Volcanism occurs when melted rock (magma) makes its way to the surface end erupt in many different forms (lava, ash, gas, rock fragments).

Depending on the composition, temperature and gas content, magmas will have great variation in physical properties. These properties will determine the type of landforms that develop in the volcanic areas.

Shield and  stratovolcanos

Lavas with low viscosity will flow easily and create wide shield volcanoes (e.g. Hawaii). Much more viscous lavas will not flow in the manner and will build the classic type of cone-shaped stratovolcano.

This type of volcanism can be explosive, e.g. Mount St Helens that erupted violently in May 1980. During such eruptions, great quantities of gas and dust may spread through the stratosphere and cause noticeable changes to the climate on Earth.

Along plate boundaries

Most of the volcanism is taking place along plate boundaries, i.e. zones of relative movement between tectonic plates. Enormous amounts of basaltic lava erupt along the mid-ocean ridges, contributing to creating new sea floor.

The large stratovolcanoes occur in continental crust above subduction zones where old sea floor is plunging into the mantle, e.g. in Japan, the Philippines, and along the Pacific coast of America.  Volcanoes may also occur within large tectonic plates; e.g. Hawaii.

A number of volcanoes worldwide constitute a hazard to human beings an infrastructure, and in areas of active volcanism substantial resources are spent on surveillance and warning.