Chapter 7-9.docx

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Western University
Earth Sciences
Earth Sciences 2240F/G
Ron Podesta

Chapter 7: V olcanoes Basics Introduction • Plate tectonic association -volcanoes are associated with plate tectonics -plate boundaries associated with volcanoes and earthquakes Pacific Ocean- ocean floors spread causing eruptions (Ring of Fire) - volcanic rock decomposes quickly to produce rich soil - copper, gold, silver deposits - contribute to earth’s release of energy Yellow Stone- erupted three times in USAcausing no summer for two years with so much ash Magma Characteristics • €Definitions Magma- molten rock, hot liquid, gases, scattered mineral crystals Lava- magma when it is out on the surface Pyroclastic- magma that blows out into the air from a violent eruption -blobs of magma or very solid blocks or fine material Ash- form of pyroclastic that is very fine but pulverized material Volcanic Rock- after the magma has been cooled or solidified Plutonic Rocks- solidified beneath the surface Igneous Rocks- produced from volcanic or plutonic magma • €Viscosity: provides resistance to flow (opposite is fluid) o Temperature: High – more fluid Low-more viscous -higher temperature the greater the atoms vibrate breaking bonds and creating a liquid -900 C liquid would be 100,000 times more fluid than 600 C o Volatiles (structure of H2O; role) -most common volatile compound -not symmetrical (one end + charge, the other end – charge) DIPOLAR -slightly positive ends of the molecules attach lightly to oxygen instead of breaking up the chain for Si-O o Composition (Si-O structures) Silica Tetrahedra (1-Si/4-O) -more, the more viscous -few at high temperatures, many at low temperatures o General order of mineral formation Dissolved Water- water attached to other molecular structures Exsolved Water- there is too much water, extra water is left in contained bubbles of water 1 Types of Magma • Main compositional types (from more primitive to more evolved) o Basaltic (basalt volcanic rocks; flows most common) 1000-1300 C Si-O –50% o Andesitic (intermediate; pyroclastic flows most common) o 800-1000 C Si-O –60% o Rhyolitic (very viscous flows, pyroclastics and ash) 600-900 C Si-O –70% AA—rubbly-looking rock; contains less water Pahoehoe—smooth surfaced, ropey-looking rock • €General eruptive style Peaceful------------------------------------------>Explosive Water Content Low------------------------------------------>High Viscosity Low------------------------------------------>High Basalt Andesite Rhyolite Volcanic Settings • €90% associated with plate boundaries; 10% with plumes/hotspots • €Spreading centers o Partial melt generation (Fig.11) -oceanic spreading center in asthenosphere – temp—1200-1300 C o -gets to surface of the cool lithosphere (3km thick) -diverging of plates means the asthenosphere rises o MORB -30-40% of asthenosphere rock will melt into Basalt -higher temperatures resist breakdown -Mid-Ocean-Ridge-Basaltmagma produced at spreading centers o Density of asthenosphere peridotite (solid rock at spreading center) 3.3 g/cc VS. partial melt (basalt from periotite/relatively buoyant) 2.9 g/cc 2 • Subduction zones o “Ring of Fire” -Pacific Ocean: up the coast of SouthAmerica, all along the west coast of North America, across theAleutian Islands toAsia, then all the way back down into the South Pacific again o Role of water to promote melting -if we add water to a relatively dry rock (at high temperatures, of course), the rock will begin to melt at much lower temperature than in a dry system. -water comes from the subducting lithosphere ocean floor saturated with water (more dense than hot asthenosphere) o Learn basic model (Fig.13) ­base of continental crust comes in contact with older low temperature rocks -potassium-rich feldspars, some micas and quartz. The result when a high-temperature basaltic magma encounters a low temperature granitic rock is melting of the granitic rock and, thus, modification of the magma. Magma Chambers- blobs moving up to the surface with spurts and stops o Modifications to basaltic melt to yield andesitic/rhyolitic melt assimilation: magma works into solid rocks, loosens blocks of rock and make them fall into the chamber then melts all of the rock -magma composition changes to an intermediate state fractional crystallization: magma gradually cools as it gets to the surface but the crystals act heavier and as they melt they fall to the base of the magma chamber where they may for a rock made of those minerals entirely 3 magma mixing: one blob of magma joining and mixing with another magma that was sitting in the chamber for a much longer time Result: produce magma of composition intermediate to that of two pots that mix Andesitic and Ryolitic rocks are produced Volcanic Eruption Hazards (see locations from Fig.17) • Subduction Volcanoes are the most dangerous Lava flows - balsaltic lava flowing from vents - andesite and rhyolite lava are very slow and you can walk ahad of them Pyroclastic flows -fragmented rock and magma ejected from a volcano -ash also belongs in this category -mixtures of hot gases and pyroclastic material nuee ardente -so hot and glow like redness (glowing cloud) -Roman City of Pompeii ash -finest partical of pyroclastic particles, thick deposits thousands of km away from the volcano are normal gas lahars (Armero) –mudflows consisting of pyroclastic material, water and anything else it picked up on the way -like natural concrete viscous sludge moving quickly until it stops and it solidifies -Nevado del Ruiz –burried the town by a lahar -1845 rebuilt the town, same thing happened and 1985 same thing Volitiles are primarly water and CO2 with small traces of sulfur dioxide, fluorine, nitrogen and others Eruption Prediction, Forecasting • €Terminology of activity state Active-volcano may not be erupting but it is active Repose- periods between eruptions Dormant- non activity for a long time Extinct-nothing will happen ever again 1% of eruptions in past 100 years caused fatalities 6-10 weeks it lasts on average nearly 80% of those in two particular days: 8 May 1902 at St. Pierre when 30,000 were overcome by a nué e ardente, and 13 November 1985 atArmero • €Predictions Short-term • €must be clear and very precise, • €time expressed in days rather than weeks, 4 • danger zones must be identified Mt. St. Helens o increase in number of earthquakes o measurements of expansion of the dome o tilt of the upper regions o monitored gas emission; consistent increase in SO2 emitted Seismic -as the earthquakes became more shallow, they were more frequent short-period earthquakes indicate the magma chamber is fracturing -long-period earthquakes indicate the magma has left the chamber and is rising -51 fully automated seismographs in Hawaii -epicenter are plotted and found to migrate along fissures just ahead of magma (point of eruption can be predicted) Deformation (Ground Tilt) -measure expansion of volcano as it brings up magma with a Tiltmeter -sensitive to “one part per million” -Global Position System (GPS) measures the expansion of volcanoes but are still under research and fine tuning Gas -SO2 monitoring was non-productive -must measure Composition of Gasses given off -scientists working in Galeras Volcano until it erupted killing one -as magma rises, lighter molecules of CO2 exsolve from liquid faster than heavier ones like SO2 -CO2 leads the magma (higher ratio of CO2 to SO2 means eruption is imminent) -remote gas sensor has been developed to detect changes in gas from low flying air planes Summit Lakes- magma degasses, temperature increases (H2O), pH decreases (add Cl and S) and the color would change Thermal -infrared satellites rising magma near rising rocks to the active vents -thermal anomalies caused by -best future prospects for predicting eruptions Earth tides -have been used successfully if the magma were high enough in the chamber or vent, then the small distortion of an earth tide could cause it to spill out Long-term Danger Assessment -danger zone is important to predict -hazard maps are important -over-estimates the risk of pyroclastic flow deposits to make sure 5 -knowledge of a volcanoes behavior are important Historic Records- not always the best Geological Records- assumption that the past is the key to the future • €Mt. Baker, • €Mt. Ranier, • €Mt. St. Helens, • €Mt. Hood, • €Mt. Shasta • €Lassen Peak all reported as dangerous (Helens the most) two amazing anomalies: 1. the dramatic decreases during both World Wars 2. what can you say to explain the dramatic rise (from 1790 to now) shown? 3. notice that in the past few years, the curve has leveled off, particularly since we have begun to monitor by satellite. Volcanic Explosivity Index -Rank from 1-9 Danger assessment Hazard maps Importance of geological record 6 Chapter 8 Killer Eruptions Introduction Catastrophic Eruptions: -subduction zone -giant resurgent calderas Explosive only if: • €it is highly viscous (high Si) • €it has a high content of exsolved volatiles (like H2O and CO2) • €the cork or cap suddenly ruptures. Magma Chamber: -rich in silica and volatile -cooling on the way up (viscous magma {rhyolite/adensite) -assimilation, magma rises (2km from surface, eruption is inevitable) magma pressure is higher than cap pressure -water is mainly dissolved in the magma -no more SiO bonds to be made therefore water bubbles form racing to the top creating pressure on the cap -air above the vent has been heated therefore it helps raise the fragments into the air • Eruption causes (see Fig.1) o High Silica: o High Volatiles o Viscous Magma o Cap Ruptures Fig.1 • €Plinean: straight up explosions Pelean: explosions that occur out the side of the volcano A: Subduction Zone Eruption Case Study 1: Cascade Range • €Ring of Fire: -N/W/E coasts of Pacific -megathrust earthquakes • Cascade Range:­subduction tectonic activity inland from the coast • €Juan de Fuca activity; no volcanism elsewhere (see Fig.4) • €Mount St. Helens 1980 –evidence there is still activity going on (refilling) o Hazard prediction that an eruption might take place before the end of the century. Three years later, in 1978, two of them produced a detailed map of the areas at risk from ash and lahars o Evidence of magma motion o Eruption event 7 Phases Products Volume erupted: 1 km3 -earthquake occurred under the volcano and seismic activity increases (magma was coming up) -ash and gas were noted and tilt meters showed a large increase in the north-side (1 meter per day) -budge grew to 150 meters -another earthquake hit 5.1 starting an avalanche -cork removed due to avalanche (pyroclastic material went high into the atmosphere -Plinean phase of eruption (Height reaching 20km) 9hours -Pelean phase occurred with a blast out the side (pyroclastic/nuee ardente[gas]) -material moving at 1080km/h -after initial blast pyroclastic material shot out 100km/h and there was 1km^3 of pyroclastic debris that was blasted out 300 degrees -10 minutes of the explosion, hot rock and snow started a lahars 120km down stream • Mount Mazama 4860 BC -still active subduction zone -3350 meters high but lost 1000 m off the top o Volume of magma erupted: 46-58 km^3 -104 km^3 of pyroclastic deposits around the volcano o Crater Lake Caldera -Wizard Island- cones and domes just above water level -eventually will be destroyed by erosion of the crater and will cause a massive flood -8km wide and 600m deep (second deepest lake in NorthAmerica) o Main worry: future lahar – dust will
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