What caused the 2004 Indian Ocean Tsunami?

On the plate boundary between the Indo-Australian plate and the Eurasian plate, the Indo-Australian plate is pushed under to form a subduction zone (this also causes an orogenic and volcanic region), simply put the plates rubbed against each other and this caused a relatively large earthquake below the surface of the Indian ocean. The shock-waves produced caused a large surge in the wave currents and (similar to the snowball effect) the wave got larger until it reached its peak, and then it struck all the countries affected.