EESA05 LEC 4.docx

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Environmental Science
Mandy Meriano

LIQUFACTION Occurs in water saturated soils and rock when ground shaking cause the water saturating the pore spaces to well up and out of the ground, causing the upper layer of the ground to behave as a fluid For this to happen, the particles that make up the soil are packed relatively inefficiently Can cause collapse of overlying structures Reclaimed soils are esp vulnerable to this because of the nature of the fill and because it tends to be saturated with water (man made land) Other areas that are vulnerable are sensitive clay on slopes It is strong clay that become weak when it is disturbed Can create landslides VOLCANOES Occur all over the solar system Their distribution and chemistry are closely related to their plate tectonic setting VOLCANIC ERUPTIONS Mankind has been on the receiving end of volcanism for millennia Oldest known picture of an erupting volcano ca. 6200 BCE reproduced from mellaart LOCATIONS OF VOLCANIC ACTIVITYAND VOLCANIC HAZARDS Activity is controlled by plate tectonics, because plate movements related to where sources of magma originate inside the earth Nearly all active volcanoes are located in one of three tectonic settings… 1. Subduction zones 2. Rift margins 3. Hot spots Seamounts: if the volcano doesn’t break out of the ground (it is just a bump) VOLCANIS HAZARDS Three main factors determine the size and explosivity of a volcanos eruptions… 1. The amount of time that has passed since the last eruption; you need time to build up the pressure 2. The viscosity (stickiness) of magma: more viscous the more explosive 3. The gas content of the magma: more water in it, the more explosive Viscosity of magma is inherently a function of the magma chemistry Both of these parameters are strongly influenced by the tectonic setting of the volcano MAGMACHEMITRY Oxygen and silicon are the two most important elements by weight in the rock forming minerals Silicon and oxygen combine to form the silicate complex ion, which has a tetrahedral structure Silica = 4 O2and 1 S Most minerals are composed of silicate tetrahedral bound to one or more metals (iron, magnesium, AL, CA) Minerals form in a magma by crystallization The order of their formation depends on their chemical composition In the intrusive rocks, the size of the minerals are larger More silica = more viscous MAGMAVISCOSITY Viscosity decreases with increasing temperature Viscosity increases with increasing silica Basaltic (low-silica) magma have a low viscosity Andesitis and rhyolitic magmas have high viscosity rhyolithic magmas almost always crystallize at depth Andesitic magmas tend to erupt explosively THE ROLE OF DISSOLVED GASES Water is the most abundant dissolved gas CO2 is the most common At depth, gases are under pressure When the pressure drops, the gases expand If the viscosity of the magma is low, the gases will expand and escape from the magma with little effect If magma is highly viscous, it resists the expansion of the gases- as the gases expand and force their way through the neck of the volcano, the magma is forced out with them VOLCANIC EXPLOSIVITY INDEX The gas content, viscosity of the magma, and the amount of magma built up in the magma chamber define the ‘explosivity’of an eruption VOLCANIC HAZARD AND TECTONIC SETTING The eruptive style, the form of the volcano and the chemistry of the magma are all linked to tectonic setting Viscous magmas tend to lead to highly explosive eruption types Thus volcanoes near subduction zones tend to be the most dangerous (very runny magma= very balsaltic) LOCATIONS OF VOLCANIS ACTIVITYAND VOLCANIS HAZARDS The most dangerous volcanoes occur near subduction zones at convergent plate boundaries Most emergent = more explosive VOLCANIS HAZARDSAND TECTONIC SETTING The volcanoes at spreading centres and at hot spots consists of low viscosity magma so that eruptions are placid Hawaiian volcanoes make a good tourist venue because eruptions are common but rarely dangerous VOLCANIS FORMS Volcanoes take several different forms, or shapes These shapes are dictated by the viscosity of the magma that emerges from them The most common forms are.. Shield volcanoes: Runnier the magma, the further it can flow which forms a kind of shield around the opening Stratovolcanoes: more anacidic more explosive and more viscous; creates taller volcanoes Calderas: massive explosion and it leaves the chamber empty and it takes away the strength of that volcano and it collapses Lava domes: very viscous stuff that stay in the chamber and if it comes out, it doesn’t go very far; the sides of the volcano starts to swell up to release pressure Scoria cones: the ash keeps b
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