Volcanic Fissure and Rifts
Volcanic Fissures and rifts are elongate cracks in the earths surface.Fissures are smaller than rifts. They typically form where the crust is under tension and is pulled apart allowing molten rock to pour out. Basalt is the predominate rock type found and fountains or curtains of "fire" can spectacularly erupt. Eruptions can pour out large volumes of basalt and eventually focus on one or two main vents. Iceland has had spectacular fissure eruptions because the crust is under extensional tension due to the rifting or spreading as the North Atlantic and European plates pull apart.
Rifts are on a larger scale and involve great extensional tensions in the crust. Successful rifting can create new oceans and split apart contients. The NW coast of Scotland has plenty of volcanics which erupted when the North Atlantic ocean was formed.