Edexcel GCSE Geography B (1GB0) — Hazardous Earth
Have you ever noticed how cold honey is incredibly hard to stir, but water splashes easily? This property—how much a fluid resists flowing—is called viscosity, and it is the master switch that decides whether a volcano oozes gently or violently blows its top.
The viscosity of magma determines how easily dissolved gases, such as and , can escape. This leads to entirely contrasting eruption styles:
The type of magma erupting directly dictates the shape and structure of the volcano that forms.
Shield volcano Found at divergent plate boundaries or hotspots, shield volcano types are formed by the partial melting of the mantle. Because they erupt low-viscosity basaltic lava, the lava flows several kilometres at high speeds before cooling and solidifying. This creates a wide base with broad, gentle slopes (less than 10 degrees). They are composed entirely of lava flows and importantly do not contain any layers of ash.
Composite volcano Found at convergent plate boundaries where subduction forces an oceanic plate to melt, a composite volcano is fuelled by sticky andesitic magma. Because the highly viscous lava solidifies quickly near the vent, it builds height rather than width, creating a steep-sided, conical shape. They are constructed from alternating layers of tephra (solid rock and ash) and lava.
Volcanic hazards are split into two categories. Primary hazards happen immediately as a direct result of the eruption. Secondary hazards occur later as a knock-on effect of the eruption, such as lahars (destructive mudflows of ash mixed with water) and jökulhlaups (massive glacial flash floods).
The two most heavily contrasting primary hazards are slow-moving lava flows and pyroclastic flow events. A pyroclastic flow is a dense, gravity-driven avalanche of superheated gas, ash, and rock that destroys everything in its path.
| Feature | Lava Flow | Pyroclastic Flow |
|---|---|---|
| Magma Type | Basaltic (Low Silica) | Andesitic (High Silica) |
| Boundary Type | Divergent / Hotspot | Convergent (Subduction) |
| Speed | Slow (usually < 16 km/h) | Extremely fast (100 - 700 km/h) |
| Temperature | Around 1200°C | 700°C to 1000°C |
| Primary Threat | Destroys property and farmland; rarely kills humans | Lethal to all life; kills by severe burning and suffocation |
Students often describe magma as simply 'thick' or 'runny'. Examiners will withhold marks for this; you must use the precise geographical terms 'high viscosity' and 'low viscosity'.
Do not confuse a pyroclastic flow (a dry, superheated cloud of gas and ash) with a lahar (a wet, concrete-like mudflow of ash and water).
In 4-mark explanation questions about volcanic explosivity, examiners specifically look for the exact causal chain: state the silica content, explain how it increases viscosity, mention that this traps gas bubbles, and conclude that the sudden release of this pressure causes the explosion.
Never use the word 'lava' to describe molten rock that is still underground; it must be called 'magma' until it physically breaches the Earth's surface.
Viscosity
A measure of a fluid's resistance to flow, primarily controlled by silica content and temperature in magma.
Magma
Molten rock found beneath the Earth's surface.
Lava
Molten rock that has erupted onto the Earth's surface.
Effusive eruption
A relatively gentle volcanic eruption characterized by the steady, non-violent flow of lava.
Explosive eruption
A violent eruption caused by the rapid release of pressure and trapped gas, ejecting rock, ash, and gas clouds.
Volcanic Explosivity Index (VEI)
A relative logarithmic scale from 0 to 8 used to measure the magnitude and explosivity of a volcanic eruption.
Shield volcano
A wide, gently sloping volcano formed by many thin layers of low-viscosity basaltic lava.
Composite volcano
A steep-sided, conical volcano made of alternating layers of ash and solidified lava (also called a Stratovolcano).
Subduction
The process at a convergent plate boundary where a denser oceanic plate is forced down into the mantle and melts.
Tephra
Any solid material, such as ash or volcanic bombs, ejected into the air during an explosive eruption.
Primary hazards
Hazards that happen immediately and directly as a result of a volcanic eruption, such as lava flows and pyroclastic flows.
Secondary hazards
Hazards that occur as an indirect consequence or knock-on effect of an eruption, such as lahars or jökulhlaups.
Lahars
Destructive secondary volcanic mudflows consisting of a mixture of water and volcanic ash.
Jökulhlaups
Sudden, massive flash floods of meltwater caused by volcanic eruptions occurring underneath glaciers.
Pyroclastic flow
A fast-moving, gravity-driven current of extremely hot gas, ash, and volcanic rock.
Basaltic magma
A silica-poor (approx. 50%), high-temperature magma that has low viscosity and typically results in effusive eruptions.
Andesitic magma
A silica-rich (60-70%+), cooler magma that has high viscosity and typically results in explosive eruptions.
Low viscosity
A fluid property where the substance is 'runny' and flows easily; in magma, this allows gas bubbles to escape.
High viscosity
A fluid property where the substance is 'thick' and sticky, resisting flow; in magma, this traps gas bubbles and builds pressure.
Put your knowledge into practice — try past paper questions for Geography B
Viscosity
A measure of a fluid's resistance to flow, primarily controlled by silica content and temperature in magma.
Magma
Molten rock found beneath the Earth's surface.
Lava
Molten rock that has erupted onto the Earth's surface.
Effusive eruption
A relatively gentle volcanic eruption characterized by the steady, non-violent flow of lava.
Explosive eruption
A violent eruption caused by the rapid release of pressure and trapped gas, ejecting rock, ash, and gas clouds.
Volcanic Explosivity Index (VEI)
A relative logarithmic scale from 0 to 8 used to measure the magnitude and explosivity of a volcanic eruption.
Shield volcano
A wide, gently sloping volcano formed by many thin layers of low-viscosity basaltic lava.
Composite volcano
A steep-sided, conical volcano made of alternating layers of ash and solidified lava (also called a Stratovolcano).
Subduction
The process at a convergent plate boundary where a denser oceanic plate is forced down into the mantle and melts.
Tephra
Any solid material, such as ash or volcanic bombs, ejected into the air during an explosive eruption.
Primary hazards
Hazards that happen immediately and directly as a result of a volcanic eruption, such as lava flows and pyroclastic flows.
Secondary hazards
Hazards that occur as an indirect consequence or knock-on effect of an eruption, such as lahars or jökulhlaups.
Lahars
Destructive secondary volcanic mudflows consisting of a mixture of water and volcanic ash.
Jökulhlaups
Sudden, massive flash floods of meltwater caused by volcanic eruptions occurring underneath glaciers.
Pyroclastic flow
A fast-moving, gravity-driven current of extremely hot gas, ash, and volcanic rock.
Basaltic magma
A silica-poor (approx. 50%), high-temperature magma that has low viscosity and typically results in effusive eruptions.
Andesitic magma
A silica-rich (60-70%+), cooler magma that has high viscosity and typically results in explosive eruptions.
Low viscosity
A fluid property where the substance is 'runny' and flows easily; in magma, this allows gas bubbles to escape.
High viscosity
A fluid property where the substance is 'thick' and sticky, resisting flow; in magma, this traps gas bubbles and builds pressure.
