Final answer:
In the student's isostasy model, water represents Earth's mantle, with the block of wood floating on top to simulate the Earth's crust.
Step-by-step explanation:
The water in the beaker of the student's model represents Earth's mantle. The student's experiment illustrates the principle of isostasy, which is the state of gravitational equilibrium between the Earth's crust and mantle such that the crust floats at an elevation that depends on its thickness and density. In the student's model, the block of wood acts like the Earth's crust, and the displacement of water mimics how the crust floats on the more fluid mantle beneath it.