How did a single event start a chain reaction that sparked world war i ?

The chain reactions in history are how one event led to another. For example, the assassination of Franz Ferdinand led to World War 1 because when Austria declared war on Serbia. The allies of both countries declared war on the allies of Austria and Serbia. Dividing Europe into two hostile sections and creating a huge international war.
(I hope this makes sense)

Cracks along the margins of plates are usually the sites of volcanoes and earthquakes. Volcanoes form because molten magma from below can squeeze up to the surface through the cracks. Plate collisions cause stresses - earthquakes - as plates rub together.

Explanation: