•However, because the oceanic crust is heavier and denser it is forced under the continental crust. 

•The result is earthquakes and volcanoes are formed.  

•Many of these mountain ranges are still getting taller today. 

•The movement causes lots of earthquakes.   

•The magma underneath the earth rises causing volcanoes to form. 

•Often earthquakes occur due to the movement of the ocean floor growing. 

•Parts of the plates can often get stuck and then eventually become free due to pressure and friction. This causes an earthquake. 

•Sometimes these plates even move in the same direction but at different speeds.  

Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that float on a hotter, softer layer in its mantle Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging, and most are found underwater.

Erupting volcanoes can pose many hazards, not only in the immediate vicinity of the eruption. One such hazard is that volcanic ash can be a threat to aircraft, in particular those with jet engines where ash particles can be melted by the high operating temperature; the melted particles then adhere to the turbineblades and alter their shape, disrupting the operation of the turbine. Large eruptions can affect temperature as ash and droplets of sulfuric acid obscure the sun and cool the Earth's lower atmosphere (or troposphere); however, they also absorb heat radiated from the Earth, thereby warming the upper atmosphere (or stratosphere). Historically, volcanic winters have caused catastrophic famines.