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• Why are different types of volcanoes different shapes? • What do volcano morphologies (shapes) tell us about their hazards? • What types of hazards can we expect from volcanoes in general? • How do we predict and prepare for volcanic eruptions? • How do we predict, evaluate, and plan for the danger of other hazards from different types of volcanoes? • What are the dangers from OUR volcanoes? Basaltic Volcanism: Chemistry • High Mg and Fe content • Low SiO2 content • Generally low volatile (gas) content Behavior • Viscosity is low • Slope of the volcano is low-angle (gently sloping) • Explosivity is low (generally) • Frequency of eruption can be constant, for years at a time. Basaltic eruptions on land occur at different tectonic settings around the world. Let’s look at 4: Hawai’i, the East African Rift Zone, the Columbia River Basalts, and Iceland. Where are they, and how do they differ? 7°10° The morphology of a volcano is strongly controlled by the viscosity of the magma. The Hawaiian shield volcano shown above is composed of interlayered basalt flows. Slope angles typcally range between 7°-10° for shield volcanoes. OCEANIC CRUST MANTLE Shield vocanoes are composed of interlayered basalt flows. Basaltic lava is erupted along linear vents lying above asthenosphere (upper mantle) hot spots. Recent pahoehoe basalt flows erupted on the “Big Island” of Hawaii. Mauna Loa is shown in the background. Note pressure ridges that formed as lava cooled and was still moving. The Hawaiian Islands and Emperor Seamount chains formed over a mantle hotspot. As the Pacific Plated moved to the northwest, new islands form above the hotspot. The age of the islands becomes progressively older to the northwest. Note that the plate changed directions ~38 million years ago from a north-south direction to northwest-southeast direction. Hawaiian basalt flows can travel great distances from their linear vent because of the low viscosity of the flow. As the oceanic plate moves away from the hotspot the crust begins to cool causing its density to increase. The crust subsides causing the island to submerge relative to sea level. The volcanic island is subject to constant wave erosion as well. In tropical oceans limestone reefs can form around the island where the water is shallow. Eventually the volcano can become completely eroded leaving a fringing reef island defined as an atoll. Over extended periods of time the island can become completely submerged and form a seamount (or guyot). The nature of volcanic hazards is strongly tied to the viscosity and gas content of the magma. Basaltic composition eruptions tend to be non-explosive (effusive). Basaltic lava is being actively erupted along the East African rift zone. A large extensional fault is shown in the foreground of the image. What tectonic setting is this? The East African rift zone is composed of basaltic lava plateaus. Steam from a recent eruption is seen in the middle of the image. Columbia River Basalts (CRBs) are widespread, covering SE Washington, Western Idaho, and Northern Oregon. They erupted over millions of years. Plateau basalts form where basalt flows are erupted along linear rifts in terrestrial settings, such as continental rift zones (e.g., East Africa) or back arc basins (e.g., Columbia Plateau). Miocene basalt flows comprise the Columbia Plateau. The low viscosity flows were erupted from linear rifts located near the border of Washington, Oregon and Idaho. Some of these flows made it to the Pacific Ocean near the modern Columbia Gorge. The deep canyon of Frenchman’s Coulee was eroded “recently” by floodwaters from the breeched ice dam occupying glacial Lake Missoula 15,000 years ago. Iceland is situated over the mid-Atlantic seafloor spreading margin. There is also a hotspot located beneath the island. Basaltic volcanism is prevalent on this island nation. Because Iceland is located at polar latitudes Arctic Circle glacial ice presents additional problems when Icelandic volcanoes erupt. In 1973 a basaltic eruption almost destroyed the important fishing port of Vestmannaeyjar on the Island of Heimaey. Groundwater and surface water mixed with magma produce gas, which resulted in large amounts of scoria being erupted during the Heimaey eruption. Water pumped from the ocean was used to quench lava flow and divert it towards the sea. Property damage may be extensive from effusive basaltic eruptions, but loss of life is rare. Two weeks after the eruption ceased the pyroclastic material (ash and cinders) were cleaned from the buildings and streets. Heimaey was saved. Andesitic Volcanism: Chemistry • Moderate Mg and Fe content • Moderate SiO2 content • Generally high volatile (gas) content Behavior • Viscosity is high • Slope of the volcano is high-angle (tall and pointy, or irregular) • Explosivity is high • Frequency is 100s of years. • Mountains can be built in 10,000s of years. 25-35° The slope angle of the of strato (composite) volcanoes is controlled by the angle of repose (25-35°) for unconsolidated pyroclasts and the viscosity properties of the silica-rich andesite and dacite flows. The volcanoes shown above are part of the Aleutian Island, Alaska. Strato (composite) volcanoes form along subduction zones where partially melted ocean crust, marine sediments and water-enriched mantle rock rise to the surface. The image shown above is a cross-section of the Japanese subduction zone. Strato (composite) volcanoes form from interlayered pyroclastic and intermediate (andesite or dacite) lava flows. Strato volcanoes can become larger over time with subsequent eruptions. Strato (composite) volcanoes become large with upbuilding. Eruptions along the flank can occur where extensional cracks can develop as magma upwells within the magma chamber. Over time magma can become more silica-rich through fractionation and the volcano’s life cycle can end with a cataclysmic eruption and emptying of the magma chamber. Cascade Volcanism: Calderas, and stratocones, and lahars… Oh MY! The Cascade volcanoes are typical strato volcanoes with slope angles between 25°-35°. Mount Rainier is shown in the foreground with Mt. St. Helens lying to upper right. Volcanism along the Cascadia Subduction Zone is a potential hazard to those residents living close to the volcanoes or adjacent to river valleys that originate from the volcanoes. Mt. Rainier has ~90% of all the glacial ice in the Cascade Mountains. Even small eruptions can cause catastrophic melting and result in lahar flows (volcanic mudflows). Lahar flows are generated when snow or glacial ice is melted by hot pyroclastics. The slurry of mud and debris will tend to flow down the slope of the volcano within preexisting drainages. Lahar flows are very dangerous and more difficult to predict their magnitude as they can be generated by relatively small eruptions. Lahar flows are the number one potential volcanic hazard generated from Mount Rainier. There have been several large lahar flows over the past 6000 years. Over the past 6000 years several major lahar flows have inundated river valleys on Mt. Rainier. The Oceola lahar occurred 5300 years ago and flowed as far as Tacoma and southern Puget Lowland. The Electron lahar occurred ~500 years ago and reached the town of Puyallup. Why are the areas of highest risk farther up the river valleys? What factors are important to consider when you think of risk factors? (Freqency and Magnitude of eruption or event). Mt. Rainier lahar deposits appear as hummocky topography within the Puyallup River valley. Lahar flows would reach the towns of Puyallup and Orting in less than 1 hour. A new development being constructed on a Mt. Rainier lahar flow (lobe outlined by white dashed lines) near Enumclaw, Washington. Large lithic fragments are preserved in the Paradise Lahar deposits. Lahar deposits may contain large volcanic clasts incorporated during an eruption and or reworking exising sediment. Prior to the 1980 eruption Mt. St. Helens stood at nearly 10,000 feet above sea level. Prior to the culminating eruption on May 18th, 1980, seismic activity was frequent as the magma moved upward beneath the volcano. Why does movement of magma produce seismic activity (minor earthquakes)? Mt. St. Helens was truly the gem of the Cascades prior to its recent eruption. Note the beautiful symmetric cone prior to the eruption. Mt. St. Helens was predisposed to landsliding on the north face of the volcano because of weakened rock. Following the landslide on the north slope of MSH the pyroclastics were erupted in a horizontal and vertical column. The “Blast Zone” on the north side of MSH was the result of the slope failure on the north face and high velocity pyroclastics and gas being ejected in the horizontal plume. The most devastating volcanic eruptions are pyroclastic eruptions that are most often associated with subduction zone volcanism. Dense clouds of super-heated gas and ash descend down the flanks of the volcano with velocities of 100-160 km/hr. Pyroclastic eruptions are also known as nuee´ ardentes (glowing avalanche). Large amounts of volcanic ash (tephra) were erupted into the atmosphere. The ash plume was blown eastward by the prevailing westerly winds. The MSH ash plume provided very beautiful sunrises and sunsets on the east side of the Cascades. Ash plumes can present very dangerous hazards to air traffic and can destroy machinery. What is the hardness of ash shards? Very few living organisms survived the catastrophic effects of the pyroclastic eruption within the “Blast Zone” on the north face of MSH. Most of the 57 fatalities from the MSH eruption occurred in the Blast Zone in restricted areas. MSH lahar flows choked streams and inundated low lying topography. MSH was almost 10,000 feet high prior to the 1980 eruption. Following the eruption it had an elevation of 8363 feet. It is rebuilding its summit today in the effusive stage. Mount St. Helens is rebuilding its dacite lava dome. The lava dome has attained almost 1400 feet of vertical relief. Large amounts of volcanic ash (tephra) were erupted into the atmosphere. The ash plume was blown eastward by the prevailing westerly winds. Isopach (“Iso” means” the same.” ash layer thickness) map for ash distribution of Mount Saint Helens (1980) eruption. Why is the ash layer thickest near Ritzville and not directly proximal to the volcano itself? Excavating a trench behind the Hyak moraine at Snoqualmie Pass (ca. 1990). Mt. St. Helens W Tephra (1500 AD) Mt. St. Helens Y Tephra (1900 B.C.) Mazama O Tephra (4800 B.C.) How do geologists predict the potential volcanic eruptions? They determine the past eruptive history of volcanoes using dated ash layers. How does ash thckness and grain size vary with distance from the volcano versus magnitude of the eruption? Tephra distribution of Cascade volcanoes (Mt. Mazama, Mt. St. Helens, and Glacier Peak). Caldera-Forming Eruptions Chemistry: • Low Fe and Mg content • High SiO2 content (felsic) • High volatile content • Large volumes of magma located in the “magma chamber.” Behavior: • Extremely explosive • “Cataclysmic” events produce ash and debris (pyroclastic) deposits 10s to100s of meters thick near the volcano, and distribute ash (tephra) globally. • Single eruptions can last for many days. • Very infrequent (100,000s to millions of years). • Following caldera formation, more mafic magma often erupts. 1 4 Crater Lake, Oregon formed from the collapse of Mt. Mazama following a cataclysmic eruption 6800 years ago. Caldera-forming volcanic centers are sometimes referred to as Supervolcanoes. What is a supervolcano? ‘The term "supervolcano" implies a volcanic center that has had an eruption of magnitude 8 on the Volcano Explosivity Index (VEI), meaning the measured deposits for that eruption is greater than 1,000 cubic kilometers (240 cubic miles).’ http://volcanoes.usgs.gov/volcanoes/yellowstone/yellowstone_sub_page_49.html For a sense of how large this is, Mt. St. Helens erupted about 0.25 cubic kilometers of material. Pyroclastic (Pyro = fiery; clastic = pieces) cones form largely from erupted pyroclasts (aka, cinders). Pyroclasts solidify in the air and fall to the ground as air-fall. High gas content is a key component of pyroclastic eruptions. Pyroclastic cones can be composed of EITHER felsic (pumice) to mafic (scoria) compositions. Pyroclastic cones may collapse to form maars following an explosive eruptive phase that empties the underlying magma chamber. Porous texture Pyroclasts, such as the pumice lapilli shown above, consist of porous textures derived from gas bubbles preserved in the rock (see inset image). Pyroclastic cones attain slope angles ~35°, which is the angle of repose for unconsolidated lapilli (cinders). Ancient rocks often tell a story of previous volcanic activity. We just need to know what we’re seeing… Columnar structures form in basalt flows as a “trade off” due to contraction during cooling and efficiency in packing. The thicker the lava flow, the thicker the diameter of the columnar structure (cooling is slower in thick lava flows). Pillow basalts on ocean floor. palagonite Pillow basalts form when basalt is erupted into water, such as along a seafloor spreading margin or into a lake. Palagonite is reddish-orange clay mineral that forms from rapidly weathered basaltic glass. It often surrounds pillow structures. palagonite Pillow basalts form when basalt is erupted into water, such as along a seafloor spreading margin or into a lake. . Palagonite is reddish-orange clay mineral that forms from rapidly weathered basaltic glass. It often surrounds pillow structures. Plutons consist of intrusive igneous rock that slowly crystallized from magma underground. Plutons are classified based on their size and orientation relative to the layered sedimentary rock in which they are intruded. How do plutons become exposed at the surface of the earth? Uplift and erosion of overlying country rock. The plutons that you observe near Mt. Rainier and Mt. Index today formed under the Cascade volcanic arc 14 to 20 million years ago and were exposed due to continued uplift and erosion of the overlying rock. Gabbro Diorite Granite Plutons are composed of coarse-grained igneous rock of all compositions. Large batholiths can form along collisional and convergent tectonic boundaries. Large batholiths underlie or are exposed within the Cascades, Rockies, Sierra Nevada, Appalachian and Himilayan Mountain Ranges. Roof pendant (dark metamorphosed sedimentary rock) comprised of remnant “country rock” is draped over the younger intruded Sierra Nevada batholith (light colored granodiorite). Why is the roof pendant metamorphosed? The Sierra Nevada batholith was intruded into the country rock over 90 million years ago. Batholiths are irregular in form and have a surface area greater than 100 km2. Dark xenoliths (mafic inclusions) are preserved in the plutonic rock and represent piece of the magma chamber that did not completely melt and become assimilated into magma body. Check out the stairs adjacent to Suzzalo library, can you see xenoliths preserved in the granodiorite rock matrix? Plutonic bodies, such as the stocks shown above, are more resistant to erosion and stand in relief as high topography. Stocks are irregularly shaped plutons which are less than 100 km2 in surface area. Note that a stock may be an extension of a larger batholith not completely exposed by erosive processes. Stocks are irregular-shaped plutons that have areas <100 km2. The stock shown above has deformed the layered country rock. The Henry Mountains, Utah, is a classic example of an exhumed laccolith. Dikes are tabular intrusive bodies that are oriented prependicular or oblique to layered country rock. Sills are tabular plutonic bodies that are oriented parallel to pre-existing sedimentary layers. Volcanic necks form when resistant magma is intruded into the vent and radial fractures of a volcano. Why do the radial fractures form as the magma rises? Over time, the less resistant rock (i.e., pyroclasts and less consolidated lava flows) comprising the flank of the volcano is eroded away leaving the resistant rock exposed in relief. Shiprock, New Mexico is a classic example of a volcanic neck. Note radial dikes projecting outward from beneath the vent. Footprints of ealry hominids are preserved in volcanic ash in East Africa. Volcanic hazards have posed a problem for humans since their earliest origins in the East African rift zone. The St. Pierre pyroclastic eruption in 1902 killed over 30,000 people on the island of Martinique in the Carribbean. There were only two survivors from this devastating eruption of Mt. Pelee. Mt. Vesuvius (seen in the background) had a cataclysmic pyroclastic eruption in 79 AD which destroyed the Roman cities of Pompeii (in foreground) and Herculaneum. The pyroclastic flows buried the city of Pompeii, killing its inhabitants, including Pliny the Elder. The modern city of Naples (seen in the background) has been constructed on top of the pyroclastic and mudflows (seen in the background above the preserved ruins. Note that the ancient Roman architecture was well preserved when entombed within the silicate ash flows. Roman columns of Pompeii’s buildings were preserved under thick ash flows. Important archaeological finds relevant to Roman culture and every day life were unearthed following their discovery in the 18th century. Casts of Roman citizens who perished in the eruption were well-preserved in the Mount Vesuvius ash. You can still see wrinkles in the cloth of the Roman toga. How do volcanologists monitor volcanoes? http://volcanoes.usgs.gov/observatories/cvo/ monitoring_videos.html • Deformation (tilt meters, GPS) • Gas emissions (sampling at the vent, airplanes, satellite remote sensing) • Earthquakes (seismometers deployed around the volcano)