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Hollow Moon Planetology: Blue Moon - Natural Disastersby Sharon Dornhoff
Lunar Volcanoes and "Fire Times"
Just as the effects of Pateran gravity upon the Hollow Moon's atmosphere are predictable, so are its effects upon the geological activity beneath the surface of Nearside land masses*. Patera's tidal pull keeps magma beneath the surface of "hot spots" in a dynamic, fluid state, counterbalancing the crystalbarrens' chilling effect upon the lower atmosphere and keeping Matera's interior from freezing. This effect is especially strong on eight separate occasions during the 18-month lunar "year", when Patera passes within 5 degrees or less of the visible moon, and Seshay-Selene manipulates its gravity to influence the elements of earth and fire, rather than air and water. These events, called "Fire Times" by Materans, can be highly destructive; yet, they are also predictable to within an incredible 3-hour interval (!), and are tracked over time as a part of the Storm Cycle which all inhabitants of the Hollow Moon plan out their lives in accordance with. Two of the annual Fire Times are quite mild, two can be devastating to large areas of the Nearside, and the remainder fall somewhere in-between. Barring a lunar total eclipse, volcanic eruptions in the HM setting will occur at these times and no others; so often do lunar volcanoes get to release their fury, on a set schedule, that there's no time for such an "out-of-season" eruption to build up.
[* - Crystalbarrens, including those covered by seas, aren't subject to this effect. With the exception of the unusually-large Fra Mauro Islands in the Midland Ocean, none of the crystalbarrens' volcano-islands are active in the present era ... unless, of course, the DM chooses to make them so.]
Not all volcanoes in the HM setting erupt, in every Fire Time, of course: that'd be overkill! Rather, individual active volcanoes are assigned a rating by the DM of One, Two, or Three. Rating One volcanoes will erupt only during a severe Fire Time; rating Two, under severe or moderate Fire Times; and rating Three (the most active) during any Fire Time on the Materan calendar. In addition to these, "semi-active" volcanoes will erupt only once every few years, always during a severe Fire Time; these may be assigned a die of the DM's choice -- anything from a d4 to a d100 -- to determine the number of lunar years between eruptions. Naturally, a semi-active volcano can erupt in any year the DM desires that it do so ... but even then, such cones should blow only during severe Fire Times. "Dormant" volcanoes haven't erupted in living memory -- which is several centuries, given the lifespans of vesper elves, gnomes, and Modrigswerg -- and are, if not geologically "dead", at least safe to live upon in the opinion of native Materans. Only the volcanoes of the crystalbarrens, and (we hope! :-D) the awesome Mount Copernicus on the Midland Ocean's western shore are truly extinct, in the Hollow Moon.
Nor are all of Matera's volcanic eruptions created equal, among those which DO occur in a given Fire Time. The Apennines, although their peaks rate a Three for activity, seldom expel anything but steam and occasional rains of hot pebbles from their vents. Taruntius on the Foecunditatis/Tranquillitatis Strait is infamous for its fumaroles, that vomit forth clouds of toxic hydrogen sulphide gas to drift over the nearby waters. Gutenberg's lava-flows follow the same path southward, year after year, building up a long ridge that reaches much of the way to Mare Nectaris while seldom endangering any lives. The very young volcano known ruefully as Hell* popped up out of a beet field in Deslandres less than 70 years ago, has grown into a full-fledged cinder cone nearly a thousand feet tall over the last several decades, and is still growing.
[* - For real! I swear! It's on the map!]
Naturally, Materans don't live on the slopes of volcanoes that are active on a regular basis ... unless the eruptions consistently direct themselves in one direction, in which case the exceptional fertility of volcanic soils may take precedence over the risk of ashfall, on the mountain's opposite flank. Cities in the Hollow Moon tend to be built in upland regions where vulcanism is minimal, in places sheltered from lava flows or mudslides and upwind from fumaroles; offshore basalt islands on the eastern seas or Midland Ocean, where volcanic activity doesn't occur, are also at a premium as locations for permanent habitation. Villages tend to be a bit less picky about their sites, as even the most-settled of cultures tend to keep relocation in mind when building smaller settlements. Many farming or fishing communities will actually occupy two different sites: a "working-village" near their crops or fishing grounds, to which able-bodied folk commute for periods of a few sleeps or a half-month at a time, and a "living-village" for children, the elderly, and their caregivers and protectors. Valuables, shrines, and the like are kept in the living-village, which is situated in a geologically-secure spot. If vulcanism threatens the croplands or mudslides foul the river, the working-village's site is packed up and abandoned, and a new place to farm or fish is established as soon as the Fire Time ends. Divinations unique to the Hollow Moon setting allow Materan priests or shamans to predict how long a site near an active or semi-active volcano will be geologically safe to occupy, although the exact nature of the disaster which will eventually render it unsafe -- lava, gas, tremors, ashfall, etc -- is not revealed. Such divinations aren't 100% reliable, but they are designed to err on the side of safety, indicating a disaster is forthcoming if there is even a shadow of a doubt; too many "false alarms", unfortunately, may cost a priest credibility, and cases of outright fraud by land-grabbing clergy are not unknown.
For the most part, Mystaran PCs will find lunar people astoundingly unintimidated, even blasť, about volcanoes. However terrifying they might seem to the people of the Known World, active volcanoes are as ordinary -- and dependable -- a fact of life, to the Hollow Moon's natives, as are the seasons of the year, to Mystara's folk! The fact that eruptions are predictable to within a matter of hours, months or years in advance, is what gives lunar folk this confidence: so long as they know what's coming, and WHEN, they've got all the time they need, to prepare for each eruption and the resulting disruption of their environment. Crops can be safely harvested, before ashfall buries the fields (and seed sown, as soon as the fertile ash cools down!); boats can be beached, or taken up a safe tributary, hours before debris sweeps downriver from an eruption-site (and then brought out again, to pick through the debris for wood and other salvageable materials!) ; people can clear out of areas where destruction is inevitable with time to spare, bringing their possessions with them (and immediately start planning what their next homes will look like ... a favourite pastime, for Cacklogallinians!). In a very real sense, each severe Fire Time is, to Materans, what winter is to Mystaran cultures -- a time to be prepared for, and to stock up supplies in readiness against; but also a time which heralds the rich renewal of the land, and leaves an explosive rebirth of vegetation and animal life in its wake. (Plus, it's only a few days out of an 18-month year, for cryin' out loud! Mystarans get stuck with whole months of harsh conditions, every winter ... a prospect which would absolutely HORRIFY Materans. ;-D)
Other Fire Time Catastrophes
Aside from eruptions, volcanic activity during Fire Times has similar consequences to that on Mystara or Earth. Here are a few possible manifestations of geological unrest, for the Hollow Moon setting:
Avalanches -- These occur quite regularly in the Mare Frigoris region, in response to Fire Time ground tremors. Active or semi-active volcanoes in the HM erupt too frequently for their peaks to accumulate much of a snow-cap, but some dormant peaks which experience tremors may unleash an avalanche as a result.
Earthquakes -- Because the Hollow Moon has no tectonic fault lines, tremors tend to be confined to the immediate vicinity of major eruptions. Characters will seldom, if ever, encounter a natural earthquake that's as destructive as the Earthquake spell; most tremors of significant size can flatten only flimsy structures such as tents, yurts, animal pens or corrals. By law, all torches, lanterns, ovens, forges and hearths in Cynidicea or the Cacklogallinian trade-cities must be completely extinguished during Fire Times, to prevent possible fires from breaking out due to spilt embers.
Floods -- One of the few dangers that tend to crop up a few days AFTER a Fire Time, flooding occasionally results from a river's getting cut off from its usual course, be it by magma or by the debris of a land- or mudslide. (At the same time, of course, another region will be suffering from the loss of its now-diverted river.)
Landslides -- Yet another response to tremors, these are especially likely in areas of collapsed calderas such as the Taurus or Caucasus Mounts, where the ground is mostly mounded rubble and boulders. The deadliest of all volcanic eruptions, pyroclastic flows, involve massive landslides of superhot debris from a volcanic cone's blown-out wall; read up on Mt. Saint Helens, for all the grisly details on these monster blasts.
Mudslides -- The bane of Hollow Moon rivers, these tend to occur several hours after an actual eruption, when ash and dislodged debris pour down the slopes of volcanoes into adjacent watercourses and are carried downstream for miles. Trees and other standing obstacles not strong enough to resist the force of flow are knocked down and carried along with the mud, and may wind up floating in the nearest lake or sea. Note that mudslides only occur in places where water is already present; unlike volcanoes in other settings, Hollow Moon volcanoes don't have snow-caps to melt and produce huge swaths of mud which can flow down their slopes (the real cause of most of the property-damage IRL volcanoes inflict).
Tsunami -- This is one disaster that virtually no area of the HM setting is at risk for, simply because there are no continental shelves along which giant waves can be uplifted! The flatness of HM seabeds, which are of level crystal rather than contoured ground, protects the vast majority of the setting's coastlines from tsunami. The only two exceptions are the mouth of the Tauruntius Strait, and Mare Humorum. The first of these locations is where the deep waters of Tranquillitatis abruptly give way to the coral-elevated seabed of Foecunditatis; tsunami can form if tremor-generated waves pass from the former sea, to the latter, but no vice versa. Such waves tend to strike the Foecunditatis coastline from the equator to the Langrenus caldera. The second spot where tsunami sometimes strike, Mare Humorum, can experience such events because eruptions along its shores cause Venturer Deep to void powerful upwellings of water, stirring up murk from the Mare's mysterious seabed and sending colossal waves crashing against every point on the steep Humorum coastline.