In contrast to the thick, gassy felsic lava that forms stratovolcanoes, mafic lava (basalt) is not very viscous and holds little gas.
Eruptions of basaltic lava are usually quiet, and the lava travels long distances to spread out in thin layers. Typically, then, large basaltic volcanoes are broadly rounded domes with gentle slopes.
They are called shield volcanoes. Most of the lava flows from fissures (long, gaping cracks) on the flanks of the volcano. The volcanoes of Hawaii are good examples.
The chain of Hawaiian volcanoes was created by the motion of the Pacific plate over a hotspot—a plume of upwelling basaltic magma deep within the mantle.
Motion of a lithospheric plate over a hotspot produces a long trail of islands and sunken islands called guyots.
A few basaltic volcanoes also occur along the midoceanic ridge, where seafloor spreading is in progress. Iceland, in the North Atlantic Ocean, provides an outstanding example.
Other islands of basaltic volcanoes located along or close to the axis of the mid-Atlantic Ridge are the Azores, Ascension, and Tristan da Cunha. Shield volcanoes show erosion features that are quite different from those of stratovolcanoes.
The gentle slopes of the original volcano are replaced by steep canyons and sharp ridges as erosion dissects the mountain mass.
If a hotspot lies beneath a continental lithospheric plate, it can generate enormous volumes of basaltic lava that emerge from numerous vents and fissures and accumulate layer upon layer.
These basalt layers, called flood basalts, can become thousands of meters thick and cover thousands of square kilometers.