What controls how a volcano erupts, and what products and landforms result?
Volcanism: the control of magma composition (silica content), viscosity and dissolved gas on eruption style; the contrast between basaltic effusive eruptions and andesitic or rhyolitic explosive eruptions; volcanic products (lava, tephra, pyroclastic flows and gases); volcanic landforms (shield volcanoes, stratovolcanoes, calderas and fissures); the link between volcanism and plate setting.
A focused answer to the OCR H414 dot point on volcanism. Covers how silica content, viscosity and gas control eruption style, the contrast between basaltic effusive and andesitic or rhyolitic explosive eruptions, volcanic products (lava, tephra, pyroclastic flows, gases), the landforms (shield, stratovolcano, caldera, fissure), and the link to plate setting.
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What this dot point is asking
OCR wants you to explain how magma composition (silica content), viscosity and dissolved gas control eruption style, to contrast basaltic effusive eruptions with andesitic or rhyolitic explosive eruptions, to describe the volcanic products and landforms, and to link volcanism to plate setting.
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 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).
- Dissolved gas. Mainly water vapour and carbon dioxide. In runny magma, gas escapes easily; in sticky magma, gas is trapped and builds up.
Effusive versus explosive
- Basaltic, effusive. Low silica, low viscosity, gas escapes; produces lava flows (pahoehoe and 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 mixtures of gas and fragments). Builds steep stratovolcanoes and, after a huge eruption empties the magma chamber, calderas.
Volcanic products
- Lava (basaltic runny; rhyolitic viscous).
- Tephra (fragmented material: ash, lapilli, bombs).
- Pyroclastic flows (the most lethal product, moving at high speed).
- Volcanic gases (water vapour, carbon dioxide, sulfur dioxide).
Landforms and plate setting
- Shield volcanoes (broad, gentle): basaltic, at constructive margins and hotspots (for example Hawaii).
- Stratovolcanoes (steep, layered): andesitic, at destructive margins (for example the Andes).
- Calderas: large collapse craters after explosive emptying of a magma chamber.
- Fissures: long cracks erupting basalt, at rifts and ridges.
The link to plate setting is direct: gentle basaltic volcanism marks constructive margins and hotspots; violent andesitic and rhyolitic volcanism marks destructive margins.
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 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 tetrahedra, giving 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 dissolved gas, so pressure builds up until the magma fragments explosively.
Q3. Name the volcano type built by runny basaltic lava and state its likely plate setting. [2 marks]
- Cue. A shield volcano; at a constructive margin or over a hotspot.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR H414/01 20196 marksExplain why basaltic magmas produce gentle (effusive) eruptions while rhyolitic magmas produce violent (explosive) eruptions.Show worked answer →
A level-of-response answer; link silica to viscosity to gas escape.
- Silica and viscosity
- Basaltic magma has low silica (about ), so few silica tetrahedra link up and it has low viscosity (it is runny). Rhyolitic magma has high silica (about ), so the tetrahedra polymerise into a stiff network and it has high viscosity (it is sticky).
- Gas escape
- In runny basaltic magma, dissolved gases (mainly water vapour and carbon dioxide) can escape easily as the magma rises, so pressure does not build up and the eruption is effusive (lava flows).
- Gas trapping
- In sticky rhyolitic magma, gases cannot escape easily; they build up under pressure until the magma fragments explosively, blasting out ash 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 link high silica to high viscosity to trapped gas to explosivity, and the reverse for basalt.
OCR H414/01 20214 marksA volcano has gently sloping sides built of many thin basaltic lava flows. Name the type of volcano, state the likely plate setting, and explain how the lava type produced this shape.Show worked answer →
Name, place, then explain the 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 solidifying, spreading out into thin, wide sheets. Repeated thin flows build a broad, gently sloping cone rather than a steep one.
Markers reward the shield identification, the constructive or hotspot setting, and the low-viscosity lava spreading into a broad gentle shape.
Related dot points
- Plate margins: the processes and features of constructive (divergent), destructive (convergent) and conservative (transform) margins; the sub-types of destructive margin (ocean-continent, ocean-ocean and continent-continent collision); the Benioff zone and subduction; the characteristic rocks, structures, earthquakes and volcanoes produced at each margin type.
A focused answer to the OCR H414 dot point on plate margins. Covers constructive (divergent), destructive (convergent) and conservative (transform) margins, the ocean-continent, ocean-ocean and continent-continent sub-types, the Benioff zone and subduction, and the characteristic rocks, structures, earthquakes and volcanoes of each.
- Igneous rocks: classification by silica content (acid, intermediate, basic and ultrabasic) and by grain size (coarse-grained intrusive, fine-grained extrusive); the relationship between cooling rate and crystal size; igneous textures (phaneritic, aphanitic, porphyritic, glassy and vesicular) and what they show about the cooling history; naming common igneous rocks such as granite, gabbro, basalt and rhyolite.
A focused answer to the OCR H414 dot point on igneous rock classification. Covers acid, intermediate, basic and ultrabasic compositions, coarse versus fine grain size, the link between cooling rate and crystal size, the main textures (phaneritic, aphanitic, porphyritic, glassy, vesicular), and naming granite, gabbro, basalt and rhyolite.
- Volcanic hazards: the hazards of an eruption (lava flows, pyroclastic flows, ash falls, lahars, volcanic gases and sector collapse) and how they relate to magma type and the Volcanic Explosivity Index; the methods of monitoring a volcano (seismicity, ground deformation, gas emissions and thermal anomalies); the use of hazard maps, exclusion zones and evacuation to mitigate risk; the comparison with earthquakes in terms of predictability.
A focused answer to the OCR H414 dot point on volcanic hazards. Covers the hazards of an eruption (lava, pyroclastic flows, ash, lahars, gases, sector collapse) and their link to magma type and explosivity, the monitoring methods (seismicity, ground deformation, gas, thermal), hazard maps, exclusion zones and evacuation, and how volcanoes compare with earthquakes for predictability.
- Earthquakes: focus and epicentre; the elastic rebound mechanism; the types of seismic wave (P, S and surface waves) and their properties; magnitude (the logarithmic Richter and moment magnitude scales) versus intensity (Modified Mercalli); the use of P and S wave arrival times and travel-time graphs to locate an epicentre by triangulation.
A focused answer to the OCR H414 dot point on earthquakes. Covers focus and epicentre, elastic rebound, P, S and surface waves, the difference between magnitude (Richter and moment magnitude) and intensity (Modified Mercalli), and how P and S wave travel times and triangulation locate an epicentre.
- Plate tectonics: the development of the theory from continental drift (Wegener's evidence) through sea-floor spreading to plate tectonics; the evidence of palaeomagnetism and symmetrical magnetic striping at mid-ocean ridges; the increasing age of oceanic crust away from ridges; the driving mechanisms of mantle convection, ridge push and slab pull.
A focused answer to the OCR H414 dot point on the development of plate tectonics. Covers Wegener's continental drift evidence, sea-floor spreading, palaeomagnetism and symmetrical magnetic striping, the increasing age of oceanic crust away from ridges, and the driving mechanisms of mantle convection, ridge push and slab pull.
Sources & how we know this
- OCR A Level Geology (H414) Specification — OCR (2017)