1) Isostasy refers to the state of gravitational equilibrium between the Earth's lithosphere and asthenosphere. The elevation and thickness of the Earth's crust depends on its density and thickness to maintain balance. 2) Alfred Wegener first proposed the theory of continental drift in 1915, which states that continents slowly drift due to convection currents in the mantle. Wegener provided evidence that all present-day continents were once joined together in a supercontinent called Pangea. 3) Plate tectonics explains continental drift as the movement of tectonic plates across the Earth's surface over geological time, caused by convection currents in the upper mantle. Pangea broke apart into smaller continents that have