The GS I syllabus names this topic directly: “important geophysical phenomena such as earthquakes, tsunami, volcanic activity, cyclone etc.” and “changes in critical geographical features (including water-bodies and ice-caps)”. It sits between physical geography (why these hazards happen and where) and disaster management in GS III (how to reduce their impact).

What UPSC asks

Prelims: seismic terms (focus, epicentre, magnitude vs intensity), plate-boundary settings of earthquakes and volcanoes, volcano types and intrusive landforms, tsunami behaviour, conditions for cyclone formation, IMD cyclone categories and naming, and active volcanoes in India.
Mains GS I: the distribution of earthquakes and volcanoes, why the Bay of Bengal gets more cyclones than the Arabian Sea, changes in glaciers, lakes and coastlines. GS III links: preparedness, early warning and resilience.

Key sub-topics

Phenomenon	Focus areas
Earthquakes	Seismic waves, magnitude and intensity, global belts, India’s seismic zones, major Indian earthquakes
Tsunamis	Causes, behaviour in deep and shallow water, 2004 Indian Ocean tsunami, early warning
Volcanoes	Types, products, intrusive and extrusive landforms, global distribution, Barren Island
Tropical cyclones	Conditions for formation, structure, IMD classification, naming, storm surge, India’s preparedness
Changing features	Glacier retreat, shrinking lakes (the Aral Sea), coastal erosion, and their effects on flora and fauna

Start with Earthquakes, Tsunamis and Volcanoes, then Tropical Cyclones.