Crustal Processes and their Effect on the Land Surface and Man

# Crustal Processes and their Effect on the Land Surface and Man ## Objectifs et prérequis This chapter aims to enable students to understand the internal processes shaping the Earth's crust, particularly volcanism and seismicity, and their direct impacts on the land surface and human activities. Students will learn to identify volcanic and earthquake hazards, explain their causes using plate tectonics theory, evaluate human responses and adaptations, and analyse case studies from Cameroon and Africa. Prerequisites include knowledge of plate tectonics (divergent, convergent, transform boundaries), basic rock types (igneous, sedimentary, metamorphic), and the structure of the Earth's interior (crust, mantle, core). ## Introduction Crustal processes such as volcanism and earthquakes are manifestations of the Earth's internal energy, driven by heat from radioactive decay and residual primordial heat. These processes reshape the land surface through the creation of new landforms (e.g., volcanic mountains, lava plateaus) and the destruction of existing ones (e.g., landslides, ground rupture). They pose significant hazards to human populations but also offer benefits like fertile soils, geothermal energy, and mineral resources. Understanding these processes is crucial for disaster risk reduction, land-use planning, and sustainable development, especially in tectonically active regions like Cameroon, which lies near the boundary of the African and South American plates and experiences volcanic activity along the Cameroon Volcanic Line. ## I. Volcanism ### Context Volcanism is the eruption of molten rock (magma) onto the Earth's surface, where it becomes lava. It occurs primarily at plate boundaries but also at hotspots. The Cameroon Volcanic Line (CVL) is a 1,600 km chain of volcanoes extending from the Atlantic Ocean (Annobón Island, Equatorial Guinea) to the interior of Africa (near Lake Chad), passing through Cameroon. It is an example of intraplate volcanism, possibly related to lithospheric thinning or mantle plume activity, though its exact origin remains debated. ### Definitions > **Volcanism**: The process by which magma and associated gases rise from the Earth's interior and are expelled onto the surface through vents or fissures. > > **Magma**: Molten rock beneath the Earth's surface, containing dissolved gases and crystals. > > **Lava**: Magma that has reached the Earth's surface. > > **Volcano**: A landform created by the accumulation of erupted materials (lava, ash, tephra) around a vent. > > **Tephra**: Fragmental material ejected during a volcanic eruption, classified by size as ash (<2 mm), lapilli (2–64 mm), and blocks/bombs (>64 mm). > > **Caldera**: A large, basin-shaped volcanic depression formed by the collapse of the ground following a massive eruption that empties a magma chamber. ### Facts, Dates, and Names - The Cameroon Volcanic Line includes active volcanoes such as Mount Cameroon (4,040 m), the highest peak in West Africa, which last erupted in 2000. - Mount Cameroon has erupted over 100 times in the last 500 years, with notable eruptions in 1909, 1922, 1925, 1959, 1982, 1999, and 2000. - The 1986 Lake Nyos disaster, though not a volcanic eruption, was caused by a limnic eruption of CO₂ from a crater lake on the CVL, killing 1,746 people and 3,500 livestock. - Other CVL volcanoes include Mount Oku (3,011 m), Manengouba (2,411 m), and Bamboutos (2,740 m), all considered potentially active. - The CVL is associated with alkaline basalts and nephelinites, indicative of low-degree partial melting of the mantle. ### Causes and Consequences **Causes**: - Magma generation due to decompression melting (at divergent boundaries or hotspots), flux melting (at convergent boundaries), or conductive heating (mantle plumes). - Rise of magma through fractures in the lithosphere due to buoyancy and pressure from dissolved gases. - At the CVL, causes may include edge-driven convection, lithospheric delamination, or a mantle plume, though no single model is universally accepted. **Consequences**: - **Constructive**: Formation of new land (volcanic islands, plateaus), fertile soils (andisols) rich in nutrients (phosphorus, potassium), geothermal energy potential, and mineral deposits (e.g., sulfur, gold, basalt for construction). - **Destructive**: Lava flows destroying infrastructure, pyroclastic flows (hot gas and ash mixtures moving at high speed), ash fall disrupting agriculture and aviation, lahars (volcanic mudflows), toxic gas emissions (CO₂, SO₂), and tsunamis from island volcano collapses. ### Concrete Examples (Cameroon and Africa) - **Mount Cameroon**: Frequent eruptions threaten the city of Buea and surrounding farms. The 1999 eruption produced lava flows that reached within 2 km of Buea, destroying plantations but causing no fatalities due to timely evacuation. - **Lake Nyos and Lake Monoun**: Both are crater lakes on the CVL prone to limnic eruptions. After the 1986 Nyos disaster, degassing pipes were installed in 2001 to slowly release CO₂ and prevent future catastrophes. Similar measures were applied to Lake Monoun after its 1984 eruption killed 37 people. - **Afar Triangle (Ethiopia)**: A divergent boundary where the African Plate is splitting, creating new crust and hosting active volcanoes like Erta Ale (persistent lava lake). - **Mount Nyiragongo (DRC)**: Part of the Albertine Rift, its 2002 eruption sent lava flows into Goma, destroying 15% of the city and displacing 400,000 people. ### Vocabulary to Remember - Vent, conduit, magma chamber, viscous vs. fluid lava, pahoehoe vs. aa lava, volcanic explosivity index (VEI), volcanic gases (H₂O, CO₂, SO₂, H₂S, HCl), volcanic landforms (shield volcano, stratovolcano, cinder cone, lava dome). ### Example Exam Question and Plan **Question**: With reference to Mount Cameroon and the Cameroon Volcanic Line, explain the causes of volcanism in the region and assess its impacts on human activities. **Plan**: I. Introduction: Define volcanism and locate the CVL. II. Causes: Discuss possible mechanisms (mantle plume, edge-driven convection, lithospheric thinning) and magma type (alkaline basalts). III. Constructive Impacts: Fertile soils supporting agriculture (e.g., banana, coffee plantations on Mount Cameroon slopes), geothermal potential, construction materials. IV. Destructive Impacts: Hazard from lava flows (1999, 2000 eruptions), ash fall, toxic gases (Lake Nyos 1986), economic disruption. V. Human Responses: Monitoring (seismic, gas monitoring), evacuation plans, degassing pipes at Lake Nyos, land-use zoning, community education. VI. Conclusion: Balance of risks and benefits; need for sustained vigilance and adaptation. ## II. Volcanoes and Man ### Context This lesson explores the dual relationship between volcanoes and human societies: how volcanic activity affects human life and how humans adapt to, mitigate, and sometimes benefit from living near volcanoes. It builds on Lesson 6 by focusing on human dimensions—risk perception, vulnerability, adaptation strategies, and sustainable livelihoods in volcanic regions. The Cameroon Volcanic Line provides a relevant case study, where dense populations live on fertile volcanic soils despite known hazards. ### Definitions > **Volcanic Hazard**: Any volcanic phenomenon (lava flow, tephra fall, pyroclastic flow, lahar, gas emission) that poses a threat to life, property, or the environment. > > **Volcanic Risk**: The potential for loss (of life, injury, damage, disruption) resulting from a volcanic hazard, dependent on the hazard's magnitude, frequency, and the vulnerability of exposed elements (people, infrastructure). > > **Vulnerability**: The susceptibility of a community or system to harm when exposed to a hazard, influenced by socioeconomic factors, infrastructure quality, and preparedness. > > **Resilience**: The capacity of a community to anticipate, prepare for, respond to, and recover from volcanic impacts. > > **Degassing**: The release of dissolved gases (especially CO₂) from magma or crater lakes, either naturally or through artificial means (e.g., pipes). ### Facts, Dates, and Names - The 1986 Lake Nyos limnic eruption released approximately 1.2 million tonnes of CO₂, forming a dense gas cloud that traveled up to 25 km, suffocating people and animals in sleeping positions due to CO₂'s higher density than air. - Following the disaster, the Cameroonian government, with international support (France, USA, UN), installed the first degassing pipe in Lake Nyos in 2001. A second pipe was added in 2011; a third is planned. - Lake Monoun underwent similar degassing after its 1984 event; pipes were installed in 2003. - The Mount Cameroon Volcanic Observatory (MCVO), established in 2003 with support from the IRD (France) and University of Buea, monitors seismic activity, ground deformation, and gas emissions. - In 2020, the MCVO reported increased seismicity beneath Mount Cameroon, prompting alert level upgrades, though no eruption occurred. ### Causes and Consequences **Causes of Human-Volcano Interaction**: - Fertile soils: Volcanic weathering produces andisols, which retain water and nutrients, supporting intensive agriculture (e.g., coffee in Ethiopia, bananas in Cameroon). - Mineral resources: Volcanic regions host sulphur deposits (e.g., at Mount Cameroon fumaroles), basalt for construction, and zeolites. - Geothermal energy: Heat from magma can be harnessed for electricity (e.g., Olkaria in Kenya) or direct use (greenhouse heating, spas). - Cultural and spiritual significance: Some communities view volcanoes as deities or ancestors (e.g., Mount Cameroon's role in Bakweri traditions). **Consequences**: - **Negative**: Loss of life and livelihoods (e.g., Nyos 1986), destruction of homes and farms, displacement, long-term soil sterilization by acidic ash, water contamination (fluoride, arsenic), psychological trauma. - **Positive**: Soil fertility enhancement, scenic landscapes boosting tourism (e.g., Mount Cameroon attracts hikers), scientific research opportunities, employment in monitoring and disaster management. ### Concrete Examples (Cameroon and Africa) - **Mount Cameroon Slopes**: Despite eruption risks, the southern and eastern slopes are densely farmed with bananas, plantains, and cocoa. Farmers accept periodic ash fall as a natural fertilizer, though heavy tephra can destroy young crops. - **Lake Nyos Survivors**: After 1986, many returned to rebuild, relying on degassing pipes and early warning systems. Community education programs teach recognition of danger signs (dead vegetation, animal deaths, gas smell). - **Ethiopian Rift Valley**: The Afar people practice pastoralism and salt mining in the harsh, volcanic Danakil Depression, adapting to extreme heat and occasional eruptions. - **Goma, DRC**: After the 2002 Nyiragongo eruption, lava cooled to form new land used for housing; however, poor planning led to reconstruction on still-hot flows, causing subsidence fires. - **Kenya**: Olkaria Geothermal Plant, Africa's largest, supplies ~20% of national electricity, reducing reliance on fossil fuels. ### Vocabulary to Remember - Hazard zonation map, alert level system (e.g., Green, Yellow, Orange, Red), evacuation route, refuge center, lahar warning system, acid rain (from SO₂), volcanic winter (global cooling from stratospheric aerosols), livelihood diversification. ### Example Exam Question and Plan **Question**: Evaluate the extent to which communities living near volcanoes in Cameroon and Africa can be considered resilient to volcanic hazards. **Plan**: I. Introduction: Define resilience in volcanic context; note the CVL as a case of long-term habitation despite risks. II. Factors Enhancing Resilience: - Indigenous knowledge (e.g., observing animal behavior, plant die-off as precursors). - Scientific monitoring (MCVO, degassing pipes). - Adaptive livelihoods (farming on slopes, accepting ash as fertilizer). - Community-based early warning and evacuation drills. III. Factors Limiting Resilience: - Poverty limiting relocation options. - High population density on fertile slopes. - Infrequent but high-magnitude events (e.g., Nyos-type disasters) hard to predict. - Limited funding for sustained monitoring and infrastructure. IV. Case Evidence: - Successful evacuation during 1999/2000 Mount Cameroon eruptions (no deaths). - Ongoing return to Nyos basin despite risks, showing trust in mitigation. - Contrast with Goma 2002: rapid return but poor land-use planning increased vulnerability. V. Conclusion: Resilience is partial and context-dependent; depends on integration of traditional and scientific knowledge, equitable access to resources, and continuous investment in risk reduction. ## III. Earthquakes and Man ### Context Earthquakes are sudden releases of energy in the Earth's crust that create seismic waves. They most commonly occur at plate boundaries due to elastic rebound theory but can also happen within plates (intraplate earthquakes) due to stresses from plate motion or glacial rebound. Cameroon experiences moderate seismic activity, primarily along the Cameroon Volcanic Line and the Gulf of Guinea transform margin, though no major historical quakes have devastated urban centers like in Haiti or Turkey. Understanding earthquakes is vital for constructing resilient infrastructure and planning emergency responses. ### Definitions > **Earthquake**: A sudden shaking of the ground caused by the passage of seismic waves through the Earth's rocks, resulting from the release of accumulated strain in the crust. > > **Focus (Hypocenter)**: The point within the Earth where an earthquake rupture initiates. > > **Epicenter**: The point on the Earth's surface directly above the focus. > > **Seismic Waves**: Elastic waves generated by an earthquake; classified as body waves (P-waves, S-waves) and surface waves (Love, Rayleigh). > > **Magnitude**: A measure of the energy released at the source of an earthquake, measured on the moment magnitude scale (Mw) or Richter scale (local magnitude, ML). > > **Intensity**: A measure of the strength of shaking produced by an earthquake at a specific location, measured on the Modified Mercalli Intensity (MMI) Scale (I–XII). > > **Fault**: A fracture in the Earth's crust along which movement has occurred. > > **Elastic Rebound Theory**: The explanation for earthquakes where strain builds up in rocks until the frictional strength of a fault is exceeded, causing sudden rupture and rebound. ### Facts, Dates, and Names - The 1983 Gouina earthquake (Mali, Mw 5.1) was one of the largest recorded in West Africa, though not in Cameroon. - Cameroon's largest instrumental earthquake occurred on June 22, 2020, near the town of Kumba (Mw 5.0, depth 10 km), causing minor damage (cracked walls, fallen ceilings) but no fatalities. - The 2020 Kumba quake was widely felt in Buea, Douala, and even parts of Nigeria, indicating efficient seismic wave transmission through the crust. - Historical accounts suggest possible stronger quakes in the past (e.g., 1862, 1906), but instrumental records are sparse before the late 20th century. - The Gulf of Guinea transform fault, where the African Plate slides past the South American Plate, is a source of seismic activity affecting Cameroon's coastal regions. - The Adamawa Plateau experiences occasional tremors linked to crustal adjustments along the CVL. ### Causes and Consequences **Causes**: - Plate boundary interactions: Convergent (subduction), divergent (rifting), and transform (shear) margins. - Intraplate stresses: Due to plate-boundary forces transmitted through the lithosphere, or glacial isostatic adjustment (less relevant in Cameroon). - Volcanic activity: Magma movement can trigger volcano-tectonic earthquakes (common on Mount Cameroon). - Human activities: Reservoir-induced seismicity (e.g., from large dams), mining, or fluid injection (e.g., geothermal wells) — documented in other regions but not yet significant in Cameroon. **Consequences**: - **Direct**: Ground shaking causing building collapse, infrastructure damage (roads, bridges, utilities), landslides, liquefaction (in saturated sandy soils), tsunamis (if underwater quake displaces ocean floor). - **Indirect**: Fires (from ruptured gas lines), epidemics (from disrupted water/sanitation), economic disruption (business closure, supply chain failure), psychological trauma (PTSD, anxiety). - **Beneficial**: Rarely, earthquakes can create new springs or alter groundwater flow beneficially; they also provide vital data for studying Earth's interior. ### Concrete Examples (Cameroon and Africa) - **Kumba Earthquake (June 22, 2020)**: Mw 5.0, shallow focus. Caused panic in urban areas; residents evacuated buildings. Damage was non-structural (plaster cracks, items falling) due to moderate magnitude and relatively good construction standards in some areas. Highlighted need for better public awareness and building code enforcement. - **Gulf of Guinea Seismicity**: Offshore quakes (e.g., Mw 6.0+ events) generate tremors felt in Kribi and Limbé. Though no tsunami has been recorded, the potential exists for submarine landslides or quakes to trigger local waves. - **Ethiopia (Afar)**: Frequent earthquakes accompany rifting; the 2005 Dabbahu rifting event involved a dyke intrusion and Mw 5.1 quake, creating a 60 km long fissure and triggering volcanic activity. - **Algeria (2003 Boumerdès)**: Mw 6.8 quake killed over 2,200 people, destroyed tens of thousands of buildings, and triggered a small tsunami—highlighting catastrophic potential in poorly constructed urban areas. - **Kenya**: The 2022 Turkana earthquake (Mw 5.8) was felt across East Africa; linked to rifting and volcanic activity in the Gregory Rift. ### Vocabulary to Remember - Seismograph, seismogram, P-wave (primary, compressional), S-wave (secondary, shear), surface waves, focal depth (shallow <70 km, intermediate 70–300, deep >300), aftershock, foreshock, seismic hazard map, building code (e.g., resistance to lateral forces), base isolation, ductility. ### Example Exam Question and Plan **Question**: Describe the causes of earthquakes in Cameroon and assess the effectiveness of current measures to reduce earthquake risk. **Plan**: I. Introduction: Define earthquake; note Cameroon's moderate seismicity linked to plate boundaries and volcanism. II. Causes: - Plate boundary forces: Transform stress from Gulf of Guinea margin, divergent rifting influence from CVL. - Intraplate transmission: Stresses from distant plate boundaries. - Volcanic triggers: Magma movement beneath Mount Cameroon and other CVL volcanoes causing volcano-tectonic quakes. - Minor anthropogenic factors (not currently significant). III. Current Risk Reduction Measures: - Seismic monitoring: National Seismic Network (limited stations; relies on international data). - Public education: Occasional drills and broadcasts after felt quakes (e.g., 2020 Kumba). - Building standards: National building code exists but enforcement is weak, especially in informal settlements. - Land-use planning: No widespread seismic zonation; critical facilities (hospitals, schools) not systematically assessed. IV. Effectiveness: - Strengths: Monitoring improved since 2020; public awareness increased after Kumba quake; some new constructions follow better practices. - Weaknesses: Sparse seismic station coverage; lack of retrofitting programs for old buildings; no national emergency response plan for earthquakes; low public knowledge of "Drop, Cover, Hold On". V. Conclusion: Current measures are rudimentary and reactive; effective risk reduction requires sustained investment in monitoring, enforcement of building codes, public education, and integration of seismic risk into development planning. ## Synthèse du chapitre - **Volcanism** is driven by magma generation and ascent, primarily at plate boundaries but also at hotspots like the Cameroon Volcanic Line, producing constructive (fertile soils, geothermal energy) and destructive (lava flows, ash, toxic gases) effects. - **Volcanoes and Man** examines the dual relationship: humans face hazards but also benefit from volcanic soils, resources, and energy; resilience depends on monitoring, adaptation, and socioeconomic factors, as seen in Cameroon’s slopes and Lake Nyos degassing efforts. - **Earthquakes and Man** focuses on seismic energy release via fault rupture, with causes tied to plate tectonics and volcanism; impacts include shaking, collapse, and secondary hazards; risk reduction in Cameroon remains limited by monitoring gaps, poor code enforcement, and low preparedness. - **Key Links**: Both volcanism and earthquakes are manifestations of crustal stress and energy release; the CVL exemplifies intraplate volcanism with seismic accompaniment; human adaptation requires scientific understanding, community engagement, and proactive policy. - **African Context**: The CVL, Afar Rift, and Albertine Rift show how tectonic processes shape hazard exposure and opportunity across the continent, necessitating region-specific disaster risk strategies. ## Glossaire > **Andisol**: A soil type formed from volcanic ash, characterized by high fertility and water retention. > > **Body Wave**: A seismic wave that travels through the Earth's interior (P-wave or S-wave). > > **Caldera**: A large volcanic depression formed by the collapse of a magma chamber roof after a major eruption. > > **Degassing**: The release of dissolved gases (e.g., CO₂, SO₂) from magma or crater lakes, either naturally or artificially induced. > > **Elastic Rebound Theory**: The model explaining earthquakes as the sudden release of stored elastic energy in rocks after frictional failure along a fault. > > **Epicenter**: The point on the Earth's surface directly above the focus of an earthquake. > > **Focus (Hypocenter)**: The subsurface point where an earthquake rupture begins. > > **Lahar**: A volcanic mudflow or debris flow composed of pyroclastic material and water, often triggered by heavy rain or lake breakout. > > **Limnic Eruption**: A rare natural disaster in which dissolved CO₂ suddenly erupts from deep lake water, forming a toxic gas cloud. > > **Moment Magnitude (Mw)**: A scale measuring the total energy released by an earthquake, based on the area of fault rupture, average slip, and rock rigidity. > > **Pyroclastic Flow**: A fast-moving current of hot gas and volcanic matter (tephra) that flows downslope from a volcano during an eruption. > > **Seismic Wave**: An elastic wave generated by an earthquake or explosion that propagates through the Earth.