Geophysical Globe: crustal thickness

These globes of the earth's topography opens to reveal the base of the crust (moho) which lies beneath it, an excellent aid for teaching and outreach in geology and geophysics. The relief at the surface and on the base of the crust are at the same scale (50x vertical exaggeration), and this is an excellent tactile way to explore the structures in the earth which lie beneath our feet

The earth's topography (i.e. where the surface of the earth is higher or lower) is controlled in large part by the thickness of the crust. Since the crust is lighter than the deeper mantle rocks, crust is buoyant and therefore "floats" at the earth's surface (if you want to know more about this - google "isostasy").

What this means is that most high places like mountains have roots! Much like an iceberg, the topography we see at the surface is small compared to the amount beneath the surface. Notice how the thickest areas of the model are found in the Himalayas, where crust reaches 80 km thick, and beneath the high Andes. Correspondingly the deep oceans, where topography is at its lowest, are associated with thin crust.

There are two models uploaded here:

• The first version is scaled correctly to crustal thickness (50x vertical exaggeration) but has no space for magnets and therefore does not have a closing mechanism. It also has a small contact area with the print bed so may need to be printed with a brim or raft if you're having adhesion issues.
• The second version is designed to be printed with embedded 5x2mm neodymium magnets, by inserting gcode to pause the print and allow the magnets to be inserted at the correct layer.(Note for teachers / geeks / pedants: This crustal thickness is not to scale - surface relief and relief of the base of crust are at the same scale, but to make this 3D printable and to have space for magnets additional constant thickness had to be added (about 3.5 mm at 100% scale)

Source data:

Topography: ETOPO1 (https://www.ngdc.noaa.gov/mgg/global/)

Moho depth: CRUST1.0 (https://igppweb.ucsd.edu/~gabi/crust1.html)

Printing:

Supports are required in the middle of the model - but be careful that you use support blockers / enforcers or fully custom supports to avoid filling in the magnet holes!

If you wish to insert magnets to make this model snap close, you should insert gcode to make the machine pause at the layer change where the print will bridge across the gap. I slice the model using PrusaSlicer and use the colour change gcode function to find the right location, then make the following edits to the gcode.**PLEASE NOTE THAT THESE MAY DIFFER FOR YOUR PRINTER**. An excellent guide to the process can be found on youtube by Joel the 3d printing nerd (h ttps://www.youtube.com/watch?v=oF1SdIR-Kow)

Gcode edits for Prusa mk3 (subsititute between <>)

M600

should be replaced with

M300 ; beep
G1 X10.000 Y200.000 E0; parking position
M1; user stop and wait for button press
M105; return to current temperature

Link to magnets I used on Amazon: https://www.amazon.co.uk/dp/B00TACMMP0/ref=cm_sw_em_r_mt_dp_U_qbJSCb6NFKJ46

How I Designed This

As with my other designs, the topography globe was created in Blender by applying a displacement map to a sphere via UV mapping. A second smaller globe of moho depth was created by the same method and then subtracted from the outer globe.

The model was sliced into two hemispheres and magnet holes added in meshmixer.

Detailed methodology here: https://github.com/jeffwinterbourne/3D_globes/

Category: Physics & Astronomy