Locations and characteristics of volcanic eruptions at plate margins

Volcanic eruptions are closely linked to the movement of tectonic plates, occurring predominantly at constructive and destructive plate margins. The type of margin influences the nature of the lava and the style of eruption.

Volcanic activity at constructive margins

• Lava type - Basaltic lava is produced, which is very hot and has low viscosity, meaning it is runny and flows easily.
• Eruption style - Eruptions are frequent and long-lasting but generally non-violent due to the fluid nature of the lava.
• Geographical features - If underwater, magma fills gaps between separating plates, creating ocean ridges. On land, plates pulling apart form rift valleys where the crust thins, allowing magma to surface.

Volcanic activity at destructive margins

• Lava type - Andesitic and rhyolitic lavas are formed, which are cooler and more viscous, meaning they flow less easily compared to basaltic lava.
• Eruption style - Eruptions occur intermittently and are short-lived but can be highly explosive due to the sticky lava blocking volcanic vents, leading to pressure build-up and violent releases.
• Geographical features - At subduction zones, one plate is forced beneath another, melting to form magma that rises to create volcanoes.

Volcanoes at hot spots away from plate margins

Not all volcanic activity is confined to plate margins. Some volcanoes form at hot spots, which are areas of intense heat beneath the Earth's crust, unrelated to tectonic boundaries.

Characteristics of hot spot volcanoes

• Lava type - Predominantly basaltic lava, which is low in viscosity and flows quickly.
• Volcanic shape - The runny lava results in shield volcanoes with gentle slopes due to the wide spread of lava.
• Location - These occur above magma plumes, fixed points of upwelling magma, often in the middle of tectonic plates, far from margins.

Primary hazards associated with volcanic eruptions

Volcanic eruptions produce immediate dangers known as primary hazards. These are direct results of the eruption itself and can cause significant destruction and loss of life.

Types of primary volcanic hazards

• Pyroclastic flows:
  • These are fast-moving mixtures of super-heated gas, ash, and rock fragments that rush down a volcano's slopes at speeds often exceeding 80 km/h, covering distances of around 10-15 km.
  • Also known as nuée ardente, they cause widespread devastation through burning and burial.
• Lava flows:
  • Lava escapes from volcanic vents and travels down slopes.
  • Speed and distance depend on viscosity, temperature, and slope steepness, with low-viscosity lava reaching speeds up to 10 km/h on steep terrain and travelling tens of kilometres.
  • Although often slow enough for evacuation, lava destroys everything in its path by burning, burying, or toppling structures.
• Volcanic gases:
  • Gases like carbon dioxide (CO₂) and sulphur dioxide (SO₂) are released during eruptions.
  • These can be toxic to humans and animals if inhaled, posing a health risk.
• Pyroclastic and ash fallout:
  • Material ejected during eruptions, also called tephra, falls back to Earth.
  • This ranges from large rocks weighing several tonnes to fine ash particles.
  • Fallout can travel thousands of kilometres, with heavier fragments landing closer to the volcano and lighter ash settling further away.
  • Large tephra can damage buildings and injure people, while thick ash layers (up to several metres) can kill vegetation, disrupt transport, and cause structural collapse.
  • Inhaling ash also poses health risks.

Secondary hazards triggered by volcanic activity

Beyond the immediate effects, volcanic eruptions can lead to secondary hazards. These are indirect consequences resulting from primary hazards and often compound the initial damage.

Types of secondary volcanic hazards

• Mudflows (lahars) - These occur when volcanic debris mixes with large volumes of water, creating fast-moving flows that can exceed 80 km/h and travel tens of kilometres. Lahars can bury or destroy habitats, settlements, and infrastructure.
• Acid rain - Volcanic gases, particularly sulphur dioxide, react with water vapour in the atmosphere to form weak sulphuric acid, which falls as acid rain. This damages ecosystems and causes deterioration of stone and metal in structures.
• Flooding - Pyroclastic flows can melt ice caps or glaciers on volcanoes, leading to sudden and severe flooding in surrounding areas.
• Tsunamis - When pyroclastic flows or volcanic debris enter the sea, they can displace water, triggering tsunamis that impact coastal regions.

Key terminology related to volcanic hazards

Understanding volcanic activity involves specific terms that describe the scale, frequency, and predictability of eruptions, aiding in hazard assessment and management.

Important volcanic hazard terms

• Magnitude - Refers to the size of a volcanic event, ranging from small, slow lava flows to massive eruptions involving vast amounts of lava, ash, and gas.
• Volcanic Explosivity Index (VEI) - A scale from 0 to 8 that measures eruption intensity based on the volume of material ejected and the height it reaches in the atmosphere.
• Frequency - Indicates how often a volcano erupts, varying from rare events every 100,000 years to frequent eruptions every few months. Generally, less frequent eruptions are larger and more destructive.
• Randomness vs. regularity - Some volcanoes erupt at predictable intervals, while others remain dormant for long periods before multiple sudden eruptions.
• Predictability - Enhanced by monitoring regular eruption patterns, tiny earthquakes, and changes in volcano shape, which can provide warnings of impending activity.