A volcano is a vent from which a combination of melted rock, solid rock debris and gas erupts. It has a reservoir of molten material below the surface (magma chamber) called magma, and when this magma rises to the surface, it is called lava. The chances of volcanoes are high in the divergent plate boundaries.

It is well known that the earth’s interior is very hot, and as the depth increases from the surface, the temperature too increases, and heat rises. Ultimately, the heated rock (magma) begins rising slowly towards the surface. The escaping gases provide the driving force for volcanic eruptions, and these volcanic gases are known as steam.
Major components of volcanoes
i. Magma chamber - A magma chamber is a cavity beneath the earth's surface filled with liquid rock - magma.
ii. Vents - An opening in the earth's surface through which air, smoke, fumes, magma etc., escape.
iii. Volcanic cone - A landform built by the magma as the ejected magma piles up around the vent in the shape of a cone.
iv. Crater - Craters are formed by the outward explosion of rocks and other materials from a volcano. It a bowl-shaped depression found
at the top of the volcano through which the magma flows out.
Based on the periodicity of eruptions, volcanoes are classified into
\((i)\) Active volcano, \((ii) \)Dormant volcano, \((iii)\) Extinct volcano.
Active Volcanoes: Constantly ejects lava, gases, ashes and fragmented materials. These types of volcanoes are found along the mid-Atlantic ridges representing divergent plate margins and convergent plate margins.
Mount St. Helens in the United States, Mount Etna in Italy and Mount Stromboli
Dormant Volcanoes: These types of volcanoes become quiet for some period after their eruptions and suddenly erupt again violently, and cause enormous damage to human health and wealth.
Mt. Fuji, Japan
Extinct Volcanoes: Volcanoes without indications of eruption or that stopped erupting permanently are considered extinct or dead volcanoes. Here, the crater is filled up with water and lakes are found.
Mt. Kilimanjaro, Tanzania Tiruvannamalai - Tamilnadu
Based on the structure and composition, volcanoes can be divided into composite volcano, shield volcano and dome volcano.
Types of Volcanoes
Three main basic types of volcanoes are:
Shield Volcano: Several flows of basaltic (intense viscous)  lava occur in a given region, and they can eventually pile up into Shield shaped large mountain called a shield volcano. They have gently sloped sides.
Mauna Loa, Hawaii
Composite Cone (Stratovolcano):Here, the eruptions are sometimes effusive and sometimes explosive. Therefore, composite cones are composed of multiple layers of hardened lava, pumice and volcanic ash. These are commonly found in the Pacific Ocean.
Mt. Fuji, Japan
Volcanic Dome: This type of volcanoes have viscous, silica-rich magma. Due to the viscous nature, the lava is prevented from flowing far from the vent and pushed into the vent of a volcanic cone. Thus, it forms a lava dome. A lava dome or volcanic dome is roughly a circular mound. 
Parícutin, Mexico