1.3a2 Most volcanism on earth occurs at mid-ocean ridges

Most volcanism on Earth takes place at the bottom of the ocean along mid-ocean ridges. These are a type of divergent plate boundary, where two oceanic plates move away from each other. The diverging plates allow hot magma to rise causing decompression melting of ultramafic mantle rock. This result in the eruption of basaltic lava.

Due to their location, this type of volcanic activity is the least observed. The eruptions tend to be slow, gentle and oozing. One exception to this is volcanic eruptions in Iceland, which was created by volcanic activity along the Mid-Atlantic ridge.

Source: Johnson, C et al (2017) An Introduction to Geology

see: 1.2 There are three types of melting that produce magma - decompression, flux and heat-induced for more on decompression melting

see: pillow lava for more on underwater basaltic eruptions

see: volcanically active regions iceland for more on Iceland's geological activity

see: 1.1b The composition of magma ranges from ultramafic to felsic for the composition of ultramafic magma