A 3-D Sliding Jigsaw

[Note: this is part of a series on the science of worldbuilding. Previous installments have covered climate and biomes and looking at habitable planets from orbit.]

Elena pulled up the map of the northern ocean on her holographic display. The ocean shimmered blue, shaded for depth, and the continental landmasses rose from the sea in progressively lighter shades of brown. She wanted to run her fingers over the mountain ridges, but she knew it was a trick of the brain. Her fingers would go right through the map. The western continent had mountains all along the coast, taller than the Andes back on earth. The eastern continent had complementary curves, separated by a widening ocean. The two had come together a few million years ago, forcing up the mountains, and were now heading apart in a complex ballet found on habitable planets across the universe.

Just as biomes fit in understandable places in relation to topography, mountains and oceans are found in understandable (and sometimes predictable) arrangements. Right now we only have one example to study closely, but scientists have figured out how it works. The Earth’s crust is made up of separate plates, and they slide slowly around the globe. When they crash together in slow motion, mountains are pushed up. When they pull apart, oceans widen. When the plates slide along each other, earthquakes occur.

How would anyone come up with such a scheme? Continents moving around, crashing into each other? Once we started making accurate maps of the globe, we quickly realized that the continents look a bit like a jigsaw puzzle, especially around the Atlantic. South America could nestle comfortably into Africa.

Geologists and explorers found other puzzling things: remains of reefs in cold areas, glacial deposits near the equator, similar fossils in widely separated areas. Minerals within rocks are aligned toward the magnetic poles when the rocks harden from magma, but in some areas the direction is wildly off from the current polar direction.

If you treat the globe as a giant jigsaw puzzle that changes over time, all of this information can be used to reconstruct what was where when. Fossil ages, rock ages, correspondences between far-away areas: all these can be used to reconstruct the patterns of continents at different times.

Here are all the pieces.

We know how they’ve moved in the past, and can predict how they will move in the future.

(More animations.)

But what carries these huge heavy pieces of rock around? Convection currents within the Earth’s mantle, just like the ones in the atmosphere. The Earth is made up of layers. At the center is a molten iron core, very hot: about 7000C at the center, and 4500C at the outer surface. Above that is the mantle, a thick layer of partially molten rock, and at the top is the crust.

As you’ve probably noticed, the crust is a lot cooler than the mantle, at least in most places. The molten rock at the bottom of the mantle is heated by the very hot core. Hot rock rises, just like hot air. It reaches the top of the mantle and cools. Cool rock sinks. These large, slow convection cells carry the crustal plates along with them.

Currents can carry two plates together at a convergent boundary. This is the most spectacular boundary. One plate slides under the other, but the force involved is great enough to push up mountains. Volcanoes form along the edge and earthquakes occur. The Andes were formed this way.

The plates can be carried apart at a divergent boundary. The thin weak crust that forms in the gap is prone to volcanic activity. The gap will eventually fill with seawater. The Gulf of California and the Rift Valley in Africa are both divergent boundaries.

Two plates can also slide along each other. This doesn’t form any impressive topographic features, but can cause earthquakes. As the plates slide along each other, the rocks bend and store up energy. Eventually the plates will slip and release the energy, sometimes violently.

Smaller convection cells can create hot spots, like that responsible for the Hawaiian volcanoes.
The Pacific plate moves slowly over this hot spot, creating a chain of volcanic islands.

So there’s some really cool planetary science. But what about the fiction? Well, the above video predicted continental positions about 300 million years into the future – want to write an accurate far-future piece? (I’ll leave biome placement as an exercise for the reader.)

Or, on a smaller scale, after the recent earthquake Concepcion, Chile, moved ten feet. There’s a science fictional premise all by itself, except real. What happens to a city’s infrastructure when it moves? All the maps are wrong, your GPS says you are somewhere else. What else might occur?

Then there’s other planets. Venus doesn’t have much in the way of plate tectonics. Mars doesn’t either, but it used to. Larger planets are likely to be more active than small ones, and younger more active than older. The planet has to have a hot core and molten mantle to carry the crust. Small planets cool faster, and old ones are cooler than young ones. Oceans appear to help lubricate things. A habitable planet with open oceans is likely to have tectonic activity.

You can follow any responses to this entry through the RSS 2.0 feed.
You can leave a response, or trackback from your own site.