07 Isostatic balance and the ecological balance of the crust
Isostatic balance (Airy's theory): the Earth's crust is in a state of permanent balance even though its rocks differ in density and thickness. This appears in the oceanic crust (sima, higher density) and the continental crust (sial, lower density and greater thickness), which are in constant equilibrium.
Mountain roots
- Large mountain chains are made of light rock with an average density of about 2.7 g/cm³.
- They have deep roots in the denser mantle rocks reaching a depth of several times their height (about 4 times, so the total depth is about 5 times the visible height).
- A mountain resembles a floating iceberg: the greater part is submerged below.
How balance is restored during erosion
- Erosion breaks up the mountain peaks and carries the debris to settle in sedimentary basins.
- The load on the mountain decreases while the load on the basin increases, producing a difference in pressure.
- Plastic mantle material moves from beneath the high-pressure zone (the basin) toward beneath the low-pressure zone (the mountain), so the mountain rises and the balance is restored.
- This repeats over millions of years.
Why the balance matters for the environment
- Isostasy keeps the crust stable and limits changes in the levels of land and sea.
- Weathering, transportation and deposition constantly reshape the surface, while the balancing mechanisms preserve its overall stability.
- It joins the internal forces (tectonic movements and earthquakes) with the external forces (weathering and erosion) in one balanced cycle.
Lesson summary
- Continental drift: one mass (Pangaea) that broke apart and drifted.
- Plate tectonics: plates float on the asthenosphere and move by convection currents — divergent, convergent and transform.
- Earthquakes: focus and epicentre; volcanic, tectonic (most destructive) and plutonic.
- Waves: P fastest through all media, S through solids only (proof of the liquid outer core), L slowest and most destructive.
- Weathering: mechanical (no compositional change) and chemical (oxidation, carbonation).
- Isostasy: light mountains with deep roots, balance restored as mantle flows under pressure differences.