Mod 6.1 - Geological Natural Disasters (Volcanoes) Flashcards
ash eruptions as a hazard
Ash clouds are typically associated with explosive eruptions, where the force of hot expanding gases can produce a roiling mushroom cloud of gas and ash. Ash fall may smother plant life, and particles from the ash cloud can enter the lungs and irritate or suffocate animal and human life. Metal and sulfates can contaminate freshwater supplies, making the water unsafe for wildlife and humans. Ash can also accumulate on buildings, causing them to collapse and potentially burying people alive.
Ash can mix with water and melting glaciers surrounding the volcano to produce lahar flows. These are flash floods of mud and ash that can destroy everything in its path.
Ash can also lead to pyroclastic flows.
lava flows as a hazard
Lava flows are generally slow moving. Despite this, they can be harmful, as shown by the ongoing eruption of Kilauea in Hawaii that has consumed whole suburbs. The movement of the flow is determined by the viscosity of the lava, the slope of the land and the rate at which the lava loses heat. As lava begins to solidify on the surface, a crust forms which is an excellent heat insulator allowing the lava on top to keep flowing while losing little heat. As such, lava flows can travel long distances. Lava flows can have a devastating effect on local communities and ecosystems as they destroy everything in their path as they advance across the landscape. On a global scale, lava flows have little effect.
pyroclastic flows as a hazard
Pyroclastic flows contain a high-density mix of hot fragmented lava blocks, pumice, ash and volcanic gas. They move at very high speed down volcanic slopes. They destroy all that is in their path because of the high temperatures of volcanic material and gases. They carry debris of destroyed vegetation and buildings in their wake.
lahars as a hazard
Lahars occur when volcanic material (such as ash, loose rubble and large boulders) is in contact with water sources (such as rivers, heavy rainfall, melting glaciers or even a dam collapse), forming a powerful mudflow. Lahars can travel long distances at high speeds (~40m/s) down a slope, and can crush, bury or destroy almost anything in their paths. They can pose a threat to people and infrastructure far beyond the volcano’s slopes.
poisonous gas emissions as a hazard
As magma rises towards the surface and pressure decreases, gases are released and continue to travel upward and are eventually released into the atmosphere. Gases can also escape continuously into the atmosphere from the soil, volcanic vents, and hydrothermal systems. By far the most abundant volcanic gas is water vapour, which is harmless. However, significant amounts of carbon dioxide, sulfur dioxide, hydrogen sulfide and hydrogen halides can be also emitted from volcanoes. Depending on their concentrations, these gases are all potentially hazardous to people, animals, agriculture, and property.
Describe an effusive volcano (eg. shield volcanoes)
Effusive volcanoes contain mafic magma with a low silica composition, resulting in it having a low viscosity. As such, dissolved gases are able to escape periodically, and hence the pressure will not build up underneath the crust, resulting in an effusive eruption. Effusive eruptions are most commonly found at divergent boundaries such as mid-ocean ridges, and oceanic hotspots such as those forming the Hawaiian Islands.
Describe an explosive eruption (eg. Stratovolcanoes)
Explosive volcanoes contain felsic magma with a high level of silica, resulting in it having a high viscosity. As such, dissolved gases are trapped in the lava, which will result in a build up of pressure. When the magma comes to the surface, the built-up pressure results in an explosive eruption. The explosive release of gas and ash are also characteristic of felsic eruptions.
Explosive eruptions usually occur at subduction zones, since the partial melting of minerals in the subducting oceanic plate will increase the silica content of the magma. This results in a more felsic magma, and as the silica crystallises, the magma will become more viscous and thus eruptions tend to be explosive.
Explain how the silica content of a magma affects the explosiveness of an eruption.
Higher silica content is linked to higher explositivity. This is because as magma moves up through a chamber, the crystallising silica causes internal friction. This makes the magma more viscous, and will result in more violent explosive eruptions as opposed to low silica magma which is less viscous and tends to flow easily out of volcanoes in an effusive eruption.
Explain how the viscosity of a magma affects the explosiveness of an eruption.
In low viscosity magma, the gas bubbles are able to escape as the magma moves up through the chamber and this will generally result in an effusive eruption. High viscosity magma tends to have high gas content, since the gas bubbles will not be able to expand easily and instead build up pressure. When the magma reaches the surface, the gas bubbles will have high pressure inside them, and upon reaching atmospheric pressure, will burst explosively resulting in an explosive volcanic eruption.
Hazard v disaster
A hazard is an event that has potential for causing injury, loss of life, and/or damage to property and environment. A disaster is an event that occurs suddenly/unexpectedly in most cases and disrupts the normal course of life in the affected area; resulting in loss or damage to life, property or environment and is beyond the coping capacity of locally affected population/society and therefore external help is required.
Therefore, for a hazard to become a disaster, the geological event, such as an earthquake or volcano, must affect humans and exceed the ability of society to cope with this hazard.
