The 2004 Indian Ocean tsunami on Boxing Day occurred as the result of an underwater earthquake, ranking 9.1 on the Richter scale, off of the west coast of Sumatra. The energy created by the shift in Earth's crust transferred to the water, leading to a devastating tsunami that was more than 20 feet tall.
Underwater earthquakes commonly occur at the boundaries between tectonic plates. As these plates shift, they release and subsequently transfer large amounts of energy. However, in the 2004 earthquake, a part of the plate broke off from the crust, sending rock shooting upward through the water. There was a forced rise in the water above the surface of the ocean. This energy from the rise then changed from vertical motion to horizontal motion due to gravity, and began crossing the ocean towards various countries' shores.
The horizontal motion of the wave formed in a ripple effect, similar to what is observed when tossing a stone into a still pond. The energy traveled faster in deep water, but as it entered the shallow water near the beaches where the tsunami broke ground, the wave slowed and built height.
According to HowStuffWorks, the earthquake involved an amount of energy equivalent to 23,000 atomic bombs. Several factors intensified the damage of this particular tsunami. It struck a highly populated area, as tourists filled the resorts in the area during the recent holiday. The death toll from this event was around 230,000 people.