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Planet earth volcano notes Document Transcript

  • 1. Prof. C.ValentiPlanet Earth Volcano Notes 1INTRODUCTION• Examine igneous rocks, chemical composition and associate them with a characteristic igneous process.• Examine processes that cause volcanic (rising magma, erupts at the surface as lava) and plutonic (magma that cools/crystallizes below the earths surface) activity.• Associate specific landforms produced as a result of volcanism and plutonism.• Examine the hazards and benefits to humans.• Know the geographic distribution of volcanoes, characteristic rock types all in association with their specific plate boundary. Volcanism. Refers to the rise of magma (igneous) and its cooling above earth’s surface. Volcanoes are conical or dome shaped initial landforms built by the emission of magma and its contained gasses from a constricted vent in the earth’s surface. Magma rises in a narrow, pipe like conduit from a magma reservoir beneath. Magma is the mixture of molten rock, suspended mineral grains, and dissolved gases that forms in the crust or mantle when temperatures are sufficiently high. Magma may be ejected: • As tephra - solid fragments ranging in size from flour sized particles to boulder sized particles which are thrown in the air due to the built up pressure of gasses. • As lavaSince volcanic eruptions are caused by magma (a mixture of liquid rock, crystals, anddissolved gas) expelled onto the Earths surface, we must first discuss thecharacteristics of magma and how magmas form in the Earth.1. Magma is characterized by a range of compositions in which silica (SiO2) is always predominant.2. Magma has the properties of a liquid, including the ability to flow, even though most magma is a mixture of suspended crystals, dissolved gases, and molten rock (often referred to as melt).3. Magma is characterized by high temperatures.Planet Earth Volcano Notes
  • 2. Prof. C.ValentiPlanet Earth Volcano Notes 2 Characteristics of MagmaTypes of MagmaTypes of magma are determined by chemical composition. Magma composition isdetermined by the common chemical elements in the Earth – silicon, aluminum, iron,calcium, magnesium, sodium, potassium, hydrogen, and oxygen. Because O2- is themost abundant anion and is therefore the anion that balances the charges on allthe cations, magma composition is usually expressed in terms of charge balancedoxides such as SiO2 (silica), the most abundant component of magma. Threegeneral types are recognized:mafic magma -- SiO2 45-55 wt%, high in Fe, Mg, Ca, low in K, Naintermediate magma -- SiO2 55-65 wt%, intermediate. in Fe, Mg, Ca, Na, Kfelsic magma -- SiO2 65-75 wt%, low in Fe, Mg, Ca, high in K, NaGases in MagmasSmall amounts of gas are dissolved in all magma and play an important role in theeruption process. The principal gas is water vapor, which together with carbondioxide, accounts for more than 98% of all gases emitted from volcanoes. Theremaining 2% is nitrogen, chlorine, sulfur, and argon.At depth in the Earth nearly all magmas contain gas dissolved in the liquid, but thegas forms a separate vapor phase when pressure is decreased as magma risestoward the surface of the Earth. This is similar to carbonated beverages which are bottled at high pressure. The high pressure keeps the gas in solution in the liquid, but when pressure is decreased, like when you open the can or bottle, the gas comes out of solution and forms a separate gas phase that you see as bubbles.Gas gives magmas their explosive character, because volume of gas expands aspressure is reduced. The composition of the gases in magma are:The amount of gas in a magma is related to the chemical composition of themagma.mafic (basaltic) magma -- SiO2 45-55 wt% and very little dissolved gas. The twocommon igneous rocks formed are basalt and gabbro.intermediate (andesitic) magma -- SiO2 55-65 wt% and a lot of dissolved gas. Thetwo common igneous rocks formed are andesite and diorite.felsic (rhyolitic) magma -- SiO2 65-75 wt% and large amounts of dissolved gas.The two common igneous rocks formed are rhyolite and granite.Planet Earth Volcano Notes
  • 3. Prof. C.ValentiPlanet Earth Volcano Notes 3Temperature of MagmasTemperature increases with depth in the earth’s GEOTHERMAL GRADIENT(temperature rises on average 2.5 degrees C for each 100 meters depth in thecrust). The mantle has a higher geothermal gradient because it is under immensepressure. The Melting point of minerals/rocks increases with increasingpressure. (Minerals will melt at higher temperatures when they are underpressure, so when under pressure, minerals will remain a solid even if thetemperature is higher).• Explanation. When an object is heated, atoms vibrate faster and spread further apart (solid, liquid, gas), increasing the volume. Melting rock requires change of state of matter from a solid to a liquid, TAKING UP MORE SPACE. As pressure is applied, this restricts volumetric expansion, so one must increase the temperature to vibrate atoms more vigorously so that they can expand (even under pressure). Therefore, a rock melting at 100 degrees at the surface (no pressure) will need to be heated to 110 degrees at some depth (under pressure) in order to change from a solid to a liquid (melt).The eruption temperature of various magmas is as follows: • mafic magma - 1000 to 1200 C o • intermediate magma - 800 to 1000 C o • felsic magma - 650 to 800 C. oViscosity of MagmasViscosity is the resistance to flow (opposite of fluidity). The more viscous amagma, the less fluid it is. Viscosity depends primarily on the composition of themagma (especially SiO2 content), and temperature. • Higher SiO2 (silica) content magmas have higher viscosity than lower SiO2 content magmas (viscosity increases with increasing SiO2 concentration in the magma). • Lower temperature magmas have higher viscosity than higher temperature magmas (viscosity decreases with increasing temperature of the magma).Thus, mafic magmas tend to be fairly fluid (low viscosity). felsic magmas tend tohave high viscosity. Viscosity is an important property in determining theeruptive behavior of magmas.Planet Earth Volcano Notes
  • 4. Prof. C.ValentiPlanet Earth Volcano Notes 4 Summary Table SolidifiedMagma Type Chemical Composition Temperature Viscosity Gas Content Rock 45-55 SiO2 %, high in Fe, Mg, Ca,Mafic Basalt 1000 - 1200 oC Low Low low in K, Na 55-65 SiO2 %, intermediate in Fe,Intermediate Andesite 800 - 1000 oC Intermediate Intermediate Mg, Ca, Na, K 65-75 SiO2 %, low in Fe, Mg, Ca,Felsic Rhyolite 650 - 800 oC High High high in K, Na.Planet Earth Volcano Notes
  • 5. Prof. C.ValentiPlanet Earth Volcano Notes 5 Volcanic Eruptions1. In general, magmas begin to rise because they are less dense than the surrounding solid rocks.2. Pressure controls the amount of gas a magma can dissolve; more gas is dissolved at higher pressure less at low. As magma rises it encounters a depth or pressure where the dissolved gas no longer can be held in solution in the magma, and the gas begins to form a separate phase (i.e. it makes bubbles just like in a bottle of carbonated beverage when the pressure is reduced).3. When a gas bubble forms, it will also continue to grow in size as pressure is reduced and more of the gas comes out of solution. In other words, the gas bubbles begin to expand.• If the liquid part of the magma has a low viscosity due to low silica content and high temperature, then the gas can expand relatively easily. When the magma reaches the Earths surface, the gas bubble will simply burst, the gas will easily expand to atmospheric pressure, and a non-explosive eruption will occur, usually as a lava flow (Lava is the name we give to a magma when it on the surface of the Earth). Characteristic of mafic (basaltic) magma eruptions.• If the liquid part of the magma has a high viscosity due to high silica content and low temperature, then the gas will not be able to expand very easily, and thus, pressure will build up inside of the gas bubble(s). When this magma reaches the surface, the gas bubbles will have a high pressure inside, which will cause them to burst explosively on reaching atmospheric pressure. This will cause an explosive volcanic eruption. Characteristic of felsic (rhyolitic) and intermediate (andesitic) magma eruptions.Nonexplosive EruptionsNon explosive eruptions are favored by low gas content and low viscosity magmas(basaltic to andesitic magmas). • If the viscosity is low, nonexplosive eruptions usually begin with fire fountains due to release of dissolved gases. • Lava flows are produced on the surface, and these run like liquids down slope, along the lowest areas they can find.Explosive EruptionsExplosive eruptions are favored by high gas content and high viscosity (andesitic torhyolitic magmas). • Explosive bursting of bubbles will fragment the magma into clots of liquid that will cool as they fall through the air. These solid particles become pyroclastsPlanet Earth Volcano Notes
  • 6. Prof. C.ValentiPlanet Earth Volcano Notes 6 (meaning - hot fragments) – a fragment of hot, shattered magma, or any other fragment of rock ejected during an explosive volcanic eruption. Tephra are unconsolidated (loose) pyroclasts ranging in size from flour to boulder sized particles which are thrown in the air due to the built up pressure of gases. Pyroclastic rock is consolidated (cemented together) pyroclasts. Ash is very fine tephra. Ash tuff is consolidated ash. Blocks are angular fragments that were solid when ejected. Bombs have an aerodynamic shape indicating they were liquid when ejected.A tremendous quantity of rock fragments, volcanic ash,and gas is blown into the air by explosions from thevolcano. Clouds of gas and tephra that rise above avolcano produce an eruption column that can rise up to45 km into the atmosphere. Eventually the tephra inthe eruption column will be picked up by the wind toform a mushroom shaped eruption cloud which carriesthe ash some distance, and then falls back to thesurface as a tephra fall or ash fall. If the eruption column collapses a pyroclastic flow (or nuee’ ardente) will occur, wherein gas and tephra rush down the flanks of the volcano at high speed. This occurs when the mixture of hot tephra and gases is too dense to rise upward. This is the most dangerous type of volcanic eruption. They are avalanches of very hot (1300-1800F) pyroclastic material (ash, rock, volcanic glass and gas) that move very rapidly (reaching speeds of 700km/hr) down the sides of the volcano incinerating everything in its path. Occurs in explosive volcanoes and is associated with gases.Lateral blasts are explosions of gas and ash from the side of the volcano. If the gaspressure inside the magma is directed outwardinstead of upward, a lateral blast can occur. Whenthis occurs on the flanks of a lava dome, a pyroclasticflow can result. The ejected material travels wayfrom the volcano at tremendous velocity. Directedblasts often result from sudden exposure of themagma by a landslide or collapse of a lava dome.Planet Earth Volcano Notes
  • 7. Prof. C.ValentiPlanet Earth Volcano Notes 7The Origin of Magmas and Igneous rocksCrystallization occurs when molten rock (contains gasses, mobile ions and possiblysuspended crystals) cools. As the melt cools, movement of ions slow down to arrangethemselves into certain minerals to the point where the crystals are closely packedtogether with no space between them.Conversely, melting requires heat, which causes ions to vibrate faster. When the forcesof the vibration exceeds the forces of the atomic bonds, the minerals will melt. ISOLATED SINGLE DOUBLE SHEET FRAMEWORK CHAIN CHAINIONIC HIGH LOWBONDING%Fe/Mg HIGH LOW%Si/O LOW HIGHCOVALENT LOW HIGHDENSITY HIGH LOWSTABILITY LOW HIGHChemical composition of the melt will determine1. The temperature it will melt or crystallize2. The order that minerals will melt or crystallize and3. Which minerals will crystallize. Silica and Oxygen will form tetrahedra first (at the highest temperatures) because ionic bonds are too weak for high temperatures.BOWENS REACTION SERIESAs a melt cools, minerals will crystallize in an ORDER based on their melting points(relative to their silicate structure) REGARDLESS OF THE COMPOSITION OF THEMELT.• The first minerals to crystallize is Olivine (isolated), Augite (single chain), Hornblende (double chain) Biotite (sheet) and finally Quartz (framework) in general. After silica tetrahedra form, iron and magnesium are sucked out of the melt to fill up the tetrahedra. Each silicate structure requires less and less iron and magnesium. The theory is that iron and magnesium are to be equally distributed among silica until all of the iron and magnesium is used up.Planet Earth Volcano Notes
  • 8. Prof. C.ValentiPlanet Earth Volcano Notes 8DISCONTINUOUS BRANCH. In the discontinuous branch, early forming minerals willcrystallize (high in iron and magnesium). If there is an excess of silica in the melt, themineral will REACT with the melt (change its crystalline structure to incorporate moresilica and oxygen) and form the next mineral on the discontinuous series whiletemperature is dropping all the while.• If the initial melt is a silica rich melt (low in iron and magnesium), as the melt begins to cool, iron and magnesium are sucked out of the melt to form OLIVINE (olivine as isolated, requires high iron and magnesium in its structure). Since there is so much excess silica remaining (without iron and magnesium), olivine reacts, incorporates more silica tetrahedra (begin to share oxygens) linking up as single chains (requiring less iron and magnesium to be neutral) forming AUGITE. At this point, olivine disappears. If there is still excess silica, augite reacts with the melt, incorporates more silica (share more oxygens) forms double chains, forming HORNBLENDE. And so on until biotite forms (sheet silicates). End discontinuous series. If the initial melt is low in iron and magnesium to start, the crystallized igneous rock will have very little iron/magnesium rich minerals in it. They would have reacted with the melt to form more silica rich minerals.• If the initial melt is high in iron and magnesium, mafic minerals would form early on, but would not have to react with the melt, because all silica would have iron and magnesium in their structure. So only mafic minerals would be present. Reacting with the melt distributes the iron and magnesium throughout all the minerals as evenly as possible. You will never find quartz and olivine in the same igneous rock.CONTINUOUS BRANCH. As the discontinuous branch runs, the continuous branchruns simultaneously. Ions such as Potassium, Sodium and Calcium are used to formfeldspars.• As the melt first begins to cool, all of the calcium is first sucked out of the melt to form calcium rich plagioclase feldspar. Once all the calcium is used up, sodium is used to form sodium rich feldspar.• The difference between the two branches is that feldspars do not react with the melt as temperature drops. Once calcium is removed, then and only then will sodium be removed.Once both branches run their courses, if there is any silica, oxygen, potassium, aluminumin the melt then, potassium feldspar, muscovite and quartz will crystallize in that order,as temperatures drop.Planet Earth Volcano Notes
  • 9. Prof. C.ValentiPlanet Earth Volcano Notes 9CHANGING MAGMA COMPOSITIONIf the earth was once a homogenized mass of molten rock, how can we get igneous rocksof different compositions. Bowen hypothesized that all melts are MAFIC in origin, sohow do we get silica rich melts to form felsic minerals? How do we get ferromagnesianrich melts to form ultramafic minerals?The cooling of the early earth resulted in it to become layered. Iron and Nickel thatcooled and crystallized first, being the densest elements, sank through the moltenearth to form the inner core (solid). Outer core remains liquid due to heat, but it ispossible that it continues to solidify. Density of minerals increases with depth in theearth (i.e. mantle is denser than oceanic crust denser than continental crust).CRYSTAL SETTLING. As a melt cools, early developed minerals in the series will form(remaining melt more silica rich). If the melt cools slowly, these minerals (solid anddense) will physically separate from the melt (settling out) so that they no longer canreact with the melt. Minerals lower on the series will crystallize in the absence of ironand magnesium.• Crystal settling is the downward movement of minerals that are denser than the magma from which it crystallized. These minerals coalesce and collect at the bottom and do not react with the melt (do not reintroduce iron and magnesium back into the melt). The remaining melt becomes more silica rich. Intermediate melt from a mafic melt. Mafic melt from an ultramafic melt.PARTIAL MELTING. Minerals crystallize in an order and they melt in an order.Minerals melt in the reverse order of Bowens reaction series. As you slowly heat uprock, silica rich minerals melt first (have the lowest melting temperature). Theseminerals have no iron and magnesium so the molten silica rich melt is less dense and risesup through the crust, separating itself from the remaining rock. The solid residuebehind when melted will have a higher composition of iron and magnesium. A mafic rock,through partial melting results in ultramafic rock.ASSIMILATION. A mafic melt, intruding into another rock may incorporate some ofthis rock into its melt. (Like melting ice cubes in coffee). Felsic country rock melted inmafic lava or magma, results in an intermediate rock. Felsic country rock dilutes theiron and magnesium concentrations.MAGMA MIXING. Two magmas intruding into one another will mix and form a magmawith a new chemical composition.Planet Earth Volcano Notes
  • 10. Prof. C.ValentiPlanet Earth Volcano Notes 10 Volcanoes and Plate Tectonics Since the upper parts of the Earth are solid, special conditions are necessary to form magmas. These special conditions do not exist everywhere beneath the surface, and thus volcanism does not occur everywhere. If we look at the global distribution of volcanoes we see that volcanism occurs four principal settings. Most of the active above sea volcanoes occur within the Ring of Fire usually in subduction zones. 80% volcanism is at convergent boundaries, 15% at divergent boundaries, and 5% at intraplate mantle plumes. 80% of the above sea volcanoes occur specifically within the Ring of Fire. Global Distribution of Volcanoes1. Along divergent plate boundaries, such as Oceanic Ridges or spreading centers.2. In areas of continental extension (that may become divergent plate boundaries in the future).3. Along converging plate boundaries where subduction is occurring.4. And, in areas called "hot spots" that are usually located in the interior of plates, away from the plate margins.Diverging Plate MarginsActive volcanism is currently taking place along all ofoceanic ridges, but most of this volcanism is submarinevolcanism and does not generally pose a threat to humans.One of the only places where an oceanic ridge reachesabove sea level is at Iceland, along the Mid-Atlantic Ridge.Here, most eruptions are basaltic in nature, but many areexplosive types. As seen in the map to the right, the Mid-Atlantic ridge runs directly through IcelandVolcanism also occurs in continental areas that are undergoing episodes of extensionaldeformation. A classic example is the East African Rift Valley, where the African plateis being split. The extensional deformation occurs because the underlying mantle isrising from below and stretching the overlying continental crust. Upwelling mantle maymelt to produce magmas, which then rise to the surface, often along normal faultsproduced by the extensional deformation. In the same area, the crust has rifted apartalong the Red Sea, and the Gulf of Aden to form new oceanic ridges. This may also bethe fate of the East African Rift Valley at some time in the future.Other areas where extensional deformation is occurring within the crust is Basin andRange Province of the western U.S. (eastern California, Nevada, Utah, Idaho, westernPlanet Earth Volcano Notes
  • 11. Prof. C.ValentiPlanet Earth Volcano Notes 11Wyoming and Arizona) and the Rio Grande Rift, New Mexico. These are also areas ofrecent volcanism.Planet Earth Volcano Notes
  • 12. Prof. C.ValentiPlanet Earth Volcano Notes 12Converging Plate MarginAll around the Pacific Ocean, is a zone often referred to asthe Pacific Ring of Fire, where most of the worlds mostactive and most dangerous volcanoes occur. The Ring of Fireoccurs because most of the margins of the Pacific oceancoincide with converging margins along which subduction isoccurring.The convergent boundary along the coasts of SouthAmerica, Central America, Mexico, thenorthwestern U.S. (Northern California, Oregon, &Washington), western Canada, and eastern Alaska,are boundaries along which oceanic lithosphere isbeing subducted beneath continental lithosphere.This has resulted in the formation of continentalvolcanic arcs that form the Andes Mountains, theCentral American Volcanic Belt, the Mexican Volcanic Belt, the Cascade Range, and theAlaskan volcanic arc. The Aleutian Islands (west of Alaska), the Kurile- Kamchatka Arc, Japan, Philippine Islands, and Marianas Islands, New Zealand, and the Indonesian Islands, along the northern and western margins of the Pacific Ocean are zones where oceanic lithosphere is being subducted beneath oceanic lithosphere. These are all island arcs.♦ Most of the magmas generated as a result of subduction are andesitic magmas, and thus volcanism in these convergent margin areas is predominantly andesitic volcanism.♦ But, through a complex process, known as magmatic differentiation, andesitic magma can change to rhyolitic magma. Thus, rhyolitic volcanism is also common in these areas.♦ Because these magmas are often gas rich and have all have relatively high viscosity, eruptions in these areas tend to be violent.♦ Volcanic landforms tend to be cinder cones, stratovolcanoes, volcanic domes, and calderas.♦ Repose periods between eruptions tend to be hundreds to thousands of years, thus giving people living near these volcanoes a false sense of security.Planet Earth Volcano Notes
  • 13. Prof. C.ValentiPlanet Earth Volcano Notes 13Hot SpotsVolcanism also occurs in areas that are not associated with plate boundaries, in theinterior of plates. These are most commonly associated with what is called a hot spot.Hot spots appear to result from plumes of hot mantle material upwelling toward thesurface, independent of the convection cells thought to cause plate motion. Hot spotstend to be fixed in position, with the plates moving over the top. As the rising plume ofhot mantle moves upward it begins to melt to produce magmas. These magmas then riseto the surface producing a volcano. But, as the plate carrying the volcano moves awayfrom the position over the hot spot, volcanism ceases and new volcano forms in theposition now over the hot spot. This tends to produce chains of volcanoes or seamounts(former volcanic islands that have eroded below sea level).Volcanism resulting from hotspots occurs in both the Atlantic and Pacific ocean, but aremore evident on the sea floor of the Pacific Ocean, because the plates here move athigher velocity than those under the Atlantic Ocean.A hot spot trace shows up as a linear chain of islandsand seamounts, many of which can be seen in thePacific Ocean. The Hawaiian Ridge is one such hot spottrace. Here the Big Island of Hawaii is currently overthe hot spot, the other Hawaiian islands still standabove sea level, but volcanism has ceased. Northwestof the Hawaiian Islands, the volcanoes have erodedand are now seamounts.The ages of volcanic rocks increase along the Hawaiian Ridgeto the northwest of Hawaii. The prominent bend observedwhere the Hawaiian Ridge intersects the Emperor Seamountchain has resulted from a change in the direction of platemotion over the hot spot. Note that when the EmperorSeamount chain was produced, the plate must have beenmoving in a more northerly direction. The age of the volcanicrocks at the bend is about 50 million years.Planet Earth Volcano Notes
  • 14. Prof. C.ValentiPlanet Earth Volcano Notes 14 Volcanic Landforms Volcanic landforms are controlled by the geological processes that form them and act on them after they have formed. Thus, a given volcanic landform will be characteristic of the types of material it is made of and on the prior eruptive behavior of the volcano. The viscosity of the melt and the rock types (tephra, lava) result in different volcanic forms. Volcanic Landforms Shield volcanoes.♦ Shield volcanoes are composed almost entirely of relatively thin lava flows built up over a central vent.♦ Are large, broad, gentle sloping shields formed by the accumulation of layers upon layers of basaltic (mafic) lava. The lava flows great distances, thinning out before it cools and solidifies.♦ Shield volcanoes are produced by mafic lava with relatively little ash and cinder. Very little pyroclastic material is found within a shield volcano, except near the eruptive vents, where small amounts of pyroclastic material accumulate as a result of fire fountaining events.♦ Mafic (basaltic) magma is low in silica so it has low viscosity and flows easily.♦ Gas content is low so eruptions are mild. Therefore they produce little pyroclastics.♦ Low gas content and gentle flowing lava (low viscosity) results in gentle eruptions.♦ Most shield volcanoes have a roughly circular or oval shape in map view.♦ Vents for most shield volcanoes are central vents, which are circular vents near the summit. Hawaiian shield volcanoes also have flank vents, which radiate from the summit and take the form of en-echelon fractures or fissures, called rift zones, from which lava flows are emitted. This gives Hawaiian shield volcanoes like Kilauea and Mauna Loa their characteristic oval shape in map view.♦ Predominant rock type is basalt♦ Located at divergent boundaries (MOR) and hot spot mantle plumes.♦ The Hawaiian Islands represent a chain of shield volcanoes that extend down to the sea floor. They form over a mantle plume called a hot spot located in the middle of the Pacific Plate. These are the tallest and largest landforms (9000 meters) on earth (from base to summit).♦ Few deaths but hazardous to property; PredictablePlanet Earth Volcano Notes
  • 15. Prof. C.ValentiPlanet Earth Volcano Notes 15 Composite volcanoes. Also known as stratovolcanoes.♦ Produced by layers of thick, viscous felsic to intermediate lava and tephra (pyroclastics). Stratovolcanoes show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called composite volcanoes. Pyroclastic material can make up over 50% of the volume of a stratovolcano. ♦ Felsic magma is high in silica so it has high viscosity and does not flow easily. The larger percentage of silica tetrahedron form long complex chains in the melt causing viscosity to be high. The chains become tangled, making the lava highly viscous and not flow very easily. ♦ Felsic magmas have larger quantities of water in it, which forms steam and causes the melt to be under great pressures. More gasses are liberated when the melt reaches the surface of the earth.♦ Viscous lava and high gas content results in violent, explosive eruptions.♦ Very steep sided, tall volcanoes (~3500 meters high) formed by the accumulation of layers of andesitic (felsic) lava and tephra. Steep slopes are due to two factors 1. High viscosity of the melt. 2. Alternating layers of tephra (unconsolidated pyroclastics) and lava flows. Both allow these volcanoes to build high, steep slopes.♦ Lavas and pyroclastics are usually andesitic to rhyolitic in composition.♦ These volcanoes can form at continental – continental collision boundaries (felsic) or continental – oceanic collision boundaries (intermediate). Examples include Mount Saint Helens, Mt Rainier, Mt. Vesuvius Italy, Mt. Fuji Japan, Andes Mountains.♦ Hazardous to humans and property.♦ Irregular eruptions, Hard to predict. Long periods of repose (times of inactivity) lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they have shown no historic activity, and people are reluctant to heed warnings about possible eruptions.Planet Earth Volcano Notes
  • 16. Prof. C.ValentiPlanet Earth Volcano Notes 16 Cinder cones.♦ Small, steep sided volcanoes measuring only 100 – 400 meters tall. Cinder cones are small volume cones consisting predominantly of tephra. They usually consist of basaltic to andesitic material. They are actually fall deposits that are built surrounding the eruptive vent. They do not build up very tall because they are made of unconsolidated pyroclastics and erode very easily.♦ Cinder cones usually occur around summit vents and flank vents of stratovolcanoes.♦ An excellent example of cinder cone is Parícutin Volcano in Mexico. This volcano was born in a farmer’s cornfield in 1943 and erupted for the next 9 years. Lava flows erupted from the base of the cone eventually covered two towns.Lava Domes (also called Volcanic Domes)♦ Volcanic Domes result from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava. Since the viscosity is so high, the lava does not flow away from the vent, but instead piles up over the vent.♦ Blocks of nearly solid lava break off the outer surface of the dome and roll down its flanks to form a breccia around the margins of domes.♦ The surface of volcanic domes are generally very rough, with numerous spines that have been pushed up by the magma from below.♦ Most dome eruptions are preceded by explosive eruptions of more gas rich magma, producing a tephra (cinder) cone into which the dome is extruded.♦ Volcanic domes can be extremely dangerous. because they form unstable slopes that may collapse to expose gas-rich viscous magma to atmospheric pressure. This can result in lateral blasts or pyroclastic flow (nuée ardent) eruptions.Planet Earth Volcano Notes
  • 17. Prof. C.ValentiPlanet Earth Volcano Notes 17Craters and Calderas♦ Craters are circular depressions, usually less than 1 km in diameter, that form as a result of explosions that emit gases and tephra.♦ Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to 50 km. Calderas form as a result of collapse of a volcanic structure. The collapse results from evacuation of the underlying magma chamber.♦ Calderas are often enclosed depressions that collect rain water and snow melt, and thus lakes often form within a caldera.♦ Crater Lake Caldera in southern Oregon is an 8 km diameter caldera containing a lake♦ Larger calderas have formed within the past million years in the western United States. These include Yellowstone Caldera in Wyoming, Long Valley Caldera in eastern California, and Valles Caldera in New Mexico.Geysers, Fumaroles and Hot Springs♦ A fumarole is vent where gases, either from a magma body at depth, or steam from heated groundwater, emerges at the surface of the Earth. Since most magmatic gas is H2O vapor, and since heated groundwater will produce H2O vapor, fumoroles will only be visible if the water condenses. (H2O vapor is invisible, unless droplets of liquid water have condensed).♦ Hot springs or thermal springs are areas where hot water comes to the surface of the Earth. Cool groundwater moves downward and is heated by a body of magma or hot rock. A hot spring results if this hot water can find its way back to the surface, usually along fault zones.♦ A geyser results if the hot spring has a plumbing system that allows for the accumulation of steam from the boiling water. When the steam pressure builds so that it is higher than the pressure of the overlying water in the system, the steam will move rapidly toward the surface, causing the eruption of the overlying water. Some geysers, like Old Faithful in Yellowstone Park, erupt at regular intervals. The time between eruptions is controlled by the time it takes for the steam pressure to build in the underlying plumbing system.Volcanic HazardsPrimary Effects of VolcanismPlanet Earth Volcano Notes
  • 18. Prof. C.ValentiPlanet Earth Volcano Notes 18♦ Lava Flows Magma is produced by the melting of solid rock beneath the surface of the earth. Molten rocks are less dense than solid rocks therefore it rises through the mantle and surfaces through vents in the oceanic and continental crust. At the surface, the molten rock is called lava. Lava flows varies depending on how much silica is present in the melt. s Less silica (mafic-basalt) indicates lower viscosity and thus faster flow. L Higher silica (felsic-rhyolite/intermediate-andesite) indicates high viscosity and thus slower flow. ♦ Although lava flows have been known to travel as fast as 64 km/hr, most are slower and give people time to move out of the way. Thus, in general, lava flows are most damaging to property, as they can destroy anything in their path.♦ Violent Eruptions and Pyroclastic Activity ♦ Hot pyroclastic flows cause death by suffocation and burning. They can travel so rapidly that few humans can escape. ♦ Lateral blasts knock down anything in their path, can drive flying debris through trees. ♦ Tephra is ejected pyroclastic material that is airborne. The molten material solidifies in the air forcing ellipsoidal bombs (volcanic bombs), glassy fragments (volcanic ash) or can include fragments of older, presolidified rock. Most tephra is fine-grained ash, and the faster the fine particles cool, the glassier the texture. ♦ Tephra falls can bury people (79 A.D eruption of mount Vesuvius buried people in Pompeii). Tephra falls can cause the collapse of roofs and can affect areas far from the eruption. Although tephra falls blanket an area like snow, they are far more destructive because tephra deposits have a density more than twice that of snow and tephra deposits do not melt like snow. ♦ Tephra falls destroy vegetation, including crops, and can kill livestock that eat the ash covered vegetation.♦ Poisonous gases. Gasses drive explosive eruptions because of the buildup of steam, but gasses occur in all eruptions. Among these poisonous gases are: Hydrogen Chloride (HCl), Hydrogen Sulfide (H2S), Sulfuric Acid, Hydrochloric Acid, Hydrogen Fluoride (HF), Carbon Monoxide (CO), Sulfur Dioxide, and Carbon Dioxide (CO2). Water and carbon dioxide make up more than 90 percent of all emitted gases.Planet Earth Volcano Notes
  • 19. Prof. C.ValentiPlanet Earth Volcano Notes 19 ♦ Volcanoes emit gases are often poisonous to living organisms. CO2 gas emission from Lake Nyos in the African Country of Cameroon killed more than 1700 people and 3000 cattle. ♦ Carbon dioxide is heavier than air and hugs the ground displacing oxygen asphyxiating animals and people.♦ Lahars. (Mudflows)Mixture of water and volcanic debris that sweeps down a volcano’s slope. Volcanoes canemit voluminous quantities of loose, unconsolidated tephra which become deposited onthe landscape. Such loose deposits are subject to rapid removal if they are exposed to asource of water. ♦ On November 13, 1985 a mudflow generated by a small eruption on Nevado del Ruiz volcano in Columbia flowed down slope and devastated the town of Armero, 50 km east of the volcano and built on prior mudflow deposits. The town had several hours of warning from villages higher up slope, but these warnings were ignored, and 23,000 people died in the mudflow that engulfed the town.♦ Debris Avalanches and Debris Flows Volcanic mountains tend to become oversteepened as a result of the addition of new material over time as well due to inflation of the mountain as magma intrudes. Oversteepened slopes may become gravitationally unstable, leading to a sudden slope failure that results in landslides, debris slides or debris avalanches.♦ Tsunamis ♦ Debris avalanche events, landslides, caldera collapse events, and pyroclastic flows entering a body of water may generate tsunamis. • During the 1883 eruption of Krakatua volcano, in the straits of Sunda between Java and Sumatra, several tsunamis were generated by pyroclastic flows entering the sea and by collapse accompanying caldera formation. The tsunamis killed about 36,400 people, some as far away from the volcano as 200 km.After an EruptionWhen a volcanic eruption spreads lava or tephra across the land, it renews the landsurface. Lava and volcanic ash, when subjected to weathering, produce very fertilePlanet Earth Volcano Notes
  • 20. Prof. C.ValentiPlanet Earth Volcano Notes 20soils. Some of the richest agricultural land in Italy, Japan, the Hawaiian Islands, thePhilippines, and Indonesia are rich volcanic soils. Remarkably, the land recovers quicklyafter an eruption, sometimes within only a year or two.INTRUSIVE IGNEOUS ACTIVITYAll bodies of what we call intrusive igneous rock, regardless of shape or size, arecalled plutons, after Pluto, the god of the underworld.• Intrusive igneous rocks crystallize below the earths surface and are only exposed by erosion and uplift. The magma that formed the pluton was squeezed upward from the place where it formed, thereby intruding the overlying rock.• Intrusive bodies vary in size and shape (classified).• All have large crystals indicative of slow cooling (its warmer down there).• Same mineralogy as volcanic (extrusive) but differs in crystal size.TERMS• Intrusive rocks obtain their name because the magma travels through (intrudes) into another rock body (country rock). The boundary is called a contact.• Pieces of country rock may become incorporated into the molten intrusion before it totally cools forming a xenolith (match country rock types).• Common Intrusive rocks include GABBRO (same composition as basalt but cooled slower), PERIDOTITE (mantle rocks) and GRANITE (felsic, making up continental igneous rocks).SHALLOW INTRUSIVE BODIES• <2 kilometers depth.• Cool quicker so smaller grain sizes.• Dikes, vertical structures cutting across sedimentary rock layers.• Sills, horizontal structures running parallel with sedimentary rock layers.• Laccoliths, Run between sedimentary layers, but forms a lens, arching overlying strata.• Veins, Runs through rocks with numerous fractures.DEEP INTRUSIVE BODIESBatholiths are large intrusive igneous bodies of irregular shape (100 square kilometers)made primarily of granite.• Larger bodies cool slower so crystal sizes are larger than shallow intrusive bodies.Planet Earth Volcano Notes