The shape of a volcano is determined by the kind of lava that has erupted.
There are many types of volcanoes including calderas, craters, fissure vents and pyroclastic cones, stratocone and shield.
Here, for simplicity, we will only look at stratocone and shield volcanoes.
Very thick, viscous lava won't flow easily and therefore builds up around the vent forming a volcano with steep sides – we call this a stratocone volcano and it has a familar triangular shape.
The Soufrière Hills Volcano, Montserrat , is a stratocone volcano that produces viscous and sticky lavas to form domes that can also produce pyroclastic flows.
Where a volcano produces thin lava it spreads far from the source forming a volcano with gentle slopes. This type is called a shield volcano.
Mauna Kea and Mauna Loa are shield volcanoes. They are the world's largest active volcanoes, rising nearly 9 km above the sea floor around the island of Hawaii.