TECTONICS EQ1 Flashcards
why are some locations more at risk from tectonic hazards?
What is a tectonic hazard?
Threats caused by the movement of tectonic plates that have the potential to cause damage to life, property, the environment or the economy.
-Includes earthquakes and volcanic activity.
What is an earthquake?
Sudden violent shaking of the ground is caused by the release of energy in the earth’s crust, typically due to the movement of tectonic plates.
What is a volcanic eruption?
-The release of magma, ash, and gases from a volcano, which is an opening in the Earth’s crust.
-This process can be either effusive (producing lava flows) or explosive (producing pyroclastic flows and ash clouds).
What is a tsunami?
-A long, high sea wave produced by a disturbance, such as a volcanic eruption, submarine earthquake or coastal landslide.
What influences the distribution of earthquakes, tsunami and volcanoes?
-Earthquakes:
This happens in distinct zones at the edges of tectonic plates (plate boundaries). Also occurs in intraplate regions, which may be caused by stress built-up, continental rifting etc.
-Volcanoes:
Active around plate boundaries. Mostly on the “Pacific Ring of Fire (75%). Found at convergent and divergent boundaries. Can also be a ‘hotspot’ in the middle of plates.
-Tsunamis:
70% in the Pacific Ocean. Caused by tectonic activity. Mostly at convergent boundaries.
What are the three main earthquake belts?
-pacific belt (ring of fire)
-Alpide belt
-Atlantic mid-ocean ridge
Oceanic crust is…
- made of denser rock types
-created at divergent plate boundaries
-thinner than continental crust
Continental crust is…
-made of less dense rock types
-created at convergent plate boundaries
-thicker than oceanic
Earts structure: crust
-Made up of oceanic and continental.
-Continental: thicker (45-50km) less dense (mostly granite)
-Oceanic: thinner (6-10km) and denser (mostly basalt)
Consists of seven major and several minor tectonic plates.
Earths structure: mantle
-between the crust and the core, the widest layer
-Upper crust:
lithosphere (crust and most solid mantle)- rigid layer above the asthenosphere
asthenosphere - semi-molten plastic-type layer which moves under high pressure. -(IMPORTANT) no movement would occur without it
- Lower mantle:
Hotter and denser than the upper mantle. The intense pressure at depth keeps the lower mantle solid
Earths structure: core
-Inner core:
The solid centre is mostly composed of iron
-Outer core:
Semi molten mostly liquid iron and nickel
What is a divergent (constructive) plate boundary?
-Plates move apart (diverge)
- convection currents push the plates apart and cause a gap
Magma rises to fill the gap and a new crust is created
- frequent low-magnitude earthquakes
-small and effusive eruptions
-basaltic eruptions ( low gas content, low viscosity, high temp)
What is a convergent (destructive) plate boundary?
-The plates move towards each other (converge) resulting in collision and the destruction or defamation of the crust.
-Friction and pressure build up in the Benioff zone causing earthquakes
-Volcanic eruptions become explosive as the magma rises to the surface.
-rhyolitic eruptions (high gas content, high viscosity, low temp)
What is a subducting plate boundary?
-Where oceanic plates subduction beneath the continental plate
- also leads to fold mountains when rocks are pushed upwards by collision
-oceanic subduction causes friction along the subducting plate.
-This is called the Benioff zone and can cause deep earthquakes far from the plate margin.
What is a conservative plate boundary?
-Where two crustal plates slide past each other, and the movement of the plates is parallel to the plate margin
-Plates can stick causing a significant build-up of pressure and powerful earthquakes
What is intra-plate/hot-spot volcanism?
-Some earthquakes and volcanic eruptions occur away from the plate boundaries. These are intra-plate earthquakes and hot spots.
-Hot spot volcanoes occur over stationary mantle plumes (columns of rising magma) in the asthenosphere. The tectonic plate moves over the plume, forming volcanic island chains (Hawaii). The oldest is the one furthest from the plume.
What are the two theories of plate tectonics?
Alfred Wegner’s theory of continental drift
Harry Hess’ theory of seafloor spreading
Describe Alfred Wegners theory of continental drift
- The continents had once formed a single landmass called Pangea, before breaking apart and drifting to different locations.
Describe Harry Hess’s theory of seafloor spreading
-Basaltic magma rises at a divergent plate to create new ocean floors at mid-ocean ridges.
-When magma rises at divergent plates, the rock begins to cool and the minerals align with the magnetic poles of the earth. We see stripes as the mineral alternatively faces north and south which could flip every 100,000 years, proving the theory of seafloor spreading.
What is paeleomagnetism?
-Paleomagnetism refers to the study of Earth’s past magnetic field, recorded in rocks and sediments
Why does the lithosphere move?
-Due to a combination of convection currents in the Earth’s mantle, slab pull, and ridge push
What is slab pull?
-Happens in the presence of subduction
-The weight of the sinking subducting plate causes a pulling action, pulling the rest of the plates down as well.
What is ridge push?
-Occurs at mid-ocean lithosphreric ridges where the lithosphere rises and newly formed plates slide sideways pushing the plate in front of them triggering ridge push.
Which scale is most commonly used to measure earthquakes
Moment Magnitude scale
What is the moment magnitude scale?
-A measure of the earthquake’s magnitude based on its seismic movement.
-Moment: product of a distance a fault is moved and the force required to move it
What is the Richter scale?
-Determines the magnitude of an earthquake from the logarithm of the amplitude of waves by seismographs. It includes varying distances between the seismographs and the epicentres of an earthquake
-Less commonly used now
What is the modified Mercalli scale?
-Describes the effect of an earthquake at a given place, on natural features, industrial investigations and human beings
-Measures intensity not magnitude
What is an earthquake fault?
-Fracture or zone of fractures between two blocks of rock.
What is the earthquake’s focus?
-Point within the earth where the rupture starts
What is the earthquakes epicentre?
-Location on the surface of the earth directly above where the earthquake starts
What are seismic waves?
-Energy released when plates move after building up stresses
What is the volcanic explosivity index (VEI)?
-A scale that describes the size of explosive volcanic eruptions based on magnitude and intensity
- takes into account the volume of products, eruption cloud height and qualitative observations
-The volcanic equivalent of the modified Mercalli scale.
Describe the three types of sisemic waves
-Primary waves: fastest, first to reach the surface, least damaging, forwards and backwards movement.
-Secondary waves: slower than P waves, more damaging, sideways movement at a right angle
-Love waves: slowest, most damaging from shaking, focus all energy on the surface
What is a primary hazard?
-Things that happen immediately as a result of a hazard
What is a secondary hazard?
-Things that happen in the hours, days and weeks after the initial hazard
What are the primary hazards of an earthquake?
-Crustal fracturing: cracks or breaks on the earth’s surface especially on fault lines.
-Ground shaking: shaking from seismic waves
What are the secondary hazards of an earthquake?
-Liquefaction: soil behaving more like a liquid during an earthquake, rearranging salt and silt grains in wet soil and water is squeezed
-Landslide: mass movement of material such as rock, earth or debris down a slope.
-Tsunami: giant waves caused by earthquakes or volcanic eruptions under the sea
-Avalanche: a mass of snow, rock, ice and soil that tumbles down a mountain
-Disease: inadequate air quality, water and poor personal hygiene after an earthquake led to increased cholera rates
What are the primary hazards of a volcano?
-Lava flow: streams of molten rock that pour or ooze from an erupting event
-Pyroclastic flow: a hot, chaotic mixture of rock fragments, gas and ash that travels rapidly away from a volcanic event
-Ash fall: a mixture of rock, mineral and glass particles are carried upwards within a volcanic plume `and downwind in a volcanic cloud but fall as the clouds cool
-Gas eruptions: eruptions of CO2 and sulphur dioxide which can poison people and animals in extreme cases. Occurs at subduction zone volcanoes (composite)
What are the secondary hazards of a volcano?
-Wildfires: uncontrollable fire that burns wildlife vegetation in open areas
-Lahars: hot or cold mixtures of water and rock fragments that flow quickly down the slopes of volcanoes. Can be dangerous and destructive and more deadly than lava flows
-Joklhlaup: glacial outburst floods occur when a lake is fed by glacial meltwater breaching its dam and draining catastrophically
