These two observations led earth scientists to the hypothesis of isostacy

• Isostacy = floating balance, in which rigid crust floats on an underlying mantle

• Mantle is more deformable and more dense than crust

• Crust floats on mantle by

  • Airy mechanism - all crust has similar density. Where elevation is higher, crust is thicker
  • Pratt mechanism - crustal density differs. Where elevation is higher, crust is less dense

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