1.1 (+ 1.5 Intro and 1.6 Summary Table)
Hazards are events that threaten or cause destruction to human lives & property.
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Hazards caused by nature are natural hazards;
Tectonic hazards include earthquakes, volcano eruptions & tsunamis.
Earthquakes are sudden shakings of land (usually less than 20 seconds).
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Occur mostly on land at destructive & conservative plate boundaries
They occur on land / under the sea;
Reason: Excessive pressure built up in plates due to plate movement causes rocks to
break and displace. The land shakes.
Volcanic Eruptions are processes by which extremely hot materials are ejected /
emitted from an opening in the Earth's surface.
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Occur mostly along the destructive plate boundaries around the Pacific Ocean;
This incomplete circle of active volcanoes is known as the Pacific Ring of Fire;
They occur on land / on seabeds.
Reason: Cracks extend from crusts towards magma chambers. Magma rises through the
cracks and reaches the Earth's surface.
Tsunamis are series of big sea waves. When they reach shallow seas at the coast, they
slow down & can tower up to over-30-metre-high waves.
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Occur mostly at destructive plate boundaries;
Coastal areas around the Pacific Ocean are most frequently affected;
May also strike islands in the middle of the oceans;
They occur along coastal areas.
Reason: Sudden vertical movement of large areas of sea floor displaces water and
generates huge sea waves.
1.2
(I) The Earth's Structure
1. Crust
(a) Continental; thick & light
(b) Oceanic; thin & dense
2. Mantle
(a) Lithosphere; formed with the crust & is solid (a.k.a. plate)
(b) Asthenosphere; plastic magma
^ Upper mantle
(c) Lower mantle; solid due to high pressure
3. Core
(a) Outer core; melted & liquid
(b) Inner core; under greatest pressure & solid
Heat from the mantle creates powerful convection currents which result in,
(II) Three types of plate boundaries
1. Constructive Plate Boundary
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Convection currents diverge;
Plates split away;
Magma wells up from asthenosphere, cools & solidifies into crust at the gap.
2. Destructive Plate Boundary
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Convection currents converge;
Plates dragged together;
Denser plate subducts into asthenosphere & melts into magma.
3. Conservative Plate Boundary
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Plates slide past each other;
Plate materials are neither created nor destroyed.
Plate tectonics describe how convection currents in the mantle cause plate
movements.
1.3
(I) Internal forces and their processes
Convectional currents generate three internal forces; compressional, tensional &
lateral force. These forces cause three processes; folding, faulting & vulcanicity.
1. Folding is the process by which rock layers are folded up under compressional force.
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The axial plane of fold varies according to differences in compressional forces;
They form fold mountains, often by folding sedimentary rocks;
Fossils of sea creatures are often found in fold mountains.
2. Faulting is the process by which rocks are displaced vertically or horizontally.
(a) Tensional Force ---> Normal Fault ---> Slides down fault plane
(b) Compressional Force ---> Reverse Fault ---> Pushed up fault plane
(c) Lateral
Force
--->
Tear
Fault
--->
slides
past
each
-
other
Compressional & tensional forces displace rocks vertically;
Landforms produced under these forces depend on dipping direction of cracks.
Block moutains (Horsts) are flat-top mountains
(a) Compressional force + cracks dipping towards each other = central block uplifted
with eroded rocks
(b) Tensional force + cracks dippping away from each other + side blocks sink,
central block remains
Rift valleys (Grabbens) are flat-bottom valleys.
(a) Compressional force + cracks dipping away from each other = side blocks pushed
up, central block depresses
(b) Tensional force + cracks dipping towards each other = central block sinks
3. Vulcanity is the process by which magma rises through the crust along cracks due to
great pressure beneath the crust.
(a) Intrusive vulcanity is the process by which magma enters cracks or chambers in
the crust from the asthenosphere, then cools down.
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Laccolith; dome-shaped feature parallel to the bedding plate
Dyke; sheet-like feature, usually vertical / tilted
Sill; sheet-like feature parallel to rock layers
Batholith; large mass of intrusive rock formed deep underground
(b) Extrusive vulcanity is the process by which magma is ejected from the crust
through a vent or fissure due to high pressure.
It is divided into two types; vent eruption & fissure eruption. In both cases, magma
that reaches the Earth's surface is called lava.
(i) Vent erruptions:
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lava, ash, broken rock fragments & gases erupt through a vent to the surface;
They cool, solidify & accumulate in height & finally form volcanoes;
They appear as cones & have circular depressions on top called craters;
Crater lakes are formed when craters are filled with rain-water;
Lava may erupt through cracks in the volcano caused by previous eruptions;
Smaller cones at the side are formed & are called parasitic cones;
Minor eruptions produce small volcanoes called secondary cones / conolets;
When formed in crater lakes, they become volcanic islands;
Violent eruptions may blow off a volcano's top, & a large depression called a
caldera is formed.
(ii) Fissure eruption is the process by which lava wells out through a linear crack or fault
on a surface.
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Lava flows in a sheet form, forming lava flow;
Sheets of lava pile forming a lava plateau.
*It is noted that there is G.S. content for volcanoes concerning its identification via
cross-sections from contour maps. Nothing new, though.
1.4
(I) Landforms due to Internal Force at Destructive Plate Boundaries
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Compressional force leads to collision of plates;
Denser / heavier plate slides below the other;
Subduction is the result of this plate subducting into the asthenosphere;
Folding, reverse faulting & vulcanity may occur;
The types of resulting landform depends on the nature of colliding crusts.
1. Continent-Continent Collision
Two plates with continental-crust-edges collide & form fold mountain ranges.
C.S. of Fold Mountain Range (Alpine-Himalayan Belt):
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Compressional force along the Alpine-Himalayan Belt causes the two continental
crusts to collide at their edges, & formed the fold mountain range Himalayas;
Magma currents brought the Indian subcontinent towards Asian continent;
The oceanic crust of the Indo-Australian plate being denser was subducted
beneath the Eurasian Plate at the destructive plate boundary;
Sediment from both crusts washed down & deposited in the coastal waters;
They accumulated, layered & compacted into sedimentary rocks;
Neither plate could subduct in the end due to their thick & light nature;
Sedimentary rock experienced great compressional force & folded up into a fold
mountain range called the Himalayas.
2. Ocean-Ocean Collision
Two plates with oceanic-crust-edges collide & form ocean trenches & volcanic
island arcs. It occurs most commonly in the western & northen Circum-Pacific
Belt.
(a) Ocean Trench (North American & Pacific Plate; Aleutian Trench):
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The denser plate bends when subducting (Pacific Plate);
Produces a deep & elongated trough called an ocean trench at the plate margin;
They form the deepest part in an ocean.
Cracks form on lighter plate & magma rises up (North American Plate);
The volcanic Aleutian Islands are formed when magma rises above sea level.
(b) Volcanic Island Arc (Eurasian & Philippine Plate; Marianas Islands):
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Cracks form on lighter plate & magma rises up (Philippine Plate);
They result in volcanic eruptions on the sea floor;
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Submarine volcanoes appear parallel to the ocean trench;
Repeated eruptions cause some submarine volcanoes to rise above sea level &
become volcanic islands;
They develop into a chain of volcanic islands called a volcanic island arc.
3. Ocean-Continent Collision
A plate with continental & oceanic crust-edges respectively collide & form ocean
trenches (Peru-Chile Trench), fold mountains (The Andes) & volcanoes (Mount
Chimborazo). They commonly occur in the eastern Circum-Pacific Belt.
(II) Landforms due to Internal Force at Constructive Plate Boundaries
Almost all constructive plate boundaries are found under the sea & are found in four
major zones: Mid-Atlantic zone, Mid-Indian zone, Antarctic zone & South-east
Pacific zone.
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Plates split instead of collide, therefore, tensional force is the main force;
New sets of landform are developed by faulting & vulcanicity.
1. Mid-Oceanic Ridges (Mid-Atlantic Ridge)
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Magma currents in the asthenosphere diverge & drag plates apart
(South American Plate from the Eurasion & African Plate);
Cracks are created where plates split;
Pressure is released from beneath the plates;
Hot lava cools, solidifies from water contact & become mid-oceanic ridges;
Movement of plates between two different sections may be opposite;
This part is called a transform fault where shearing occurs.
2. Rift Valleys (northern Mid-Atlantic Ridge)
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Cracks develop under newly extended ocean floor;
Normal faulting occurs under tensional force;
The central block of the ridge sinks & forms a flat-bottom rift valley;
The mid-oceanic ridge continues to extend sideways as the process repeats itself;
#A rift valley is like an "over-extended mid-oceanic ridge".
3. Submarine Volcanoes & Volcanic Islands (Easter Island)
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Cracks form in plates when they move apart;
The pressure of magma is released when cracks extend into the asthenosphere;
Vulcanicity occurs & produces active submarine volcanoes;
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Repeated eruptions form volcanic islands when above sea levels;
Easter Island is located on the East Pacific Ridge as a result of the splitting of the
Pacific & Nazca Plate;
Old ridges & their submarine volcanoes are dragged in opposite directions as new
ones form in the zone of rising magma;
The ocean floor thus expands at the zone of divergence;
The ocean floor expands a few cm a year & is called sea-floor spreading.
1.5
(I) Reasons for tectonic hazards to occur at plate boundaries:
1. Earthquakes
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When plates move, friction may hold them back;
Pressure slowly builds on the plates;
When the pressure exceeds the strength of the rock, it breaks & displaces;
Therefore, earthquakes are found along all plate boundary types.
2. Volcanic Eruptions
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Convergence of magma currents build pressure on destructive plate boundaries;
Subducted plate material intensifies this pressure;
Magma rises through cracks that extend downward to the magma chamber;
Along constructive plate boundaries, water pressure at the seabed suppresses
volcanic activity, making it less violent.
3. Tsunamis
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Caused by sudden vertical movement of a large area of seafloor.
Submarine earthquakes, sumbarine volcanic eruptions & submarine landslides
are the 3 major causes for tsunamis.
(a) Submarine Earthquakes
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Tsunamis caused by strong submarine earthquakes originated close to a seabed;
Destructive tsunamis are associated with vertical displacement of seabed rocks.
(b) Submarine Volcanic Eruptions
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Violent submarine volcanic eruptions may displace water & cause tsunamis.
(c) Submarine Landslides
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Any large-scale collapse of coastal materials into water can displace water;
They travel in all directions;
Tsunamis hit islands in the Pacific Ocean before they arrive at the opposite coast;
Therefore, the Hawaiian Islands is still a tsunami-prone area.
(II) Reasons for earthquake & volcanic zones far from plate boundaries:
1. Hot Spots
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Hawaii is affected by both earthquakes & volcanic eruptions;
The Hawaiian islands are located above a hot spot beneath the Pacific Plate;
It is an area above the asthenosphere where great heat accumulates;
Hot spots are fixed locations, therefore creating a volcanic island arc;
Volcanic eruptions may be accompanied by earthquakes;
Hot spots can be found all over the world underneath oceans / continents.
2. Other
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New convection currents of magma in the asthenosphere may develop, cracking
up, & forming new uncertain plate boundaries;
The African Plate is breaking up in East Africa under tensional force, causing
the central block to sink below the two splitting plates. Friction accumulated
at the fault plane causes earthquakes.
(III) Destruction brought about by tectonic hazards
1. Earthquakes
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Broken / displaced rock released stored energy & produces seismic waves;
Generated from the focus, the point where energy is released;
Crustal surface directly above the focus is the epicentre;
Intensity of the earthquake is greatest here.
(a) Describing an earthquake in three ways
i.
ii.
Focus depth: shallow (0-70km), intermediate (70-300) deep (>300km)
Intensity: indicated by Modified Mercalli scale. Measured by degree of
damage by the earthquake. Ranges from I - XII.
Magnitude: indicated by Richter scale. Measured by amount of energy
released by the earthquake. Ranges from 1 - 9.
iii.
(b) The earthquake's effects on human beings & the environment
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Damages buildings, bridges, dams & other structures;
Collapsing structures injure & kill people;
Fires are the result of broken underground gas pipes & power lines;
Earthquakes cause landslides, avalanches, & may trigger tsunamis;
Aftershocks occur after major earthquakes;
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Breakdown of communication links & damage to hospitals & utility
networks make relief work more difficult.
2. Volcanic Eruptions
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Lava, steam, carbon dioxide & sulphuric gases are the most common
materials ejected from a volcano;
Strong eruptions may eject ash & broken rock fragments;
Major damage is caused by lava, volcanic ash & gases.
(a) Lava
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Molten lava is extremely hot & burns almost everything in its path.
(b) Volcanic ash
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When mixed with rainwater, mudflows which bury villages are formed;
Ash in the air makes breathing difficult & blocks the sun, lowering visibility;
Crop failure is a result of large ash coverage of farmland.
(c) Gases
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Sulphuric gases can poison people;
Acid rain may result, killing vegetation over large areas;
Carbon dioxide released into the air intensifies the greenhouse effect.
3. Tsunamis
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They are less common than earthquakes & volcanic eruptions;
When a huge wall of water rushes towards coastal areas, it can drown people,
uproot trees, crush buildings & highways, wash away beaches & sweep
heavy objects inland.
1.6
(I) Five important conditions to help reduce losses during tectonic hazards
1. Effective Monitoring & Warning Systems
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Monitoring tectonic hazards & issuing warnings give time for evacuation.
(a) For Earthquakes
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Scientists analyse past records of earthquakes to predict time, place & scale of
future earthquakes;
Use of seismographs to monitor earthquakes by measuring seismic waves;
(b) For Volcanic Eruptions
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There are usually signs of imminent activity from a volcano before eruption;
Scientists install instruments to detect this activity
Studying the history & geological structure of the volcano helps prediction.
(c) For Tsunamis
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Measured by buoys, satellites & ocean-bottom tsunami gauges;
An international system called the Pacific Tsunami Warning System;
The Regional Tsunami Warning & Mitigation System for the Indian
Ocean was established in 2006.
2. Hazard-Proof Structures
(a) For Earthquake-Prone Regions
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Earthquake-proof designs for buildings prevent collapse;
Examples include shock-proof tech, steel frames & reinforced foundations.
(b) For Eruption-Prone Regions
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Roofs built with strong structures & steep sloping sides to prevent volcanic
ash from collapsing into structures;
Barriers are erected when direction of lava flow is known (e.g. Hawaii).
(c) For Tsunami-Prone Regions
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Sea walls are built along coastal shores.
3. Land Use Planning
Land use policies implemented in tectonic hazard-prone areas:
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

Avoidance of building facilities with potential dangers;
Imposition of laws governing height & structure of buildings;
Evacuation routes with adequate road signs.
(a) For Earthquake-Prone Regions
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Large open spaces (e.g. parks) are designated safety evacuation areas;
Schools / indoor stadiums are designated evacuation / shelter centres.
(b) For Volcanic Regions
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Volcanic hazard-zone maps show lava / mud flow-prone areas.
(c) For Tsunami-Prone Regions
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Subsidies encourage people living in coastal areas to move to higher ground;
Use of evacuation routes, channels & woodland reserves.
4. Education & Drills
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Regular large-scale earthquake drills for general public;
Educated & hazard-aware public minimises potential losses.
5. Well-Trained & Well-Equipped Rescue Teams
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Victims have better chance of being saved;
Rescue teams reduce disease, dirty water, starvation, etc.-related casualties;
Rescue equipment includes advanced life detectors, medicine & wireless
communication networks.
6. Other Measures
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Disaster aid reduces losses during hazards;
Governments set up emergency funds that strengthen rescue equipment;
Insurance provided against hazards.
(II) Effectiveness of the five measures
1. Monitoring & Warning Systems
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Dependent on inputs of capital & technology;
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Poor countries cannot afford them;
Rural areas do not normally have access to radios.
2. Hazard-Resistant Structures
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Dependent on inputs of capital & technology;
Corrupt countries have ineffective building laws;
Housing built with inferior materials ---> low resistance.
3. Land Use Planning
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The accuracy of identification of high risk areas;
Wrong decisions lead to loss of more lives & property.
4. Education & Drilling
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Traumatic experiences of some people make their education ineffective.
5. Well-Trained & Well-Equipped Rescue Teams
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Delayed in less developed countries due to buried roads, collapsed
communication systems, corrupt management & political barriers.
(III) Factors affecting the vulnerability of places to tectonic hazards
A. Regional Factors
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Higher population density exposes more people to hazards;
More economic infrastructure ---> more value loss during hazards;
Extreme post-hazard weather raises death tolls in exposed areas;
Relief of land may mean that low-lying areas are more prone to tsunamis, &
steep slopes create faster lava flows & more landslides after earthquakes;
Small inlets of shallow water ---> coastal relief is vulnerable to tsunamis.
B. Tectonic Hazard Factors
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Higher magnitudes release more energy & destruction;
Longer hazard durations decreases resistibility of people & structures;
The distance from source of destruction varies indirectly with its strength;
Some areas will experience less destruction due to its travel path;
More / fewer casualties depending on time of occurrence.
(IV) Less developed countries V.S. more developed countries
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More developed countries suffer less to tectonic hazards of similar magnitude.
Hazard Resistance of Less Developed Countries
1. Economic Development
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Lack capital & technology;
Insufficient funding for research, warning system development &
improvement of rescue equipment;
Poorly developed transport networks.
2. Strength of Buildings
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Stone, brick & wooden structures cannot withstand heavy shaking / fires
which occur after earthquakes.
3. Population Density
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Higher.
4. People's Awareness
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Strong traditional values, ignorance & little education make a population
less prepared.
5. Government Efficiency
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Corruption is common.
What less developed countries do with limited resources:
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Receive disaster aid and relief work;
Find infrastructural resistance improvement with limited resources.
*Table 7 on P.83 is another great studying tool
1.7
Four main factors to consider when deciding to live in hazard-prone areas:
1. Risks
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Perception of risk of hazards vary greatly among people;
Perception differences are a result of difference in education level, past
experience, religious background & character of people.
2. Opportunities
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Vulcanicity, folding & faulting provide valuable resources, job
opportunities & weath to local people. Desirable in less developed areas.
(a) Geothermal Power
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Hot water & steam found in volcanic areas are used to generate power.
(b) Metals & Minerals
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Solidified magma below the Earth's surface forms rocks containing them.
(c) Fertile Soil
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Volcanic ash is rich in minerals;
Weathering of solidified lava produces fertile soil;
Fertile soil provides nutrients for crop growth & attracts farmers.
(d) Tourist Attractions
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Volcanic areas & areas with folds and faults are scenic spots;
Hot springs & geysers are flows of hot mineral-containing water;
Geysers are spectacular natural fountains;
Job opportunities, higher incomes & better living standards are given.
3. Social Links
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Ties to one's motherland & ancestors prevent people from leaving;
Culture, tradition and personal relationships are strong.
4. Limitations on Migrations
a. Political barriers; population & immigration policies
b. Physical constraints; extreme environment