What controls how a volcano erupts, and what products and landforms result?
Volcanic activity and eruption styles: the control of silica content, viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions (shield volcanoes and fissures) and andesitic or rhyolitic explosive eruptions (stratovolcanoes, pyroclastic flows and calderas); the volcanic products (lava, tephra and pyroclastic material); the link between eruption style and plate setting.
A focused answer to the Eduqas Geology statement on volcanic activity. Covers how silica content, viscosity and dissolved gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, the products (lava, tephra and pyroclastic material), the landforms (shield, stratovolcano, caldera and fissure), and the link to plate setting.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Eduqas wants you to explain how magma composition (silica content), viscosity and dissolved gas control the style of an eruption; to contrast basaltic effusive eruptions with andesitic or rhyolitic explosive eruptions; to describe the volcanic products and landforms; and to link the eruption style to plate setting. The chemistry here follows directly from the plate-margins statement, since the magma type is set by how the magma formed.
The answer
What controls eruption style
The violence of an eruption is set by three linked properties of the magma:
- Silica content. More silica means more polymerisation of the silica tetrahedra, which makes the magma stiffer.
- Viscosity. Resistance to flow. Low-silica basaltic magma is runny (low viscosity); high-silica rhyolitic magma is sticky (high viscosity). Higher temperature lowers viscosity, but silica is the dominant control.
- Dissolved gas. Mainly water vapour and carbon dioxide. In runny magma the gas escapes easily; in sticky magma it is trapped and the pressure builds.
Effusive versus explosive
- Basaltic, effusive. Low silica, low viscosity, gas escapes freely; produces fluid lava flows (the smooth ropy pahoehoe and the rough blocky aa) and lava fountains. Builds broad, gentle shield volcanoes and fissure flows.
- Andesitic or rhyolitic, explosive. Higher silica, high viscosity, gas trapped; produces ash, pumice and deadly pyroclastic flows (fast, hot, ground-hugging mixtures of gas and rock fragments). Builds steep, layered stratovolcanoes, and after a huge eruption empties the magma chamber the summit can collapse to form a caldera.
Volcanic products
- Lava (basaltic runny; andesitic and rhyolitic viscous).
- Tephra (any fragmented material thrown out: ash, lapilli and larger bombs and blocks).
- Pyroclastic material and pyroclastic flows (the most lethal product, racing downslope at high speed and high temperature).
- Volcanic gases (water vapour, carbon dioxide and sulphur dioxide).
Landforms and plate setting
- Shield volcanoes (broad, gently sloping): basaltic, at constructive margins and hotspots (for example Hawaii).
- Stratovolcanoes (steep, layered cones of lava and ash): andesitic, at destructive margins (for example the Andes).
- Calderas: large collapse craters formed after explosive emptying of a magma chamber.
- Fissures: long cracks erupting basalt, at rift valleys and mid-ocean ridges.
The link to plate setting is direct: gentle basaltic volcanism marks constructive margins and hotspots, while violent andesitic and rhyolitic volcanism marks destructive (subduction) margins, where flux melting and interaction with continental crust make the magma silica-rich.
Examples in context
Example 1. Hawaii (hotspot, basaltic). Runny basaltic lava from a hotspot spreads in thin flows to build the broad shield volcanoes of Hawaii, with gentle effusive eruptions and lava fountains rather than violent explosions.
Example 2. Andean stratovolcanoes (destructive margin). Subduction generates viscous andesitic magma that traps gas, producing explosive eruptions, ash falls and pyroclastic flows that build the steep stratovolcanoes of the Andes.
Try this
Q1. State how silica content affects the viscosity of a magma. [2 marks]
- Cue. Higher silica means more polymerisation of the tetrahedra and so higher viscosity (a stickier magma); lower silica gives lower viscosity (a runnier magma).
Q2. Explain why high-silica magmas erupt explosively. [2 marks]
- Cue. High viscosity traps the dissolved gas, so pressure builds up until the magma fragments explosively, blasting out ash and pyroclastic flows.
Q3. Name the volcano type built by runny basaltic lava and state its likely plate setting. [2 marks]
- Cue. A shield volcano; found at a constructive margin or over a hotspot.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20196 marksExplain why basaltic magmas produce gentle (effusive) eruptions while rhyolitic magmas produce violent (explosive) eruptions.Show worked answer →
A levels-of-response answer; link silica to viscosity to the ease of gas escape.
- Silica and viscosity
- Basaltic magma has low silica (about ), so few silica tetrahedra polymerise and the magma has low viscosity (it is runny). Rhyolitic magma has high silica (about ), so the tetrahedra link into a stiff network and the magma has high viscosity (it is sticky).
- Gas escape
- In runny basaltic magma, dissolved gases (mainly water vapour and carbon dioxide) escape easily as the magma rises and the pressure falls, so pressure does not build up and the eruption is effusive (lava flows and fountains).
- Gas trapping
- In sticky rhyolitic magma, the gases cannot escape easily; they build up under pressure until the magma fragments explosively, blasting out ash, pumice and pyroclastic flows.
- Result
- Low viscosity and easy gas escape give gentle basaltic eruptions; high viscosity and trapped gas give violent rhyolitic eruptions.
Top-band answers connect high silica to high viscosity to trapped gas to explosivity, and the reverse chain for basalt.
Eduqas 20214 marksA volcano has gently sloping sides built of many thin basaltic lava flows. Name the type of volcano, state its likely plate setting, and explain how the lava type produced this shape.Show worked answer →
Name the volcano, place it, then explain its form from the lava.
- Type: shield volcano
- Gently sloping sides built of many thin basaltic flows describe a shield volcano.
- Plate setting
- Basaltic, effusive volcanism occurs at constructive margins (mid-ocean ridges) and at hotspots (for example Hawaii), so this volcano is most likely at a constructive margin or over a hotspot.
- Why the shape
- Basaltic lava has low viscosity, so it flows long distances before it solidifies, spreading out into thin, wide sheets. Many such thin flows pile up into a broad, gently sloping cone rather than a steep one.
Markers reward the shield identification, the constructive or hotspot setting, and low-viscosity lava spreading far to give a broad, gentle shape.
Related dot points
- Plate margins and their features: the processes and characteristic features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-ocean island arcs, ocean-continent margins and continent-continent collision); the Benioff zone, subduction and decompression melting; the diagnostic rocks, structures, earthquakes and volcanoes of each margin type.
A focused answer to the Eduqas Geology statement on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-ocean, ocean-continent and continent-continent sub-types, the Benioff zone, subduction and decompression melting, and the diagnostic rocks, structures, earthquakes and volcanoes that identify each margin in the exam.
- The lithosphere, mantle plumes and hotspots: the structure and composition of oceanic and continental lithosphere; the principle of isostasy and isostatic adjustment; mantle plumes and hotspots as a cause of intraplate volcanism; hotspot tracks and the age progression of volcanic island chains (for example Hawaii); the basis of the Component 3 geology of the lithosphere option.
A focused answer to the Eduqas Geology statement on the lithosphere, mantle plumes and hotspots. Covers oceanic and continental lithosphere, the principle of isostasy and isostatic rebound, a worked density calculation, mantle plumes and hotspots as a cause of intraplate volcanism, hotspot tracks and the age progression of island chains such as Hawaii, and the Component 3 lithosphere option.
- Earthquakes and seismic waves: the focus and epicentre; the elastic rebound mechanism; the P, S and surface waves and their properties; the difference between magnitude (the logarithmic Richter scale and its saturation, and the moment magnitude scale) and intensity (the Modified Mercalli scale); the use of P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.
A focused answer to the Eduqas Geology statement on earthquakes. Covers the focus and epicentre, the elastic rebound mechanism, P, S and surface waves and their properties, the difference between magnitude (Richter saturation and moment magnitude) and intensity (Modified Mercalli), a worked example using the P-S travel-time gap, and how triangulation from three stations locates an epicentre.
- Plate tectonics theory and evidence: the development of the theory from Wegener's continental drift, through Hess's sea-floor spreading, to plate tectonics; the evidence of palaeomagnetism and the symmetrical magnetic striping of the ocean floor; the increasing age of oceanic crust away from mid-ocean ridges; the driving mechanisms of mantle convection, ridge push and slab pull.
A focused answer to the Eduqas Geology statement on the development of plate tectonics. Covers Wegener's continental drift evidence and why it was rejected, Hess's sea-floor spreading, palaeomagnetism and symmetrical magnetic striping, the increasing age of oceanic crust away from ridges, a worked spreading-rate calculation, and the driving forces of mantle convection, ridge push and slab pull.
- Earth structure: the layered internal structure of the Earth (crust, mantle, outer core and inner core) and the mechanical layers (lithosphere and asthenosphere); the seismic evidence for the layering from changes in P and S wave velocity at boundaries such as the Moho; the P and S wave shadow zones as evidence for a liquid outer core; the use of meteorites and density as evidence for the composition of the core and mantle.
A focused answer to the Eduqas Geology statement on Earth structure. Covers the crust, mantle, outer and inner core, the lithosphere and asthenosphere, the seismic evidence from P and S wave velocity changes and the Moho, the shadow zones proving a liquid outer core, the meteorite analogy for the core, and locating an earthquake epicentre from the P-S travel-time gap.
Sources & how we know this
- Eduqas A Level Geology Specification (A220QS) — Eduqas (2017)