In other words, a tsunami is an incredibly massive ocean wave that is caused by an underwater earthquake or volcanic eruption and infrequently causes extreme destruction once it strikes land. Tsunamis can have heights of up to 30 m (98 ft) and reach speed of 950 kilometers (589 mi) per hour. They are described by long wavelengths of up to 200 kilometers (124 mi) and long periods, typically between 10 minutes and an hour.