They occur in Hawaii, because of the climate. Volcanoes are caused by built pressure. They occur in Hawaii because of it's tropical climate. Volcanoes are caused by an eruption in the Earth's surface due to running water that carve out mountains. They occur on islands, like Hawaii where the climate is warm.
Volcanoes occur when the mountains get too hot and they erupt.
I think the mixture of heat and sand causes volcanoes. Volcanoes occur in the mountains and deserts because it is hot and there is a lot of sand. Heat and gasses from the Earth's core push their way up, bringing lava with them. When things such as fossil fuels are burned and the emmissions break down the ozone layer which is the Earth's protection from the Sun. That allows the Sun to warm the planet, causing climate change which also causes an increase in natural disasters.