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Tectonic Activity - A general introduction

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´╗┐Tectonic Activity & Hazards PURPLE ? KEYWORDS & DEFINITIONS BLUE ? EXPLANATION RED - EXAMPLE -Tectonic activity generates a wide range of natural hazards. -The fundamental cause of these is plate tectonics, and thus the hazards have a distinct geography, linked to different tectonic settings. -Tectonics is a key landscape-forming process which produces distinctive landforms in active regions. -Tectonic hazards pose significant risk to human populations and their possessions, related to their vulnerability and the magnitude and frequency of hazardous events. -Risk varies due to many factors, including level of economic development, preparedness and education. -Hazard impacts can be short- or long-term and people respond in a variety of ways. Response depends on knowledge, technology and the availability of financial resources. -?A hazard is a perceived natural event that has the potential to threaten both life and property? (Whittow). -Therefore, an event is not hazardous without human occupancy of the area. Since hazards occur at the interface between natural and human systems, it is unlikely that any hazard is truly natural. -There is a continuum from natural to quasi-natural to man-made: -Dregg?s model: of the relationship between hazard, disaster and human vulnerability. ...read more.


Cold lahars ? when heavy rainfall loosens ash that was deposited on slopes. For example, following the eruption of Mount Pinatubo, Philippines, 1991, destroyed 100 000 homes. Climate change ? material ejected into atmosphere can reduce solar insolation and lead to cooling. For example, following eruption of Krakatau, 1883. Plate Tectonic Theory What causes plates to move? 1. Slab-pull: sinking cool, dense oceanic crust pulls the rest of the plate. 2. Ridge rises: material deposited on top of mid-ocean ridges slides down, pushing on the plate. 3. Convection currents in the asthenosphere. Oceanic crust (sima) Silica and magnesium dominated Mafic ? silicate rock rich in magnesium and iron Basalt ~ 6-10 km thick At its deepest temperature is ~ 1200 oC Young ? mainly < 200 million years Dense and heavy (~ 3 g/cm3) Dark in colour Few types Continental crust (sial) Silica and aluminium dominated Felsic ? silicate rock rich in aluminium, oxygen, sodium and potassium Granite ~ 35-40 km thick (60-70 km thick under mountain chains) Old ? mainly > 1500 million years Less dense and lighter (~ 2.6 g/cm3) Light in colour Numerous types Lithosphere Consists of the crust and the rigid upper layer of the mantle. ...read more.


This causes differential movement and further seismic activity. The subducting lithosphere melts due to increased pressure and friction with both the overlying lithosphere and the semi-molten asthenosphere. It carries with it sea-floor sediments (less dense) and water in pores. The water reduces the melting temperature, and creates high gaseous content. Volcanic activity is explosive, due to great compressional tension (pressure) and the gaseous content of the subducting lithosphere. At ~ 100 km depth the newly formed magma is less dense than the surrounding asthenosphere, so rises through weaknesses in the lithosphere above it. Seismic activity is often deep but magnitude can be very high due to intense compressional forces. Conservative/transform boundaries It is called conservative because plate material is neither created nor destroyed at these boundaries, but rather plates slide past each other. The classic example of a transform plate boundary is the San Andreas Fault in California. The North American and Pacific Plates are moving past each other at this boundary, which is the location of many earthquakes. These earthquakes are caused by the accumulation and release of strain as the two plates slide past each other. Another example of a transform boundary is seen at the mid-ocean ridges, where the spreading centres are offset by transform faults anywhere from a few meters to several kilometres in length. ...read more.

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