Natural Hazards Flashcards
What are the causes of earthquakes?
They are caused by a sudden release in tension in the earths crust. Often subduction causes friction between plates which can be accredited for building up this tension.
What are intra and inter plate earthquakes?
An intra plate occurs within a plate whereas an inter plate occurs at a plate boundary.
Global distribution of earthquakes
pacific ring of fire. Alpine- Hymilaya collision zone. African rift valley. Mid Atlantic ridge.
Pacific ring of fire
A seismically active area around the edge of the Pacific Ocean.
What is the Alpine Himalaya collision zone?
Where content plates are crushing into themselves and causing seismicity over Western Europe.
African rift valley
An area in Southern Africa where part of the continent is pulling away from itself.
The mid Atlantic ridge
Underwater mountain range formed from a constructive plate margin.
What causes a volcano
Volcanoes are formed from a gap emerging in the crust where magma forces its way to the surface as seen in constructive volcanoes. The violence of a volcano is determined by the magma and how easily gasses can escape.
Distribution of volcanoes
Volcanoes are situated in linear clusters. There is a clustering of volcanoes around the edge of the pacific which is where 75% of the worlds volcinicity. This is the pacific ring of fire.
Sima
Oceanic plates- younger, heavier and denser.
Sial
Continental plates, thicker, heavier and lighter.
collision plate boundary
Plates of similar weight and size, through slab pushed, are thrust into each other. This causes the plates to buckle as they compress into each other, forcing earth upwards and creating fold mountains.
Constructive
when two oceanic plates move away from one another creating a gap. From this gap magma rises, cools and solidifies. As this process continues enough sediment is deposited that the, now shield volcano, breaches the surface.
Destructive
When a heavier and younger oceanic plate is forced under an older and lighter contental plate. In the subduction zone a sea trench is formed. As the plate is forced under it begins to melt in the magma and it is forced out as there is no room. It escapes through a composite volcano.
Conservative plate boundary
When two plates move in the same direction but at different speeds or when two plates move past one another, tension is accrued and eventually released in the form of earthquakes.
What is a mantle plume
A mantle plume in the mantle which then melts through the lithosphere. This is then a hotspot and this magma leaks through the crust and behaves as a constructive plate boundary. As the plate moves, as the plume is deeper, the plume remains and a chain of islands is created like Hawaii.
Who first proposed plate tectonic theory
Alfred Wegner
What is mantle convection?
when cool mantle sinks to the earth’s core as cool mantle is less dense. This mantle which then gets heated rises to the top in a circular fashion. The mantle is heated by the earth’s core which is heated by radioactive decay. As the convection currents reach the top of the current the current pulls down subducting plate creating slab pull.
What is paleo magnetism
confirmation of repeated reversals of the earth’s magnetic field in the geological past.
Sea floor spreading
Recycling of oceanic crust, the sea floor spreading theory. Underwater volcanoes create a constructive plate margin and magma rises and locks in the earths magnetism from the time this took place. This pushes apart the two plates when it cools and as the two plates are pulled apart a new gap is formed where more magma locks in the earth’s magnetism of the time in which it set. From this we can see changes in when the earths magnetism
Subduction
when 2 plates meet and the heavier one is pushed under the main plate. As this plate is pushed under it pulls the rest of the plate and the other side of the plate is pulled under.
Def Locked fault
A fault which is not slipping because the frictional resistance on the fault is greater than the shear stress across the fault
Destructive
Oceanic plate subducts where the continental plate is pulled further under by convection currents. Earthquakes take place at different depths. Convection currents push the plates. The plate that is subducting is being pulled the convection current which causes slab pull at the other end of the plate.
Constructive
palaeomagnetism is locked in as two oceanic plates are pulled away from one another.
Collision
2 continental plates are pushed into one another. As the plates are the same weight and density neither subduct nor they collide. Fold mountains and earthquakes are created but no volcanism takes place. Most violent earthquakes- alpine Himalaya collision zone.
Transform
(conservative) Slab push is taking place as one slab is pushed alongside one another. Either they are moving at different rates or in different directions. Friction builds up until it is released in the form of an earthquake.
Benioff zone
The Benioff zone is an area of seismicity corresponding with the slab being thrust downwards in a subduction zone. The different speeds and movements of rock at this point produce numerous earthquakes. It is the site of intermediate/ deep focused earthquakes. This theoretical framework is an important framework in determining the position of the hypocentre. This is the zone of melting.
Focal depth
The depth at which the earthquake starts
Where do deeper earthquakes take place.
Subducting plates often lead to deeper earthquakes.
Composite volcano is destructive margin, and a shield volcano is a constructive margin.
P Waves
P waves are primary waves which are vibrations caused by compressions in the crust. They spread quickly from the fault at a rate of about 8 km/sec. these are body waves which pass along the crust. These waves travel the fastest and travel in longitudinal ways. Least destructive but they are the fastest.
S waves
S waves are secondary waves and are transverse (cross section of the wavelength looks like an S). These waves travel though the surface on the crust. Also, very destructive.
L Waves
L waves or love waves occur in the horizontal plane. These waves are the most damaging. These are surface waves
Seismic waves
Waves of energy that travel In all directions.
Liquefaction
Soil liquefaction is a phenomenon in which the strength and stuffiness of a soil is reduced by earthquake shaking or other rapid loading. This occurs in saturated soils, that is, soil which the space between particles is full of water. Before the earthquake the particles support the weight of the of the water however vibrations break down this structure which results in flooding and may leave cavities in the soil. Under the strong shaking of an earthquake this soil behaves and flows as water.
Landslides
where a mass of rock, soil or sediment moves down an angle. This starts where slopes weaken and fail. Rarely occur when the magnitude is less than four. Half the earthquake deaths in Japan are linked to earthquakes.
Lava flows
The faster moving this is the greater risk that is posed to human life. The viscosity of lava depends on the percentage of silicone dioxide it contains. On steep slopes some lava flows can reach 15m/ sec. The greatest lava flow was in 1873 in Iceland and following the eruption over 1/5th of the population died in the following famine.
Pyroclastic flows
These are responsible for most primary volcanic deaths. Pyroclastic flows are the result of a frothing of molten magma in the vent of the volcano. The bubbles burst explosively to eject hot gas and material such as pumice, crystals and ash. These clouds are most dangerous when they come out of the volcano sideways as famously happened in Pompeii. The temp of these clouds can be upwards of 1000 degrees.
