The Holmes Theory of Mountain Building, proposed by Arthur Holmes in the late 1920s, attributes the formation of mountains primarily to the action of thermal convection currents in the Earth’s mantle.

Holmes suggested that radioactive decay within the Earth generates heat, producing convection currents within the semi-fluid mantle. These currents operate like boiling water: hot material rises toward the crust, spreads out, cools, and then sinks again, creating circular convective motions.

Mountain Building Mechanism

• Where convection currents descend, they create compressional forces under the crust, pushing continental crusts together.
• The meeting and compression of continental and oceanic crusts along geosynclines (long, narrow depressions) lead to the accumulation and thickening of sediments.
• The compressional forces fold and uplift these sediments, beginning the stages of mountain building: lithogenesis (accumulation of sediments), orogenesis (folding and uplift), and gliptogenesis (further uplift due to thermal expansion, e.g., melted eclogite).
• Holmes’ model explained that the largest oceans would exert the greatest compressive force, creating the biggest mountains along continental margins, as seen around the Pacific.

Holmes’ convection current theory provided a much-needed mechanical explanation for crustal movement and mountain building, paving the way for the modern theory of plate tectonics. While some details were later modified, the idea that heat-driven mantle convection drives crustal movement remains central in geology today.

Three Stages of Mountain Building:

1. Formation of Geosynclines: Rising convection currents create large sedimentary basins where sediments accumulate and increase density.
2. Orogenesis: Faster convection currents compress sediments in geosynclines, leading to folding and mountain formation.
3. Gliptogenesis: Weakened convection currents reduce pressure, allowing uplift of folded sediments and heavier materials, resulting in gradual mountain rise and surface shaping.

Link(s) and Source(s):

Asthenosphere

Lithosphere