Pet Planet

This science toy offers you an opportunity to use the scientific principles of plate tectonics to design and build your own pet planet. We'll start by reviewing the two types of crust that make up the surface of a planet, and discuss how the crust is broken up into tectonic plates. We'll show how the plates move, and introduce the three types of plate boundaries. Once we establish some basic outlines for planet building, you'll get to work, making the building blocks that you'll use to illustrate the tectonic forces at work on your planet.

What is plate tectonics? in simple terms, it is the process that forms and shapes the crust of our planet. The earth's crust is made up of huge "tectonic" plates that interact, one with the other. The plates slide, tear apart, collide, and buckle, creating mountain ranges, ocean basins, and volcanic islands chains.

With apologies to professional geologists, the following bare-bones primer on plate tectonics introduces concepts that are actively shaping planet Earth, but which could apply equally well to any planet with active tectonic processes. Your pet planet doesn't have to follow Earth's tectonic model, but understanding the process, as it happens on Earth, will serve you well in designing and building your own planet. Think of this instructable as a guide for the care and feeding of your pet planet.

The guide begins with "technical" information about plate tectonics, plate boundary types, and other tectonic processes that will be the building blocks for your planet. The technical steps are followed by a few steps with practical information about working with the colored vinyl material, and putting the pieces together to build your planet. Let's get to world building!

Blank World

Colored Vinyl Sheets

Scissors

Utility Knife

Large Plastic Jar Lid (for stand)

Hair Dryer (optional)

Tape

Black fine-point marking pen

To begin building your planet you will need to acquire a bank globe. I wanted to build this using an inexpensive plastic playground ball, like the ones you see in a big wire cage at the supermarket, but those appear to be a seasonal item, and I couldn't find one. I wound up ordering a ten-inch diameter, rigid plastic ball, originally intended to be a dog toy. It wasn't overly expensive, but I would say that any smooth ball would do, the bigger the better. Try to find one in shades of blue. If you can't find a blue ball, perhaps your planet will have pink oceans.

The continents, plate boundaries, and other features of your planet will be built up from pieces of adhesive-backed vinyl. These vinyl sheets come in a range of colors, and you'll need to decide which colors to use to represent surface topography, and which combinations of colors you'll use to represent the three types of plate boundaries.

While you're reading the tectonics section and assembling the Legend and the symbol tapes, you should begin thinking about what features you want for your planet. The number, size, and distribution of continents is limited only by your imagination. Don't feel overwhelmed by the vastness of the empty globe. Spend some time daydreaming about your planet, and ideas will come to you. Any decision you make can be changed by simply peeling up the vinyl and putting it somewhere else. Start planet building and make adjustments as you go.

Let me state at the outset that this instructable provides the bare minimum of scientific information to get you started on building your pet planet. The information leaps across volumes of geologic research and study. I beg the forgiveness of generations of geologists and scientists whose tireless work and devotion has revealed the mystery of plate tectonics. Although much has been discovered, there is much left to learn. If this instructable inspires just one student to study geology, I hope they will forgive me.

I encourage anyone who finds this instructable to use this as a stepping off point for further study in the exciting field of geology. Geology happens on other planets, too, and there is no limit to what future geologists and planetary geologists will discover. You might be one of them.

Types of Crust:

In simplest terms, our planet is covered with two types of crust: oceanic crust (basaltic rocks); and continental crust (granitic rocks). Oceanic crust is generally thinner and more dense than continental crust. You could think of continental crust like the froth that forms on a boiling pot of jelly. It's too light to sink back into the jelly, but it gets broken up and pushed around by the currents underneath. [If you've never made jelly, I can't help you.]

The Earth's crust is divided up into about a dozen major "tectonic" plates, and many smaller ones. Most of the major plates include land masses, but when we talk about continents moving, we understand that it's actually the plate that the continent rides on that is moving.

Plate Boundaries:

There are three possible interactions that can occur between crustal plates: spreading apart (rifting), sliding past in opposite directions (lateral faulting), and diving under the edge of an adjacent plate (subduction). Each type of plate boundary is discussed below, with and a couple of additional concepts thrown in to give you some ideas to work with.

Rifts:

Rifting (illustrated in the cartoon, above) begins with the crust thinning and spreading above a mantle plume. The spreading causes fractures, and magma rises to fill these fractures, as the magma cools, the rocks shrink, causing more fractures, which become filled with more magma. As the process continues, the rift widens, eventually forming an ocean basin. This is why many ocean basins have a "ridge" running down the middle. Rifts are also referred to as mid-oceanic ridges, or spreading centers; we'll refer to them as rifts going forward.

There are a couple of things to remember about rifts: first, rifts can grow at different rates in different places, but they don't make sharp bends. Bends along the trace of a rift are accomplished by offsets along lateral faults (discussed below). The second thing to know about rifts is that new crust is created on each side of the rift, and as the rift grows you need to compensate for that growth by consuming crust somewhere else.

Subduction Zones:

Crust is created at rifts, and it is consumed in subduction zones (trenches). Subduction is where a slab of oceanic crust descends into the mantle beneath an adjacent plate of crust. But how does subduction start (you may ask)? Consider a crustal plate that includes both oceanic crust and continental crust. The margin where the continent is attached to the oceanic crust is called a passive margin; (Note: this is not a boundary between plates, but a "margin" between two types of crust within the same plate.) .

As rivers wash sediments off of the continent, into the ocean basin, a wedge of sediments forms on top of the oceanic crust. This wedge is thickest at the edge of the continent, and the weight of the sediments warps the crust downward. This down-warping eventually causes the crust to break loose from the edge of the continent.. The oceanic crust is more dense, and begins sliding into the mantle, beneath the edge of the continent (or another plate of oceanic crust), forming a deep trench.

This process of one plate sliding beneath the edge of another is called a subduction, and the dipping boundary between the two plates is called a subduction zone. As the plate of oceanic crust (and sediments) descends into the mantle, the cold, wet rocks and sediments react with the hot mantle, producing magma that rises to the surface of the overriding plate. The resulting range of volcanic mountains is referred to as a volcanic arc (or magmatic arc) because the range of volcanoes typically has an "arc" shape, mimicking the shape of the subduction zone. The west coast of South America, with the Andes mountain range, is an example of this type of plate boundary.

A subduction zone can also form when a plate of oceanic crust becomes subducted beneath the edge of another plate of oceanic crust. In this instance, the volcanic mountains that form behind the subduction zone are referred to as an island arc.

In California, the volcanic arc that formed when the western margin of the continent was a subduction zone, was located in the area we now know as the Sierra Nevada mountain range. The volcanic rocks have mostly eroded away, leaving the roots of the mountains exposed as vast granitic "batholiths" that originally intruded into the overlying plate, beneath volcanic arc. Going forward, we'll use the terms subduction zone and trench more or less interchangably.

One other thing about subduction, that we'll discuss again later, is what happens when a landmass (or island arc) on the subducting plate gets swept into the subduction zone. The landmass is too large to slide beneath the continent, so the collision pushes up mountains. Subduction cannot continue at that location, and the landmass becomes part of the continent. In time, a new subduction zone may form on the seaward side of the new mountains.

Lateral Faults:

The third type of plate boundary is a lateral fault boundary, where two plates slide past each other, in opposite directions. These faults are sometimes called transform faults or transform boundaries. Lateral faults connect segments of other types of boundaries, and are the main mechanism for accomplishing bends in rifts. On land, lateral faults often have bends, due to obstacles, or stresses caused by other plate movements. These faults may splay out over a broad area called a shear zone, often with significant uplift or down dropping of the rocks adjacent to the shear zone.

Lateral faults can be either Right Lateral Faults, or Left Lateral Faults. Say you are standing on one side of a lateral fault, looking across the fault at the opposing plate, if the rocks on the opposite plate move to your right, this is a Right Lateral Fault; if the rocks on the opposite side plate move to your left, this is a Left Lateral Fault. The San Andreas fault in California, extending for hundreds of miles, is an example of right lateral fault, with the rocks on the west side moving north, and rocks on the east side moving south. Note: the sense of movement (left or right) on a lateral fault refers only to the movement between the two plates at that point. The overall direction of plate motion may be different than the relative direction of movement between two plates at a particular point.

Oblique Subduction:

We think of subduction as one plate sliding beneath the edge of another, with the movement of each plate being more or less perpendicular to the boundary, but this is rarely the case. Subduction can occur at any angle to the plate boundary. As the angle between the plate boundary and the direction of plate convergence becomes narrower and narrower, the relative movement between the plates becomes more lateral than convergent, and boundary can transition from a subduction zone to a lateral fault depending on the shape of the boundary and direction of motion.

Don't overthink this, just be aware that the orientation of a subduction zone does not have to be perpendicular to the direction of movement for the decending plate.

In addition to the tectonic forces acting at plate boundaries, there are also tectonic processes at work within plates. The Hawaiian Island chain is an example of intra-plate tectonics. A mantle hot spot (plume of rising magma) beneath the Pacific plate results in volcanoes forming in the middle of the plate. As the plate moves above the hot spot, volcanoes form, making an "island chain" on the surface of the moving plate. Older islands sink and erode as they move away from the hot spot, many falling beneath the waves entirely. A bend in the chain of Hawaiian volcanoes and seamounts shows that the plate movement has changed direction at some point in time. Perhaps due to a plate collision or rifting somewhere else.

Mantle hot spots can also occur beneath continental crust (Yellowstone National Park is believed to be above a mantle hot spot). In some cases a hot spot can develop into a continental rift, swelling the crust until it splits, as illustrated in the cartoon drawing shown in the previous step, eventually the hot spot-rift may grow wide enough to form an ocean basin. The Red Sea, at the eastern edge of the African continent, is an example of an active rift has evolved into an oceanic basin, A branch of the rift extends to the south, on land, as the great Rift Valley of east Africa.

We've looked at boundaries between two plates, but when a third plate becomes involved the point where the three plates come together is called a triple junction. Triple junctions can occur in a variety of configurations; they occur where a rift system branches, and whenever a plate boundary (of any kind) becomes dragged into a subduction zone. The cartoon, above, illustrates some examples of triple junctions from planet Earth.

There isn't anything particular you need to know about triple junctions, just be aware of the relative direction of movement between the plates at triple junctions.

We previously mentioned the problem of a landmass getting swept into a subduction zone. When an island arc or other landmass overlies a plate that is moving toward a subduction zone, the landmass will be blocked from descending beneath the overriding plate. The landmass, instead, "docks" to the edge of the continent, becoming part of the overriding plate. This docking process is called "accretion," with the new crust becoming an "accreted terrane."

As convergence continues, a new subduction zone will form on the oceanward side of the newly accreted terrane. The former subduction zone, now a "fossil subducton zone," is known as a "suture zone," where rocks that formerly existed at great depth in the subduction zone have been pushed to the surface. Suture zones typically include quantities of metamorposed seafloor rocks, called "serpentine," that typically occur in a band between the original continental crust and the new accreted terrane.

I'm using purple vinyl to indicate serpentine in suture zones, where I've added landmasses to continents (apologies if the purple color does not show up well in the photos). You may want to identify suture zones when building your planet to show how much of a continent is made up of accreted terranes.

Working with the adhesive-backed vinyl can be frustrating, until you figure out the technique for removing the backing material from the vinyl. The trick is to FIRST, fold a corner (about a quarter-inch or less) toward the vinyl side, creasing the fold on the paper side. Then, fold the corner back the other direction (paper-side together), without pinching the fold. You just want to push the corner back around far enough that the paper will stay bent toward that side. With a little teasing, the vinyl should just "pop" loose from the paper backing. Otherwise, a little help from the edge of your scissors or a sharp fingernail should do the trick.

If you can't get that trick to work for you, try attaching a small piece of tape to each side of the cut piece of vinyl (leaving finger tabs on each piece), and pull the tape apart. The backing should come off easily when you pull the tape pieces apart. Then you just need to figure out how to detach the tape from the vinyl.

Resist the urge to peel the backing piece away, entirely. It's much easier to position the piece where you want it, while the backing is mostly attached.

When building up continents, you will quickly see that a large piece of flat vinyl does not easily wrap around your globe. This is not a problem. You will be building your continent from pieces, and if you limit the width of any particular piece to a few inches, you will be able to get the piece to lie flat. You may find using a hairdryer aids in stretching the vinyl a bit to get it to lay down, but don't stretch it too much or it will retract when it gets cold.

You will still have many little wrinkles, where the vinyl buckles along the edges. Smooth the vinyl down so it sticks to the globe between the wrinkles (minimizing the number of wrinkles if you can). Using the very tip of a sharp pair of scissors, slide the lower blade of the scissors down the length of the fold, and snip, extending the cut a tiny bit past the end of the fold (to avoid puckers). As you remove the scissors, hold one cut edge up slightly while the other edge is smoothed down, then let the flap fold down over the other edge, and smooth it down firmly. Repeat this "snipping" procedure wherever there is a wrinkle (bubbles can be pierced with the tip of a utility knife).

This snipping process also applies to laying out the boundary symbol tapes. Where it is necessary to curve the tapes (for all trenches and some lateral faults) focus on adhering the outside edge of the strip of tape as it bends around the turn, then snip evenly on the side of the tape that is standing up, pressing each segment flat as you go.

Lastly, when you build up continents with different layers (indicating different surface elevations), you can use the backing piece from one layer as a guide to help you cut out the next overlying layer.

The first step in building your pet planet will be designing the "Legend." The legend will define what colors of vinyl you want to represent the various elements of your planet. Each color represents a different elevation, except for the mid-range of elevations, which are divided into three environments (desert-grassland-forest). We also have colors to represent sub-sea topography, such as shallow inland seas and continental shelf areas.

The Legend will also show which combinations of colors you'll use to represent the three types of plate boundaries. If you want to imagine some other features, revise the legend to suit your planet's needs.

If you've been daydreaming about your planet, now is the time to put those ideas to work. Think about how much of your planet you want to cover with land, and how much will remain ocean basin. The arrangement and distribution of land masses, rifts, trenches, and faults are all up to you. It can be daunting, but keep in mind you can remove and reapply the vinyl, so changes are relatively easy.

Get familiar with your world. It's a good idea to place small red dots representing the poles of your planet. These handy reference points will help when you're building continents and laying out plate boundaries.

Place your hands on the globe, and slide them around to visualize how the plates interact. You will note that any time you move a hand, it will interact in one of three ways with any other adjacent plate. The movement will either leave a gap (rift) behind the moving plate; scrape past the plates on either side (faults); or slide beneath an opposing plate. It can be confusing when you're thinking about all the interactions, focus on one boundary at a time.

Regarding whether to place boundary types first or continents first, the answer is sure. Some relationships become more clear after you apply continental land masses, and some may be easier to understand without the continents in the way. You can always re-apply any boundaries that get covered up, so don't overthink the problem.

To get started, print out a copy of the Legend file (PDF and MSWord versions are provided), and decide whether you want to use the colors in this example or define your own symbology. Either way, fill in your legend and keep it handy as you build your planet. Of course, feel free modify the legend to suit your imagination.

You've got the legend constructed, now make strips of vinyl "symbol tapes" for each of the plate boundary types from your legend. When layering the sticky vinyl strips, it's best to attach the upper most layer to the next piece down, and so on, to build up the symbols. You will note that the left and right lateral fault symbols are just the same symbol, but inverted.

Once you have prepared some symbol tapes, it's time to start building your planet. It's hard to know where to begin, but I've been mulling over a few ideas; I'm going to start by laying out some free-style rifts, just letting my imagination wander around the globe. Remember that rifts don't make bends, you need to make stair-steps, using lateral faults to connect rift segments where bends are needed. I recommend cutting the tape segments at 45-degree angles for making these stair-steps.

Take care that you orient the lateral fault symbol tapes correctly to indicate left or correct sense of movement on these faults. If the bends in your rift seem a little crowded, this is a good time to reconsider how wide to make your symbol tapes. I made all these tapes very wide (5/8-inch) so they would show up better in the pictures. You may want to make your boundary symbol tapes much narrower, to avoid a crowded appearance.

For my first continent, I think I'll start with a super-continent with a large inland sea. I might crash some other continents and island arcs into it later, just to make it interesting. Oh wait, what's happening here, the super-continent is bulging... wait, it's... it's... a MANTLE HOT SPOT... OH NO, it's rifting, noooooooooo.

Whatever you do, be sure to have fun with it, maybe you need symbols for "bolide impacts," or "kimberlite pipes;" (look them up) make it your own world.

For my pet planet, I'm generally using Earth-style topographic conventions for the colors representing different elevations and environments. By all means, for your pet planet feel free to design whatever environments you want, and choose colors that represent your vision. Note: there are other combinations of vinyl colors available, but be sure that you buy the stuff that is removable. The vinyl shown here has a "mat" finish. I think glossy would also work.

You may never feel quite finished with your planet, tectonics can be tricky. You may find yourself coming back to make revisions from time to time. You can add, remove, or rearrange continents and plate boundaries as much as you like (the vinyl can be re-positioned many times). See what happens when an island arc crashes into a trench. Rift that continent in half and think about what effect that would have on other plates.

You may find it easier to understand your pet planet if you use a fine-point marker to draw arrows, indicating direction of movement, on the plate boundary markers.

Finally, you've put a lot of work into building your planet, honor that effort by giving your pet planet a name. You can also name as many of the features on your planet as your imagination can bear. See if you can explain how tectonic processes shaped the surface of your planet. If you can do that (and still keep your friends), you may be on your way to a becoming a geologist.

[I've included a couple of pictures of my first pet planet "Runcible-8" as another example. I made this one with a six-inch ball, adhesive-backed foam (colored with markers), and symbol tapes that I printed on sticky labels (colored with markers). Sorry it's a little tattered. I'm attaching the file with the "arrow" symbols, in case somebody wants to try using them for their planet. I don't know whether you can print onto the colored vinyl, but that might be an option.]