The most explosive type of volcano is a stratovolcano, also known as a composite volcano. These volcanoes are characterized by their steep, conical shape and are built up by layers of hardened lava, tephra, and ash. A prime example of a stratovolcano is Mount St. Helens in the United States.
The explosiveness of stratovolcanoes comes from their viscous lava, which tends to trap gases. When these gases build up under the surface, they can cause tremendous pressure to develop. Eventually, this pressure can lead to explosive eruptions, sending ash and pyroclastic flows high into the atmosphere, which can be devastating to the surrounding areas.
In contrast, shield volcanoes have more fluid lava that allows gases to escape more easily, resulting in less explosive eruptions. Thus, while all volcanoes can erupt, stratovolcanoes are renowned for their potentially catastrophic eruptions due to their structure and lava composition.