Overview
This lecture explains the process of volcanic eruptions, the factors influencing eruption types, the effects of eruptions, and introduces the main types of volcanic eruptions.
The Process of Volcanic Eruption
- Magma forms beneath Earth's surface and can push through holes or cracks, causing eruptions.
- When magma reaches the surface, it's called lava; once cooled, it becomes igneous rock.
- The temperature of magma ranges from 700°C to 1300°C.
Factors Affecting Volcanic Eruptions
- The nature of an eruption depends on magma temperature, chemical composition, and dissolved gas content.
- Viscosity refers to a liquid's resistance to flow; higher viscosity means thicker and slower-flowing material.
- Higher magma temperature lowers viscosity (more fluid); lower silica content also lowers viscosity (more fluid).
- More dissolved gases in magma generally increase its ability to flow; loss of gases makes magma more viscous.
Mechanisms Behind Eruptions
- Rising magma heats and develops gas bubbles, increasing pressure as gases expand.
- When pressure exceeds the rock strength, fracturing and eruption occur.
- Eruption style varies by magma chemistry, temperature, gas/water content, and volcanic structure.
Types of Volcanic Eruptions
- Icelandic: Effusion of molten basaltic lava from fissures, building lava plateaus.
- Hawaiian: Fluid lava flows from summits and fissures, forming large shield volcanoes.
- Strombolian: Moderate, frequent bursts of gas and incandescent lava (e.g., Stromboli volcano).
- Vulcanian: Moderate explosions of gas and ash forming dark eruption clouds.
- Pelean: Explosive outbursts with pyroclastic flows (hot volcanic fragments and gas).
- Plinian: Intensely violent, gas-rich eruptions sending ash clouds into the stratosphere.
Volcanic Materials and Hazards
- Volcanic bombs: Large molten rocks ejected during eruptions.
- Eruption clouds: Ash clouds that can extend 12 miles high and travel great distances.
- Tephra: Fragmented material from explosive eruptions, ranging from ash to boulders.
- Pyroclastic flows: Fast-moving hot gases and volcanic matter, highly destructive.
- Lahars: Volcanic mudflows formed when hot material mixes with water.
- Lava flows: Molten rock flowing on the surface after eruption.
Effects of Volcanic Eruptions
- Positive effects: Fertile volcanic soil, mineral release, geothermal energy, valuable materials like ash.
- Negative effects: Destruction of cities, loss of lives/property, immediate environmental damage, toxic gases, and ash.
Key Terms & Definitions
- Magma — Molten rock beneath Earth's surface.
- Lava — Magma that reaches the Earth's surface.
- Viscosity — Resistance of a liquid to flow.
- Tephra — Fragmented volcanic material from explosive eruptions.
- Pyroclastic flow — Fast-moving current of hot gas and volcanic matter.
- Lahar — Volcanic mudflow formed from volcanic material and water.
Action Items / Next Steps
- Prepare for the next lesson on how volcanic energy can be harnessed for human use.