The central theme of Pratt's concept of Isostasy was:

Question

Accepted Answer

The central theme of Pratt's concept of Isostasy was:

**Option 3: Uniform depth with varying density**

**Explanation:**

Pratt's theory of isostasy proposes that the Earth's crust is composed of columns of varying densities. These columns all have the same depth (or level of compensation) at which the pressure is equal. Imagine a series of wooden blocks of the same height but made of different types of wood (different densities) floating in water. The denser blocks will sink lower, but the tops of all the blocks will still be at the same level. Similarly, in Pratt's model, mountains are thought to be made of less dense material than the surrounding plains, allowing them to "float" higher on the denser mantle. The key is that the *depth* to which these columns extend is uniform, and the *density* varies to maintain equilibrium.

**Why the other options are incorrect:**

*   **Option 1: Uniform density with varying thickness:** This describes Airy's theory of isostasy, not Pratt's. Airy proposed that the crust has a uniform density, but mountains have deeper "roots" extending into the mantle, while plains have thinner crust.

*   **Option 2: Law of floatation:** While the law of floatation is a general principle underlying isostasy, it doesn't specifically define Pratt's *unique* contribution. Both Pratt's and Airy's theories rely on the law of floatation, but Pratt's specifically focuses on density variations at a uniform depth.

*   **Option 4: Root formation:** Root formation is a key component of Airy's isostatic model, where mountains have deep roots of lower-density crust extending into the mantle. Pratt's model does not rely on root formation; instead, it focuses on density variations within columns of uniform depth.