Extrusive Volcanism. Features and Hazards of volcanoes Flashcards
how do natural hazards happen
it is caused by plate movement and includes earthquakes and volcanic eruptions
how do volcanoes occur
when magma reaches the earth surface and causes volcanic eruptions leading to formation of various volcanic features
Distribution of active volcanoes in three different places
Divergent plate: occurs mainly around mid ocean ridges when crust is spreading. some active volcanoes are also found when two continental plates diverge like the eastern africa
convergent plate: convergent plate margin’s subduction zone account for almost all of the explosive volcanoes. most eruptions occur at the pacific ring of fire. although it only producese 10-13% of the magma it is responsible for 88% of volcanic eruptions with fatalities
mantle hotspots/mantle plume: mantle hotspots can also lead to volcanism in locations that are not along plate margins. one notable example is in the middle of the pacific plate beneath the Hawaiian Islands.
structure of volcano
the magma chamber is a large space within the lithosphere where magma rising from a deeper level accumulates until it rises to the surface of the site of volcano.
vent is a opening at which volcanic material flows and crater is a circular depression at the top of the volcano
what are the 5 volcanic hazards
lava flow
pyroclastic flow
mudflow
tsunami
ash fall
what is lava flow
magma is known as lava as it reaches the surface of the earth. there are three type of magma, basaltic, andesitic and granitic magma. granitic magma has the highest silica content while basaltic magma has the least, hence granitic magma is the most vicious.
lava from more vicious magma such as granitic and andesitic magma usually solidify underground and does not flow easily even if it manages to reach the surface.
less vicious magma like basaltic magma is more likely to make its way to the surface forming lava flow. their low silica content the explosion less violent.
what are mudflows
mudflows. in more vicious andesitic and granitic magma, gases are less able to escape resulting is pressure build up which is capable of producing a violent eruption.
these superheated gases expand a thousand folds and blows pulverized rocks and lava from the vent forming pyroclasts.
ash is much finer hence mudflow can form when it becomes saturated with water and flows down slope of volcano. some mudflows occur at time of eruption and are usually generated by eruptions at crater lakes or melting of glaciers.
other mudflows occur when rain falls on accumulated ash and occurs after eruption.
mudflows can attain a speed of up to 100km/h when it accelerates downslope and is able to stuff that gets in its way.
pyroclastic flows
volcanoes can emit thousand of tons of gas per day. pyroclastic flow is formed when these burning air combine with pyroclasts. large volcanic eruption lead to the eruption of many cubic kilometers of pyroclast mostly ash and pumice. these are initially carried upwards as billowing eruption column which subsequently collapses forming avalanches. because of hot buoyant gas emitted, ash can travel downslope in a almost frictionless environment attaining a high speed of 200km/h and spread up to 100km. pyroclastic flows are dangerous as it is around 300 degree Celsius hot and causes horrific death tolls as anything that gets knocked down by it will burn. most people die as they chock on the ash/ gas.
ash fall
occurs when ash is ejected into atmosphere and settles to the surface over a wider area. it brings total darkness for hours, suffocating animals, smothering plant lif, preventing use of machinery and if thick enough, causing roofs of houses to collapse. it is scorching hot and capable of starting a fire
Tsunamis
can be caused by underwater explosive volcanic eruptions, submarine caldera collapses and pyroclastic flows enter the sea.
formation of shield volcanoes
as magma rises to the top of the volcano, the pressure of magma body is greatly reduced, hence dissolved gas form bubbles and can escape more easily through the vent. less vicious magma, these gases migrate upwards and escape the vent easier. thus less explosive leading to formation of shield volcanoes
explain more about shield volcanoes
commonly found at oceanic areas at the OO divergent plate boundary. Built primarily by successive flows of basaltic lava from non explosive volcanoes and usually contain a small amount of pyroclastic material.
shield volcanoes are domed shape because their slopes and convex. the slope near the summit is usually gentler as lava exiting from the vent is still hot and fluid and will readily run down a gentle slope. However as lava cools and loses gas, it becomes more vicious hence steeper slope is required for it to flow.
basaltic lavas are fluid and capable of flowing great distances down gentle slopes which accounts for its wide base and low gradient with base of over 100km and gentle slopes of an average of 10 degrees
formation of stratovolcanoes
more viocious magma like andeisitic magma and granitic magma hinder the upward migration of air, collecting bubbles that increase in size and pressure, until they eject the semi molten rock in the form of pyroclast, leading to formation of stratovolcanoes with alternating layer of lava
stratovolcanoes
stratovolcanoes lie above subduction zones at the pacific ring of fire. produced when more vicious lava of andesitic composition is extruded. built of alternate layers of lava and pyroclastic deposits.
finer pyroclasts ejected have lower angle of rest, they will form gentler slopes. decreasing particle size of pyroclasts account for the decreasing angle of volcano slopes.
calderas
some volcanoes have depressions hat are nearly circular called calderas that exceed one km in diameter. forms when summit of volcanic structure collapses into partially emptied magma chamber.
