northeast, which is the downhill side of the Tharsis Bulge, and is naturally the side that the pressured rock would be pushed out to. Have any of you visited the Olympus Mons aureole?” Several of them nod. “Fascinating region.”
“Okay,” Arthur says, “but why wouldn't that just sink the whole area? I would think that there would be a depression circling the edge of the volcano, rather than this cliff.”
“Exactly!” Stephan cries.
But Hans is shaking his head, a smile on his face. He gestures for the brandy flask again. “The point is, the lava shield of Olympus Mons is a single unit of rock—layered, admittedly, but essentially one big cap of basalt, placed on a slightly soft surface. Now by far the greatest part of the weight of this cap is near the center—the volcano's peak, you know, still so far above us. So—the cap is a unit, a single piece of rock—and basalt has a certain flexibility to it, as all rock does. So the cap itself is somewhat flexible. Now the center of it sinks the farthest, being heaviest—and the outside edge of the shield, being part of a single flexible cap, bends upward.”
“Up twenty thousand feet?” Arthur demands. “You're kidding!”
Hans shrugs. “You must remember that the volcano stands twenty-five kilometers above the surrounding plains. The volume of the volcano is one hundred times the volume of Earth's largest volcano, Mauna Loa, and for three billion years at least it has been pressing down on this spot.”
“But it doesn't make sense that the scarp would be so symmetrical if that was what happened,” Frances objected.
“On the contrary. In fact that is the really telling aspect of it. The outer edge of the lava shield is lifted up, okay? Higher and higher, until the flexibility of the basalt is exceeded. In other words, the shield is just so flexible and no more. At the point where the stress becomes too much, the rock sheers off, and the inner side of the break continues to rise, while what is beyond the break point subsides. So, the plains down below us are still part of the lava shield of Olympus Mons, but they are beyond the break point. And as the lava was everywhere approximately the same thickness, it gave way everywhere at about the same distance from the peak, giving us the roughly circular escarpment that we now climb!”