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Orbital photo of Capsica, a small world hotter and drier than Earth Orbital photo of Capsica, a small world hotter and drier than Earth. Capsica: the Tlasi Calderas

by Chris Wayan, 2019

Outline only!


The Tlasi Calderas are huge, cliffwalled volcanic lakes just northeast of the rugged rift zone called the Arch. Whether on Earth, Mars, Venus or Capsica, most vulcanism on this scale occurs in part of a chain along a plate boundary or spreading zone, and Tlasi is no exception. Two peculiarities make this complex unique:

  1. Its breadth. Tlasi Caldera itself, the widest by far, is an oval 250 km long and nearly 200 wide (150 by 120 mi). Mars and Venus both have calderas wider than any on Earth's but this is outsized even for Mars.
  2. Its low profile. Elsewhere, big calderas are atop shield volcanoes that may not be steep but are still tall--Earth's Hawai'i is 10 km, Venus's Mount Ma'at over 8, Mars's Olympus 26, and on Capsica volcanic chains like Kurai and Nohaa reach 14-15 km. But Tlasi Caldera's walls are just 2-3 km high (6-10,000'), and the central caldera is below sea level and flooded, as are the outer slopes. Hawai'i is largely flooded too, of course, but it rises from an abyss 6 km deep (20,000'!); Tlasi's 'abyss' is just a kilometer or two. Base to top, Tlasi is just 4-5 km tall. For a low-gravity world like Capsica, that's modest.
Map of the Tlasi Calderas, large volcanic lakes in the Northeast Arch, on Capsica, a small world hotter and drier than Earth.

What's going on here? Why isn't the caldera at the summit of a great shield volcano? Blame the nature of this hot spot: low or intermittent pressure, but high heat, or local chemistry causing unusually fluid magma at normal heat--though it looks like ordinary basalt. Whatever the cause, the result is, eruptions create a very big lava pool, but when pressure subsides, the land does too. Hawai'i has arcs of deeper abyss around it, where the weight of all that piled-up rock has bowed down the sea floor; clearly that's happened here on a larger scale (or maybe Capsica's lower seas just expose these arcuate trenches better). Plus, deep 'blue holes' in the craterlake show that lava retreated far down several vents before congealing. This hot spot is fickle.

Parallels do exist. Many vents on Jupiter's manic moon Io are broad and low, with calderas this deep and proportionately this wide. What's intriguing is that such holes-to-hell yawn on the same little world that has peaks rivaling the Martian giants, and quite Terran volcanic cones in other spots. Temperature and chemistry of magma vary locally, causing wildly different landforms.

So think of this region as part Hawai'i, with its huge flows but relatively fluid lava and thus low relief, and part Io, with its even more fluid magma often ebbing to create great pits as often as peaks. Or you could see it as very big mud-pots bubbling away but not building up.

What does this strange topography do to life? Well... in the short term, not much. The whole Arch is a spreading zone, after all--its crust is new, moving a lot, subject to quakes, full of faults and fissures, and under pressure, corrugating as it spreads. Tlasi is just a bigger vent than most. Instead of linear ridges and valleys, as in most of the Arch, concentric ridges and trenches. But it's in the same altitude-range as standard Arch landforms; the life-zones are much the same.


Tlasi is different from the rest of the Arch, but not in a way Terrans will think of readily: Capsicans find it a frustrating region to travel in. In Capsica's low gravity and fairly dense air, most natives fly. Riding the winds along ridges if the easiest way around, and the long straight ridges flanking the Arch's central rift are major flyways. From them, fracture zones offer 'onramps' and 'exits' leading to other continents. This huge, simple, consistent flyway system, way more regular than the Outer Hemisphere's spotty flyways full of gaps and sea-passages, led the Arch to dominate culturally for centuries. Communication is civilization! But Tlasi breaks the pattern. Its concentric caldera walls generate great updrafts--flyways lead hundreds of kilometers. But they all curve inward, lead nowhere! You have to circle and hop, as if the calderas are gigantic atolls in an unfriendly sea.

Not just the natives. To survive, tourists must stick to the cooler heights. And in your strap-on wings, you'll be even more dependent on those updrafts--subtly, relentless curving off-course.

So while the climate is quite typically Capsican, and we'll be following the same routes Capsicans use, we may not see many natives. They prefer the straight ridge-zones elsewhere. Most of the folks you do see will be heading to or from the northern continent of Bel--curved ridges or not, the strait between Tlasi and southwest Bel is narrow--by far the safest route. Such travelers avoid the main caldera, hopping instead between lesser peaks and ridges to the northwest--from Last Ridge to the cliffwall of sulfurous Lake Lems, to Mount Detalosi Crater, to coastal Mount Ressel (these two peaks are quite Hawai'ian shield volcanoes), to Krasha Island (not volcanic; a tilted fault-block), and on into Bel.

These ridges, cones and caldera rims are high enough so Earth tourists can tour in relative comfort in winter, though it'll be muggy all the way--30-35°C (86-95°F). It might drop to 25 at night if you're lucky (77°F). Don't bother packing that coat. If you're touring in fall or spring, beware; even high ridges will be 35-40°C (95-104°F). Don't pack clothes at all.

Summer? Don't. Just don't.

Tlasi Caldera is just 19-22° north, so the lowlands, wet or dry, summer or winter, are warm even by Capsican standards, and fatal to Terrans most of the year--40-45°C (104-113°F) in winter (and humid, too!) and up to 65°C (149°F) in summer (and humid, too!) They'd be hotter yet if they were dry; but clouds often cover them. Except in the rift valley of the Arch to the southwest. Most days there are sunny; those low, bare flats can hit 75°C (167°F). Though nearly all this region is is viewable from some high viewpoint or other, fully 90% of it is simply untourable, unreachable, untouchable. Well, not quite as bad as snow-country is for Capsicans--snow burns them at a touch. Just slow death.

SCENERY Map of Tlasi Caldera, a flooded shield volcano in the Northeast Arch, on Capsica, a small world hotter and drier than Earth.

The cliffs of Tlasi are impressive but severe. The world just ends! Highland rainforest one second, cliffs two miles high the next. Capsica's low gravity makes such gigantic scarps stabler than on Earth. Still, cliff-sheltered water to the horizon, deep blue--this is the deepest lake on Capsica, with holes as deep as 2.4 km (7500'). The far side can't be seen--the curvature of this little world hides them. The cliff seems almost straight--the arc is modest in any hundred-kilometer slice.

Since the wall blocks storms from every direction, the inner cliffs are relatively dry; more rock than forest. Little runoff into the lake. It's at sea level, and quite brackish; evidently lava tubes connect the lake with the sea, and salt water percolates in to offset evaporation. "Water, water everywhere, nor any drop to drink." Though you will find SOME--creeks in the forests on the outer slopes. But the scenic inner cliffs are biologically rather sterile--drier, saltier, and then there's that little problem, every few millennia, of getting bathed in lava again, and having to reseed from scratch. Oh, and the lake unpredictably burps carbon dioxide, harmless enough to vegetation but fatal to animals, whether Capsican or Terran. So no, you can't go swimming.


Tlasi Caldera has long been the heart of an ancient (but strictly local) cult teaching equanimity in the face of life's catastrophes. That knife-blade contrast between flourishing cloud forest one moment and stark crater the next is, to the Capsican eye, a sort of physical sermon on ephemerality. Why "strictly local"? Remember, this spiritual retreat destroys its own temples and chases out its devotees every few millennia! If it's the voice of the volcano goddess, she mainly says "Go away."

But in between eruptions, once the lava cools enough for trees and watertables to be re-established, the temples always get rebuilt. Crazy Capsicans! Gluttons for rejection. Unlike Terrans, who have the good sense not to rebuild in zones devastated by flood, fire, tornado, quake, hurricane, blizzard, or eruption. Never.

At least, when Tlasi does erupt, there's plenty of warning--the lava in that huge caldera has a long way to rise before it spills out onto living lands. Few will die--if any. When a population's winged, eruptions may swallow homes, temples, and history... but not lives.

Map of Capsica, a hot planet.
Nohaa Island Ralopa Islands Arctic Is. off Bel Notahi Peninsula Eastern Bel Fulisse Peninsula NW Bel (upper right) SW Bel, deserts Chai Cape Corona Kurai Peninsula Hi and Vepra Yaku and Az Isle of Goret The Eel Prath Peninsula Kifura Isle of Valiha Ri Kshen Isles Tlasi Caldera in NE Arch Arch: NE Mt Artho, NE Arch Arch, dry central Arch, north coast Arch: tropical west Ekurre Range Arch: east down to canal Arch: right, canal down to volcano Arch: right base Arch: Left base. Lower left map-edge, southern Arch South Pole Giant World Map

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