John Harris's List: Dynamic Planet

• A shield volcano is a large volcano with shallow-sloping sides.
• Shield volcanoes are formed by lava flows of low viscosity — lava that flows easily. Consequently, a volcanic mountain having a broad profile is built up over time by flow after flow of relatively fluid basaltic lava issuing from vents or fissures on the surface of the volcano. Many of the largest volcanoes on Earth are shield volcanoes. The largest in terms of area covered is Mauna Loa of Hawaii; the tallest measured from its base under the ocean, however, is Mauna Kea of Hawaii. All the volcanoes in the Hawaiian Islands are shield volcanoes.
• Shield volcanoes can be so large that they are sometimes considered to be a mountain range

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• A cinder cone or scoria cone is a steep conical hill of volcanic fragments that accumulate around and downwind from a volcanic vent.^[1] The rock fragments, often called cinders or scoria, are glassy and contain numerous gas bubbles "frozen" into place as magma exploded into the air and then cooled quickly.^[1] Cinder cones range in size from tens to hundreds of meters tall.^[1] Cinder cones are made of pyroclastic material.
• Many cinder cones have a cereal bowl-shaped crater at the summit. Lava flows are usually erupted by cinder cones, either through a breach on one side of the crater or from a vent located on a shank.^[1] If the crater is fully breached, the remaining walls form an amphitheatre or horseshoe shape around the vent. Lava rarely issues from the top (except as a fountain) because the loose, uncemented cinders are too weak to support the pressure exerted by molten rock as it rises toward the surface through the central vent.^[1]
• Cinder cones are commonly found on the flanks of shield volcanoes, stratovolcanoes, and calderas.^[1] For example, geologists have identified nearly 100 cinder cones on the flanks of Mauna Kea, a shield volcano located on the Island of Hawaii.^[1] These cones are also referred to as scoria cones, cinder, and spatter cones.^[1]

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• A stratovolcano, sometimes inappropriately called a composite volcano, is a tall, conical volcano with many layers (strata) of hardened lava, tephra, and volcanic ash. Stratovolcanoes are characterized by a steep profile and periodic, explosive eruptions. The lava that flows from them is viscous; it cools and hardens before spreading far. The magma forming this lava is classified as felsic, having high to intermediate levels of silica (as in rhyolite, dacite, or andesite), unlike less viscous mafic magma, which forms shield volcanoes (such as Mauna Loa in Hawaii), having wider bases and more gently sloping profiles.^{[citation needed]}

   

  Although stratovolcanoes are sometimes called composite volcanoes because of their composite layered structure built up from sequential outpourings of eruptive materials, volcanologists prefer stratovolcano. The term composite volcano, is best reserved for those edifices with more than one eruptive peak. Stratovolcanoes are one of the most common types of volcanoes.
• Stratovolcanoes are common in subduction zones, forming chains or 'arcs' along tectonic plate boundaries where oceanic crust is drawn under continental crust (Continental Arc Volcanism, e.g. Cascade Range, central Andes) or another oceanic plate (Island arc Volcanism, e.g. Japan, Aleutian Islands). The magma that forms stratovolcanoes rises when water, which is trapped, both in hydrated minerals and in the porous basalt rock of the upper oceanic crust, is released into mantle rock of the asthenosphere above the sinking oceanic slab. The release of water from hydrated minerals is termed "dewatering," and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths. The water freed from the rock lowers the melting point of the overlying mantle rock, which then undergoes partial melting and rises due to its lighter density relative to the surrounding mantle rock, and pools temporarily at the base of the lithosphere. The magma then rises through the crust, incorporating silica-rich crustal rock, leading to a final intermediate composition (see Classification of Igneous Rock). When the magma nears the surface, it pools in a magma chamber under the volcano. There, the relatively low pressure allows water and other volatiles (CO₂, S^2-, Cl^-) dissolved in the magma to escape from solution, as occurs when a bottle of carbonated water is opened. Once a critical volume of magma and gas accumulates, the obstacle provided by the volcanic cone is overcome, leading to a sudden explosive eruption.
• In recorded history, explosive eruptions at subduction zone (convergent-boundary) volcanoes have posed the greatest hazard to civilizations.^[1] Subduction-zone stratovolcanoes like Mount St. Helens and Mount Pinatubo typically erupt with explosive force, because the magma is too stiff to allow easy escape of volcanic gases.

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• In volcanology, a lava dome or plug dome is a roughly circular mound-shaped protrusion resulting from the slow eruption of felsic lava (usually rhyolite or dacite) from a volcano, or from multiple lava episodes of different magma types. The characteristic dome shape is attributed to high levels of silica in the magma, causing the magma to be quite viscous and thick. The viscosity of the lava prevents it from flowing far from the vent that it extrudes from, causing it to solidify quickly and build on previous volcanic extrusions, creating a dome-like shape. Domes may reach heights of several hundred meters, and can grow slowly and steadily for months or years. The sides of these structures are composed of unstable rock debris. Due to the possibility of the building up of gas pressure, the dome can experience more explosive eruptions over time. When part of a lava dome collapses while it still contains molten rock and gases, it can produce a pyroclastic flow, one of the most lethal forms of a volcanic event. Other hazards associated with lava domes are the destruction of property, forest fires, and lahars triggered by pyroclastic flows near snow and ice. Lava domes are one of the principal structural features of many stratovolcanoes worldwide.
• Characteristics of lava dome eruptions include shallow, long-period and hybrid seismicity, which is attributed to excess fluid pressures in the contributing vent chamber. Other characteristics of lava domes include their spherical dome shape, cycles of dome growth over long periods, and sudden onsets of violent explosive activity.^[1] The average rate of dome growth may be used as a rough indicator of magma supply, but it shows no systematic relationship to the timing or characteristic of lava dome explosions.^[2]

• An explosive eruption is a volcanic term to describe a violent, explosive type of eruption. Mount St. Helens in 1980 was an example. Such an eruption is driven by gas accumulating under great pressure. Driven by hot rising magma, it interacts with ground water until the pressure increases to the point at which it bursts violently through the overmantle of rock. In many cases the rising magma will contain large quantities of partially dissolved gas. Sometimes a lava plug will block the conduit to the summit, and when this occurs, eruptions are more violent. With the sudden release of pressure following the initial explosion, the gas comes out of solution violently and explosively. This secondary explosion is often far more violent than the first one; the rocks, dust, gas and pyroclastic material may be blown 20 km into the atmosphere at rate of up to 100,000 tonnes per second,^{[citation needed]} traveling at several hundred meters per second. This cloud will then collapse, creating a pyroclastic flow of hot volcanic matter.

• Effusive eruptions are a volcanic phenomenon; in some ways the opposite of explosive eruptions. An effusive eruption is characterized by an outpouring of low viscosity lava which has a fairly low volatile content. Usually, shield volcanoes have effusive eruptions. Basaltic magma erupted in an effusive manner may become an aa or a pahoehoe and silicic magma blocky lavas and domes.

• A submarine eruption is a type of volcanic eruption where lava erupts under an ocean. Most of the Earth's volcanic eruptions are submarine eruptions, but few have been documented because of the difficulty in monitoring submarine volcanoes. Most submarine eruptions occur at mid-ocean ridges.^[1]

• A pyroclastic flow (also known scientifically as a pyroclastic density current)^[1] is a common and devastating result of some volcanic eruptions. The flows are fast-moving currents of hot gas and rock (collectively known as tephra), which travel away from the volcano at speeds generally as great as 450 mi/h (700 km/hr).^[2] The gas can reach temperatures of about 1,000 °C (1,830 °F). The flows normally hug the ground and travel downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope.

• Causes

   

  There are several scenarios which can produce a pyroclastic flow:

   
   
  • Fountain collapse of an eruption column from a plinian eruption (e.g., Mount Vesuvius's destruction of Pompeii, see Pliny the Younger). In such an eruption, the material ejected from the vent heats the surrounding air and the turbulent mixture rises, through convection, for many kilometres. If the erupted jet is unable to heat the surrounding air sufficiently, there will not be enough convection to carry the plume upwards and it falls, then to flow down the flanks of the volcano.
   
  • Frothing at the mouth of the vent during degassing of the erupted lava at the mouth. This can lead to the production of a type of igneous rock called ignimbrite. This occurred during the eruption of Novarupta in 1912 which produced the largest flows to be generated during recorded history.
   
  • Gravitational collapse of a lava dome or spine, with subsequent avalanches and flow down a steep slope e.g., Montserrat's Soufrière Hills volcano.
   
  • Fountain collapse of an eruption column associated with a vulcanian eruption e.g., Montserrat's Soufrière Hills volcano has generated many pyroclastic flows and surges.
   
  • The directional blast (or jet) when part of a volcano explodes or collapses (e.g. the May 18, 1980 eruption of Mount St. Helens) As distance from the volcano increases, this rapidly transforms into a gravity-driven current.
• The kinetic energy of the moving boulders will flatten trees and buildings in their path. The hot gases and high speed make them particularly lethal. The towns of Pompeii and Herculaneum, Italy, for example, were famously engulfed by pyroclastic surges in 79 AD with many lives lost.
• A recent documentary film showed tests by a research team at Kiel University, Germany of pyroclastic flows moving over water[1]. The tests revealed that hot ash traveled over the water on a cloud of superheated steam, continuing to be a pyroclastic flow after crossing water; the heavy matter precipitated out of the flow shortly after initial contact with the water, creating a tsunami due to the precipitate mass.

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• A lahar is a type of mudflow or landslide composed of pyroclastic material and water that flows down from a volcano, typically along a river valley.
• Lahars have the consistency of concrete: fluid when moving, then solid when stopped.
• Lahars can be extremely dangerous, because of their energy and speed. Large lahars can flow several dozen meters per second and can flow for many kilometres, causing catastrophic destruction in their path.
• Snow and glaciers can be melted by a pyroclastic flow during an eruption

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• Volcanic gases include a variety of substances given off by active (or, at times, by dormant) volcanoes. These include gases trapped in cavities (vesicles) in volcanic rocks, dissolved or dissociated gases in magma and lava, or gases emanating directly from lava or indirectly through ground water heated by volcanic action.

   

  The sources of volcanic gases on Earth include:

   
   
  • primordial and recycled constituents from the Earth's mantle,
   
  • assimilated constituents from the Earth's crust,
   
  • groundwater and the Earth's atmosphere.
   
   

  Substances that may become gaseous or give off gases when heated are termed volatile substances.

• Composition

   

  The principal components of volcanic gases are water vapor (H₂O), carbon dioxide (CO₂), sulfur either as sulfur dioxide (SO₂) (high-temperature volcanic gases) or hydrogen sulfide (H₂S) (low-temperature volcanic gases), nitrogen, argon, helium, neon, methane, carbon monoxide and hydrogen. Other compounds detected in volcanic gases are oxygen (meteoric), hydrogen chloride, hydrogen fluoride, hydrogen bromide, nitrogen oxide (NO_x), sulfur hexafluoride, carbonyl sulfide, and organic compounds. Exotic trace compounds include methyl mercury, halocarbons (including CFCs), and halogen oxide radicals.

   

  The abundance of gases varies considerably from volcano to volcano. However, water vapor is consistently the most common volcanic gas, normally comprising more than 60% of total emissions. Carbon dioxide typically accounts for 10 to 40% of emissions.^[1]

   

  Volcanoes located at convergent plate boundaries emit more water vapor and chlorine than volcanoes at hot spots or divergent plate boundaries. This is caused by the addition of seawater into magmas formed at subduction zones. Convergent plate boundary volcanoes also have higher H₂O/H₂, H₂O/CO₂, CO₂/He and N₂/He ratios than hot spot or divergent plate boundary volcanoes.^[1]
• Hazards
• Volcanic gases were directly responsible for approximately 3% of all volcano-related deaths of humans between 1900 and 1986.^[1] Some volcanic gases kill by acidic corrosion; others kill by asphyxiation. The greenhouse gas, carbon dioxide, is emitted from volcanoes, although volcanic emissions account for less than 1% of the annual global total.^[3] Some volcanic gases including sulfur dioxide, hydrogen chloride, hydrogen sulfide and hydrogen fluoride react with other atmospheric particles to form aerosols.

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• Volcanic ash consists of small tephra, which are bits of pulverized rock and glass created by volcanic eruptions,^[1] less than 2 millimetres (0.079 in) in diameter.
• Ash deposited on the ground after an eruption is known as ashfall deposit. Significant accumulations of ashfall can lead to the immediate destruction of most of the local ecosystem, as well the collapse of roofs on man-made structures. Over time, ashfall can lead to the creation of fertile soils.

• he most devastating effect of volcanic ash comes from pyroclastic flows. These occur when a volcanic eruption creates an "avalanche" of hot ash, gases, and rocks that flow at high speed down the flanks of the volcano. These flows can be impossible to outrun.^[12] As well as being impossible to outrun, they are almost as difficult to predict. In many cases prediction has been based on the topography of a region, only to see a valley fill and overflow^[13]. In 1902, the city of St. Pierre in Martinique was destroyed by a pyroclastic flow which killed over 29,000 people.^[14]

   

  Volcanic ash (by itself) is not poisonous, but inhaling it may cause problems for people whose respiratory system is already compromised by disorders such as asthma or emphysema. The abrasive texture can cause irritation and scratching of the surface of the eyes. People who wear contact lenses should wear glasses during an ashfall, to prevent eye damage. Furthermore, the combination of volcanic ash with moisture in the lungs can create a substance akin to liquid cement.

   

  Therefore, people should take caution to filter the air they breathe with a damp cloth or a face mask when facing an ashfall. Ash is very dense, as only 100 millimetres (3.9 in) of ash leads to the collapse of weaker roofs. A fall of 300 millimetres (12 in) leads to the death of most vegetation, livestock, the wiping out of aquatic life in nearby lakes and rivers, and unusable roads.^[15] Accompanied by rain and lightning, ashfall leads to power outages, prevents communication, and disorients people.^[8]
• Volcanic ash jams machinery. This poses a great danger to aircraft flying near ash clouds. There are many instances of damage to jet aircraft as a result of an ash encounter.

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• A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption. They are sometimes confused with volcanic craters.

• A crater lake is a lake that forms in a volcanic crater, caldera, or maar. Incoming precipitation fills the depression to form a deepening lake, until an equilibrium is reached between the rate of water coming in and the rate of water loss due to evaporation, subsurface drainage, and possibly also surface outflow if the lake fills the crater up to the lowest point on its rim.
• While many crater lakes are picturesque, they can also be deadly. Gas discharges from Lake Nyos suffocated 1,800 people in 1986, and crater lakes such as Mount Ruapehu's often contribute to destructive lahars.

• A volcanic plateau is a plateau produced by volcanic activity. There are two main types: lava plateaus and pyroclastic plateaus.
• Lava plateaus are formed by highly fluid basaltic lava during numerous successive eruptions through numerous vents without violent explosions (quiet eruptions).
• Pyroclastic plateaus are produced by massive pyroclastic flows and they are underlain by pyroclastic rocks: agglomerates, tephra, volcanic ashes cemented into tuffs, mafic or felsic.

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• A geyser is a hot spring characterized by intermittent discharge of water ejected turbulently and accompanied by a vapour phase (steam).
• The formation of geysers is due to particular hydrogeological conditions, which exist in only a few places on Earth, and so they are a fairly rare phenomenon. Generally all geyser field sites are located near active volcanic areas, and the geyser effect is due to the proximity of magma. Generally, surface water works its way down to an average depth of around 2,000 metres (6,600 ft) where it meets up with hot rocks. The resultant boiling of the pressurized water results in the geyser effect of hot water and steam spraying out of the geyser's surface vent.

• A volcanic plug, also called a volcanic neck or lava neck, is a volcanic landform created when magma hardens within a vent on an active volcano. When forming, a plug can cause an extreme build-up of pressure if volatile-charged magma is trapped beneath it, and this can sometimes lead to an explosive eruption. If a plug is preserved, erosion may remove the surrounding rock while the erosion-resistant plug remains, producing a distinctive landform.

• Arc volcanism is situated above subduction zones on consuming plate boundaries. The arcs produce andesitic and minor basaltic magmas.
   
   Rift-related volcanism is developed where a spreading centre within a continent sinks partially into the mantle and begins melting. The resulting magmas are typically bimodal, including melted crustal material as rhyolitic to rhyodacitic magma, and some mantle material, usually basaltic.
   
   Mid ocean ridge volcanism is developed within the rifted spreading centre. It receives magma directly from the mantle, and this is typically basaltic, with a typical mid ocean ridge chemistry.
   
   Hot spots are developed anywhere within a plate, and within both continental and oceanic crust. They form where the plate passes across a mantle convection cell. Where a crustal weakness is present, the heat produced from the hotspot melts some of the lower crust and upper mantle, which move upward along the crustal weakness to erupt. These are typically basaltic magmas, but some lower crustal material can produce rhyolites.

• A seismogram is a graph output by a seismograph. It is a record of the ground motion at a measuring station. The energy measured in a seismogram may result from an earthquake or from some other source, such as an explosion.A recording of earth motion as a function of time is called a seismogram.
• Seismograms are essential for measuring earthquakes using the Richter scale.

• Seismic waves are waves that travel through the Earth or other elastic body, for example as the result of an earthquake, explosion, or some other process that imparts forces to the body.
• P waves (primary waves) are longitudinal or compressional waves, which means that the ground is alternately compressed and dilated in the direction of propagation. In solids, these waves generally travel almost twice as fast as S waves and can travel through any type of material. In air, these pressure waves take the form of sound waves, hence they travel at the speed of sound. Typical speeds are 330 m/s in air, 1450 m/s in water and about 5000 m/s in granite. When generated by an earthquake they are less destructive than the S waves and surface waves that follow them, due to their bigger amplitudes.
• S waves (secondary waves) are transverse or shear waves, which means that the ground is displaced perpendicularly to the direction of propagation. In the case of horizontally polarized S waves, the ground moves alternately to one side and then the other. S waves can travel only through solids, as fluids (liquids and gases) do not support shear stresses. Their speed is about 60% of that of P waves in a given material. S waves are several times larger in amplitude than P waves for earthquake sources.

• Love waves are surface waves that cause horizontal shearing of the ground. They are named after A.E.H. Love, a British mathematician who created a mathematical model of the waves in 1911. They usually travel slightly faster than Rayleigh waves, about 90% of the S wave velocity.

   

  Seismic waves travel through water because gravity pulls the waves down causing friction of the waves.
• When an earthquake occurs, seismographs near the epicenter, out to about 90 km distance, are able to record both P and S waves, but those at a greater distance no longer detect the high frequencies of the first S wave.
• In the case of local or nearby earthquakes, the difference in the arrival times of the P and S waves can be used to determine the distance to the event. In the case of earthquakes that have occurred at global distances, four or more geographically diverse observing stations (using a common clock) recording P-wave arrivals permits the computation of a unique time and location on the planet for the event.
• A quick way to determine the distance from a location to the origin of a seismic wave less than 200 km away is to take the difference in arrival time of the P wave and the S wave in seconds and multiply by 8 kilometers per second. Modern seismic arrays use more complicated earthquake location techniques.

  • WRITE THIS DOWN!

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