Final answer:
A tsunami can be caused by underwater earthquakes, with the most destructive effects occurring when the waves strike coastal areas. The 2004 Indian Ocean tsunami, resulting from an earthquake, claimed hundreds of thousands of lives.
Step-by-step explanation:
A tsunami is a powerful series of sea waves, commonly referred to as 'tidal waves', primarily caused by underwater earthquakes. These waves can also be triggered by other events such as landslides, volcanic eruptions, or the impact of a large meteor. Tsunamis are distinguished from normal ocean waves by their immense scale and energy, capable of crossing entire ocean basins at high speeds.
When a tsunami travels through the open ocean, its speed can reach up to 700 km/h. However, as it approaches the shallower coastal waters, the wave slows down and increases in height—sometimes dramatically. The most devastating impact occurs when the waves reach the shore, potentially rising to heights of up to 30 meters, causing significant destruction to coastal communities and landscapes.
The correct answer to the question provided is (a) earthquakes, as they are a common cause of tsunamis. The massive 2004 Indian Ocean tsunami, for example, was triggered by an undersea earthquake off the coast of Sumatra, leading to an estimated 283,000 fatalities.