How do volcanoes form, and what are their main features?
Explain how volcanoes form at plate boundaries and describe the main features and types of volcano
A clear, scaffolded answer to the N(A)-Level Geography outcome on volcanoes. How magma reaches the surface, the main parts of a volcano, the difference between shield and composite volcanoes, and what they erupt.
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What this dot point is asking
This outcome asks you to explain how volcanoes form, mainly at plate boundaries, and to describe their main parts and the difference between the two common types. Volcanoes are a key tectonic hazard, so understanding how they work prepares you for the hazards topic. The central idea is that a volcano is a place where molten rock from inside the Earth reaches the surface, and the kind of lava decides what the volcano looks like and how dangerous it is.
The answer
How a volcano forms
A volcano forms where magma (molten rock from the mantle) reaches the surface. This happens mainly at plate boundaries:
- At a convergent boundary, the sinking (subducting) oceanic plate melts in the hot mantle, forming magma. Because magma is less dense than the rock around it, it rises through cracks and erupts, building a volcano.
- At a divergent boundary, plates pull apart and magma rises into the gap, erupting to form new crust, often gently.
When magma reaches the surface it is called lava, and the volcano grows as erupted lava and ash pile up over time.
The main features of a volcano
A typical volcano has several parts:
- Magma chamber: the store of molten rock deep beneath the volcano.
- Vent: the main pipe through which magma rises to the surface.
- Crater: the bowl-shaped opening at the top, around the vent.
- Cone: the mountain built up from layers of erupted lava and ash.
- Lava flows: streams of molten rock that pour out and cool on the slopes.
Two types of volcano
The thickness of the lava (its viscosity) decides the shape:
- Shield volcano: built from runny lava that flows far before cooling, giving gentle, wide, sloping sides. Eruptions are usually gentle. An example is in Hawaii.
- Composite (strato) volcano: built from thick lava and layers of ash that cannot flow far, giving steep, tall, cone-shaped sides. Eruptions are often violent and explosive. Many of the volcanoes around the Pacific and in Indonesia are this type.
What volcanoes erupt
Volcanoes release lava (molten rock), ash (fine fragments), gases (such as steam and sulphur), and sometimes fast, deadly clouds of hot gas and ash. Explosive composite volcanoes produce the most ash and gas, which makes them more hazardous.
Examples in context
Example 1. Mount Merapi, Indonesia. Merapi is a steep composite volcano formed where the Indo-Australian Plate subducts beneath the Eurasian Plate. Its thick lava and trapped gas produce explosive eruptions and dangerous fast-moving clouds of hot ash and gas. Despite the danger, fertile volcanic soils on its slopes mean many people farm nearby, linking it to the hazards topic.
Example 2. Hawaiian shield volcanoes. The volcanoes of Hawaii, such as Mauna Loa, are broad, gently sloping shield volcanoes built from very runny lava that flows long distances. Their eruptions are usually gentle enough for people to watch from a safe distance, the opposite of the explosive composite volcanoes found around the Pacific.
Try this
Q1. Name three features of a volcano. [3 marks]
- Cue. Any three of: magma chamber, vent, crater, cone, lava flows.
Q2. Explain why a shield volcano has gentle sloping sides. [2 marks]
- Cue. It is built from runny (low-viscosity) lava that flows far before cooling, so the material spreads out into a wide, gentle slope.
Q3. Explain why composite volcanoes erupt explosively. [2 marks]
- Cue. Their lava is thick and traps gases; pressure builds until it is suddenly released in a violent, explosive eruption of ash, gas and rock.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original6 marks(a) Name three features of a volcano. (b) Explain how a volcano forms at a convergent plate boundary.Show worked answer →
(a) Three features: the crater (the opening at the top), the vent (the pipe through which magma rises), and the magma chamber (the store of molten rock beneath the volcano). Other acceptable features include the cone (the mountain built of erupted material) and the lava flows.
(b) At a convergent boundary, the denser oceanic plate is forced down (subducted) beneath the other plate. As it sinks into the hot mantle it melts, forming magma. Because magma is less dense than the surrounding rock, it rises through cracks and the vent. When it reaches the surface it erupts as lava, ash and gas, building a volcano over time.
What markers reward: three correct features named, subduction and melting to form magma, and the magma rising and erupting to build the volcano.
Original4 marksExplain two differences between a shield volcano and a composite volcano.Show worked answer →
A shield volcano has gentle, sloping sides and is wide, because it is built from runny (low-viscosity) lava that flows far before cooling. Its eruptions are usually gentle.
A composite volcano has steep sides and a tall, cone shape, because it is built from thick (high-viscosity) lava and layers of ash that do not flow far. Its eruptions are often violent and explosive.
What markers reward: two clear differences, linking shield volcanoes to runny lava and gentle eruptions, and composite volcanoes to thick lava, ash layers and violent eruptions.
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