Plate Tectonics and Volcanism
Tectonic features: Rift valleys and continental
plates, chain volcanoes, uplifted/fractured regions of
the curst, landslides from "earth"quakes, fault
zones, sinkholes, subsidence faults.
Volcanic Features: Shield ("hot
spot") volcanoes with calderas, lava flows, lava
tubes, outgassing vents, flooded craters, ash clouds
- Plate tectonics creates chains of volcanoes.
- Faster plates yield more, smaller volcanoes in a
- Volcanic eruptions can occur years to tens of
thousands of years apart. If the magma from a
previous eruption has time to cool and solidify,
it forms denser, stronger rock than
shield/slopes. So next eruption will often form
- Lava must flow downhill, and can do so rapidly
and for great distances if the cooling time is
Fluvial Erosion (Erosion due to water)
Fluvial features:streamlined beds, sinuous
"braided" channels, Oceans, lakes, or dried-up
river deltas and sand bars, (tear-drop shape islands,
- If liquids exist on the surface of the planet,
they can transport particles, bury and/or erode
- Again liquid always runs downhill. Sediment
carried from one spot must be deposited someplace
else, usually in a characteristic delta pattern.
- It takes a lot of liquid to move a small amount
of solid material.
Aeolian Erosion (Erosion due to wind)
Aeolian Features: Dunes, downwind dust tails,
dust storms, undercut cliffs
- If the planet's atmosphere is substantial enough,
winds can transport small particles, bury
features or erode them through sandblasting.
- Wind transport can be uphill as well as downhill.
- It can create some features similar to fluvial
erosion, but typically broader, shallower, and
- Tails form downwind of positive or sometimes
negative relief features as turbulence causes
particles to drop.
- Dune ridges are perpendicular to wind flow, with
sharp edge of ridge downwind. Rounded dunes are
formed by alternating wind direction.
Glacial Erosion (Erosion due to ice)
Glacial Features: Layered terrain, straight
V-shaped valleys, plains scoured flat with material
deposited at edge.
- If planet has liquids and regions of low
temperature (polar regions or mountain peaks)
repeated snowfall can build up large masses of
ice. If temperatures vary, these can expand and
recede or flow downhill.
- If a layer of ice or permafrost is buried under
normal soil and melts suddenly, fretted terrain
can be produced.
- Mountain glaciers must flow downhill. Polar
glaciers spread radially outwards from poles.
- During winter at Maritan pole, it gets cold
enough to freeze carbon dioxide out of the air as
"dry ice" snow. Similarly, this snow
and ice evaporates in summer. Since the
atmosphere itself is disappearing, this leads to
strong planetwide seasonal winds.
- When explaining a feature through an erosion
process, be sure to account for elevation, slope
and sheer volume removed. Ask where the eroded
Gradual bombardment by meteors and asteroids produces
impact craters, can overturn, transport and mix soil, and
trigger dramatic climate or tectonic changes.
Gardening Features: impact craters, eject
- Planets with thicker atmospheres disintegrate
smaller meteors, so that gardening is less
- Current bombardment is very slow; larger meteors
and asteroids are much less common. Any
significantly cratered surface is probably very
old. Bombardment is assumed to be uniform over
the surface of a planet, so less cratered terrain
- When age dating, topmost feature is youngest, and
surface with fewest craters is youngest.
- Random chance alignment of independent impacts
- Fragmentation of a large body just before impact;
possible for smaller chains and more likely if
there is an atmosphere.
- A ray of debris from a larger impact; possible
for smaller chains near/radially aligned with a
- Skipping from an impact at a grazing angle;
possible for a small chain, but you should see a
sequence of progressively smaller craters, first
and largest distorted along length of chain.
Models rarely produce more than a few hops.
- Collapse of a lava tube; possible for small
diameter craters, especially of uniform size.
Craters must lead down hill, and there must be
nearby volcanic activity.
- Subsidence along an underground fault. If an
underground fault spreads, surface material can
collapse, producing sinkholes along the fault;
look for evidence of other faulting/tectonic
activity. A water table drop can also cause
- Explosive outgassing along an underground fault.
If an underground pocket of gas is quickly
compressed or heated, it may vent explosively
along a fault line or similar weakness in the
surface. Such explosions are more forceful in
planets with light gravity and little atmosphere.
Look for evidence of other faulting or volcanic