Sunday, February 25, 2007
Thursday, February 22, 2007
PART THREE: Types of Volcanic Eruptions
The diagram below demonstrates how the various eruption types can be expressed as a function of height of eruption column and explosiveness. These parameters can be quantified in ancient eruptions by measuring the dispersal and fragmentation of the erupted tephra deposits.
modified from Cas and Wright, 1988
Fissure eruptions are generated at several contemporaneous sites along a linear fracture, or along a parallel, but offset fracture system, such as that shown in the picture here.
Regional fracture systems can appear where the Earth's crust is broken and pulled apart by tensional forces. If these regions are underlain by reservoirs of basaltic magma, this low-viscosity melt will utilize the fractures and ascend through the crust to generate a fissure eruption. For example, Mid-oceanic ridges (divergent plate margins) typically extrude basaltic magma from fissure eruptions because these are areas where global-scale extension is coincident with the rise of par tially molten asthenosphere.
Southwest Rift Zone, Kilauea
Eruption Style: "The Curtain of Fire"
As fluid, gas-poor basaltic magma rises up through a fissure, it is extruded at the surface as a wall of incandescent, liquid-to-plastic fragments known as a curtain of fire. Two such eruptions are shown below from extrusive events on the Kilauea volcano, Hawaii. Fissure eruptions are quiescent, and the height of the airborne eruptive material is small, often only a few tens of meters. The basaltic fragments in the curtain of fire thus remain largely liquid when they hit the ground. These coherent lumps of hot, fluid lava are called spatter. When they land, they can be hot and fluid enough to fuse together to form an aggregate called agglutinate, or agglutinated spatter. Spatter commonly builds up as banks along the fissure sides to produce spatter ramparts.
Fissure eruptions are also common on the flanks of many large volcanoes and, therefore, they are not restricted to areas undergoing regional extension. Magma-filled fissures radiating from the summit regions of active volcanoes like Mt. Etna, Mauna Loa, and Kilauea propagate outward from the central vent system. Extrusion from these propagating fissures can produce elongate volcano morphologies, such as those that are typical of many Hawaiian shield volcanoes. Note, for example, the axial elongation of the Mauna Loa shield volcano shown in the image to the left. Mauna Loa Fissures
When fissures cease to erupt, the remaining magma residing in the fissure will cool and crystallize into an igneous rock intrusion. The resulting rock structure is called a dike. Dikes are tabular in shape, and they cut discordantly across adjacent rock layers. In areas of ancient volcanism, dikes are often delineated as resistant walls standing above more easily eroded rock types.
Dike from Shiprock, New Mexico
Hawaiian Eruptions
The calmest of all the eruption types, they are characterized by the effusive emission of highly fluid basalt lavas with low gas contents. The relative volume of ejected pyroclastic material is less than that of all other eruption types. The hallmark of Hawaiian eruptions is steady lava fountaining and the production of thin lava flows that eventually build up into large, broad shield volcanoes. Eruptions are also common in central vents near the summit of shield volcanoes, and along fissures radiating outward from the summit area. Lava advances downslope away from their source vents in lava channels and lava tubes.
Fire Fountains
Central-vent Hawaiian eruptions are noted for their spectacular jet-like sprays of liquid lava called fire fountains. These incandescent jets ascend hundreds of meters into the air. They can occur in short spurts, or last for hours on end.
The top of fire fountains are often carried away downwind to produce an airborne curtain of glowing fragments that showers downward. The indivudual liquid-to-plastic fragments (clasts) generally cool quickly by radiating their heat into the atmosphere. Thus, they are chilled and solid by the time they hit the ground, where they accumulate as cindery fragments called scoria. However, during very high eruption rates, the fire fountains become so dense that the clasts can no longer radiate heat freely into the atmosphere. These clasts are kept hot by the heat of surrounding clasts. Under these conditions the molten clasts, spatter, may hit the ground and fuse together to form agglutinated spatter cones and spatter ramparts. If the eruption rates are high enough, spatter-fed flows (clastogenic lavas) may develop as hot spatter fragments blend together on the ground and flow away.High Lava Fountain
Pu'u O'o volcano, Big Island of Hawaii, 1985
Lava Lakes
The fluid basalt associated with Hawaiian eruptions sometimes ponds in vents, craters, or broad depressions to produce lava lakes. In some cases, lava may erupt from a vent located within a crater, or surface lava flow may pour into a crater or broad depression. As lava lakes cool, they produce a grey-silver crust
that is usually only a few centimeters thick, as shown here in the image of the Kupaianaha lava lake. Active lava lakes contain young crust that is continually destroyed and regenerated. Convective motion of the underlying lava causes the crust to break into slabs and sink. This then exposes new lava at the surface that cools into a new crustal layer which will again break up into slabs and be recycled into the circulating lava beneath the crust.Pyroclastic particles like Pele's tears, Pele's hair, and reticulite, which are common in Hawaiian eruptions, are not present in Strombolian eruptions. Spatter-fed flows are minor. Instead, Strombolian eruptions are dominated by scoria fragments, which are highly vesiculated clasts of basalt with a cindery appearance. Tephra bombs and lapilli accumulate around the vent to produce well-bedded, and often well-sorted, scoria-fall deposits
In contrast to Hawaiian eruptions, true Strombolian eruptions produce little or no flowing lava. However, during the end stages of scoria-cone formation, it is not unusual for Strombolian activity to wane and give way to the calm extrusion of basaltic lava flows. As a general rule, a'a lava flows appear to be more common than the more
fluid pahoehoe types. As the vesiculating lava is de-gased toward the end of the eruption, it may ooze out from under the volcanic edifice to produce a lava flow, or pond in the vent to produce a lava lake. This will only occur if the underlying basalt is fluid enough to flow, which has not proved to be the case at Stromboli itself.Volcanian Eruptions
In contrast to basaltic strombolian eruptions, vulcanian eruptions are most often associated with andesitic to dacitic magma. The high viscosity of these magmas makes it difficult for the vesiculating gases to escape. This leads to the build up of high gas pressure and explosive eruptions. The ejected lava fragments do not take on the aerodynamic shapes common to Strombolian eruptions. This is partly due to the higher viscosity of the erupting magma, but also because the ejecta often incorporates a high proportion of crystalline material broken away from the rock plugging the throat of the volcano. These eruptions are often associated with growing lava domes, such as that at Mt. Pelée in 1902, and with the genration of pyroclastic flows from dome collapse.
bundant. These resemble a crusty loaf of bread broken by deep cracks that often expose a frothy interior. These large pyroclastic fragments form when viscous, gas-rich magma is ejected from the vent to produce a bomb whose exterior chills quickly to a glassy or fine-grained crust while in flight. The interior of the bomb, however, continues to vesiculate on the ground, which leads to expansion of the interior and cracking of the brittle outer crust.Although blocks and bombs are common in proximal deposits, the bulk of Vulcanian deposits is very fine grained and dominated by ash. The abundance of ash indicates a high degree of fragmentation, which can only be generated by magmas with high gas contents. In some cases, these high gas contents are derived from heated meteoric water. It is likely, therefore, that many vulcanian eruptions are at least partially hydrovolcanic. Although the ash-fall deposits generated by volcanian eruptions are highly fragmented, they are only moderately dispersed. This suggests a high degree of explosiveness (high fragmentation) associated with the development of eruptive columns that are of only moderate heights (moderate dispersal).
Plinian Eruptions
These spectacularly explosive eruptions are associated with volatile-rich dacitic to rhyolitic lava, which typically erupts from stratovolcanoes. The duration these eruptions is highly variable, from hours to days. The longest eruptions appear to be associated with the most felsic volcanoes. Although Plinian eruptions typically invlove felsic magma, they can occasionally occur in fundamentally basaltic volcanoes where the magma chambers become differentiated and zoned to create a siliceous top. An example of this was the Hekla eruption (Iceland) of 1947-48. Over the past 800 years, Hekla has had a history of generating violent initial eruptions of pumice, lasting a few hours, followed by prolonged extrusion of basaltic lava from the lower part of the chamber.
Rather than producing the discrete explosions that are typical of Vulcanian and Strombolian eruptions, Plinian eruptions generate sustained eruptive columns. Although they differ markedly from nonexplosive Hawaiian eruptions, Plinian eruptions are similar to Hawaiian fire fountaining in that both of these eruption types generate sustained eruption plumes. In both, the eruption plumes are maintained because the growing bubbles rise at about the same rate as the magma moves up through the central vent system.
Plinian eruptions generate large eruptive columns that are powered upward partly by the thrust of expanding gases, and by convective forces with exit velocities of several hundred meters per second. Some reach heights of ~45 km. These eruptive columns produce widespread dispersals of tephra which cover large areas with an even thickness of pumice and ash (see pumice-fall deposits). The region of pyroclastic fall accumulation is generally asymmetric around the volcano as the eruptive column is carried in the direction of the prevailing wind, as shown here in this NASA image of the Klyuchevskaya eruption in 1994.
The regions surrounding Plinian eruptions are not only subject to large volumes of pumice airfall (from 0.5 to 50 km3), but they are also subject to the most dangerous types of volcanic phenomena: pyroclastic flows and lahars. The occasional collapse of the eruptive column will generate hot, pyroclastic flows that advance down the volcano flanks at hurricane-force speeds. In addition, large volumes of water are often generated by the melting of snow banks and alpine glaciers during the eruption. The mixing of this water with unconsolidated tephra can generate volcanic mudflows (lahars). These features have the consistency of wet concrete, yet they can advance down slopes at the same rate as a rapidly moving stream.
PART TWO : Volcano Hazards!
There are many different kinds of volcano hazards. As observed from the picture above, it mostly involves the expelling of molten, hot rock in either molten, or solid. Some hazards are more severe than others depending on the size and extent of the event taking place and whether people or property are in the way.
Volcanic activity since 1700 A.D. has killed more than 260,000 people, destroyed entire cities and forests, and severely disrupted local economies for months to years. Even with our improved ability to identify hazardous areas and warn of impending eruptions, increasing numbers of people face certain danger. Scientists have estimated that by the year 2000, the population at risk from volcanoes is likely to increase to at least 500 million, which is comparable to the entire world's population at the beginning of the seventeenth century! Clearly, scientists face a formidable challenge in providing reliable and timely warnings of eruptions to so many people at risk.
Lahar
Lahar is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys. When moving, a lahar looks like a mass of wet concrete that carries rock debris ranging in size from clay to boulders more than 10 m in diameter. Lahars vary in size and speed. Small lahars less than a few meters wide and several centimeters deep may flow a few meters per second. Large lahars hundreds of meters wide and tens of meters deep can flow several tens of meters per second--much too fast for people to outrun.
As a lahar rushes downstream from a volcano, its size, speed, and the amount of water and rock debris it carries constantly change. The beginning surge of water and rock debris often erodes rocks and vegetation from the side of a volcano and along the river valley it enters. This initial flow can also incorporate water from melting snow and ice (if present) and the river it overruns. By eroding rock debris and incorporating additional water, lahars can easily grow to more than 10 times their initial size. But as a lahar moves farther away from a volcano, it will eventually begin to lose its heavy load of sediment and decrease in size.
What triggers a lahar?
Eruptions may trigger one or more lahars directly by quickly melting snow and ice on a volcano or ejecting water from a crater lake. More often, lahars are formed by intense rainfall during or after an eruption--rainwater can easily erode loose volcanic rock and soil on hillsides and in river valleys. Some of the largest lahars begin as landslides of saturated and hydrothermally altered rock on the flank of a volcano or adjacent hillslopes. Landslides are triggered by eruptions, earthquakes, precipitation, or the unceasing pull of gravity on the volcano.
Effects of lahars
Lahars racing down river valleys and spreading across flood plains tens of kilometers downstream from a volcano often cause serious economic and environmental damage. The direct impact of a lahar's turbulent flow front or from the boulders and logs carried by the lahar can easily crush, abrade, or shear off at ground level just about anything in the path of a lahar. Even if not crushed or carried away by the force of a lahar, buildings and valuable land may become partially or completely buried by one or more cement-like layers of rock debris. By destroying bridges and key roads, lahars can also trap people in areas vulnerable to other hazardous volcanic activity, especially if the lahars leave deposits that are too deep, too soft, or too hot to cross.
After a volcanic eruption, the erosion of new loose volcanic deposits in the headwaters of rivers can lead to severe flooding and extremely high rates of sedimentation in areas far downstream from a volcano. Over a period of weeks to years, post-eruption lahars and high-sediment discharges triggered by intense rainfall frequently deposit rock debris that can bury entire towns and valuable agricultural land. Such lahar deposits may also block tributary stream valleys. As the area behind the blockage fills with water, areas upstream become inundated. If the lake is large enough and it eventually overtops or breaks through the lahar blockage, a sudden flood or a lahar may bury even more communities and valuable property downstream from the tributary.
Look at the amount of debris the house is covered in!
Lahars are responsible of a large number of deaths in many tectonically unstable countries, due to their sudden and violent nature. They may be the deadliest of all volcanic hazards.
Volcanic Gases
Magma contains dissolved gases that are released into the atmosphere during eruptions. Together with the tephra and trapped air, volcanic gases can rise tens of kilometers into Earth's atmosphere during large explosive eruptions. Once airborne, the prevailing winds may blow the eruption cloud hundreds to thousands of kilometers from a volcano. The gases spread from an erupting vent primarily as acid aerosols (tiny acid droplets), and may condense in clouds to form acid rain. The volcanic gases that pose the greatest potential hazard to people, animals, agriculture, and property are sulfur dioxide, carbon dioxide and hydrogen fluoride. Locally, sulfur dioxide gas can lead to acid rain and air pollution downwind from a volcano. Globally, large explosive eruptions that inject a tremendous volume of sulfur aerosols into the stratosphere can lead to lower surface temperatures and promote depletion of the Earth's ozone layer. Because carbon dioxide gas is heavier than air, the gas may flow into in low-lying areas and collect in the soil. The concentration of carbon dioxide gas in these areas can be lethal to people, animals, and vegetation. A few historic eruptions have released sufficient fluorine-compounds to deform or kill animals that grazed on vegetation coated with volcanic ash; fluorine compounds tend to become concentrated on fine-grained ash particles, which can be ingested by animals.
Landslides
Landslides are large masses of rock and soil that fall, slide, or flow very rapidly under the force of gravity. These mixtures of debris move in a wet or dry state, or both. Landslides commonly originate as massive rockslides or avalanches which disintegrate during movement into fragments ranging in size from small particles to enormous blocks hundreds of meters across. If the moving rock debris is large enough and contains a large content of water and fine material, the landslide may transform into a lahar and flow downvalley more than 100 km from a volcano!
How are they generated?
Landslides are common on volcanoes because their massive cones (1) typically rise hundreds to thousands of meters above the surrounding terrain; and (2) are often weakened by the very process that created them--the rise and eruption of molten rock. Each time magma moves toward the surface, overlying rocks are shouldered aside as the molten rock makes room for itself, often creating internal shear zones or oversteepening one or more sides of the cone. Magma that remains within the cone releases volcanic gases that partially dissolve in groundwater, resulting in a hot acidic hydrothermal system that weakens rock by altering rock minerals to clay. Furthermore, the tremendous mass of thousands of layers lava and loose fragmented rock debris can lead to internal faults and fault zones that move frequently as the cone "settles" under the downward pull of gravity.
These conditions permit a number of factors to trigger a landslide or to allow part of a volcano's cone to simply collapse under the influence of gravity:
- intrusion of magma into a volcano
- explosive eruptions (magmatic or phreatic--steam-driven explosions)
- large earthquake directly beneath a volcano or nearby
- intense rainfall that saturates a volcano or adjacent tephra-covered hillslopes with water, especially before or during a large earthquake
Effects of volcano landslides
A landslide typically destroys everything in its path and may generate a variety of related activity. Historically, landslides have caused explosive eruptions, buried river valleys with tens of meters of rock debris, generated lahars, triggered waves and tsunami, and created deep horseshoe-shaped craters.
By removing a large part of a volcano's cone, a landslide may abruptly decrease pressure on the shallow magmatic and hydrothermal systems, which can generate explosions ranging from a small steam explosion to large steam- and magma-driven directed blasts. A large landslide often buries valleys with tens to hundreds of meters of rock debris, forming a chaotic landscape marked by dozens of small hills and closed depressions. If the deposit is thick enough, it may dam tributary streams to form lakes in the subsequent days to months; the lakes may eventually drain catastrophically and generate lahars and floods downstream.
Lava flows. (Yes, it does.)Lava flows are streams of molten rock that pour or ooze from an erupting vent. Lava is erupted during either nonexplosive activity or explosive lava fountains. Lava flows destroy everything in their path, but most move slowly enough that people can move out of the way. The speed at which lava moves across the ground depends on several factors, including (1) type of lava erupted and its viscosity; (2) steepness of the ground over which it travels; (3) whether the lava flows as a broad sheet, through a confined channel, or down a lava tube; and (4) rate of lava production at the vent.
Effects of Lava Flows
Everything in the path of an advancing lava flow will be knocked over, surrounded, or buried by lava, or ignited by the extremely hot temperature of lava. When lava erupts beneath a glacier or flows over snow and ice, meltwater from the ice and snow can result in far-reaching lahars. If lava enters a body of water or water enters a lava tube, the water may boil violently and cause an explosive shower of molten spatter over a wide area.
Lava destroys and burns everything in it's path.
What are pyroclastic flows?
Pyroclastic flows are high-density mixtures of hot, dry rock fragments and hot gases that move away from the vent that erupted them at high speeds. They may result from the explosive eruption of molten or solid rock fragments, or both. They may also result from the nonexplosive eruption of lava when parts of dome or a thick lava flow collapses down a steep slope. Most pyroclastic flows consist of two parts: a basal flow of coarse fragments that moves along the ground, and a turbulent cloud of ash that rises above the basal flow. Ash may fall from this cloud over a wide area downwind from the pyroclastic flow.
Explosive eruption of magma and solid-rock fragments or the collapse of a vertical eruption column of ash and larger rock fragments may generate pyroclastic flows.
Effects of pyroclastic flows
A pyroclastic flow will destroy nearly everything in its path. With rock fragments ranging in size from ash to boulders traveling across the ground at speeds typically greater than 80 km per hour, pyroclastic flows knock down, shatter, bury or carry away nearly all objects and structures in their way. The extreme temperatures of rocks and gas inside pyroclastic flows, generally between 200°C and 700°C, can cause combustible material to burn, especially petroleum products, wood, vegetation, and houses.
Pyroclastic flows are violent as well as dangerous. As they can move very quickly, up to 120km/hr, they can come suddenly and swiftly, destroying and engulfing everything in their path with hot volcanic ash. Many deaths have been attributed to these fast flowing hot ash flows, which leaves animated corpses in their wake ( as the extreme heat causes muscles to contract and thus, many unfortunate burn victims have their bodies locked in a curl or bent over position ) .
Tephra
Tephra is a general term for fragments of volcanic rock and lava regardless of size that are blasted into the air by explosions or carried upward by hot gases in eruption columns or lava fountains. Such fragments range in size from less than 2 mm (ash) to more than 1 m in diameter. Large-sized tephra typically falls back to the ground on or close to the volcano and progressively smaller fragments are carried away from the vent by wind. Volcanic ash, the smallest tephra fragments, can travel hundreds to thousands of kilometers downwind from a volcano.
Potential Effects of Volcanic Ash
Volcanic ash is highly disruptive to economic activity because it covers just about everything, infiltrates most openings, and is highly abrasive. Airborne ash can obscure sunlight to cause temporary darkness and reduce visibility to zero. Ash is slippery, especially when wet; roads, highways, and airport runways may become impassable. Automobile and jet engines may stall from ash-clogged air filters and moving parts can be damaged from abrasion, including bearings, brakes, and transmissions.
And that about sums up most of the volcanic hazards many volcanoes post to our immediate surroundings. Next time, we will go into further detail, giving specific examples and case studies, truly showing why volcanoes are really scary.
We would like to acknowledge the U.S.G.S. website as a major source for most of work above.
Part One: Volcanoes- Volcanism at plate tectonic boundaries
Volcanoes are formed at plate boundaries of the earth’s crust. Plate boundaries mark the sites where two plates are either moving away from one another, moving toward one another, or sliding past one another. There are three types of boundaries:
1) Divergent plate boundaries -- Plates diverge from one another at the site of thermally buoyant mid-oceanic ridges. Oceanic crust is created at divergent plate boundaries.
→gives rise to Sea-floor spreading/ Spreading center volcanism
2) Convergent plate boundaries -- Plates converge on one another at the site of deep oceanic trenches. Oceanic crust is destroyed at convergent plate boundaries.
→gives rise to Subduction zone volcanism
3) Transform plate boundaries -- Plates slide past one another.
→ lack of significant volcanism
Additionally, Intraplate volcanism/ hotspot describes volcanic eruptions within tectonic plates.
As the hot asthenosphere rises to shallow levels, it decompresses and melts to produce basalt magmas. These magmas pond in crustal magma chambers where they are tapped by vertical fractures to allow the rapid rise of magma to the surface. Fissure eruptions often result in erupting basalt generating vast lava fields. The lava quenches quickly against the bottom waters to produce characteristic bulbous shapes called pillow basalt.
The crustal portion of the subducting slab contains a significant amount of surface water, as well as water contained in hydrated minerals within the seafloor basalt. As the subducting slab descends to greater depths, it encounters greater temperatures pressures which cause the slab to release water into the mantle wedge overlying the descending plate. Water lowers the melting point of the mantle, causing it to melt. The magma produced varies from basalt to andesite in composition.
The magma rises upward to produce a series of volcanoes parallel to the oceanic trench, also known as volcanic islands. The chain of volcanic islands is called an island arc.
If the oceanic plate subducts beneath continental plate, then a belt of volcanoes called a volcanic arc. E.g. the Cascade volcanic arc of the U.S. Pacific northwest, and the Andes volcanic arc of South America.
Fig.1.4. Formation of a volcanic arc
The most volcanically active belt on Earth is known as the Ring of Fire, a region of subduction zone volcanism surrounding the Pacific Ocean.
The interior regions of plates with voluminous volcanism are called hotspots. Most hotspots are thought to be underlain by a large plume of anomalously hot mantle. These mantle plumes appear to be generated in the lower mantle and rise slowly through the mantle by convection. They rise as a plastically deforming mass with a bulbous plume head fed by a long, narrow plume tail. As the head impinges on the base of the lithosphere, it spreads outward into a mushroom shape. Such plume heads have diameters between ~500 to ~1000 km. Decompressional melting of this hot mantle source can generate huge volumes of basalt magma.
Intraplate seamount chains can be attributed to volcanism above a mantle hotspot to form a linear, age-progressive hotspot track. Mantle plumes appear to be largely unaffected by plate movements. As lithospheric plates move across stationary hotspots, volcanism will generate volcanic islands that are active above the mantle plume, but become inactive and progressively older as they move away from the mantle plume in the direction of plate movement. Thus, a linear belt of inactive volcanic islands and seamounts will be produced. E.g. The "Big Island" of Hawaii.
Fig.1.3 How Hawaii is formed above the stationary mantle plume, and becomes progressively older to the northwest.
Courtesy of the USGS.
:) Steph