Interactive Explainer
What causes tsunamis?
A tsunami usually starts when the seafloor moves suddenly and lifts or drops a large column of water. In deep water the wave can be fast and broad but not especially tall, then it grows steeper and more dangerous as it slows in shallow water.
Most tsunamis begin with underwater earthquakes, landslides, or volcanic activity that displace the ocean surface.
A tsunami is not just a giant wind wave. Its wavelength is far longer and the whole water column is involved.
The severe flooding often happens near shore, where the fast deep-ocean wave is forced to pile upward.
Try It Yourself
Tsunami Lab
Increase the seafloor displacement, deepen the ocean basin, or steepen the coastline to see why the wave you barely notice offshore can become destructive at landfall.
What changes the fastest
What is driving the result
The Big Idea
What is actually happening?
An interactive explainer about how sudden seafloor motion displaces huge volumes of water, why tsunamis race across deep oceans so quickly, and why the biggest danger often appears near shore.
The seafloor moves suddenly
An underwater earthquake, landslide, or volcanic collapse can push a large section of seafloor up or down in seconds.
The ocean surface is displaced
That sudden motion lifts or drops the water above it, creating a long wave that starts spreading across the basin.
The wave travels fast in deep water
Because the wavelength is huge, the disturbance can move across the ocean quickly even when the surface height offshore looks modest.
Shallow water forces the wave upward
As the front slows near shore, the energy compresses into a steeper, taller, more dangerous surge and flood.
Good Follow-Up Questions
The details are where this gets interesting
The short answer helps, but the edge cases and comparisons are what make the topic memorable.
Wave height offshore can be misleading
A tsunami in deep water may pass a ship as a broad, subtle rise and fall. The real hazard builds as the water column interacts with the coastline.
The first wave is not always the biggest
Tsunamis arrive as a train of waves, and local basin reflections can make later arrivals stronger than the first one.
Tides can change the damage pattern
A tsunami on top of a high tide can push farther inland, even if the tsunami source itself did not change.
Compare Scenes
The same source can look very different in deep water versus near shore
Coastline geometry decides whether the incoming energy spreads out, reflects, or stacks up into a much more dangerous surge.
Fast Answers
Questions people usually ask next
Good science pages should answer the obvious follow-ups without making the reader hunt for them.