Driving the Rock Cycle: Bowen's Series and Metamorphic Grades

Introduction

Two engines, one cycle

You already know the rock cycle's three destinations — igneous, sedimentary, and metamorphic. Now ask a more precise question: what energy drives each transformation?

The rock cycle runs on two completely separate engines. The first is geothermal heat — thermal energy released by the radioactive decay of uranium, thorium, and potassium deep inside Earth. This engine powers everything that involves melting or high-pressure transformation. The continental geothermal gradient averages 25–30 °C per kilometre of depth — so at the base of a 35 km crust, temperatures reach roughly 875–1,050 °C (USGS Volcano Hazards Program). That is hot enough to melt crustal rocks.

The second engine is solar radiation. Sunlight drives evaporation, rainfall, and freeze-thaw cycles — the agents of physical and chemical weathering that break rocks apart and mobilise sediment. Without the Sun, all sedimentary processes would halt. Gravity links the two engines: it transports eroded material downslope toward depositional basins, but it generates no energy of its own.

Knowing which engine controls which step is what separates reading rock history from simply labelling rock types.

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