What event commonly triggers tsunamis?

• A. Lava flow
• B. Undersea earthquake
• C. Moon tides
• D. Storm surge

Answer: (B)

Tsunamis happen when a large amount of seawater is moved suddenly, usually by an undersea earthquake that makes the ocean floor snap and shift vertically. That abrupt seabed motion lifts or drops the water above it, sending a long, fast wave spreading across the ocean. In deep water these waves are spread out and may be barely noticeable, but as they approach shore and the water becomes shallower, they slow down and grow taller, producing the huge waves that crash on coastlines. The other events listed don’t generate that rapid, large-scale vertical displacement: lava entering the sea can cause local disturbances, but not the widespread tsunami that earthquakes create; moon tides are gradual, regular gravitational effects that cause daily sea-level changes; and storm surge is a weather-driven rise in water level, not a seismic ocean-floor displacement.