What are the two energy sources that drive the hydrologic and rock cycles?

• A. Internal energy and external energy
• B. Chemical energy and tidal energy
• C. Solar energy only
• D. Gravitational potential energy only

Answer: (A)

Energy driving both the hydrologic and the rock cycles comes from two sources: heat from inside Earth and energy from the Sun. The Sun powers the hydrologic cycle by driving evaporation of water from oceans and land, fueling atmospheric circulation that moves and condenses moisture to form clouds and rain. Gravity helps rivers and water to flow downhill, but the input energy that lifts water into the atmosphere and keeps the cycle moving is solar energy. For the rock cycle, internal heat from Earth’s interior provides the energy for melting rocks, forming magma, and driving metamorphism and plate tectonics that recycle materials deep underground. Solar energy also shapes the rock cycle at the surface by promoting weathering and erosion, which transport and break down rocks. So the two fundamental energy sources are internal energy and external (solar) energy.