Physical Geography

Plate Tectonics

The Structure of the Earth

The Earth is composed of concentric layers. The crust and the uppermost part of the mantle form the lithosphere, a rigid outer shell approximately $100\;\mathrm{km}$ thick. Below the lithosphere lies the asthenosphere, a semi-fluid zone of the upper mantle. The lithosphere is divided into tectonic plates that float on and move over the asthenosphere.

The crust exists in two types:

• Continental crust: granitic composition, lower density ($\approx 2.7\;\mathrm{g/cm^3}$), thickness $30$--$70\;\mathrm{km}$
• Oceanic crust: basaltic composition, higher density ($\approx 3.0\;\mathrm{g/cm^3}$), thickness $5$--$10\;\mathrm{km}$

Plate Boundaries

Destructive (convergent) boundaries occur where plates move towards each other.

• Oceanic-oceanic convergence: the denser oceanic plate is subducted beneath the other, forming a deep ocean trench and a volcanic island arc (e.g., the Marianas Trench, the Japanese islands).
• Oceanic-continental convergence: the denser oceanic plate is subducted beneath the continental plate, forming a trench and a volcanic mountain range on the continent (e.g., the Andes, the Peru-Chile Trench).
• Continental-continental convergence: neither plate subducts due to similar density; instead, both buckle and fold, forming extensive mountain ranges (e.g., the Himalayas, the Alps).

Constructive (divergent) boundaries occur where plates move apart. Magma rises from the asthenosphere to fill the gap, creating new oceanic crust at mid-ocean ridges (e.g., the Mid-Atlantic Ridge) or continental rift valleys (e.g., the East African Rift).

Conservative (transform) boundaries occur where plates slide horizontally past each other. Crust is neither created nor destroyed, but friction causes stress to build up, released as earthquakes (e.g., the San Andreas Fault).

Evidence for Plate Tectonics

• Continental fit: the jigsaw match of South America and Africa (Wegener, 1912)
• Fossil distribution: identical fossil assemblages (e.g., Mesosaurus) found on separated continents
• Geological matching: rock types and mountain chains align across oceans (e.g., the Appalachians and the Scottish Highlands)
• Palaeomagnetism: magnetic minerals in ocean floor basalts record reversals of Earth's magnetic field in symmetrical stripes either side of mid-ocean ridges
• Seafloor spreading: age of oceanic crust increases with distance from mid-ocean ridges

Volcanism

Types of Volcanoes

Shield volcanoes form at constructive boundaries and over mantle plumes (hotspots). They are characterised by low-viscosity basaltic lava, gentle slopes ($2$--$10^{\circ}$), and effusive eruptions. Examples: Mauna Loa (Hawaii), Eyjafjallajokull (Iceland).

Composite (stratovolcanoes) form at destructive plate boundaries. They produce high-viscosity andesitic lava, steep slopes ($30$--$35^{\circ}$), and explosive eruptions with pyroclastic flows, ash clouds, and lahars. Examples: Mount Pinatubo (Philippines), Mount St. Helens (USA).

Dome volcanoes form from highly viscous rhyolitic lava that solidifies near the vent, creating steep-sided mounds prone to violent explosive eruptions. Example: Mount Pelee (Martinique).

Volcanic Hazards

• Lava flows: destroy infrastructure and habitats but typically move slowly enough to permit evacuation
• Pyroclastic flows: fast-moving ($> 700\;\mathrm{km/h}$), high-temperature ($> 700^{\circ}\mathrm{C}$) mixtures of gas, ash, and rock fragments; the most lethal volcanic hazard
• Ash falls: disrupt air travel, damage crops, contaminate water supplies, and cause respiratory problems
• Lahars: volcanic mudflows formed when hot pyroclastic material melts glacial ice or mixes with water; highly destructive to settlements downstream
• Volcanic gases: $\mathrm{SO_2}$, $\mathrm{CO_2}$, and $\mathrm{H_2S}$ can cause acid rain, respiratory illness, and (in high concentrations) asphyxiation

Supervolcanoes

Supervolcanoes produce eruptions exceeding $1000\;\mathrm{km^3}$ of ejecta (compared to $< 1\;\mathrm{km^3}$ for typical eruptions). They are not mountainous but rather large calderas formed when the ground collapses after a massive eruption. Example: Yellowstone (USA), Toba (Indonesia). The Toba eruption ($\approx 74000$ years ago) may have caused a volcanic winter and a human population bottleneck.

Earthquakes

Causes and Mechanics

Earthquakes occur when stress builds up along a fault until the frictional resistance is overcome and the rocks suddenly slip. The point of initial rupture is the focus (hypocentre); the point on the surface directly above is the epicentre.

Seismic waves:

• P-waves (primary): compressional, fastest ($6$--$8\;\mathrm{km/s}$ in the crust), travel through solids and liquids
• S-waves (secondary): shear, slower ($3.5$--$4.5\;\mathrm{km/s}$), travel through solids only (their absence beyond a certain distance indicates a liquid outer core)
• Surface waves (Love and Rayleigh): slowest, travel along the surface, cause the most damage due to their amplitude and rolling motion

Measuring Earthquakes

• Magnitude: the Richter scale measures the amplitude of seismic waves on a logarithmic scale (each whole-number increase represents a tenfold increase in amplitude and approximately $31.6$ times more energy). The moment magnitude scale ($M_w$) is now preferred as it more accurately measures the total energy released.
• Intensity: the Mercalli scale measures the observed effects and damage at specific locations, ranging from I (not felt) to XII (total destruction).

Factors Affecting Earthquake Impact

• Magnitude: higher magnitude generally causes greater damage, but the relationship is non-linear
• Depth: shallow focus ($< 70\;\mathrm{km}$) earthquakes cause more surface damage than deep-focus ones
• Distance from epicentre: intensity decreases with distance due to energy dissipation
• Geology: soft, unconsolidated sediments amplify seismic waves (liquefaction); bedrock transmits waves with less amplification
• Population density and urbanisation: more people and infrastructure in hazard zones increase vulnerability
• Building quality: earthquake-resistant design (base isolation, flexible structures) dramatically reduces casualties and damage
• Time of day: nighttime earthquakes may reduce outdoor casualties but increase indoor ones due to sleeping populations
• Preparedness and response: early warning systems, evacuation plans, and emergency services capacity

Mass Movement

Mass movement is the downslope movement of material under the influence of gravity. It is classified by type of movement, rate, and material involved.

Classification

Falls: material detaches from a steep slope or cliff face and falls freely or bounces. Triggered by weathering, undercutting, or earthquakes. Fast and often catastrophic.

Slides: a coherent mass moves along a defined plane of weakness (slip surface). Includes rotational slides (curved failure surface, common in clay) and translational slides (planar failure surface, common along bedding planes).

Slumps: a rotational form of slide where the mass rotates backwards as it moves downslope, creating a crescent-shaped scarp at the top.

Flows: material moves as a viscous fluid. Includes mudflows (saturated soil), debris flows (mixture of soil, rock, and water), and earthflows (slow-moving saturated soil on moderate slopes).

Creep: the very slow ($< 1\;\mathrm{m/year}$), continuous downslope movement of soil and regolith. Caused by repeated cycles of wetting and drying, freezing and thawing, or heating and cooling. Evidence includes tilted fences, bent tree trunks, and cracked walls.

Factors Influencing Mass Movement

• Slope angle: steeper slopes provide a greater component of gravitational driving force
• Material properties: cohesion, permeability, and angle of internal friction determine resistance to movement
• Water: saturation reduces cohesion and adds weight; pore water pressure in slopes can trigger failure
• Vegetation: roots bind soil and intercept rainfall; deforestation increases slope instability
• Human activity: excavation at the base of slopes, loading the top of slopes, and poor drainage increase risk
• Geological structure: bedding planes, joints, and faults provide planes of weakness

Coastal Processes

Wave Types and Characteristics

Constructive waves have a long wavelength, low amplitude, and strong swash. They deposit sediment and build beaches.

Destructive waves have a short wavelength, high amplitude, and strong backwash. They erode cliffs and remove sediment.

Wave energy depends on wind speed, wind duration, and fetch (the distance over which the wind blows across open water).

Coastal Erosion Processes

• Hydraulic action: the force of water compressed into cracks, prising apart rock
• Abrasion (corrasion): sediment carried by waves scrapes and wears away the cliff face
• Attrition: rock particles collide and break down into smaller, smoother pieces
• Solution (corrosion): weak acids in seawater dissolve soluble rocks (limestone, chalk)

Coastal Erosion Landforms

• Headlands and bays: differential erosion of alternating resistant and less resistant rock
• Cliffs and wave-cut platforms: waves undercut the cliff, forming a notch; the cliff collapses, retreats, and leaves a flat platform at low tide
• Caves, arches, stacks, and stumps: waves exploit weaknesses (joints, faults) in headlands; erosion enlarges cracks into caves; erosion through a headland forms an arch; arch collapse leaves a stack; further erosion reduces the stack to a stump

Coastal Deposition Landforms

• Beaches: accumulations of sediment between the lowest and highest tide marks
• Spits: elongated ridges of sand or shingle extending from the coastline into open water, formed by longshore drift; the end may curve hookwards due to wave refraction
• Bars: ridges of sediment that extend across a bay, partially blocking it
• Sand dunes: formed when sand is blown inland from the beach and trapped by vegetation (embryo dunes, foredunes, yellow dunes, grey dunes)
• Tombolos: ridges of sediment connecting an island to the mainland

Longshore Drift

Waves approach the coast at an angle due to prevailing wind direction. Swash carries sediment up the beach at an angle; backwash carries it straight back down under gravity. Net sediment transport is along the coast in the direction of the prevailing wind.

Weathering

Mechanical (Physical) Weathering

• Freeze-thaw (frost shattering): water enters cracks, freezes, and expands by approximately $9\%$, exerting pressure that widens cracks. Repeated cycles break rock apart.
• Thermal stress: repeated heating and cooling causes differential expansion and contraction in rocks, leading to granular disintegration.
• Salt crystallisation (haloclasty): saltwater enters pores, evaporates, and salt crystals grow, exerting pressure on rock.
• Exfoliation (onion weathering): outer layers of rock peel off due to pressure release when overlying material is removed (common in igneous rocks).

Chemical Weathering

• Carbonation: $\mathrm{CO_2}$ dissolved in rainwater forms weak carbonic acid, which reacts with calcium carbonate in limestone: $\mathrm{CaCO_3 + H_2CO_3 \to Ca(HCO_3)_2}$. Produces limestone pavements with clints (blocks) and grykes (gaps).
• Oxidation: iron-bearing minerals react with oxygen and water, producing iron oxides (rust), weakening the rock.
• Hydration: water is absorbed into the crystal structure of minerals, causing expansion and disintegration (e.g., the conversion of anhydrite to gypsum).
• Hydrolysis: minerals react with water, forming new (often weaker) minerals. Feldspar in granite hydrolyses to kaolinite (clay), a key process in granite weathering.

Biological Weathering

• Plant roots grow into cracks, physically prising rock apart
• Lichens and mosses secrete organic acids that dissolve rock surfaces
• Burrowing animals and plant roots create passages for water entry, accelerating chemical and physical weathering

Common Pitfalls

• Confusing the focus and epicentre of an earthquake. The focus is the point of rupture within the Earth; the epicentre is the point on the surface directly above it.
• Stating that oceanic crust is "heavier" than continental crust. The correct term is "denser." Mass and density are distinct properties.
• Confusing constructive and destructive boundaries in terms of hazard type. Destructive boundaries produce explosive volcanism and deep earthquakes; constructive boundaries produce effusive volcanism and shallow earthquakes.
• Describing longshore drift without specifying the direction of net sediment transport, which depends on the prevailing wind and wave approach angle.
• Confusing weathering with erosion. Weathering is the in-situ breakdown of rock; erosion is the removal and transport of weathered material.

Practice Problems

Problem 1: Plate Boundary Identification

The Pacific Plate is moving north-west relative to the North American Plate at approximately $60\;\mathrm{mm/year}$. Describe the type of plate boundary, the geological processes occurring, and the associated hazards.

This is a transform (conservative) boundary where the Pacific Plate slides horizontally past the North American Plate along the San Andreas Fault.

Processes: friction between the plates causes stress to accumulate along the fault. When the stress exceeds the frictional resistance, the plates suddenly slip, releasing energy as an earthquake.

Hazards: shallow-focus earthquakes with significant surface shaking. The lack of volcanic activity distinguishes this from convergent boundaries. Secondary hazards include landslides, liquefaction, surface rupture, and fires from broken gas lines.

Problem 2: Earthquake Magnitude and Energy

An earthquake measures $6.0$ on the moment magnitude scale. How much more energy does it release than an earthquake of magnitude $4.0$?

The relationship between magnitude and energy is approximately:

$\log_{10}(E_2 / E_1) \approx 1.5 \times (M_2 - M_1)$

$\log_{10}(E_2 / E_1) = 1.5 \times (6.0 - 4.0) = 3.0$

$E_2 / E_1 = 10^{3.0} = 1000$

A magnitude $6.0$ earthquake releases approximately $1000$ times more energy than a magnitude $4.0$ earthquake. The amplitude difference would be $10^{(6.0 - 4.0)} = 100$ times, but the energy release difference is far greater.

Problem 3: Coastal Erosion Calculation

A cliff is retreating at an average rate of $1.5\;\mathrm{m/year}$. A farmhouse is currently $45\;\mathrm{m}$ from the cliff edge. Calculate how many years it will take before the cliff reaches the farmhouse, and discuss the uncertainties in this estimate.

$\mathrm{Years} = \frac{45}{1.5} = 30 \mathrm{ years}$

Uncertainties:

• The retreat rate is an average; actual rates vary significantly between years depending on storm frequency and severity
• Climate change may increase storm intensity and frequency, accelerating erosion
• The cliff may experience a sudden, large collapse event rather than steady retreat
• Coastal defences (existing or future) could alter the retreat rate
• The geology may not be uniform along the cliff face, with some sections more resistant than others

Problem 4: Volcanic Hazard Assessment

Mount X is a composite volcano with a population of $500000$ living within $30\;\mathrm{km}$. The last eruption was $200$ years ago and produced pyroclastic flows travelling at $600\;\mathrm{km/h}$ reaching $25\;\mathrm{km}$. Assess the risk and recommend monitoring strategies.

Risk assessment: High. The large population within the hazard zone ($30\;\mathrm{km}$), the volcano's history of violent pyroclastic flows, and the long repose interval (which may allow magma to accumulate and increase eruption explosivity) all indicate significant risk.

Monitoring strategies:

• Seismology: monitor seismic activity (harmonic tremor indicating magma movement, swarms of small earthquakes indicating fracturing rock)
• Ground deformation: GPS and tiltmeters to detect swelling as magma accumulates beneath the volcano
• Gas monitoring: measure $\mathrm{SO_2}$ emissions (rising levels suggest magma is approaching the surface)
• Thermal imaging: satellite and ground-based sensors to detect temperature anomalies
• Groundwater monitoring: changes in water temperature, pH, or chemistry can indicate magma interaction
• Evacuation planning: establish exclusion zones, evacuation routes, and early warning systems

Problem 5: Weathering Process Comparison

Compare freeze-thaw weathering and carbonation in terms of their mechanism, rock type affected, climate requirements, and resulting landforms.

Aspect	Freeze-Thaw Weathering	Carbonation
Mechanism	Physical: water freezes and expands in cracks	Chemical: carbonic acid reacts with calcium carbonate
Rock type	Any rock with cracks or joints; particularly effective on jointed rocks	Limestone and chalk
Climate	Cold environments with frequent freeze-thaw cycles	Warm, humid climates (higher $\mathrm{CO_2}$ solubility and reaction rates)
Rate	Relatively fast per cycle but limited to freeze-thaw zones	Slow and continuous; rate increases with temperature and moisture
Landforms	Scree slopes, talus fans, angular rock fragments	Limestone pavements (clints and grykes), caves, swallow holes, grikes

Both processes often work together in upland limestone areas (e.g., the Yorkshire Dales), where freeze-thaw widens joints that are then enlarged by carbonation.