BotEC: Walking to the Center of the Earth

Peter Kresan


Question: The earth's interior is composed of three main concentric zones: the crust, the mantle, and the core. The outermost layer, the crust, averages 40 km thick on the continents and is thinner (averaging 8 km thick) under the oceans. The middle layer, the mantle, is an average of 2900 km thick, and the core, the innermost layer, has an average radius of 3470 km, about the size of the planet Mars.
  1. Assuming that you can walk 10 miles in a day (and that you can stand the heat and pressure!), how many days would it take you to walk to the crust-mantle interface, if you started from the land surface? Round off to the nearest whole day. (Hint: 1 mile =1.6 km)
  2. How many more days would it take to get to the center of the earth?


  1. 3 days
  2. 396 days

References and Resources

This SERC page describes the use of Back of the Envelope Calculations

A View from the Back of the Envelope (more info) : This site has a good number of easy simulations and visualizations of back of the envelope calculations.

The Back of the Envelope : This page outlines one of the essays in the book "Programming Pearls" (ISBN 0-201-65788-0). The book is written for computer science faculty and students, but this portion speaks very well to back of the envelope calculations in general.