Initial Publication Date: July 6, 2009

Density, Isostasy, and Topography

Anne Egger, Stanford University

The original activity Density, Isostasy, and Topography already exists within the SERC website. This page describes how this activity can be used to teach about the process of science.


In this activity students develop an explanation for the bimodal topography of the earth through measuring densities of rocks and wood blocks, deriving the isostasy equation, and applying their knowledge to estimate crustal thicknesses.

How does this activity lend itself to teaching the process of science?

This activity or one of many variations on it is a common component of any introductory geology course. Though it may seem simplistic I use it not only to teach the concept of isostasy and isostastic equilibrium but to highlight the methods that geologists must use in order to study the earth. The things that I highlight are:
  1. The use of analogous materials - Geologists and geophysicists make models and use materials other than rocks in order to study how the crust behaves. The use of models and analogous materials allows us to do experiments on the time scale of a single class rather than millions of years.
  2. Mathematical equations are models of reality that allow us to make predictions - We derive the isostasy equation from empirical data then use the equation to predict something we can't measure. These equations are useful tools in the scientific tool belt.
  3. Error and variability in geologic data - Why are our measured densities different than published averages We discuss the limits of accuracy for the tools we are using and the fact that all granites are not created equal. We take lots and lots of measurements on different rocks in order to achieve a realistic average.

Specific adaptations to modify this activity to teach the process of science

There are no specific adaptations other than highlighting the points mentioned above.

Assessment adaptations to focus on the process of science

As a follow-up to this assignment students read a Scientific American article and a journal article about mountains erosion and isostasy and are asked several questions. This allows them to see how this relatively simple relationship is being actively used in research today.