A volcano is formed when magma reaches the Earth's surface

A volcano is any place where magma erupts onto the Earth's surface, or has been known to erupt in the past several million years. As well as lava, an erupting volcano also produces ash and volcanic gases.

Most volcanoes are found at active plate boundaries, created by volcanism at subduction zones, mid-ocean ridges and continental rifts. These are known as interplate volcanoes.

Intraplate volcanoes can be found away from plate boundaries, where a hotspot in the mantle has melted through the crust. The Hawaiin island chain is an example of intraplate volcanism.

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: Earle, S (2015) Physical Geology#Chapter 4 Volcanism

Previous: Magma is molten rock within the Earth's crust

Deeper: 1.3b classification of volcanoes depends on shape, eruption style and magmatic composition for different types of volcano