Volcanoes

Where the interior reaches the surface.

Two styles

Runny lava at spreading ridges and hotspots builds gentle shield volcanoes. Sticky, gas-rich magma at subduction zones traps pressure and erupts explosively.

Hawaii is a hotspot; the Pacific Ring of Fire is subduction.
Large eruptions inject aerosols that cool global temperatures for a year or two.

Benefits

Volcanic soils are exceptionally fertile and volcanic gases supplied much of the early atmosphere. That is why people keep living near them despite the risk.