A volcano's shape and eruption style are set mostly by its magma's viscosity — how easily it flows — together with how much gas is dissolved inside it.
- Viscosity — higher values resist flow, keeping lava near the vent and steepening the cone; it also raises gas pressure before an eruption.
- Gas content — dissolved volatiles (mostly water and CO2) expand explosively as magma nears the surface; more gas plus more viscosity means a more explosive, ash-rich eruption.
- Eruption rate — how fast magma reaches the surface; fast, gassy, low-viscosity eruptions favour lava fountaining and cinder cones.
Runny, low-gas basaltic magma spreads into broad, gently sloped shield volcanoes. Thick, gassy magma traps pressure until it fails catastrophically, building steep stratovolcanoes from alternating lava and ash. Very gassy magma erupting fast from a single vent sprays cinders into a small, steep cinder cone.