The formation and general features of volcanoes

Volcanoes are geological structures typically, though not always, shaped like a cone. They form when material from the Earth's mantle is pushed through an opening in the crust, known as a vent. This process results in the accumulation of various substances on the surface, creating the volcanic structure over time.

Key features of volcanic formation

• Mantle material eruption - Molten rock and other materials are forced upwards from deep within the Earth through the vent.
• Variety of ejected substances - These can include ash, gas, solid rock fragments called volcanic bombs, and molten rock, which is termed magma when underground and lava once it reaches the surface.
• Surface accumulation - Ejected materials cool and solidify, building up to form the volcano's shape.

Classification of volcanoes as extinct, dormant, or active

Volcanoes are categorised based on their eruption history and likelihood of future activity.

Categories of volcanic activity

• Extinct volcanoes - These are volcanoes that are not expected to erupt again. An example is Arthur's Seat in Edinburgh, Scotland.
• Dormant volcanoes - These have not erupted in over 2000 years but could potentially become active again in the future.
• Active volcanoes - These have erupted in recent times and are likely to erupt again. A prominent example is Mount Vesuvius in Italy, known for its frequent activity.

Examples of major active volcanoes:

Characteristics of composite volcanoes

Composite volcanoes, also known as stratovolcanoes, are one of the most common types of volcanoes. They are built from alternating layers of different materials ejected during eruptions, creating a distinct structure.

Structure and composition of composite volcanoes

• Layered formation - Made up of alternating layers of lava and ash, along with other volcanic debris.
• Crater - The main opening at the top through which eruptions occur.
• Secondary cone - Smaller cones that can form on the sides of the main volcano due to additional vents.
• Main vent - The central channel through which magma rises from the magma chamber to the surface.
• Magma chamber - A large underground reservoir of molten rock that feeds the volcano.
• Example - Mount Fuji in Japan is a well-known composite volcano, showcasing the classic steep, conical shape formed by layers of ash and lava.

Differences between shield and dome volcanoes

Not all volcanoes are composed of both ash and lava. Shield and dome volcanoes are formed primarily from lava, but they differ significantly in their lava characteristics and resulting shapes.

Shield volcanoes

• Lava type - Composed of alkaline, runny lava that flows easily and quickly.
• Shape and spread - The fluid lava spreads over wide areas, creating broad, gently sloping structures.
• Example - Mauna Kea in the Hawaiian Islands, known for its expansive, flattened profile due to the flow of runny lava.

Dome volcanoes

• Lava type - Made of acidic, thicker lava that flows slowly and hardens more rapidly.
• Shape and structure - The viscous lava results in steep-sided, conical shapes as it does not travel far from the vent before solidifying.
• Example - Mount Lassen in the USA, which exhibits the characteristic steep slopes of a dome volcano due to its thick lava.