Geophysical Hazards Flashcards
What is a tectonic/Lithosphere Plate?
Thin, rigid outer layer of earths crusts broken up into 8 major and minor sections. They are inflexible and float on the mantle.
What is the aesthenosphere?
The base of lithospheric plates and the upper mantle
What is the Benioff zone?
Area where plates are subducting, causing sloping zone of earthquake activity
Who was Alfred Wegener?
Helped develop the plate tectonic theory and this provided the explanation for volcanic and earthquake activity and proved continental drift.
What is sea floor spreading?
Formation of new oceanic crust at a mid-ocean ridge as plates move apart, causing sea floor to widen
What is a rift valley?
A rift valley is a lowland region that forms where Earth’s tectonic plates move apart. e.g East African rift or Thingvellir in Iceland
What are the 2 types of crust?
The oceanic crust, which is thinner (5–10km) but heavier and denser
The continental crust, which is thicker (25–90km) but is older and lighter
The oceanic crust is denser and so subducts under the continental crust
The continental crust is not destroyed and so is much older than the oceanic crust
How do tectonic plates move?
Large scale convection currents and the flow of heat is from radioactive decay of materials in the mantle and heat loss by the earth as it continues to cool from its original formation
What is subduction?
The plunging of one of the plates beneath another. The subduction zones form where an oceanic lithospheric plate collides with another plate whether continental or oceanic.
What is rifting and how does it occur?
It’s the production of rifts at constructive plate boundaries e.g East Africa Rift valley or the rift at Thingvellir in Iceland where north american plate and eurasian plate are moving away from each other
Main cause is hotspot activity
Upwelling convection in mantle causes the oceanic crust to form a ridge. Lateral tension develops, causing rift faulting and downward movement of the central block as magma intrudes along faults giving surface lava. Lateral movement continues with further intrusions parallel to original rift faults. main rifting sequence repeats periodically as upwelling continues.
What are the 3 types of volcanoes and what are the characteristics of each?
Shield- Occur when no explosive activity and are formed form very hot, runny basaltic lava that flows great distances so shield volcanoes have gentle sloping sides, a shallow crater and large circumference. Frequent eruptions e.g Mauna Loa (Hawaii)
Composite- Most common and formed by alternating layers of ash and lava. Slopes of 30 degrees near summit and 5 degrees near base and made by viscous lava solidifying and ash being ejected and forming a layers on the side e.g mount Etna and Versuvius
Cinder- Formed by fragment of material which accumulate around vent to form a cone. Typically concave at material spreads out near base and has slope angle of 30-40 degrees. Typically not very tall e.g Paricutin (Mexico)
What are the types of volcanic eruptions?
How can volcanoes form?
Formed by:
Subduction
Rifting
Hotspots
Pacific ring of fire where volcanoes must commonly found
What are the primary and secondary volcanic hazards?
Primary:
Lava flow
Ash fallout
Pyroclastic flows (700 degrees at 500km/h)
Gas emissions - occurred at lake Nyos, a volcanic crater lake, released huge volumes of gas in 1986 and suffocated 1700 people and 3000 cattle
Secondary:
2 types of landslides:
Lahars - combination of heavy rain and unstable ash. In nicaragua lahar killed 2000+
Debris avalanches - 1980 St Helena
and avalanches can aid the eruption to occur
Both landslides can trigger tsunamis as debris falls into the sea. Could occur in La palma and could create ‘mega tsunami’ that would hit Americas
Global dimming can occur due to ash cloud and debris blocking sun. Mt Pinatubo in ‘91 cooled global temp by 0.5-0.6 degrees
What is an earthquake?
Series of seismic vibrations or shock waves that originate from the focus/ point at which plates release tension or compression suddenly
What is the epicentre?
Point on the surface immediately above the focus of the earthquake
What are the two types of waves that occur from an earthquake?
Primary (P) waves shake the earth backwards and forwards and can move through liquids
Secondary (S) waves cause horizontal movement and can’t move through liquids
When P + S waves reach surface, some transformed into Love waves (side to side) and Rayleigh waves (up and down). These travel slower but cause most damage
What causes earthquakes?
Earthquakes can occur anywhere but typically occur near plate boundaries and can happen at all boundaries. Plate movement leads to build up of pressure and when this is released an earthquake occurs.
What are the 4 plate boudaries?
Constructive - plates move apart forming new land
Destructive - oceanic forced to subduct below as is more dense into Benioff zone. Typically causes both EQ and volcanoes
Collision - Plates of similar density move towards each other forming mountain ranges
Conservative - plates moving next to each other either in different directions or at different speeds in same direction e.g San Andreas Fault
What are human triggers of Earthquakes?
Dam building - Added weight of water in reservoir adds stress to faults risking possible fracture and water can go into cracks changing pressure. E.g Three Gorges Dam weighs over 38 trillion kg.
The sichaun EQ partly caused by Zipingpu dam construction as reservoir contained 315 million metric tons and lead to EQ that killed 80,000+
Mining and fracking - Causes cracks within rock leading to instability and was cause of 2.9 magnitude Lancashire EQ in 2019
What are primary hazards of Earthquakes?
Ground shaking
Infrastructure damage
Deaths
What are secondary hazards of earthquakes?
Tsunami - release of pressure leads to water displacement and wall of water e.g Boxing day tsunami 2004
Liquefaction - shaking of ground causes particles to move further apart and behave more like a liquid than a solid. Causes buildings to collapse and occurs when saturated, unconsolidated land affected by s-waves
Landslides, rockfalls and mud flows
Disease e.g cholera outbreaks as water contaminated
Fire - EQ can lead to damage to gas pipes and electrical lines leading to large fires.
What is a mass movement?
Any large-scale movement of the earth’s surface that are not accompanied by a moving agent e.g river, glacier and is the movement downhill under the influence of gravity
What factors increase Shear stress, increasing likelihood of mass movements movement?
Removal of lateral support by undercutting or slope steepening and removal or underlying support. Caused by river and glacier erosion, previous rockfalls and slides, loss of strength due to exposure to sediments
Loading of slope as vegetation and debris accumulate and weight of water mounts
Lateral pressure as freeze thaw action occurs in cracks
Transient stresses typically by EQs or movement of vegetation by wind
