The Isostasy Balance
Erode a kilometre off a mountain range and the summits barely drop — the range floats back up.
Phase 0 · The phenomenon
The mountains that will not go down
A cross-section showing continental crust about 35 km thick beneath lowlands, thickening beneath a mountain range: the mountains rise a few kilometres above sea level, and a much larger root of crust bulges down tens of kilometres into the orange mantle below. An iceberg drawn beside it shows the same shape — a small tip above water and most of its bulk below.
Over a few million years, rivers and glaciers strip an average of a kilometre of rock from a mountain range and carry it to the sea. You might expect the mountains to end up a kilometre lower.
They do not. Measured carefully, the land surface drops only a fraction of that — and some of the highest peaks, untouched by the rivers, end up higher than before.
Mountains are not stacked on a rigid table. In this lab you will find out what they are floating on, and why taking weight off makes them rise.
Why do mountains barely get lower when erosion removes a kilometre of rock — and how can the highest peaks even rise?
What this lab works toward
HS-ESS2-1Model how Earth's internal and surface processes, working at different sizes and speeds, build continents and ocean-floor features.HS-ESS2-2Analyze data to show how one change to Earth's surface can trigger feedbacks that change other Earth systems.
Plain-English summaries. The course maps these performance expectations to this lab; the official wording is published at nextgenscience.org.
The stations
- Floating BlocksWhat sets how much of a floating block sits below the surface?
- Crustal RootsHow deep a root does a mountain range need?
- Erosion ReboundWhen erosion removes rock evenly, how much lower does the land actually get?
- Peak UpliftHow can the highest peaks end up higher after erosion?
Each station runs the same loop: predict before you see any data, investigate the model and record runs, reveal what the measurements show, then explain the gap between your prediction and the result. A claim-evidence-reasoning checkpoint and an exit ticket close the lab.
- Each column of crust floats on its own (Airy isostasy). Real plates are stiff and spread a load over hundreds of kilometres, so narrow features are partly held up by the plate's strength.
- The mantle behaves as a fluid only over thousands to millions of years. The rebounds here are the final balanced state, not something that happens overnight.
- Crust and mantle each have one density (2.8 and 3.3 g/cm³). Real crust varies, and the rocks carried away are replaced by nothing but air.
- The mountain range is not being pushed up by plate collision during the erosion; the stations isolate the floating effect.