Volcanism By Hans Ulrich Schmincke Presented By Mahmuda Afrin Badhan Mount Holyoke College ‘11 Volcanism By Hans Ulrich Schmincke http://www.fukubonsai.com/images3/VolcanoFlowAug2002.jpg
The motivation to study volcanoes comes from wanting to know what happens beneath volcanoes and why they erupt the way they do --- as well as the processes leading to it and how they behave afterwards. Details of recent eruptions are available at www.volcano.si.edu/gvp . (The Global Volcanism Network of the Smithsonian Institution and the US Geological Survey) A VOLCANO is not made on purpose to frighten superstitious people into fits of piety and devotion, nor to overwhelm devoted cities with destruction; a volcano should be considered as a spiracle to the subterranean furnace, in order to prevent the unnecessary elevation of land, and fatal effects of earthquakes; and we may rest assured, that they, in general, wisely answer the end of their intention, without being in themselves an end, for which nature had exerted such amazing power and excellent contrivance. - James Hutton, Theory of the Earth, Codicote, 1795 http://www.photopumpkin.com/wp-content/uploads/volcanoes-1.jpg
The Volcano-Magma System is divided into four zones for simplicity purposes.
Root zones – magma generated by partial melting of pre-existing older rocks.
Processes in the root zones explain why a volcano forms at a particular place on the Earth and not somewhere else at any given time, the characteristic magma composition, the way it erupts, i.e. quietly, gushy, highly explosive.
The intensity of the eruption depends on:
Viscosity of the magma and rise speed
Interaction with external water
Expansion and bursting of bubbles formed when the magma saturates with volatile compounds.
Also… All volcanoes emit gases, sometimes tens of thousands of tons without erupting explosively. For example, The Merapi volcano is a lava dome oozing out slowly over time and it appears to have lost most of it’s volatility by the time the magma reaches the surface. On the other hand, some volcanoes have ejecta rising as high as 40 km into the atmosphere and gases that rise even higher.
The volumes, heights and forms of volcanoes fundamentally depend on the physical and chemical properties of the magma.
In other words, they depend on the processes in the root zones of the volcanoes whose dynamics is determined by their plate tectonic setting.
Because of the motion due to plate tectonics, a single volcano does not tell much about its local origin.
Morphology and architecture does not tell the type of tectonic setting; e.g. caldera volcanoes (an irregular to subspherical collapse feature several km to tens of kilometers in diameter within a volcano – formed by roof subsidence over an evacuated magma chamber) form in very different types of tectonic environments.
Some volcanoes have forms governed by near-surface processes (e.g. interaction of magma and water) hence unsuitable to associate with any particular tectonic setting.
Most volcanoes on Earth form either along convergent or divergent plate margins or in the continental or oceanic plate interiors.
The magmas of volcanoes in each of these settings are characterized by specific chemical compositions.
The volatile contents are well reflected in the mode of eruption.
For example, volcanoes over subduction zones are highly explosive because there the water-rich sediments and oceanic crust are dehydrated at depth and the processes of magma formation are strongly governed by fluid release from the subducted slab.
Magma composition and volcanic morphology show more complex characteristics in hybrid plate tectonic settings.
Most magmas have basaltic composition and eruption temperatures of 1100 to 1250 degrees celcius, which is too high for them to have generated in the crust (where the temperature is about 500 degrees celcius).
Magma is generated by partial melting of rocks in the Earth’s mantle or, in much smaller amounts, in the lower crust. Volcanoes are basically features on the surface where the magma can erupt.
Why do magmas rise?
They rise because of their lower density compared to the surrounding rocks.
They also rise because of dynamic triggers, like the pressure due to rising mantle plumes.
Eruption of Mt. Pelee (Martinique, 1902) – Nuees Ardentes - A French term applied to a highly heated mass of gas-charged ash which is expelled with explosive force and moves at hurricane speed down the mountainside.
(Definition from http://volcano.und.edu/vwdocs/glossary.html )
Eruption of Taal Volcano (Philippines, 1965) – magma-water contact, base surges.
Eruption of Mt. St. Helens (Washington, USA, 18 May 1860) – lasted six years; sector collapse, lateral blasts, eruption forecast.
Eruption of El Chichon (Mexico, 1982) – sulphuric acid aerosols, climate impact.
Eruption of Pinatubo (Philippines, 19 June 1991) – another milestone in eruption prediction and effective mitigation and understanding volcanic climate forcing.
Montserrat (Lesser Antilles, 1995 – present) – mechanisms of dome growth, pyroclastic density currents.
Mt. Usu (Japan, March 31 – August 2000) – Phreatic eruptions and major ground deformation)
Miyakejima (Japan, July-August, 2000) – lateral magma withdrawal, caldera collapse and ensuing phreatomagmatic eruption
The Kilauea volcano on the island of Hawaii is know to exhibit more activity than any other volcano on Earth.
Its study has lead into more new insights into the architecture and dynamics of active volcanoes, flow and crystallization of lava and gas evolution than any other volcano.
Modern analytic instruments used to study volcanoes include mass spectrometers, electron microprobe, broad-band seismometers, remote sensing, GPS, high resolution aerial laser scanning and computer power.
Newer methods are the use of ion-probe, single crystal dating, and analytic probing into crystals to determine trace element, isotopic composition, and focused study of gas and fluid intrusion. They also help us look into the origins of magma and their evolution prior to eruption.
Observations that help us to detect, quantify and predict processes and eruptions include tracking of airborne ash clouds, deformational evolution of volcanic edifices by radar, infrared radiation of higher temperature areas on active volcanoes, quantitative detection of gas emissions – especially S0 2 (using total ozone mapping spectrometer), mapping aerosol clouds resulting from major Plinian eruptions, and mapping the surface of volcanoes with spatial resolution of better than 10m.
Volcanoes are best studied in interdisciplinary fields, although this has proven to be very difficult.
The Impact of Volcanic Activity on the Environment and on Society.
Media tends to only report volcanic activity when people or buildings have been harmed, usually because of the social and political problems arising and need of evacuation.
People are being informed fully of the potential natural hazards in advance where applicable so that they can prepare themselves for a possible crisis. This is quite a revolution because traditionally the responsible authorities retained such info in order to prevent panic amongst the population.
One major task of hazard-focused work is assembling hazard maps which is particularly analysis and mapping of
Products of previous volcanic eruptions.
Modern theoretical insight into transport mechanisms.
Energy involved in the eruption and transport mechanisms.
Forecasting likely energies released.
Pathways based on analysis of older deposits.
Public education has helped increase awareness of the importance of advanced preparation for such a crisis.
Most people are reluctant to evacuate unless given strict orders or convinced by widespread gossips.
Volcanologists are often consulted about the impact on climate from eruptions. We have the global warming and greenhouse effect issues but are climatic changes also caused by volcanic eruptions? Scientists study the volcanic forcing of climate to get the answers.