Firstly I think I must try and define the word hazard with respect to geology. I found many different definitions in books and on the Internet, but the definition that describes it best in my opinion was by Darren Gravley a Ph.D. student in volcanology at the University of Canterbury in New Zealand. He defined a hazard as an interaction between humans and an extreme natural event with respect to cultural perceptions and value systems.
Many people believe the mankind has only one enemy, itself. But our planet has a huge range of hazards that have and could cause huge devastation to human life. Earthquakes, volcanoes, landslides, avalanches and floods are just a few geological hazards.
In this essay I will talk about two hazards that most people think of when this topic is brought up. Volcanoes and earthquakes are probably the most common hazards, many cities and well populated areas are in close proximity to these areas.
The Earth's crust is broken into plates. These rigid plates "float" on the Asthenosphere. As the plates move about they push together or pull apart. It is at the plate boundaries that most geological hazards take place.
Volcanoes.
Most of the world's volcanoes are found on a subduction zone. These are areas where plates push together and one plate slides beneath the other. When this plate goes deep enough inside the mantle, the rock on it melts and forms magma. This moves upwards and erupts at the surface of the Earth. When plates move apart magma usually comes to the surface and erupts. These are called rift zones. Hotspots are another area a where volcanoes occur. These are in the middle of plates and are places where magma melts through the plate and erupts.
Volcanoes have many aspects in which they can be hazardous, all of which make standing beside an active volcano one of the most dangerous places in the world. Typically people have believed that the super hot lava that erupts from a volcano was the only danger.