Tectonic Processes and Hazards EQ1 Flashcards
What is the global distribution of earthquakes?
Most earthquake zones are found at or close to tectonic plate boundaries in clusters.
- 70% of all earthquakes are located in the ‘Pacific Ring of fire’ in the Pacific Ocean
- most powerful found ad convergent (destructive) and conservative boundary’s
- rare intro-plate earthquakes can occur
What patterns do the distribution of earthquakes follow?
- The oceanic feature zone (OFZ)
- The continental fracture zone (CFZ)
- Scattered earthquakes in continental interiors (old fault lines e.g. Church Stratton Fault, Shropshire)
What is the OFZ?
The oceanic fracture zone is a belt of tectonic activity through the oceans along mid-ocean ridges. They come onshore in Africa, the Red Sea, the Dead Sea rift and California.
What is the CFZ?
The continental fracture zone is a belt of activity following the mountain ranges. From Spain, via the alps to the Middle East, the Himalayas to the East Indies and the finishing in the Pacific.
What is the cause of earthquakes?
Earthquakes are formed from the sudden releases of stored energy. As tow plates move past each other they inevitably get stuck. This results in the build up of strain and pressure putting the plates under increasing stress. The sudden releases of this ‘stress’ (‘stick and slip’) causes a pulse of energy to radiate out from the earthquake focus.
What is the definition of a intra-plate earthquake?
These occur in the middle or interior of tectonic plates and are much fares than boundary earthquakes.
What is the global distribution of volcanoes?
Volcanoes tend to occur at or close to tectonic plate boundary’s with exceptions for hotspots. There are 500 active volcanos with an average of 50 erupting each year.
What is the definition of a volcano?
A landform that develops around a weakness in the Earths crust from which molten magma, volcanic rock and gases are ejected or extruded.
What is the global distribution of Tsunamis?
Tsunamis are fairly predictable with then happening along plate margin activity.
- 90% of all event happen in Pacific basin
- most are generated at destructive boundary’s at subduction zones
What causes a Tsunami?
Tsunamis are generated what a sub-marine earthquake displace the sea bed vertically. As a result a large volume of water is displaced in the ocean water column which then moves outwards in all directions from the point of displacement.
What tectonic hazards can be found at constructive (all types) plate boundary’s)?
- volcanoes
- earthquakes
Convections currents forces the two oceanic plates away from each other allowing magma to rise in a small basaltic eruptions creating new oceanic plate. Minor earthquake are also found.
E.g. Mid-Atlantic ridges, Iceland
What tectonic hazards can be found at destructive (oceanic - oceanic) plate boundary’s)?
- frequent earthquakes
- volcanos
One oceanic plate is subducted beneath another due to slap pull. This generates frequent earthquakes and a curving chain of volcanic islands, which erupt.
E.g. The Aleutian Islands, Alaska
What tectonic hazards can be found at destructive (continental-continental) plate boundary’s)?
- infrequent major earthquakes
- rare volcanic eruptions
The collision of two continental plate boundary’s create a mountain belt as they crumple together. Magma is generated at depth meaning it has time to cool and solidify making eruptions rare. Major earthquakes are created over a large area.
E.g. the Himalayas with the Indo -Australian and Eurasian plates creating Nepal 2015)
What tectonic hazards can be found at destructive (oceanic - continental) plate boundary’s)?
- earthquakes
- volcanos
1. Mantle convection move the two plates together.
2. Denser oceanic plate is subducted beneath the lighter continental the density of the plate pulls in self into the mantle (slap pull).
3. The oceanic plates begins to melt into the mantle generating a high pressures and causing it to erupt explosively (high gas and silica).
What tectonic hazards can be found at conservative plate boundary’s)?
Plates slide past each other causing
- frequent earthquakes (shallow focal depth)
E.g. San Andreas Fault
What are the three types of plate boundaries?
- constructive (divergent)
- destructive (convergent)
- conservative
What is a constructive plate margin?
These are plate margins are moving away from each other driven by convection current. There are most clearly displayed at mid-ocean ridges. Create shallow and low magnitude earthquake event (majority sub marine).
What is a destructive plate margin?
This is where plates are moving together creating actively deforming collision. This causes frequent earthquakes and volcanic eruptions.
What is a conservative plate margin?
This is where one plate is sliding against another in sinistral (left) or dextral (right). Lithosphere is nether created nor subduncted resulting in extensive high magnitude earthquakes.
What does the type of plate margin depend on?
- motion (are the plates moving apart, colliding or sliding last one another)
- plate type (continental or oceanic)
What is a hotspot volcano?
Volcanic eruptions that can occur intra-plate meaning they are at a distance from plate margins.
E.g. Hawaii within the Pacific Plate
How does a hotspot volcano form?
- An isolated plume of convecting heat called a mantle plume rises towards the surface.
- This high heat and pressure causes the lithosphere to melt
- This allows molten magma to rise through the cracks and erupts as an active volcano (basaltic)
- The mantle plume stays stationary but the tectonic plates moves slowly over it creating a chain of volcanic islands.
What is an intra-plate earthquakes?
This is when earthquakes o cud in a mid plate setting, it usually happen on major fault lines being reactivated by tectonic stress.
E.g. New Madrid Seismic Zine generates high magnitude earthquake but is miles away form a plate boundary
What are the key characteristic of plate tectonics?
- move at speeds of 2-5 cm annually
- they are split into 7 very large plates (Pacific) and smaller minor plates (Nazca)
- they fit together in a constantly moving jigsaw
- made up of two types of crust
What are the two types of crust?
Oceanic crust - thin but very dense. It underlined the ocean basins and is composed primarily of basalt
Continental crust > thicker but much lighter. It underlines the continents and is composed of primarily granite.
What are the key discoveries about plate tectonics throughout the years?
- 1912 Alfred Wegener’s came out with continental drift hypothesis and postulated that now sap rates continents had once been joined.
- Arthur Homes in 1930 theory was the the earths internal radioactive heat was that driving the mantle movement (convection currents)
- 1950, discovery of the rocks in the lower mantle being plastic and flow allowing the rigid lithosphere to move.
- 1960s discovered paleomagnetic signals to prove the processes of sea floor spreading at mid ocean ridges
- 1965, Tuzo Wilson recognised transform faults
