While the carries across the ledge are being made, and Camps Two and Three are being dismantled, Arthur and Marie have begun finding the route above. Roger goes up with Stephan to supply them with rope and oxygen. The climbing is “mixed,” half on rock, half on black ice rimed with dirty hard snow. Awkward stuff. There are some pitches that make Roger and Hans gasp with effort, look at each other round-eyed. “Must have been Marie leading.” “I don’t know, that Arthur is pretty damn good.” The rock is covered in many places by layers of black ice, hard and brittle: Years of summer rain followed by frost have caked the exposed surfaces at this height. Roger’s boots slip over the slick ice repeatedly. “Need crampons up here.”
“Except the ice is so thin, you’d be kicking rock.”
“Mixed climbing.”
“Fun, eh?”
Breath rasps over knocking heartbeats. Holes in the ice have been broken with ice axes; the rock below is good rock, lined with vertical fissures. A chunk of ice whizzes by, clatters on the face below.
“I wonder if that’s Arthur and Marie’s work.”
Only the fixed rope makes it possible for Roger to ascend this pitch, it is so hard. Another chunk of ice flies by, and both of them curse.
Feet appear in the top of the open-book crack they are ascending.
“Hey! Watch out up there! You’re dropping ice chunks on us!”
“Oh! Sorry, didn’t know you were there.” Arthur and Marie jumar down the rope to them. “Sorry,” Marie says again. “Didn’t know you’d come up so late. Have you got more rope?”
“Yeah.”
The sun disappears behind the cliff, leaving only the streetlamp light of the dusk mirrors. Arthur peers at them as Marie stuffs their packs with new rope. “Beautiful,” he exclaims. “They have parhelia on Earth too, you know—a natural effect of the light when there’s ice crystals in the atmosphere. It’s usually seen in Antarctica—big halos around the sun, and at two points of the halo these mock suns. But I don’t think we ever had four mock suns per side. Beautiful!”
“Let’s go,” Marie says without looking up. “We’ll see you two down at Camp Five tonight.” And off they go, using the rope and both sides of the open-book crack to quickly lever their way up.
“Strange pair,” Stephan says as they descend to Camp Five.
The next day they take more rope up. In the late afternoon, after a very long climb, they find Arthur and Marie sitting in a cave in the side of the cliff that is big enough to hold their entire base camp. “Can you believe this?” Arthur cries. “It’s a damn hotel!”
The cave’s entrance is a horizontal break in the cliff face, about four meters high and over fifteen from side to side. The floor of the cave is relatively flat, covered near the entrance with a thin sheet of ice, and littered with chunks of the roof, which is bumpy but solid. Roger picks up one of the rocks from the floor and moves it to the side of the cave, where floor and roof come together to form a narrow crack. Marie is trying to get somebody below on the radio, to tell them about the find. Roger goes to the back of the cave, some twenty meters in from the face, and ducks down to inspect the jumble of rocks in the long crack where floor and roof meet. “It’s going to be nice to lie out flat for once,” Stephan says. Looking out the cave’s mouth, Roger sees a wide smile of lavender sky.
When Hans arrives he gets very excited. He bangs about in the gloom hitting things with his ice axe, pointing his flashlight into various nooks and crannies. “It’s tuff, do you see?” he says, holding up a chunk for their inspection. “This is a shield volcano, meaning it ejected very little ash over the years, which is what gave it its flattened shape. But there must have been a few ash eruptions, and when the ash is compressed it becomes tuff—this rock here. Tuff is much softer than basalt and andesite, and over the years this exposed layer has eroded away, leaving us with our wonderful hotel.”
“I love it,” says Arthur.
The rest of the team joins them in the mirror dusk, but the cave is still uncrowded. Although they set up tents to sleep in, they place the lamps on the cave floor, and eat dinner in a large circle, around a collection of glowing little stoves. Eyes gleam with laughter as the climbers consume bowls of stew. There is something marvelous about this secure home, tucked in the face of the escarpment three thousand meters above the plain. It is an unexpected joy to loll about on flat ground, unharnessed. Hans has not stopped prowling the cave with his flashlight. Occasionally he whistles.
“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.