Tectonic Hazards PDF

Earth Sciences and Plate Tectonics. Earth Sciences and Plate Tectonics. How thick are the layers of the earth? Lithosphere- 100 km thick Asthenosphere- 100-700 km thick Oceanic Continental Younger (400m) Older(1.4b years) Basalt Granite Thinner(6-7km...

Earth Sciences and Plate Tectonics. Earth Sciences and Plate Tectonics. How thick are the layers of the earth? Lithosphere- 100 km thick Asthenosphere- 100-700 km thick Oceanic Continental Younger (400m) Older(1.4b years) Basalt Granite Thinner(6-7km) Thicker(45-50km) More dense less mass Less dense more mass Plate formation Oceanic: BIRTH-Magma will rise from asthenosphere, cool and solidify. DEATH- Plate is forced into the asthenosphere where it melts and becomes magma. Continental: BIRTH - If oceanic plate survives long enough it becomes continental. DEATH- erosion and weathering Density of plates oceanic has more mass but is less dense so it sinks into the asthenosphere continental has less mass but is more dense so it sits on top of the asthenosphere. Exceptions to usual laws of tectonics - Earthquakes and Volcanoes Magma plumes and hotspots Occurs when radioactive decay in the core unusually creates concentrated spots of heat hotter than usual forming thermal currents. 1 / 18  Earth Sciences and Plate Tectonics. This cause magma plumes to rise breaking through faults in the lithosphere creating volcanoes. Plumes does not move but the plate can move so it create a chain of volcanoes like Hawaii. Interestingly, Iceland sits on top of a plume and is constantly expanding due to it being on a divergent plate boundary. Intraplate earthquakes These occur away from plate boundaries. Scientist aren't sure what cause these but they are thought to form when stresses build up in ancient faults like the Rhine Rift valley. These do have the potential to be quite deadly as the spatial predictibiliy is very low as they don't occur on plate boundaries increasing the vulnerability of the population. 2001 Gujarat earthquake- 13,800 dead Plate boundaries Reasons for increased tectonic activity speed of plates direction of plates type of plates. Convergent boundaries Oceanic Vs Continental - subductive destructive Oceanic is denser so when it collides with continental it sub-ducts into the asthenosphere and melts. Deep ocean trenches can be observed where the oceanic starts to subduct under the continental. The oceanic plate will melt after being sub-ducted over 100km and is destroyed completely at 700km. This occurs at the Benioff Zone. Melting occurs from the heat of the asthenosphere and the constant friction of the subduction. 2 / 18  Earth Sciences and Plate Tectonics. This friction might create stresses which can be released as earthquakes. The melted oceanic plate breaks down into magma which rises through the asthenosphere and find any weaknesses in rock faults in the lithosphere. If they reach the surface, volcanoes can form Landforms Fold mountains can occur as the continental plate is being pushed up due to friction. Volcanoes can form here often leading to very explosive eruptions due to the immense amount of friction and volume between the two plates- an example of this is in japan where the pacific plate is sub-ducting under the Eurasian plate. Oceanic vs Oceanic Simply whichever plate is denser ( usually younger) or faster will subduct under the older one. Deep ocean trenches occur where the subduction melts and again oceanic plate melts which causes the magma to rise up into the asthenosphere as it is less dense and reach the surface forming volcanoes. Repeated eruptions Over millions of years these underwater volcanoes can rise above sea level to form island arcs. Very powerful earthquakes can form here as they range from shallow to deep. Landforms Trenches like the Mariana trench is formed here where the Mariana plate is subducting under the Pacific plate. Volcanic island arcs can form here as the plates converging with each other causes the crust to melt and magma to form which can rise to the ocean floor and if it reaches above sea level can form Volcanic island arcs *-Aleutian Islands off the coast of Alaska* Continental Vs Continental 3 / 18  Earth Sciences and Plate Tectonics. A collision margin forms as both plates have the same density and are less dense dense than the asthenosphere below them so neither is sub-ducted. The plates buckle as they collide fracturing and breaking the plate to form fold mountains. Subduction can occur here though when the denser sediments which are compressed cause plate subduction beneath them. Landforms Fold mountains can occur here such as the Indo-Australian Plate colliding with the Eurasian Plate forming the Himalayas. No magma formed as plate is sub-ducted but earthquakes that occur here can have a shallow focus. Constructive Boundaries Divergence under the ocean Volcanic island arcs is a prime example of this. Plates move apart and magma rises to fill the gap due the pressure of the asthenosphere. Magma continues to rise here widening the rift valley. landforms earthquakes can occur here and are often quite shallow (