Hazardous Earth Flashcards
What is the structure of the earth?
The earth is made up of an inner core, outer core, mantle and crust
What is the crust like?
-0-100km long
-Consists of the oceanic(more dense) and continental(less dense) plate
-oceanic thickness is 5-10km
-continental thickness is 30-70km
What is the mantle like?
-100-2900km long
-The upper mantle contains the lithosphere and belown it the asthenosphere(100-300km)
-Lithosphere (semi-core) The top part of the mantle
Rigid and stuck to the underside of the crust Varies in thickness Boundary with asthenosphere difficult to define (melts and becomes incorporated in asthenosphere)
-Asthenosphere extends 100-300km Semi-molten/viscous - Allows rock to move due to high pressure in mantle. Flowing slowly
-It also has a lower mantle
What is the core like?
-Consists of a liquid outer core and solid inner core
-Liquid outer core is 2883km to 5140km thick
-Solid inner core is 5140km to 6371km thick
Where do convection currents exist and What do these do?
They occur in the Asthenosphere and are caused by vast amounts of heat generated in mantle.They pull on the underside of the lithosphere causing it to move, resulting in plate movement
Explain the movement of the crust caused by convection currents in the mantle:
-Hot rock rises from lower to upper asthenosphere.
-Hot rock spreads and cools, pushing plates apart.
-Cool rock sinks back down towards core.
-As (oceanic) plate subducts at ocean trenches, gravity pulls it under
-Rising mantle pushes crust upwards at mid-ocean trenches, while gravity pulls it back down
Who is Alfred Wegner?
Alfred Wegener (1880-1930), a German meteorologist and geologist, was the first person to propose the theory of continental drift
What was Wegners idea of continental drift?
He proposed that in the carboniferous period, 250 million years ago, a large single continent pangea existed. This slowly broke apart into 2 large land masses. This movement slowly continued to the present day as the continents separated and spread across the globe.
What is the geological evidence for continental drift?
-The fit of continents such as South America and Africa on either side of the Atlantic
-Evidence from about 290million years ago of the effects of contemporaneous glaciation in southern Africa , Australia, South America, India and Antarctica, suggesting that these land masses were joined at this time, located close to the south pole
-Mountain chains and some rock sequences on either side of Oceans show great similarity, e.g. northeast Canada and northern Scotland
What is the biological evidence for continental drift?
-Similar fossil brachiopods (marine shellfish) found in Australian and Indian limestones
-Similar fossil animals found in South America and Australia, especially marsupials
-Fossils from rocks younger than the Carboniferous period, in places such as Australia and India, showing fewer similarities , suggesting that they followed different evolutionary paths
Who is Marie Tharp?
Marie Tharp is a celebrated cartographer who contributed to a revolution in geological thinking - mapping the seafloor revealing the existence of mid-ocean ridges around the globe.
What 4 things can be used as evidence of the sea floor spreading?
-Paleo magnetism
-Ruggedness (mountains)
-Age
-Volcano and earthquake distribution
How can paleomagnetism be used as evidence of the sea floor spreading?
-During the history of the Earth the direction of magnetic north has frequently changed/reversed.
Thus, sometimes rocks are magnetised to the North and sometimes to the South
-Igneous rocks, form part of the oceanic crust and ocean floor. These rocks contain iron particles and as lava erupts, it cools and the magnetic orientation of the iron is locked into the rock, depending on the Earth’s polarity at the time. Earth’s polarity is not constant, it changes every 400,000-500,000 years and this is recorded in the rocks on the ocean floor
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How can age be used as evidence for the sea floor spreading?
-During the 1960s, an ocean-drilling programme was established to investigate ocean sediments and crustal rocks in the deep ocean.
-Drilling recovered cores in water up to 7,000m deep and revealed a spatial pattern of sediments. This supported sea-floor spreading theory.
-Thick and old sediments were found near continents.
-Nowhere in the oceans was rock older than 200 million years – confirming that ocean crust was constantly recycled over this period
What is sea floor spreading?
Lateral movement of new oceanic crust away from a mid-ocean ridge (constructive plate boundary)
How does sea floor spreading provide evidence for continental drift?
-Sea-floor spreading moves material across ocean floors.
-Newest rock is found in the middle (at mid-ocean ridge), therefore new rock is being created here - these palaeomagnetic stripes show that each one is mirrored either side of the mid-ocean ridge, showing that new crust is being created here before it moves apart
-Rocks are not the same age as the earth (4.5 billion years) meaning oldest rock is being destroyed and replaced by newer rocks - also shows new crust is being created
-Indicates that, as fresh molten rock from asthenosphere reaches ocean bed, ‘older’ rock is pushed away from the mid-ocean ridge.
-Oldest rock would be located furthest from the point it is created (nearer land).
-Eventually, the sea floor reaches an ocean trench where material is subjected into the asthenosphere and becomes semi-molten (caused by convection currents).
How does the global patterns of plate boundaries provide evidence for continental drift?
-Earthquakes are concentrated in narrow bands at plate boundaries (rigid lithosphere and crust broken up into tectonic plates that are moving) - there is an obvious pattern that most volcanic eruptions and earthquakes occur around the edges of plates, e.g. the Pacific Ring of Fire, where the most movement occurs (evidence for continental drift)
-Although there are some in the centre of plates, there is an obvious pattern
How do subduction zones provide evidence for continental drift?
-Where plates collide, fold mountains and ocean trenches prove that the plates are moving towards each other
-The Benioff zone in ocean trenches, where oceanic crust is being subducted, experiences earthquakes occurring below sea level - this proves that plates are subducted
-Volcanic activity also occurs on these boundaries due to the melting of the subducted plates - volcanic activity proves plate movement
How do hot spots provide evidence for continental drift?
-Hot spots are mantle plumes - a convection current must be set up as lava rises. If there is a heat differential, a convection current will be set up
-Hot spots are able to record the movement of plates over a mantle plume, e.g the Hawaii hotspot movement
-There is little evidence for this, as convection currents can’t be modelled in the mantle, only on a smaller scale in labs.
-For this theory to be correct, we are assuming convection currents in the mantle will act the same way as on a small scale - this makes the evidence unconvincing due to the lack of support
How do plates floating on the mantle provide evidence for continental drift?
-Seismic profiling allows you to identify layers beneath the surface. The wave will be reflected if there is a sudden change in rock type, indicating that there are different layers. However, this provides little evidence as to how many layers are beneath the surface, and doesn’t prove the mantle is a liquid layer.
-Ophiolite sequences are a full section of the ocean crust, found in crumple zones in fold mountains. However, these sequences contain variably altered oceanic rocks, with only a small part of the mantle to examine, so these are often unconvincing.
-Shield volcanoes reveal what is under the crust
How is evidence from ancient glaciations used to support the theory of continental drift?
-Glacial striations and till deposits, found in now-tropical regions such as South America, Africa, India, Australia, and Antarctica, indicate that these areas were once covered by ice
-During the Late Carboniferous to Early Permian a massive ice sheet spread across the supercontinent Gondwana, aligning perfectly when these landmasses are reconstructed in their past positions
- This provides strong geological evidence that these continents were once connected and located near the South Pole
-The presence of glacial deposits in warm climates, where glaciers cannot form today, highlights a clear paleoclimatic mismatch, which can only be explained if the continents have moved over time
How is evidence from fossil records used to support the theory of continental drift?
- Identical fossils of extinct plants and animals, such as the Mesosaurus (a freshwater reptile) and Glossopteris (a seed fern), have been found on continents that are now separated by vast oceans, including South America, Africa, India, Antarctica, and Australia
-Since these species could not have crossed such large bodies of water, their distribution suggests that these continents were once joined together as part of the supercontinent Pangaea
-The presence of similar fossil species across widely dispersed landmasses provides strong biogeographical evidence that these regions were once connected before drifting apart
What is the global pattern for plate boundaries?
Earthquakes are concentrated in narrow bands at plate boundaries (rigid lithosphere and crust broken up into tectonic plates that are moving)
What are the global pattern of plates and plate boundaries?
The Earth’s lithosphere is divided into seven major plates, including the Eurasian, North American, South American, African, Indo-Australian, Pacific, and Antarctic Plates, along with several smaller plates such as the Nazca and Philippine Plates. These plates interact at four main types of boundaries: constructive (divergent), where plates move apart, forming new crust at mid-ocean ridges like the Mid-Atlantic Ridge; destructive (convergent), where one plate subducts beneath another, creating deep-sea trenches (e.g., Peru-Chile Trench) and volcanic arcs (e.g., Andes Mountains); conservative (transform), where plates slide past each other, leading to earthquakes, as seen at the San Andreas Fault; and collision zones, where two continental plates converge, forming mountain ranges like the Himalayas. These interactions drive the global distribution of earthquakes and volcanoes, with the most tectonically active regions forming the Pacific Ring of Fire, where numerous earthquakes and volcanic eruptions occur due to intense plate activity.
What are the 3 types of plate boundaries?
-Divergent (constructive)
-Convergent (destructive)
-Transform (conservative)
What are the features and processes of a divergent plate boundary?
-Plates move apart, magma from the mantle fills the gap forming new oceanic crust
-E.g mid Atlantic ridge
-Landforms: Rift valleys, mid ocean ridges and shield volcanoes
-Hazards: Volcanic activity, minor earthquakes
What are the features and processes of a convergent plate boundary?
- (Oceanic-continental) Denser oceanic plate sub-ducts under continental plate, forming trenches and volcanic mountain ranges e.g. Andes mountains
-(Oceanic-oceanic) One oceanic plate subducts under the other, creating deep ocean trenches and volcanic island arcs e.g. Mariana Trench
-(Continental-continental) Collision with no subduction due to similar densities, forming high mountain ranges e.g. Himalayas
-Hazards: Earthquakes, volcanic eruptions, tsunamis
What are the features and processes of a conservative plate boundary?
-At transform boundaries, plates slide past each other horizontally
-Crust is neither created nor destroyed, but the friction between plates often leads to earthquakes. E.g San Andreas fault in California where the pacific and North American plates slide past each other, producing frequent earthquakes and the Alpine Fault in New Zealand where the pacific and ind-Australian plates slide past each other.
What are explosive eruptions?
-Found at convergent plate boundaries, where magma is viscous and rich in silicia, leading to violent explosions
-Volcano types include strato-volcanoes (composite Cone volcanoes) with layers of ash and lava, which can build up intense pressure
-Calderas are volcanic craters usually more than 2km in diameter and from when an explosive eruption causes the volcano to collapse, leaving a large crater
-E.g. The 1883 eruption in Krakatoa in Indonesia left a caldera 7km wide
What are effusive eruptions?
-Occur at divergent plate boundaries where magma as low viscosity and low silicia content, allowing lava to flow more easily
-Shield volcanoes with gentle slopes form through effusive eruptions as seen in Iceland
-Lava Plateux form when large volumes of low viscosity lava flow over extensive areas, such as the Deccan plateau in India which covers more than 500,000km squared known as the large igneous province while the Colombia plateau in the northwest USA covers 130,000km squared
What are the volcanic hazards associated with effusive/explosive eruptions?
-Lava flow
-Pyroclastic flows
-Ash flows
-Lahars
-Volcanic gases
-Volcanic bombs and tephra
What are lava flows?
-Lava flows are streams of molten rock that emerge from a volcanic vent
-They can be slow moving, giving people time to evacuate, but they cause extensive destruction=tin by incinerating buildings, roads and vegetation
-Basaltic lava is fluid and spreads far while andestic and rhyolitic lavas are more viscous and flow less readily
What are pyroclastic flows?
-These are fast moving , ground hugging clouds of hot gas, ash and volcanic debris that can reach over speeds of 100km/hr and temperature around 400-700 degrees Celsius
-They can destroy everything in their path and can travel several km from the eruption site
-Highly lethal de to their speed, temperature and density
What is ash fall?
-Volcanic ash consists of fine particles of volcanic rock and glass
-When ejected into the atmosphere, it can settle over wide areas, disrupting transportation, damaging machinery, contaminating water supplies and affecting health
-Ash fall can also lead to building collapse if it accumulates heavily on rooftops and disrupt agriculture by burying crops
What are Lahars?
-Lahars are volcanic mudflows that occur when volcanic debris mixes with water from rainfall, melted snow or ice
-These flows move rapidly down river valleys, carrying boulders, ash and mud
-Lahars can be highly destructive, burying entire communities and they can persist for years after an eruption due to repeated rainfalls
What are volcanic gases?
-Volcanoes release gases such as sulfur dioxide, carbon dioxide, hydrogen sulfide and water vapour
-Sulfur dioxide can lead to acid rain and respiratory issues, while CO2 in high concentrations can cause suffocation, especially in low-lying areas
-These gases can also impact the climate, as sulfur dioxide can reflect sunlight, causing temporary cooling
What are volcanic bombs and tephra?
-Volcanic bombs are large fragments of lava that cool into solid rock before hitting the ground, while tephra includes all airborne volcanic material
-These materials can cause injuries or fatalities on impact and damage structures within the blast zone
Why do some eruptions not occur near plate boundaries?
Not all volcanic eruptions occur at plate boundaries; some take place at hot spots, where magma rises from deep within the mantle. Hot spots are areas of anomalous volcanic activity caused by mantle plumes, where hot, buoyant material from the Earth’s interior melts through the lithosphere. A key example is the Hawaiian Island chain, formed as the Pacific Plate moves over a stationary hot spot, creating a series of volcanoes. As the plate moves, older volcanoes become extinct, while new ones form, producing a linear island chain. Another example is the East African Rift Valley, where continental rifting causes crustal thinning and magma upwelling, leading to volcanic activity in areas like Mount Kilimanjaro and Nyiragongo. Unlike traditional plate boundary volcanism, hot spot eruptions can occur in the middle of tectonic plates and are not directly linked to plate interactions
Why are volcanoes different shapes and sizes?
Shield volcanoes, such as Mauna Loa in Hawaii, are broad with gentle slopes, formed by low-viscosity basaltic lava that flows over long distances. In contrast, composite (stratovolcanoes), like Mount Fuji and Mount St. Helens, are steep-sided and cone-shaped, built from alternating layers of lava and ash due to explosive eruptions. Cinder cone volcanoes, such as Parícutin in Mexico, are small with steep slopes, created from pyroclastic material ejected during short-lived eruptions. Lava dome volcanoes, like Mount Pelée, have a rounded, dome-like shape formed by slow-moving, high-viscosity lava. The largest and most catastrophic type is the supervolcano, such as Yellowstone in the USA, which forms massive calderas rather than peaks and erupts with immense destructive power, altering global climates. These different volcanic forms highlight the link between magma properties, eruption dynamics, and tectonic processes.
What is the VEI index?
The Volcanic Explosivity Index (VEI) is a scale used to measure the explosiveness of volcanic eruptions, ranging from 0 to 8. It considers factors like the volume of erupted materials, eruption height, and duration. VEI 0-1 represents gentle eruptions with little ash, such as those from shield volcanoes like Hawaii, while VEI 6-7 and VEI 8 correspond to catastrophic eruptions, such as Krakatoa (1883) and Toba (74,000 years ago), which can cause global climate impacts. The VEI helps assess volcanic activity and predict potential hazards, making it a key tool in understanding volcanic risks.
What are earthquakes?
The release of stress that has built up within the Earth’s crust caused by tension, compression or the shearing of rocks. Release of energy causes seismic waves to be released from the focus.
Why do earthquakes happen?
-Pressure builds up within the crust.
-Pressure reaches threshold.
-Energy is released as an earthquake.
What are the parts of an earthquake?
-Focus point at which seismic waves originate from (point where stress is released).
-Epicentre directly above the focus on the surface (where most severe impacts will be).
-Seismic waves energy released travels as waves, in the form of S waves or P waves.
-Fault line where movement in plates occur.
Where do earthquakes happen?
-Mid-ocean ridge/rift valleys (constructive boundary) - tensional forces
-Ocean trenches (subduction margins)/island arcs (destructive boundary) - compressive forces
-Collision zones (collision boundary) = compressive forces
-Conservative margins - shearing force
-Away from plate margins
what are the features and characteristics of shallow earthquakes?
-0-70km deep
-Occur in cold, brittle rock resulting from fracturing of rocks due to stress within the crust
-Very common
-Releasing low levels of energy
-High-energy, shallow quakes are capable of causing severe damage
-Shallow earthquakes cause the most damage.
what are the features and characteristics of deep earthquakes?
-70-700km deep
-Increasing depth = extreme temps/pressures
-Rarely cracks rock - strong but does less damage
Features of primary(p) waves?
-Fast-travelling
-Low-frequency
-Compressional waves
-Vibrate in the direction they’re travelling
-Travel through Earth’s interior (through solids and liquids)
Features of secondary(s) waves?
-Half the speed of P waves
-High-frequency
-Vibrate at right angles to the direction they’re travelling
-Cannot pass through liquids (so cannot travel through outer core)
-Cause more surface damage that P waves, hitting the area slightly delayed
Features of surface(L) waves?
-Slowest waves
-Low-frequency
-Rolling movements that move the surface vertically
-Travel through outer crust only
What is the modified Mercalli scale?
-Qualitative measure of damage experienced related to ground movement (impacts felt and seen by people).
-Attempts to measure both intensity and impact.
-Based on observation and description.
What are the advantages of the MMS?
- No specialist equipment or training required
- Quick and easy
- Used to quickly identify areas for assistance
What are the disadvantages of the MMS?
- Subjective
- Score would be much greater in a city compared to the countryside for the same earthquake
- Inaccurate
What is the Richter scale?
Measure of energy released on a logarithmic scale (each whole number represents a 10-fold increase in amplitude).
Largest record quakes are magnitude of around 9.
Damage is determined by magnitude but also by population density and level of preparedness
What are the advantages of the Richter scale?
- Logarithmic
- Objective
- Based on ground movement, so is more accurate and scientific
What are the disadvantages of the Richter scale?
- Only accurate for magnitudes between 3 and 7
- Can only calculate it with a seismogram and a known distance from the epicentre
What is the moment magnitude scale?
-Measures energy released more accurately than Richter.
-Measures amount of movement of the ground - a direct function of energy.
-Accurate for large earthquakes but not useful for smaller ones.
-Measures direct energy that causes ground movement so can be used to predict damage.
What is an escarpment?
An escarpment is a steep slope or cliff that forms as a result of faulting (the breaking and displacement of Earth’s crust). Earthquakes often occur along tectonic faults, and the vertical movement along these faults can lead to the formation of escarpments. Earthquakes can trigger both uplift (where one side of a fault is pushed upwards) and subsidence (where land sinks), contributing to the development of escarpments.
What is a rift valley?
A rift valley is a large, elongated depression or valley that forms as tectonic plates move apart due to extensional forces. The Earth’s crust stretches, and the land in between subsides or sinks, often as a result of earthquake activity along normal faults.
What happens during ground shaking and displacement?
Vertical and horizontal movement of ground.
Severity depends on:
- Earthquake magnitude
- Distance from epicentre
- Local geology
Swaying of buildings impacts their stability
Displacement of rocks along fault lines can:
- Rip apart pipelines and sewers
- Sever rigid structures (railway tracks and roads)
- Cause building collapse
- Disrupt drainage systems - can have serious implications for public water supply and irrigation for agriculture.
What are the hazards generated by earthquakes?
-ground shaking and ground displacement
-liquefaction
-landslides and avalanches
-tsunamis associated with sea-bed uplift and underwater landslides
-flooding.
What happens during liquefaction?
-Vibrations of quake cause surface materials with high water contents (fine-grained sands, alluvium, landfill) to behave like liquids.
-Materials lose strength; slopes (river banks) collapse and structures tilt and sink as foundations give way.
What happens during landslides and avalanches?
-Shaking loosens slopes on mountains causing slope failure.
- Blocks transport routes where accessibility is already difficult. - hinders rescue efforts
- Block rivers - natural dams create temporary lakes which lead to flooding when dam fails.
-The huge amount go damage caused by landslides in the example, in terms of both monetary value and the destruction of homes means that it poses a greater risk to humans than ground movement does on its own.
-Avalanches are one of the smallest hazards, with only a very small number of deaths in the example.
What happens during tsunamis?
-Underwater earthquakes
Cause sea bed uplift - displaces water above, producing powerful waves at surface which spread outwards from epicentre.
Wave height increases as they approach shoreline and shallower water.
-When a large volume of rock is displaced underwater it slumps down, displacing water, creating a wave which radiates outwards
What happens during flooding?
Has there been changes in the Frequency of Tectonic Hazards Over Time?
-Tectonic hazards (earthquakes, volcanic eruptions, and tsunamis) occur due to natural plate movements, so their frequency has remained relatively constant over geological time.
-However, improved monitoring and reporting (e.g., via seismic sensors, satellite imagery) mean more hazards are now recorded.
Has there been changes in the impacts of Tectonic Hazards Over Time?
-Increased Exposure: Rising global population and urbanisation mean more people live in hazard-prone regions (e.g., Tokyo, Jakarta).
-Development Levels: LICs and NEEs often suffer more due to weak infrastructure, poor emergency response, and high vulnerability (e.g., Haiti 2010 vs. Japan 2011).
-Climate Change: Indirectly worsens secondary hazards (e.g., melting glaciers increasing lahar risks from volcanic eruptions).
-Globalisation: Disruptions from tectonic events now have wider economic impacts due to interconnected global supply chains (e.g., Japan’s 2011 tsunami disrupted car and electronics industries worldwide).
What is the disaster risk equation?
Risk= Hazard×Vulnerability÷CapacitytoCope
-Hazard: The magnitude, frequency, and type of tectonic event.
-Vulnerability: Factors such as population density, infrastructure quality, governance, and economic status.
-Capacity to Cope: The ability of a community/country to respond (e.g., emergency services, early warning systems).
What are the possible Future Strategies to Cope with Risks from Tectonic Hazards
-Adaptation Strategies (reducing vulnerability):
Improved land-use planning to prevent building in high-risk zones.
Investment in infrastructure (e.g., aseismic buildings, tsunami sea walls).
Education and community preparedness (e.g., earthquake drills in Japan and California).
-Mitigation Strategies (reducing hazard impact):
Early warning systems (e.g., ShakeAlert in the US).
Disaster insurance to reduce economic burden.
International aid and disaster response teams for LICs.
-Prediction and Monitoring:
Seismic activity monitoring (e.g., GPS, radon gas emissions).
Tsunami warning systems using deep-sea sensors (e.g., Pacific Tsunami Warning Center).
What is the relationship between Disaster and Response (The Park Model)?
-Pre-Disaster (Preparation & Risk Reduction):
Includes hazard mapping, evacuation plans, and preparedness drills.
-Event (Hazard Occurs):
The initial shock phase where primary and secondary impacts happen.
-Relief (Hours to Days After the Event):
Emergency response, search and rescue, humanitarian aid.
HICs recover faster (e.g., Christchurch 2011 vs. Haiti 2010).
-Rehabilitation (Weeks to Months After the Event):
Restoring basic services, temporary shelters, and rebuilding efforts.
-Reconstruction (Months to Years After the Event):
Long-term rebuilding with improved hazard-resistant infrastructure.
Some areas may undergo long-term decline (e.g., Haiti still struggling after 2010 earthquake).
