Lithospheric Processes
The Theory of Plate Tectonics explains the dynamics of the Earth and attributes tectonic processes and landforms to the Earth’s internal energy and the movement of tectonic plates.
Structure of the Earth (Overview): core, mantle, crust (Figure references in the transcript).
The Lithosphere vs. the Asthenosphere:- Lithosphere: the rigid outer shell, consisting of the crust plus the solid upper mantle; broken into large units called lithospheric plates.
Asthenosphere: lies beneath the lithosphere; hotter, ductile, capable of slow flow (a few centimetres per year), enabling plate movements.
Continental and Oceanic Crusts:- Crust types: Continental and Oceanic (Figure 2 in the transcript).
Differences (Table 1):
Thickness: Continental, Oceanic
Age: Continental: very young to about four billion years; Oceanic: less than two hundred million years
Type: Continental: Granite; Oceanic: Basalt
Density: Continental approximately 2.7 g/cm$^3$; Oceanic approximately 3.0 g/cm$^3$
Colour: Continental: pale-coloured; Oceanic: dark
The fact that basaltic oceanic crust is denser than granitic continental crust explains subduction at convergent margins; oceanic crust is generally recycled through subduction, contributing to the absence of very old oceanic crust (often less than 200 Myr).
Mechanism for Plate Movement:- The lithosphere is made of a few large plates and many smaller ones; plates move slowly (a few cm/yr).
Two major theories describe plate movement:
2.1 Mantle Convection Currents: energy from internal heat (core–mantle boundary); hot mantle rocks rise, drag lithospheric plates at the base of the lithosphere, and cycle heat by cooling and sinking—drives plate motion via divergence and convergence of convection currents.- Process: heat causes expansion and reduced density, rock rises; at plate bases, hot mantle rock moves laterally, cooling and sinking; sinking is balanced by upwelling elsewhere; convection cells maintain motion.
2.2 Slab-pull Force: more recently considered as a key active driving force; dense oceanic plates subduct under lighter plates, pulling rest of the plate along due to the weight of the subducting slab; plate motion partly driven by sinking dense plates in subduction zones (ocean trenches).
Tectonic Plate Boundaries – Processes and Resultant Landforms:- Three kinds of plate boundaries: divergent (constructive), convergent (destructive), transform (conservative).
1 Divergent Plate Boundary Landforms (constructive): plates move apart, new crust formed, zone of tension.
1.1 Rift Valleys and Block Mountains: continental–continental divergence; vertical displacement along faults forms elongated lowlands (rift valleys) and up-standing blocks (block mountains).- East African Rift system highlighted; features include fault scarps up to 600 m+, step faulting vs grid faulting, and volcanoes along the rift floor (e.g., Kilimanjaro, Kenya). Lakes may form along Rift Valleys (salt lakes like Natron or freshwater lakes like Malawi).
1.2 Mid-Oceanic Ridges: oceanic–oceanic divergence; extensive ridges (tens of thousands of km long; hundreds of km wide) with central rift valleys; pillow lava forms from rapid underwater cooling; seafloor spreading creates new crust; older crust moves away toward trenches and is subducted.- Example: The Mid-Atlantic Ridge; great transform faults create the ridge’s zig-zag path; volcanic islands can be located near ridges (Azores, Ascension); Iceland is a major site above a mantle plume causing thicker crust and uplift.
2 Convergent Plate Boundary Landforms (destructive): boundaries where plates move toward each other; different landforms depend on crust types involved.
Possible Types of Convergence:
Continental-Oceanic Convergence: A denser oceanic plate subducts beneath a lighter continental plate. This process leads to the formation of ocean trenches, volcanic mountain ranges (Andean-type fold mountains), and associated volcanism on the overriding continental plate.
Oceanic-Oceanic Convergence: One oceanic plate subducts beneath another oceanic plate. This results in the formation of deep ocean trenches and volcanic island arcs parallel to the trench, with explosive volcanic eruptions.
Continental-Continental Collision: Two continental plates collide. Since continental crust does not subduct, the intense compression leads to significant folding and uplift, forming vast mountain ranges (Himalayan-type fold mountains). Volcanism is generally absent in these zones.
2.1 Ocean Trenches: long, narrow deep-sea troughs. Formed at subduction zones where denser plate descends.
Continental–Oceanic convergence example: Peru-Chile Trench along South America; Nazca plate subducts beneath South American plate.
Oceanic–Oceanic convergence example: Mariana Trench (deepest on earth); subduction creates trench and a volcanic island arc (e.g., Marianas Islands).
2.2 Volcanic Island Arcs: arc of volcanic islands parallel to the trench formed when one oceanic plate subducts beneath another; water in subducting basalt lowers melting point of mantle, magma rises to surface producing explosive eruptions; magma typically andesitic; islands formed about ~100 km from boundary.- Eruptions are violent due to increased viscosity as magma ascends; examples: Japan, Mariana Islands.
2.3 Andean-type Fold Mountains: continental–oceanic convergence; oceanic plate subducts beneath continental plate; mantle melts due to water release and magmas migrate; volcanism forms composite/stratovolcanoes; folding forms mountain ranges (orogeny).- Example: Andes along western coast of South America; Nazca plate subducting under South American plate.
2.4 Himalayan-type Fold Mountains: continental–continental collision; no subduction; intense compression leads to folding and uplift of continental crust (e.g., Himalayas); absence of volcanism due to lack of subduction.
Key points for converging boundaries (summary):- Subduction occurs only with oceanic crust; denser plate subducts; continental plates/crust do not subduct.
Folding occurs only with continental crust; oceanic crust does not fold.
Ocean trenches form where subduction occurs.
Volcanoes form on the overriding plate when subduction occurs.
3 Transform Plate Boundary Landform: plates slide past one another along transform faults; limited creation/destruction of crust; also called conservative boundaries.
Example: San Andreas Fault (North American Plate and Pacific Plate), about 1,300 km long; movement causes major earthquakes (e.g., 1906 San Francisco earthquake).
Extrusive Volcanism – Features and Associated Hazards:- A natural hazard is a naturally occurring event threatening lives and property; tectonic hazards include earthquakes and volcanic eruptions.
Extrusive volcanism occurs when magma reaches the surface, causing eruptions and various volcanic features.
Distribution of major active volcanoes (Figure 15 reference).
Active volcano locations: most explosive volcanism occurs at subduction zones (convergent margins); these volcanoes account for roughly 88% of eruptions with fatalities despite representing only about 10–13% of magma reaching the surface.
Divergent margins hosts volcanism mainly along mid-ocean ridges; some active volcanoes occur where two continental plates diverge (e.g., eastern Africa).
Mantle plumes can cause volcanism away from plate margins (e.g., Hawaiian hotspot).
Volcano anatomy (Figure 16): magma chamber, vent, crater.
1 Volcanic Hazards:
1.1 Lava Flow: magma becomes lava at surface; magma types influenced by silica content and viscosity.
1.2 Mudflows: gases trapped in viscous magma lead to violent eruptions; pyroclasts and ash can mix with water to form mudflows; two main types: primary (eruptive) and secondary (rainfall on ash deposits).
1.3 Pyroclastic Flows: hot gases plus pyroclasts travel rapidly; can travel up to ~200 km/h and extend beyond 100 km; extremely lethal; examples include Mount Pelée (1902) where ~29,000 people died; pyroclastic flow composition: bombs/blocks (>64 mm), lapilli (2–64 mm), ash (<2 mm).
1.4 Ash Falls: ash clouds darken skies, suffocate vegetation and animals, can cause roof collapses when wet; ash clouds affect machinery and aviation (e.g., Eyjafjallajökull 2010 disruption in Europe; costs ~US$1.8 billion to airlines). It spreads widely relative to lava flows and pyroclastic/mudflows.
1.5 Tsunamis: volcanic eruptions can generate tsunamis via underwater explosions, submarine caldera collapses, or ash/magma entering the sea; Krakatau (1883) linked to devastating tsunamis that killed tens of thousands.
2 Types of Volcanic Eruptions and Landforms:
2.1 Shield Volcanoes: built by successive low-viscosity basaltic lava flows; typically broad, gently sloped domes; broad bases (>100 km), gentle slopes (<10° near summit, increasing with distance); examples: Hawaiian volcanoes (e.g., Mauna Loa), Iceland at some ridges; associated with mantle hotspots or divergent boundaries; lava flows produce limited pyroclastic material.
2.2 Composite Volcanoes / Stratovolcanoes: common above subduction zones (Pacific Ring of Fire); tall, symmetrical structures with steeper summits; formed by alternating layers of lava and pyroclastics; eruptions highly explosive due to viscous andesitic/granitic magma; examples: Nevado del Ruiz (Colombia), Mount Fuji (Japan).
2.3 Calderas: large circular depressions (>1 km diameter) formed when the summit collapses into an emptied magma chamber; example: Crater Lake, Oregon.
3 Benefits of Living Near Volcanoes:
3.1 Fertile soil for agriculture: volcanic deposits enrich soils with Mg, K; ash acts as natural fertilizer and porosity retains moisture; many volcanic regions support agriculture (Java, Bali).
3.2 Mining of precious stones, minerals and building materials: hydrothermal alteration can form mineral deposits (e.g., diamonds from kimberlite pipes); sulphur mining near active volcanoes for industry.
3.3 Tourism: volcanoes attract visitors for hiking, scenery, hot springs (e.g., Vesuvius near Pompeii).
3.4 Geothermal energy: volcanic regions host geothermal systems; Iceland and New Zealand harness geothermal heat for district heating and power; Reykjavik as example; geothermal energy contributes to low fossil fuel use.
Seismic Activities - Associated Hazards and Their Management- 1 Causes and Features of Earthquakes:
95% of earthquakes are inter-plate (at plate boundaries); intra-plate earthquakes also occur and can be deadly (e.g., Tangshan 1976, Sichuan 2008).
1.1 The Cause of Earthquakes: due to compression, tension or shearing as plates move; energy release when deformation exceeds strength; sequence at transform boundary (frictional locking, stress buildup, rupture, release).
1.2 Earthquake Focus and Epicentre: focus is the origin underground; epicentre is the point on the surface directly above the focus.
Depth categories (Table 7): shallow < 70 km (85%); intermediate 70–300 km (12%); deep > 300–700 km (3%).
2 Measuring Earthquakes:
2.1 Mercalli scale: qualitative; measures intensity (ground shaking, damage, secondary effects); used to map isoseismal (areas of equal intensity).
2.2 Richter scale: quantitative; magnitude scales with energy release; each unit increase corresponds to ~10x amplitude increase and ~32x energy increase; ranges listed (e.g., 2.0, 5.0, 6.0–6.9, 7.0, 7.4, 7.6, 8.3, 9.3, 9.5 with notable events).
3 Factors Affecting Earthquake Casualties and Damages:
3.1 Population Density: higher casualties in densely populated urban areas; many of the world’s largest cities lie in or near earthquake zones.
3.2 Distance from Epicentre: stronger waves near the epicentre increase damage; attenuation reduces impact with distance.
3.3 Depth of Earthquake Foci: shallow earthquakes release more surface energy; deeper earthquakes have less surface shaking; subduction zones feature intermediate/deep events with high energy.
3.4 Time of Occurrence: nighttime quakes trap people indoors; e.g., Sun Moon Lake region (Taiwan) 1999 near midnight.
3.5 Geology: loose, unconsolidated, water-saturated sediments amplify seismic waves; liquefaction can undermine foundations (Mexico City 1985).
3.6 Level of Preparedness: building codes, drills, emergency plans reduce casualties; developed nations typically better prepared.
4 Earthquake Hazards:
4.1 Ground Motion: amplitude/frequency of surface seismic waves determines damage; Rayleigh waves (elliptical motion) and Love waves (horizontal shear).
4.2 Tsunamis: transmission across oceans; generated by submarine earthquakes; 2004 Indian Ocean tsunami example; speed in deep water ~800 km/h; near shore height up to ~15 m; first low trough then high crest.
4.3 Liquefaction: water-saturated sediments lose strength during shaking; buildings sink or tilt (Kobe 1995 example).
4.4 Landslides: triggered on steep slopes, particularly near convergent margins (e.g., Sichuan 2008).
4.5 Fires: ruptured gas lines, electrical fires; Kobe 1995 had widespread fires due to damaged infrastructure.
5 Managing Earthquakes: Earthquake Prediction:
Prediction remains elusive; probability estimates use recurrence intervals and monitoring animal behavior, though not foolproof (Haicheng 1975 evacuation vs. Tangshan 1976 failure).
5.1 Recurrence Intervals: average return period for major ruptures (historical LA area example); Parkfield project highlighted unpredictability of exact timing.
5.2 Monitoring Animal Behaviour: some historical successes (Haicheng 1975 evacuation) but not consistently reliable (Tangshan 1976).
6 Managing Earthquakes: Mitigation Measures:
6.1 Alarm and Automated Systems: seismic sensors can trigger automatic responses if epicentre > 100 km; may cut power, halt trains, shut gas/water lines; cost and false alarms are concerns.
6.2 Hazard Resistant Designs: aseismic designs to minimize damage; base isolation (flexible supports) to decouple structure from ground motion; deep foundations; limitations include cost and enforcement, especially in less developed nations.
6.3 Hazard Mapping: seismic hazard maps identify vulnerable geologies; guide land-use planning, avoid building on high-risk zones; inform building codes.
6.4 Earthquake Evacuation Measures: drills (e.g., Japan), shelters, first aid training; national disaster rehearsals; effectiveness varies by country.
6.5 Tsunami Monitoring and Warning Systems: networks of sensors and detectors to forecast tsunamis; limitations include false alarms and timing; high costs for developing nations.
7 Managing Earthquakes: Response to Earthquakes:
Short-Term Response (Table 11): emergency aid, shelter, food, water, medical care; rescue operations (72-hour window as critical for locating survivors); e.g., Tohoku 2011, Afyon 2002, Haiti 2010.
Long-Term Response (Table 12): infrastructure rebuilding, stricter building codes, economic recovery measures, health support, compensation and land restoration, recovery timelines (e.g., Sichuan 2008; Kobe 1995; Christchurch 2011).
Notes on connections and implications:- The content links plate tectonics theory to real-world hazards and planning, emphasizing the ethical and practical importance of preparedness and resilient infrastructure.
Real-world relevance shown through case studies (e.g., Andes, Himalaya, Pacific Ring of Fire, Iceland, Mexico City, Kobe, Christchurch, Haiti, Sumatra 2004), highlighting the interplay of geology, hazard management, and socio-economic outcomes.
Paleogeography and plate interactions have long-term implications for resource distribution (fertile soils, mineral deposits, geothermal energy) and for human settlement decisions.
Mathematical and numerical references from the transcript (formatted as text):- Crustal thickness/age ranges: Continental thickness approximately 25–70 km; Oceanic thickness approximately 5–8 km; Oceanic crust age less than 200 Myr.
Plate movement rates: average rate of a few cm/yr (mantle convection and plate motion).
Depth categories for earthquakes: shallow less than 70 km; intermediate 70–300 km; deep more than 300–700 km.
Ocean trench depths: up to 7,500–10,000 m.
Tsunami speeds: deep-water velocity approximately 800 km/h; near shore height up to approximately 15 m.