How Thick is the Crust?

Earth's Structure (Wikipedia)

Earth's Structure (Wikipedia)

I think the first time I really heard much about the Earth's crust was on the TV show "Bill Nye the Science Guy." (In fact, I was obsessive about not missing an episode as a child and I was ecstatic when I got to see Bill speak at Penn State last year.) He talked about earthquakes and Earth's structure, cut in with funny segments of a family telling their son, "Ritchie, eat your crust."

The crust in an interesting thing - it's what we live on top of and there are lots of interesting places where it's different due to geologic processes that concentrate certain types of materials. The crust is broken up into around a dozen major tectonic plates that move at about 4-6"/year. These plates are either oceanic or continental crust. Oceanic crust is generally relatively thin ~6 km (4 miles) and oceanic crust is much thicker at ~35 km (22 miles). The thin oceanic crust is also more mafic and dense than the felsic continental crust.

These differences create complex interactions when the plates meet each other at plate boundaries. We did a whole show on plate tectonics over at the Don't Panic Geocast recently, so if you'd like to hear about the discovery and arguments over plate tectonics you should check it out.

Today, I'd like to share a tool and Dr. Charles Ammon and I have made to visualize a crust model and allow anyone to explore the crust. All you need is Google Earth! We used a model called Crust 1.0 by Laske et al. that has how thick the crust (broken up into a few divisions) is for  64,800 points on the Earth along with some other crustal properties. That's every one degree of latitude and longitude! They put a lot of work into making this model. Generally we would use a Fortran program to get values out of the model, but Dr. Ammon had an idea to visualize the data in a more intuitive way with Google Earth. Over the Thanksgiving holiday I wrote a Python utility to access the model values and then we wrote a simple script that generates a Google Earth KML file based on the model.

All you have to do is head over to the project's GitHub page and click the "Download ZIP" button. While you're waiting on the download you can scroll down and read all about the development, the model, and find activities to try. Next, open folder you downloaded (most operating systems will automatically unzip it for you now) there will be several files, but the only one you need is the CRUST_1.0.kmz file.

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As long as you have Google Earth installed, double click that file and you'll see the Earth appear covered in red dots. If you zoom out too far, they will disappear though!

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Each red dot is a location where the model has the average crustal properties like how fast P and S seismic waves can travel and the density. All of these are explained in more detail on the project webpage. You should also try some of the projects we have listed there! As a starter, let's look at oceanic and continental crust and verify my assertion about their 3x thickness difference.

Clicking out in the Atlantic Ocean (make sure you are not on the continental shelf) we see about 13 km thick crust (the top of the mantle number). The water depth is also handy to have on-hand sometimes.

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Clicking well onto the North American plate we see about 36 km thick crust. Next you should head over to mountainous regions and basins and see how the structure of the crust is different - why is that? Sorry, no homework answers here!

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This is a really fun way to learn about the crust and a good reference tool as well! There are flyers in the docs folder that you can print to use as teaching aids or handout to students! We had a lot of fun making this 1-day project and hope that you'll explore it and let us know what you think! A big thanks to the folks that did the massive amount of work making the model - we just made it visible in Google Earth! Everything is open-source as always.

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