Plate Tectonics Flashcards
Francis bacon
Noticed fit of s America and w Africa 1620
Batholiths
Form deep within the surface when large masses of magma cool and solidify
Metamorphic areole
The area around the batholiths is altered by heat and pressure of the intrusion
Batholiths
Form deep within the surface when large masses of magma cool and solidify > dome of igneous rock
Dykes
Vertical intrusions with horizontal cooling cracks that cut across bedding planes
Sills
Horizontal intrusions along the lines of bedding planes with vertical cooling cracks
Laccoliths
Smaller injections of magma from a lens shape that is intruded between layers of rock - forces the overlying strata to arch upwards forming a dome
Basaltic lava
Constructive boundaries
Low silica and viscosity
Not violent eruptions - frequent long and escapes easily
Andesite lava
Formed at destructive margins
Medium silica and viscosity
Violent eruptions - pressure builds up due to blockages (solidification) forming in vents - intermittent and short.
Rhyolitic lava
Formed at destructive margins
High viscosity and silica content
Violent eruptions - pressure builds up due to blockages (solidification) forming in vents - intermittent and short.
Solfataras
Small volcanic areas without cones, produced by gases escaping to the surface
Caldera
Destructive margin
Build up of gases becomes extreme and a huge explosion removes the summit Which may be able to flood
Central part of volcano collapsed as magma chamber emptied
Layers of lava, ash and cinder - circular crater (kms across)
Geysers
Occur when water, heated by volcanic activity, explodes into the surface
Hot springs where water and steam erupts
Form in areas of intense volcanic activity
Groundwater heated above bpt by magma
Becomes pressurised and forces to surface through cracks which spray out from vents
Erupt periodically - pressure has built up sufficiently
Primary waves
Travel fastest and are compressional - vibrating in the direction they are travelling
Hot springs
Groundwater emerges at the surface 20-over 90 degrees
Close to an area of recent intrusive activity - water heated
High mineral content (doesn’t explode)
Secondary waves
Travel at half the speed of P waves and sheer rock by vibrating at right angles to the direction of travel
Surface waves
Travel slowest and near to the ground surface. Some surface waves shake the ground at right angles and some have a rolling motion - vertical ground movement
Evidence for theory of plate tectonics
- Fit of S America and Africa - jigsaw - continental shelves, drop sea level by 100m in some areas
- Fossils of Triassic reptiles found on separate continents with no land bridge eg Brachiopods India and Australia
- Earthworms species NZ, Asia and N America
- Similar rock geology - laid under same conditions in one location eg Appalachain mountain chain N America similar N Scotland
- Coal found in Antarctica - no tropical climate/dense vegetation that is needed to form
- Similar glacial deposits Antarctica, Africa, S America, India and Australia > fit
- 1940s-1960s > extensive mapping of ocean floor > mid-ocean rich > Ewing more recent than imagined, volcanic, 1000 miles wide, 2500m high, ocean trenches proved; Harry Hess 1960s - centre of ridge newest - sea floor spreading 5cm/yr; Hugo Benioff - subduction
- 1962 Fred Vine - direction of magnetism of rocks changed at regular intervals > paleomagentism = piles reversed. Recorded in rock due to basaltic lava cooling and iron crystals forming
Continental crust
Thicker (30-70km)
Older
Less dense
Oceanic crust
Thinner (6-10km)
Newer > constantly renewed
Denser
Crust components
Si, O, Al, K, Na
Lithosphere
Rigid part of the mantle (silicate rocks) that includes the crust
Athenosphere
Semi-molten part of the mantle (silicate rocks)
Outer core
Ni, Fe
Semi-molten
Inner core
Ni, Fe
Over 5000 degrees
Solid
How do convection currents in the mantle occur?
Radioactive decay generates heat
Lower parts of asthenosphere heat up > less dense so rise
Move to top > cool, dense and sink
Circular movement = convection current
Creates drag on the base of the tectonic plates, causing them to move
Constructive margins
Diverging
Mantle is under pressure from plates above - pressure released when plates move apart
Causes mantle to melt > magma > less dense than plate so rises > volcano
Plats don’t move uniformly, causing build up of pressure > cracks = fault lines > earthquakes
Mid-ocean ridges
Rift valleys
How are mid-ocean ridges formed?
Oceanic-oceanic constructive margin
Under water volcanoes erupt, cool and new crust is formed which can build up to above sea level eg Iceland
How are rift valleys formed?
Continental-continental constructive margin
Rising magma under land causes continental crust to bulge and fracture > fault lines
Crust between parallel faults drop eg East African Rift System
Volcanoes also found eg Mt Kilimanjaro
Destructive margins
Converging
Oceanic-Continental > deep sea trench, fold mountains as well as volcanoes (oceanic crust heated by friction and contract with upper mantle > magma rises) and earthquakes (plates get stuck > build up of pressure and jerk past each other)
Continental-Continental > deep sea trench, island arcs as well as earthquakes and volcanic eruptions
How are deep sea trenches formed?
Destructive continental- oceanic More dense oceanic crust forced under continental = subducted Up to 10km Also at continental-continental More dense plate subducted
How are island arcs formed?
Clusters to volcanic islands that occur in a curved line eg Mariana Islands > very gradual > sea mounts form and break surface in a line parallel to plate boundary (50-100km away)
Collision margin
Destructive continental-continental
Neither subducted = no volcanoes
Pressure can build = earthquake
Fold mountains can form eg Himalayas
How are fold mountains formed?
Form where plates meet > made of sediments accumulated on continental crust > folded upwards along edge of continental crust
Conservative margin
Plates move past each other
Become locked together > pressure builds
Plates jerk = fault lines = earthquake
eg Pacific/N American plate San Andreas Plate
Hot spots
Volcanic activity away from plate margin
Magma plume - vertical column of magma that rises up from the mantle.
Volcanoes form above magma plumes.
Magma plume remains stationary, but crust moves above it
Volcanic activity that was above hot spot decreases as it moves away
New volcanoes form in new part of the crust
Chain of volcanoes forms eg Hawaii (proven by turn of islands = change in crust movement)
Primary impacts of vulcanicty
Tephra - solid material ejected into the atmosphere
Pyroclastic flows - very hot, gas charged, high velocity flows made up of gases and tephra
Lava flows
Volcanic gases
Secondary impacts of vulcanicty
Lahars - volcanic mud flows
Flooding - melting of glaciers and ice caps
Tsunamis
Volcanic landslides
Climatic change - volcanic debris can reduce global temps
Intrusive volcanic activity
Beneath earth's surface High pressure = semi-molten mantle Pressure releases = molten magma Less dense than rock so rises up Forced to surface through cracks and solidifies
Boiling mud
If hot springs mix with surface deposits
Very fine grained soil
Brightly coloured - minerals
Acid/dome volcano
Destructive margin
Steep sides due to high viscosity of lava - short distances and convex
Composite cone = andesitic lava and layers of ash and lava
Shield/basic volcano
Constructive margin/hotspots
Gentle slope - low viscosity of lava - flows long distances
Fissure volcano
Constructive margin
Fairly flat surface due to low viscosity
Layers of lava and long linear vent
Lava plateaux
Volcanic and seismic monitoring
Gas emissions Ground deformation Thermal monitoring Satellite images Remote sensing Mass movements and failures (landslides) Seismometers > seismographs
Volcanoes case study 1: Mount Etna > Etna
Catania, Sicily, Italy
MEDC > GDP = $35926 per capita
3323m
Constructed over an old shield volcano > stratovolcano
Truncated by several small calderas eg Valle del Bove 5-10km horse-shoe shaped caldera/depression open to earth (collapsed during eruption > landslide)
Persistent eruptions with minor lava emissions from one of 3 prominent summit craters
Not regarded as particularly dangerous - fertile volcanic soils
Volcanoes case study 1: Mount Etna > Nature, impacts and management
End of 1991, lava began to pour from vents high on eastern flank in Valle del Bove and advanced on settlement of Zafferana
- Large earth barrier 10m high and over 40m long which held back lava for several months
- Spring 1992 lava spilt over barrier towards Zafferana. Smaller barrier built and were overwhelmed by advancing lava, destroying orchards and a few small buildings
- Dropping concrete blocks by helicopter blocked the primary feeder channel - dropped through roof of upper lava tube
- May 1992 engineers blasted openings in the lava tube to encourage a new direction of flow. This stopped lava advancing on Zafferana and eruption ended 10 months later in 1993
Volcanoes case study 1: Mount Etna > Success of stopping flows due to…?
Low effusion rates during eruption
Would lava ever have reached Zafferana?
High elevation of eruptive vents - 2200-2350m - well away from inhabited areas. Would have had to flow 8km more to become a serious threat
Large possibility of diverting flow into uninhabited areas - at least 7km from nearest village
However it is not always possible to stop lava flows on Mount Etna eg 2002 a more serious eruption destroyed the ski station of Piano Provenzana and part of another station at Rifugio Sapienza, clouds of ash rained down on the area, particularly affecting the city of Cantania
Volcanoes case study 2: Mount Pinatubo > general
Philippines > generally LEDC > GDP = $2765 per capita
1991-1993
Destructive margin - oceanic crust moves towards and is subducted by continental Eurasian plate
Oceanic plate > magma > volcano
Mt. Pinatubo not erupted since 1380 - not considered a hazard by locals who work on fertile soils
Volcanoes case study 2: Mount Pinatubo > nature of hazard and management
- June 1991 first signs of eruption
- Thousands evacuated from Angeles and 15000 from Clark American air base
- 12 June explosion > steam and ash 30km up > 3rd largest eruption in the century
- 50cm ash fell nearby and over 10cm within 600km
- Followed by earthquakes and torrential rain (combined with ash > thick mud)
- Destroyed all crops and caused buildings to collapse > 200000 including local hospitals and factories
- Power supplies cut off for 3 weeks and water supply contaminated
- Roads and bridges destroyed
Volcanoes case study 2: Mount Pinatubo > impacts and management
Ash ruined 1991 harvest and 1992 planting was impossible
Over 1 million farm animals died - lack of grass
Farmers refuge in large cities > shelter and foods in shanty-town style refugee camps > disease spread rapidly
1993 typhoons - heavy rainfall and lahars (mud flows)
700 deaths - only 6 as a direct result of eruption - others from disease and suffocation in lahars
Some did not return top former homes - everything planted is destroyed
Others decided to return as mountains were still their home
Shanty refugee camps deemed safer than returning to an area where eruptions, earthquakes and lahars still occurred until the regrowth of vegetation stabilised the slopes after many years
Causes of seismicity
Built up tension at boundary > pressure released when plates jerk past each other, sending out seismic waves from the focus in the lithosphere/crust
Shallower focus causes greatest damage as epicentre is closer to focus
Human activity may cayse eg large reservoirs puts pressyre on surface rocks
Also reactivation of old fault lines eg 23/09/02 Dudley UK 4.8
Seismic waves
Travel through earth’s interior, radiate from focus line ripples on water
Magnitude of earthquakes
- Richter scale - amplitude of seismic waves, logarithmic, energy release proportional to magnitude
- Mercalli scale - intensity of event and impact, 12 point scale, 1=2 on Richter - detected but hardly felt, 12=8.5 on Richter - total destruction
Effects of earthquakes
Ground shaking (initial), soil liquefaction - shaken soils with high water content lose mechanical strength and behave as a liquid, landslides/avalanches, collapsing buildings, destruction of road systems and service provisions eg gas, fries from ruptured gas pipes etc, flooding, disease, food shortages, disruption to local economy
Factors affecting earthquake impacts
Earthquake magnitude and focus depth Location and boundary Bedrock Population density Buildings and structural vulnerability Level of development Earthquake preparedness
Tsunami causes
Giant sea waves generated by shallow-focus underwater earthquakes, volcanic eruptions, underwater debris slides and large landslides into the sea
Long wavelengths in open ocean with low wave heights > 700kmph
On reaching shallower water bordering land, they rapidly increase in height
Drawdown - wave trough infront causing a reduction in sea level
Can be over 25m high (number of waves)
Effects of tsunamis
Wash structures inland and backwash takes back out
Water and debris cause drowning and injury
Effects felt 500-600m inland
Most at convergent (subduction) boundaries, 90% within Pacific basin
Factors affecting tsunamis
Height of waves and distance travelled Length of event causing tsunami Extent to which warnings can be given Coastal physical geog - offshore and incoastal area Coastal land use and population density
Earthquakes case study 1: Haiti > general
12 Jan 2010 4:53pm
LEDC > GDP = $820 per capita
7 Richter scale
13km depth
Seismically active area > complex plate margins surrounding > 1 destructive and 2 conservative
Enriquillo-Plantain Garden Fault System locked for over 250 years
Earthquakes case study 1: Haiti > nature of hazard and impacts
Energy released along 65km of fault
Land-movement 1.8km
By Jan 24, 52 aftershocks of over 4.5 Richter scale
Epicentre 25km SW capital in Leogane
Killed 230000 (8.8 Chilean 1 month later killed only 800 due to clear disaster response plan)
Destroyed 60% capital Port-au-Prince - 70% buildings collapsed, 86% in slums, only 50% access to latrine, 2`% leftover buildings needed demolishing
4000 amputees, 1.5m homeless, 4000 prisoners escapes, $8bn damage and losses
Earthquakes case study 1: Haiti > Aid and effects
Dominican republic > water, food, heavy machinery for rescue, hospital use
Iceland emergency team in 24 hours
No emergency plan - lack of awareness of what to do
Lack of co-ordination > 100 UN personnel, many health workers and 1/4 civil servants killed
Priority initially given to security troops not aid
Money was spent on aid workers’ accommodation and transport
Few NGOs spoke French
Some projects not addressed
1500 camps in Port-as-Prince had only 1 delivery of water per week - untreated water drunk > October 2010 first case of cholera > endemic due to lack of expertise, medical staff, clean drinking water or disposal of human waste
6900 deaths and 500000 cases reported
Rape and sexual attacks not uncommon in camps due to lost husbands, tents easy to break into, little privacy at washing facilities and latrines, badly lit at night and no policing
Unretrived corpses decay in mass graves
Little communication > roads and elec damaged
Aid ships turned away from devastated port
Earthquakes case study 1: Haiti > suffered badly due to…
2004 tropical storm Jeanne killed 3000
In a hurricane belt > 2008 hurricanes killed 800, 60% harvested destroyed and many landslides - not yet recovered > services from NGOs and UN > destroyed 700000 homes
Unstable gov
Poor - 2008 32% GDP from family abroad, 70% on less that $2 a day, 1% Haitians control half wealth, limited developed infrastructure and health services
Poor planning, governance and shortage of health nurses and underfunded health system > 25 doctors and 11 nurses per 100000 - urban
Population 9.8 million - high incidence of AIDS/HIV (2.2% aged 15-49) > highest outside sub-Saharan Africa
LE 60
IM 86/1000 - malnourishment
Earthquakes case study 1: Haiti > recovery
World Bank cancelled half debt and gave 5 years before repayments should begin - also gave food for 200000 6-23 months, funding for 140000 schools within camps, new water supply system in 6 rural locations > encourage to stay there, work with locals to rebuild homes in a hazard resilient way
Cash for work programmes allowed some Haitians to support themselves - 20% jobs lost and 85% agricultural, workers - destroyed
July 2010, 98% rubble remained uncleared and 1.6million in camps
Jan 11 relief appeal at over £1bn - 72% of what is required - 810000 in camps, 95% children in earthquake zone returned to school
Need for a long-term strategy - many traumatised, amputees no jobs, some nurses work elsewhere for better pay
Earthquakes case study 2: Northridge > general and nature of hazard
17 Jan 1994 4.30am, federal holiday
Los Angeles, USA
MEDC > GDP = $53042 per capita
6.7 on Richter scale, focus 18.4km
Result of movement along a previously unknown thrust fault - ground acceleration
Highest ever instrumentally recorded in a North American EQ
Earthquakes case study 2: Northridge > Impacts
57 died - federal holiday and early morning so most in homes
Over 1500 seriously injured
12500 buildings moderate - serious damage
]11 major roads seriously damaged and closed for rebuilding
Other roads damaged up to 32km from epicentre
More than 11000 landslides - block roads and damaged water lines
Landslide damaged homes - Pacific Palisades area
Over 20000 immediately homeless
11 hospitals suffered some structural damaged and unable to serve
600 recorded aftershocks - 5.6 11 hours after, weakening already damaged buildings
80km south, score board collapses onto seating at Anaheim stadium
Several days after, 9000 premises without electricity, 20000 without gas, 48500 little/no water
Over $30 billion damage and over 700000 applications to federal and state programmes for financial assistance
Earthquakes case study 2: Northridge > Recovery
Demonstrated that some buildings do not perform well eg multi-storey wood-frames building and those with a weak first floor (due to parking on ground floor)
New building must conform to building codes eg reinforced schools and hospitals
New law: by 2005, hospitals must have earthquake-proof acute care units and emergency units
