How is it that a volcano (like Krakatau) can cause a massive tsunami?

Answer 1

A tsunami is caused by the displacement of the entire water column, so it is much different than just a wave. A wave is the upper layer of water that is moving. As the entire water column is displaced, it pushes the water around it, which in turn bushes the water around it. Picture a group of people in an arena doing the wave. Anyway, as this water is moving in the open ocean, there is not much surface evidence that something is going on, but below the surface, the entire water column, from top to bottom is moving. As this water column approaches land, it starts to rise up, because the bottom of the ocean is rising, as you get closer to shore. When the wave hits the dry ground, it has all the water, from top to bottom, pushing it forward. This is what gives it the ability to cover so much land.

A volcano has the ability to cause a tsunami if it is close to water, and it displaces a large enough volume of debris into the water to displace the entire water column. There are several volcanic islands, some of which are not volcanically active, that pose a threat of a tsunami. As these volcanic islands weather and erode, large sections are breaking away and slipping toward the ocean. If these large chunks break off and move into the ocean quickly, they will displace enough water to cause a tsunami.

Answer 2

Krakatau actually caused a localized tsunami due to the shallow sea in its area. Some waves made it to Africa but not much. When the mountain exploded it left a hole in the ocean which the water suddenly filled. This generated tsunami-like waves that devastated the villages on the coast in the Sunda Strait. This is different from the great quake that devastated northern Sumatra, Sri Lanka and the islands in between two years ago. In that case a long piece of the continental shelf slid down, creating a water displacement that was followed by a tsunami. When I was in Aceh last summer I saw the result, a two hundred mile strip of coast swept clean 3 km inland.

Answer 3

According to what I saw on the Discovery Times Channel, when the volcano cone of Krekatau collapsed (because it was now hollow after spewing out off of its innards), avalanches slammed into the ocean. At least, that is one theory, because no one who saw it up close survived.