Since tsunamis involve the ocean floor, it makes sense that they form when the ocean floor moves. NOAA said most tsunamis form when tectonic plates knock against or move over each other, otherwise known as an earthquake. When one of the tectonic plates sublimates or goes under the other, it displaces water around it, and according to Geosciences Australia, the resulting waves move away from the source, building up energy. NOAA, citing data from the Global Historical Tsunami Database, noted that 80 percent of all tsunamis since the 1900s were generated by earthquakes.

But tsunamis can also form in other ways. Massive landslides under the sea can move around water as well. This happens when steep areas of the ocean floor, filled with sediment, sand, and mud, drops and draws surrounding water down, causing it to travel across the sea bed. Most often, undersea landslides occur around the continental slope. Volcanic eruptions have also been known to cause earthquakes and landslides and, subsequently, tsunamis. This is true for both land-based and underwater volcanoes. Sometimes an eruption creates pyroclastic flows — a mixture of ash, gas, lava, rocks, and water — which pours onto the ocean, thereby displacing water.

NOAA also mentioned a very rare weather phenomenon that causes tsunamis. A meteotsunami occurs when air pressure disturbances generate fast-moving waves that reach the ocean floor. Meteotsunamis are more common than previously thought and can occur in several regions of the planet.