## THREE/2 Tectonic Activity

STEP ONE: Generate the tectonic activity factor by using the formula in 3.2.1. Note the factor, it will be used later.

STEP TWO: Roll on Table 3.2.2 to determine the tectonics of the world.

Table 3.2.1 Base Tectonic Activity Factor

TFactor = (1d10 + 5) x M0.5 / A

 ... where: M is the mass of the planet (in earths) A is system age (in Gy)

Differential Tidal Stress: If the world has significant tidal influence from anything other than the primary (large moon, moons in multiple lunar systems, etc), multiply with 1.0 + (Tidal Force x 0.25). For Earth this would mean 1.25.

Icy Worlds: For worlds made up mostly of ice, multiply by density.

Rotation: If the world has a shorter day than 18 hours, multiply by 1.25. If the world has a longer day than 100 hours, multiply by 0.75. If the world has a longer day than its year or is tidally locked, multiply by 0.5.

Table 3.2.2 Tectonic Activity Generation

AGE MODIFIERS:

As a world ages, it cools off. The radioactivity in the core which generate internal heat die down, etc.

SIZE MODIFIERS:

Smaller worlds cool off faster, and they also have more surface area compared to the heat-generating core.

TIDAL INFLUENCE:

Stress from different tidal forces, like a primary and a moon, tend to increase tectonic activity. If a world rotate faster it also generates more stress.

TYPES OF TECTONICS:

On 3.2.2. there are six different "types" of tectonics listed. These are just suggestions, and they are listed in general order of increasing tectonic activity.

DEAD: This world has none or almost none tectonic activity. Small quakes may be possible, or brief activity after a big impact. Dead tectonic worlds once were active, and their surface tend to show signs of older tectonic activity as erosion is very slow. Dead worlds do not recycle atmosphere lost unless by impacts.

HOT SPOT: This world has volcanic activity in a few distinct areas, generally as large volcanoes. It is common on smaller worlds. Much of the world is not active, though, and craters and similar old signs remain.

PLASTIC: The world has a thick crust which deforms plastically. Unlike Hot Spot worlds, this tend to affect the entire world and creates distinct "continents" of higher lying terrain. There generally is some very weak plate activity too. Volcanoes are concentrated in hot spot regions, typically the highest areas of the surface.

PLATE TECTONIC: The crust moves as plates. Some plates are thin and young, (on Earth this would be the ocean plates), other are thick and old. Mountain ranges form when plates collide.

PLATELET TECTONIC: As for plate tectonic, but the crust is thinner, plates smaller and the plates recycle themselves much faster. These worlds have much volcanic activity.

EXTREME: These worlds have so abundant tectonic activity as they do not really fall into any of the categories. It can be seen as a mix of Platelet and Hot Spot tectonics. These worlds are likely not very habitable unless they have an extensive ocean cover. Moons severely deformed by tidal forces fit into this category.

ANOMALIES:

Large impacts can start at least short-lived tectonic activity. On smaller worlds impact craters are the important way of surface shaping, as the internal tectonic activity is too small. Worlds with high-density (radioactive) cores can also maintain tectonic activity longer than those with cores depleted in heavy elements. Other options include tectonic activity from the shrinking of the core or mantle (as it cools off), forming scarps and ridges. This is most likely on smaller rocky worlds.

GRAVITY EFFECTS:

High gravity does not really influence tectonic activity, but it affects how high mountain ranges (volcanic, impact based or plate-tectonic) can be - as the rocks which builds mountains cannot support an infinite amount of weight. Thus, on a world with half the gravity of Earth mountains can be about twice as high.

LACK OF HYDROSPHERE:

A world without liquid oceans (typically water) cannot sustain normal plate and platelet tectonics as well as a world with a hydrosphere can. Continental plates tend to grow fast and thicken the crust, but as heat is still generated from the interior one gets "melt-throughs" instead and wide-scale volcanism, such as basaltic flooding. These melt-throughs can shape the surface radically. In effect, the world behaves much as world with a plastic crust but more actively.

ICY WORLDS:

As icy worlds have a very different composition their tectonics are also different. Icy mantles and crusts made up of ices and frozen gasses need less heat to be active, and on these worlds water and ammonia may take the place of lava on rocky worlds. Such activity could be fueled both by internal energy and sunlight. Icy worlds in the inner area of the Outer Zone tend to have flat surfaces (as ice at those temperatures can't support much vertical surface features). Other types of tectonics can be seen when the watery part of a icy world's mantle freezes and expands, cracking the surface, forming parallel ridges and flooding craters.

TECTONIC EVOLUTION:

Worlds of any size above a few kilometers across have a period of tectonic activity - the process of formation and cooling. The larger a world is the longer time it will be active. Primarily during the first hundreds of million years of a system, impacts will also serve to remodel the surface of worlds, and sometimes to remodel their interior composition too. On small worlds the tectonic activity dies down rather quickly, though. In a billion years, chunk-size worlds will lose activity. In another 2-3 billion years, small terrestrial worlds will also become inactive. Plate tectonics slow down, perhaps to be replaced by plastic or hot spot tectonics, which in turn fade. This mean that the geological recycling of a world slows and finally stops. Erosion becomes less effective, water freezes or binds, lost atmosphere will not be replaced by new tectonic activity. These worlds can still support life, but not in the same way an geologically active world can. Of course, if the system is rich in stray asteroids of a small enough size, a certain replacement of atmosphere and elements may be possible anyway.