Page 53 - Laboratory Manual in Physical Geology

E. REFLECT & DISCUSS How is a mountain like the iceberg in FIGURE 1.10B ?

F. REFLECT & DISCUSS Clarence Dutton was not the first person to develop the concept of a floating crust in equilibrium
balance with the mantle, which he called isostasy in 1889. Two other people proposed floating crust (isostasy) hypotheses in
1855 (See illustration below). John Pratt (a British physicist and Archdeacon of Calcutta) studied the Himalaya Mountains
and hypothesized that floating blocks of Earth’s crust have different densities, but they all sink to the same compensation
level within the mantle. The continental blocks are higher because they are less dense. George Airy (a British astronomer
and mathematician) hypothesized that floating blocks of Earth’s crust have the same density but different thicknesses. The
continental blocks are higher because they are thicker. Do you think that one of these two hypotheses (Pratt vs. Airy) is
correct, or would you propose a compromise between them? Explain.

Continent Continent
Ocean Ocean Ocean Ocean

2.8 g/cm 3
2.9 g/cm 3 2.9 g/cm 3 2.9 g/cm 3
2.9 g/cm 3 2.9 g/cm 3 3.0 g/cm 3 3.0 g/cm 3
3.1 g/cm 3
Compensation level

Mantle 3.3 g/cm 3 Mantle 3.3 g/cm 3

Airy Hypothesis: Pratt Hypothesis:
• Blocks of crust have the same density. • Blocks of crust have different densities.
• Blocks of crust have different thicknesses. • All blocks of crust sink to the same level
• Continental blocks are higher than ocean blocks because (compensation level).
they are thicker than the ocean blocks. • Continental blocks are higher than ocean blocks because
they are less dense than the ocean blocks.

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