To understand what earthquakes are, you need to understand the different parts of the Earth. First, there is the inner core; which is made out of Iron and Nickel, then the Outer Core; which is made of hot, melted Iron. Then comes the Mantle; composed of a big fat layer of flexible rock, and last of all, the Crust; which is a pretty thin layer of Basalt and Granite. The crust is really weird, because it is broken up into large pieces, called Tectonic plates. These plates move, about as fast as your fingernails grow. OK, now I can explain how earthquakes happen. The big plates of earth’s crust sometimes run into each-other and start pushing together. Eventually, they get so stressed that they break. That is an earthquake. Earthquakes can also happen underwater. If they are big enough, the form tsunamis (sue-nom-ees), which are huge walls of water coming out of the ocean. The biggest earthquake in the 20th century was the Valdivia earthquake in Chili. It was rated at 9.5 on the Richter scale.