Geosphere Dynamics: Geological Processes & Volcanic Hazards
Geosphere Dynamics and Processes
External Geological Processes
These processes occur on the most superficial layer of the lithosphere. External agents like atmospheric gases, water, ice, and wind drive geological processes such as:
- Weathering: The physical or chemical alteration of rocks.
- Erosion: The movement of weathered materials to lower areas.
- Transport: The carrying of eroded materials.
- Sedimentation: The settling of transported materials, which can eventually transform into sedimentary rocks.
Internal Geological Processes
These processes are driven by geothermal energy, the residual heat from the planet’s interior combined with energy from radioactive decay. This energy primarily drives internal geological processes.
The oceanic lithosphere is created at mid-ocean ridges from magma and destroyed in subduction zones. Sediments accumulate in these zones and can follow two paths:
- Ascending: When convergent plates collide, sedimentary rocks can be uplifted and folded, forming mountains.
- Descending: Sediments carried into trenches are dragged down with the subducting oceanic lithosphere. Increased pressure and temperature cause structural changes, leading to the formation of metamorphic rocks. If temperatures rise further, rocks can melt to form magma. The cooling of magma gives rise to two types of igneous rocks:
- Plutonic (Intrusive): Formed from slow cooling beneath the surface.
- Volcanic (Extrusive): Formed from rapid cooling on the surface.
Geohazards Explained
A geohazard refers to any geological condition, process, or event (natural or induced) that can cause economic or social harm. Geological risk is a function of:
- Hazard: The probability and intensity of a potentially damaging geological event.
- Vulnerability: The susceptibility of exposed elements (people, infrastructure) to damage.
- Exposure: The elements (people, property) located in hazard-prone areas.
Volcanic Risk Assessment
Volcanoes are fractures or vents through which magma erupts.
Volcano Distribution
Volcanoes are primarily found at:
- Plate boundaries (convergent and divergent)
- Hotspots (areas of volcanic activity away from plate boundaries)
Parts of a Volcano
- Crater: The opening through which lava and volcanic materials erupt.
- Volcanic Cone: The mound formed by the accumulation of erupted materials.
- Magma Chamber: The underground reservoir where magma is stored.
- Conduit (Pipe/Fireplace): The passage through which magma rises to the surface.
- Lava Flow: Streams of molten rock flowing from the vent.
- Eruption Column: A cloud of gas, ash, and rock fragments ejected vertically.
Volcanic Risk Factors
- Exposure: Volcanic areas are often densely populated because volcanoes can create fertile soil and provide geothermal heat.
- Vulnerability: Measures the susceptibility of people and infrastructure to damage from volcanic events.
- Hazard: Depends on the magnitude, type, geographic distribution, affected area, and frequency of eruptions.
Primary Volcanic Hazards
- Gases: The ease or difficulty with which gases escape influences eruption explosivity. Dangerous gases include sulfur dioxide, carbon dioxide, and fluorine.
- Lava Flows: The hazard depends on viscosity (resistance to flow). More viscous (acidic, silica-rich) lavas are generally slower but more destructive. Less viscous (basic, fluid) lavas, like pahoehoe, flow more easily, often associated with shield volcanoes (e.g., at ridges). Destructive plate margin volcanoes tend to have more viscous, acidic lavas.
- Pyroclastic Falls (Tephra): Fragments ejected into the air, classified by size:
- Ash: Fine particles (<2 mm).
- Lapilli: Pea-to-walnut sized fragments (2-64 mm).
- Bombs/Blocks: Larger fragments (>64 mm).
- Explosions: More viscous lavas trap gases, leading to more explosive eruptions. Volcanoes can be broadly classified as effusive (lava flows dominate) or explosive (pyroclastics dominate). If water enters the magma chamber, a highly explosive phreatomagmatic eruption can occur (often larger).
- Pyroclastic Flows (Nuées Ardentes / Burning Clouds): Occur when an eruption column collapses or lava dome explodes, sending a fast-moving, high-temperature mixture of gas and volcanic debris down the slope. When these fragments solidify, they form pyroclastic flow deposits (ignimbrites).
- Volcanic Domes: Formed when highly viscous lava extrudes and piles up around the vent, potentially plugging it. The sudden explosion or collapse of a dome can cause enlargement of the crater and trigger dangerous pyroclastic flows.
- Caldera Formation: Occurs after a massive eruption empties the magma chamber, causing the overlying volcanic structure (roof) to collapse inward, forming a large depression (caldera).
Indirect Volcanic Dangers
- Lahars: Fast-moving mudflows or debris flows composed of volcanic material, rock debris, and water, often triggered by heavy rain or melting snow/ice during an eruption.
- Tsunamis: Can be generated by large coastal or submarine volcanic explosions, caldera collapses, or volcano-triggered landslides entering water.
- Landslides/Debris Avalanches: Collapse of unstable volcanic slopes.
Types of Eruptions and Volcanoes
It is important to know that any volcano can change its eruption style at any time. Volcanoes built up by alternating layers of lava flows and pyroclastic material are called stratovolcanoes (or composite volcanoes).
Volcanic Hazard Prediction and Prevention
Prediction Methods
Effective prediction requires understanding the complete history of each volcano, including the frequency and intensity of its past eruptions. Monitoring techniques include:
- Seismic monitoring (earthquakes)
- Ground deformation measurements (tilt, GPS)
- Gas emission monitoring
- Thermal monitoring (infrared)
- Hydrological monitoring
Prevention and Correction Methods
- Diversion or chilling of lava flows (limited success).
- Reducing water levels in crater lakes or nearby reservoirs to prevent lahars or phreatic eruptions.
- Installing early warning and alarm systems.
- Land-use planning and restricting construction in high-risk zones.
- Public education and evacuation plans.