4.1 Tectonics

Question 1

What are hazards? What are the types?

Answer 1

Threats that could injure people and damage environments

Natural hazards occur when humans interact with the physical world.

Disasters are hazards which cause so much damage that recovery without help is impossible

Risk is the exposure to a hazard

Vulnerability is the degree to which conditions make a population more likely to experience a hazard

Question 2

What is the perception of risk?

Answer 2

Responses to an event depend on experience, material well being and personality

3 choices in an event - do nothing, adjust to the situation or leave the area.

Adjustment may occur by identifying hazards, estimating the risk and evaluating the cost caused by the hazard

Question 3

What are the ways of reducing risk?

Answer 3

• Hazard mapping uses a body of theory where the build up to a disaster and its aftermath are assessed.
• Rehabilitation refers to people being able to make homes safe to live in again
• Government use rehabilitation, reconstruction and aid focused on reducing the impact of future events to improve safety of the region.

Question 4

What is the global distribution of seismic hazards?

Answer 4

• 84% of seismic events occur as plate boundaries, in clear linear patterns determined by them.
• Collision and destructive plate boundaries result in linear chains and conservative and constructive result in lines of narrow earthquakes.
• The remaining 15% are intraplate earthquakes caused by human activity
• Broad belts associated with subduction zones
• Narrow belts associated with constructive plate boundaries as new material is formed and plates are moving apart.
• Collision boundaries are broad and narrow

Question 5

How can humans trigger earthquakes?

Answer 5

• Groundwater extraction changes stresses and can trigger earthquakes
• Disposing liquid waste by injecting into underlying rock lubricates and reactivates faults
• Underground nuclear testing
• Fracking of rock
• Increased crustal loading - weight of water behind dams

Question 6

What is the global distribution of volcanos?

Answer 6

• Most found at plate boundaries, some in hotspots such as Hawaii
• 3/4 of 550 volcanos in the ring of fire in the Pacific.
• Found along major plate boundaries - Pacific Rim is often related to subduction either below oceanic or continental crust.
• Chains of volcanic islands such as the Aleutian Islands are found here.
• Where oceanic crust subducts continental crust, young fold mountains and composite volcanos are formed, such as the Andes
• Plumes or hotspots are jets of hot material rising from deep within the mantle, such as Hawaii.

Question 7

What are the different types of volcano and where they are found?

Answer 7

• At subduction zones, more viscous andesitic lava is found which is more explosive.
• Mid ocean ridges have more basaltic fluid lava deep from the mantle rising from convection currents, melting and forming new oceanic crust or shield volcanos/island arcs.
• At subduction zones cold ocean floor slides down and warms up slowly due to hydrogen in the slab. When water meets the hot rock there are explosions due to pressure. The eruption is generally influenced by processes producing magma at different compositions - mainly andesitic

Question 8

What is found at Constructive O/O boundaries

Answer 8

Earthquakes - shallow foci, generated by magma movement and tensional forces. Submarine and not very threatening, may cause tsunami

Volcano - gentle, non explosive eruptions due to low silica content, very runny, submarine eruption eventually can build high enough to create islands

Question 9

What is found at constructive c/c boundaries?

Answer 9

Earthquakes: Continental rift valleys can be made, causing rifting and rift valleys, cause shallow foci earthquakes.

Volcanos: Horizontal forces drag plates apart, magma rises and cools, forming blocks in the gap which rise and fall, creating volcanos. An example is the Graben valley of blocks. Transform faults may also occur at right angles to plate boundaries due to different spreading rates

Question 10

What is found at convergent c/c boundaries?

Answer 10

Earthquakes: fold mountains form and focus of earthquake at the point inside the crust where pressure is released

Question 11

What is found and convergent boundaries?

Answer 11

Earthquakes: friction builds up at subduction zone, shallow foci earthquakes when pressure released. Compressional forces cause crustal stresses and deep earthquakes in the Benioff zone line, subject to major earthquakes deep at an angle to the subduction zone

Volcanos: friction causes partial melting, rises through fissures and cracks to form volcanos. Lava has high silica content and is andesitic but low temperature. It is viscous and gassy, building highly explosive eruptions.

Question 12

What is found at conservative boundaries?

Answer 12

Earthquakes: friction builds and when released creates frequent shallow foci earthquake events

Question 13

What are intraplate boundaries?

Answer 13

Areas where earthquakes and volcanos do not occur on plate boundaries

Earthquakes may occur due to human activity

Hawaiian islands are a good example of a series of volcanic islands not on a plate but instead of hotspots and mantle plumes where magma rises from the mantle.

Question 14

What are the types of magma?

Answer 14

All magma contains oxygen and silicon

Basic lava: low silica content, low gas and low viscosity, high mean temperatures. Lava cools in basalt and is dark in colour due to iron and magnesium

Acidic lava: highest silica content, high gas content and viscosity, low mean temperatures. Traps gas bubbles in magma chambers causing explosive and destructive eruptions which violently eject lava into the air, cooling into rhyolitic and andesitic rock.

Question 15

What causes earthquakes?

Answer 15

Result from movement along faults. Crustal stressed occur from the movement as tension builds up over time when rocks are locked together

When this becomes too great, there will be a sudden release of the stress as rocks shift along the fault.

As the fault moves, shock waves span out, known as seismic waves. The point of origin is called the focus and the epicentre is the point directly above the focus.

Question 16

What are the primary and secondary hazards from earthquakes?

Answer 16

Primary: ground shaking, surface faulting

Secondary: liquefaction, tsunamis, injury, pollution, damage to property, flooding, fires, disease, loss of infrastructure - e.g. electricity, sewage, communications

Question 17

What are the processes of an earthquake?

Answer 17

• May be preceded by small tremors known as foreshocks and followed by aftershocks, particularly damaging as damage buildings damaged by initial shock
• Seismic waves travel along surface of the earth and through the body, moving energy without moving material.
• The first waves are Primary and Secondary waves. Primary waves travel through compression and expansion, passing through solids, liquids and gases. S waves travel side to side, passing only through solids - take longer, have longer wavelength and cause more destruction than P

-When a free surface is present, they combine to form complex surface waves - Love and Rayleigh waves.
Love waves cause the earth to move at right angles to the direction of movement whereas Rayleigh waves are the slowest, causing it to move in a rolling motion causing large amounts of damage in an earthquake as they produce horizontal and vertical ground movement, causing huge building damage to foundations

Question 18

How does liquefaction occur?

Answer 18

• Unconsolidated sediment under the surface shakes in a less predictable way, so the damage is far greater to foundations.
• P waves can turn solid sediment into fluids like quicksand by disruption sub-surface water conditions as unconsolidated sediment falls and water is forced up above ground, causing the water to mix with the sediment and form liquid like surfaces.
• The soil loses its cohesive strength and becomes weak and liquid like.
• It flows like a liquid causing structures to tilt and shake, leading them to fall and be unstable and structure failure and instability.
• This leads to buckling - piles are embedded into the group due to loss of soil support, spreading as soil moves down, loss of foundation support between liquefied and non liquefied soil, bridges lose support.

Question 19

What are the resultant hazards of earthquakes?

Answer 19

• Damage to buildings, infrastructure and transport systems
• Collapsing stuff causes injuries and deaths as well as effect of being able to help
• More damage caused by aftershock as they shake already weak structures
• Surface displacement occurs along fault lines which fracture gas pipes and cause damage to communication lines

Question 20

What are tsunamis?

Answer 20

• 90% of Tsunamis occur in the Pacific as they are associated with subduction zones
• Sudden uplift from the sea floor occurs during subduction
• Underwater earthquakes can snap the sea floor at a fault line, lifting a column of water above it. As gravity pulls the water down, waves fan outwards
• Travel 600-900km/h
• Tectonic up thrust causes vast quantities of water to be displaced by coastal uplift, creating large energy waves and a train of oscillatory waves over the ocean surface.
• Wavelengths continue to grow with the wave’s periods. As it approaches the coastline, they are slowed dramatically by friction, causing the amplitude to increase to large levels of damage
• This may also occur due to underwater volcanos and landslides, particularly along destructive plate margins where two plates move towards each other causing pressure
• Volcanic eruption - Tonga - huge displacement of water from eruption
• Landslides into water - La palma, Vajont dam or landslides underwater - Boxing Day

Question 21

How are earthquakes classified?

Answer 21

The Richter Scale looks at the amount of energy released on a logarithmic scale, so an earthquake of 5.0 is 10x stronger than 4.0. It uses the momentum magnitude scale measuring the amount of energy released and produces figures from the richeter scale

The modified mercali scale relates ground movement to observations of damage done in the environment and around people. This may be advantageous as it allows ordinary eyewitnesses to provide information on the earthquake?

Question 22

What factors affect earthquake damage?

Answer 22

• Strength and depth of earthquakes
• Number of aftershocks increases damage
• Population density - type of buildings - HICs tend to have better quality buildings
• time of day
• Distance from centre
• Type of rock and sediments - unconsolidated material damaged
• Secondary hazards such as mudslides, disease, tsunami, fires, hunger
• Level of economic development influences preparedness, technology and ability to recover

Question 23

What can be measured to predict earthquakes?

Answer 23

• Ground surface changes
• Uplift of subsidence
• Ground tilt
• Changes in rock stress
• Micro-earthquake activity
• Anomalies in magnetic fields
• Radon gas concentration
• Strain meters
• Laser measures
• Magnetometers

Question 24

What are the methods of predicting earthquakes?

Answer 24

-Satelitte surveying - topography, movement, shape, emf uses motion detectors - INSAR - stored digitally, mapped and analysed for risk mapping, accurate, large time interval, relies of land deformation, often not sensitive enough

Laser reflection - movement in earth and elevation, accurate, tiny variations may not be enough however

Seismometer - records ground motions with detectors, minor shocks precede a larger earthquake, graphed

Gravity meter - measures changes in gravity in rocks as stress increases in build up, rocks stressed

Radon gas - sensor as radon gas product of geothermal heat generated underground and released through cracks and breaks in the rock. Cracks allow radon gas to escape to surface, indicating earthquakes - unreliable, hard to position and may be due to other factors

Strain meter - measures stretching and compression of crust using silicon oil. Makes stresses visible, reliable gives evac time, hard to place, inaccessible once placed

Water table level - drops prior to earthquakes as when rocks crack create new spaces in crust for groundwater to flow. Easy to monitor but may occur for other reasons.

Question 25

What are the methods of earthquake proofing

Answer 25

• Flexible foundations
• Suspension points
• Vibrational control devices convert energy into heat
• Pendulum power (ball)
• Concrete and plastic rings divert energy
• Reinforced structures with steel and concrete
• Cross brazing
• Horizontal frames
• Moment resistant frames
• Earthquake resistant materials
• Raised buildings to collapse into ‘cushions;

Question 26

What are other methods of earthquake management

Answer 26

• FEMA playbook for preparation for work and families to quickly maintain and establish business, increased knowledge of safety policies.
• Families encouraged to create plans in emergency with disaster kits.
• Worldwide awareness education days to practice drills and safety in event of earthquakes encouraging people to develop plans
• In Japan learn to use fire extinguishers and med kits - practice drills away from desks and buildings
• LIC safe zones from EQ resistant material for people to go to - concrete walls reinforced with steel
• Development of new building codes to be more resistant to earthquakes

Question 27

How are tsunamis managed?

Answer 27

• Impossible to precisely predict, only warn
• Areas where common sometimes are measured
• Coastal buoys
• Hazard mapping - know where it happens on sea floor
• Tidal gauges in region can establish whether a tsunami has formed and investigates the earthquake or cause
• Investigating earthquakes - seismograph and magnitude
• Early warning systems then used to warn areas at risk - takes a while
• LICs lack warning systems - in Indonesia the early warning system failed as it was vandalised
• Red cross tsunami prep - go as far and high as possible, have emergency preparedness plan and list, evacuation plans - be informed of risk and response plans - has emergency app to warn
• Safe plus floating shelters build designed to carry occupants with the tsunami - cost effective strapped in with harnesses
• In the US Pacific Tsunami Warning Centre has 15 staff working 24/7 to monitor tsunamis for the Indian, South China and Caribbean sea to send out watches, advice and warnings.

Question 28

What are volcanic hazards?

Answer 28

• Depends on type of lava it contains
• Runny basaltic lava is hot and produces shield volcanos
• Hot andesitic and rhyolitic lava forms strato volcanos associated with destructive plate boundaries.

-Volcanos can cause explosive eruptions with volcanic gases, lava flows, lahars, mudflows, landslides, pyroclastic flow, ash fallout, lava bombs

Question 29

What are the types of eruption?

Answer 29

Icelandic - basaltic build up

Hawaiian - central runny basaltic lava down sides - gases and pyroclastic flow

Strombolian - frequent gas eruptions blast out of plugs of sticky cooled lava which build up into cones of ash or pumice. There is viscous lava which solidifies fast after an explosion, clearing a blocked vent and spews volcanic ash

Vesuvian - powerful blasts of gas pushing ash clouds high into the sky

Plinian eruptions - gas rushes up through sticky lava, ash and fragments into big explosion - violent and immense gas clouds and debris

Question 30

What is the volcanic explosivity index?

Answer 30

Relative measure of explosiveness, based on amount of material ejected, height of the cloud and damaged caused

Question 31

What are the 4 types of lava flow?

Answer 31

Lava flow - from a vent, fountains of lava jets and lava domes - piles of highly viscous lava. They are on the surface, slow and more viscous flows clump up near vents without a large spatial impact.

Aa flow - few metres thick, brittle crust, lower part cool and flows, crust torn into irregularly shaped cinder blocks

Pahoehoe - Basaltic, basic lava which cools slowly and has a rope appearance. Very slow wit cool surface and flows under the surface. Surface is cool and smooth and glossy.

Pillow flow - basaltic or basic, cool fast, encounter sea water and has glassy edge. Inside flows as it cools, blocky lava is cool and stiff and doesn’t travel far from the vent of the volcano

Question 32

What are the 2 types of lava?

Answer 32

Basaltic - least viscous, hot at 1200 degrees, low silica content, high magnesium and iron content, gas release easily and creates non explosive, effusive eruptions

Acidic - rhyolitic - most viscous, low temperature, high silica, gases trapped inside, cools into granite, light in colour, thick, hot gas ejection - highly explosive

Question 33

What are the ways to reduce lava flows?

Answer 33

• Water spraying
• Bombing
• Seeding lava with foreign nuclei
• Diversion
• Afforestation

Question 34

What are the primary volcanic hazard?

Answer 34

-Pahoehoe lava flow - first to erupt, thin and fluid, low viscosity, downslope in smooth rolling motion. Overriding lavas and breakout on the flow top and sides produce compound flows. Some look like ropes and become more viscous, creating plastic substance. They are slow, not dangerous however destructive and destroy buildings and release volcanic gases

Aa flow - more viscous, thicker and cools quickyl with brittle curst on top. Flow underneath and crust torn, creating mass of debris which tumbles forward. Jagged cinder blocks break off and overriden by the massive lava core of flow which pushes through

Question 35

What are the hazards of volcanos?

Answer 35

Direct: pyroclastic flow, volcanic bomb, lava flow, ash fallout, volcanic gas, EQs

Indirect: atmospheric ash fallout, landslides, tsunami, lahars, acid rain

Human: settlements, loss of life, loss of farmland, destroyed infrastructure and comms

Question 36

How do the volcanic hazards form in the air

Answer 36

• Ash and debris fall from cloud, blanketing ground with pyroclastic deposit which can be dangerous and damage lungs and cover buildings, killing livestock.
• Can cause buildings to collapse and havoc with global climate. Pyroclastic material can also be known as tephra
• Some of it forms when the vent is shattered or when lava solidifies in the air.
• All ejections of pyroclastic material are dangerous as usually hot.

Volcanic bombs - lava form in air and fall to the ground, hot and explosive however some of the lighter particles may rise into the atmoshpere and damage aircraft, such as at Eyjafjallajokull

Question 37

How do volcanic hazards form on the surface?

Answer 37

When lava domes collapse, hot and dry fragments move rapidly away from the vent down slopes and valleys by gravity, forming pyroclastic flows which cause over half the deaths.
Dense mass of material builds up, gathering mud and rock fragments which destroy everything - fast moving up to 100km/h and hot - 700 degrees

Lahars are mudflows which area combination of heavy rain and unstable ash, causing mudslides down the volcano. People are at risk of debris, ash falls and poisonous gases close to the volcano, and further are at danger to pyroclastic flow, mudflow and debris.

When water mixes with loose pyroclastic material, wet concrete flows may flow down the volcano - dense and viscous but fast containing volcanic material

Volcanic landslides may occur when masses of rock and soil move downslope under gravity which form lahars. Magma forces it s way to the surface and pushes outwards, causing the rock to break or over steepen and then collapse by gravity. These may also trigger volcanic explosions as they remove the surface allowing gas and magma to escape

Question 38

How do hazards differ between boundaries?

Answer 38

-Constructive and hotspots - shield volcanos, erupt on ocean floor, basaltic, fire fountains and ash flows - flooding and cloud disruption

Destructive - strato volcanos, dome volcanos rise through silica rich continents so rhyolitic, andesitic so explosive, pyroclastic flow, laharas and ash affect wide areas - may also flood

Question 39

What actions may be taken during eruptions?

Answer 39

• Cold water hosed to cool and reduce lava flow length
• Lava flow bombed and diverted
• Aircraft measure amount of gas
• Tiltmeters detect when volcano swells
• Boreholes measure water tempeartures
• Hot springs monitored
• Seismometers in volcanos
• Lahar channels to divert mudflows
• Evacuation routes

Question 40

Why may people live near volcanic hazards?

Answer 40

• Soils rich deep and fertile
• Not been predicted
• Intense agriculture
• Tourism and money
• Culturally symbolic
• Unaware of dangers
• Low perception of risk
• Acceptance of the hazard - people reluctant to eave jobs
• Sulphur mined there
• Geothermal heat and energy