5: Chapter Volcanoes and Other Igneous Activity
Volcanoes can either extrude magma violently or gently A) depending on: 1) Composition 2) Temperature 3) Amount of dissolved gases B) All factors influence viscosity (magma's mobility) 1) More viscous=greater resistance to flow=more explosive eruption 2) Strongly influenced by temperature--As lava cools its mobility decreases 3) Directly related to silica content a) More silica=greater viscosity b) Silica structures link together into long chains even before crystallization begins 4) Dissolved gases tend to increase fluidity of magma a) Escaping gases provide force to propel molten rock from volcanic vent II) During an eruption: A) Before eruption--summits begin to inflate 1) Magma is migrating into shallow reservoir inside cone 2) Volatiles migrate upward and accumulate near top of chamber 3) Upper portion of magma enriched in gases B) During eruption--Gas-charged magma moves from magma chamber and rises through conduit or vent 1) Pressure drops--gases suddenly released and expand a) Fluid basaltic magmas allow gases to migrate and escape with ease--can create lava fountains b) Viscous magmas explosively expel eruption columns (jets of ash-laden gases that evolve into buoyant plumes) i) Magmatic differentiation occurs--iron rich minerals crystallize and settle out leaving magma enriched in silica and dissolved gases Gases begin to collect as tiny bubbles Transformed into gas jet that is explosively ejected from volcano ii) Pressure of molten rock below drops--why there is often series of eruptions Explosive event usually followed by quiet emissions of "degassed" lavas 1. Soluble gases migrate slowly 2. Only uppermost portion of magma has enough gas to trigger explosive eruptions iii) After eruptive phase, gas buildup starts anew--explains sporadic eruptive patterns of viscous lavas III) Materials extruded during eruptions A) Lava flows 1) Over 90% of lava is basaltic a) Usually fluid b) Flow in thin, broad sheets or stream-like ribbons c) Can travel very far before congealing 2) Pahoehoe flows--relatively smooth skin that wrinkles as still-molten subsurface lava continues to advance a) Fluid basaltic lavas of Hawaiian type b) Resembles twisting braids of ropes c) Lava tubes: tunnels that once were horizontal conduits carrying lava from volcanic vent to flow's leading edge i) Develop in interior of flow when temperatures are high I)
ii) Still-molten lava within conduits continues forward motion leaving behind lava tubes iii) Allow fluid lavas to advance great distances from source iv) Rare in andesitic/rhyolitic lavas d) Hotter, richer in gases, and faster than aa flows on same slope e) Most Hawaiian flows start this way but can change into aa flows when moving downslope 3) Aa flows--surface of rough, jagged blocks with sharp edges and spiny projections a) Made by basaltic lavas b) Relatively cool/thick and advance at rates of 5-50m/hr c) Gases escaping produce voids and sharp spines d) As molten interior advances the outer crust is broken giving flow appearance of an advancing mass of rough, clinkery rubble 4) Block lavas--detached, blocks with slightly curved surface that cover unbroken lava in interior a) Made by andesitic and rhyolitic magmas b) Smoother surfaces than aa flows 5) Pillow lavas--composed of elongated structures resembling large bed pillows stacked atop one another a) Occur at ocean floor b) Flow's outer skin quickly congeals--lava usually able to move forward by breaking through hardened surface B) Large volumes of gas 1) Dissolved gases held in molten rock by confining pressure 2) Usually makes up 1-6% of total weight--usually water vapor 3) Contribute significantly to gases in planet's atmosphere 4) Hawaiian eruptions usually: a) 70% water vapor b) 15% carbon dioxide c) 5% nitrogen d) 5% sulfur dioxide e) Lesser amounts of chlorine, hydrogen, argon 5) Volcanoes are natural source of air pollutants 6) Play an important role in creating narrow conduit that connects magma chamber to surface a) High temp. and buoyant force from the magma body cracks the rock above b) Hot blasts of high pressure gases expand and crack surface--develop passageway to surface c) Hot gases with rock fragments erode wall producing larger conduit--volcanic pipes usually circular in shape d) As it enlarges--magma moves upward to produce surface activity e) After eruptive phase volcanic pipe gets choked with material not thrown clear of vent--new surge of gases may clear conduit C) Pyroclastic material (broken rock, lava bombs, fine ash/dust) 1) Propelled outward when basaltic lava is extruded and dissolved gases escape freely and continually a) Some may land near ventbuild cone-shaped structure b) Small particles may be carried great distances 2) Viscous magmas are highly charged with gases--expand thousand fold so particles go everywhere 3) Ejected materials that range in size from dust to pieces weighing several tons 4) Ash/dust particles produced from gas laden magma during explosive eruption
a) Froth is blown into very fine glassy fragments b) When hot ash falls glass shards fuse together to form welded tuff 5) Lapilli--small beads to walnut sized (AKA cinders) 6) Particles larger than 64mm diameter are: a) Blocks--when made of hardened lava b) Bombs--ejected as incandescent lava. Often take streamlined shapes c) Usually fall onto slopes of cone 7) Scoria--vesicular ejecta produced by basaltic magma a) Black to reddish-brown fragments b) Size of lapilla and resemble cinders/clinkers 8) Pumice--vesicular ejecta produced by andesitic or rhyolitic magma--lighter in color and less dense than scoria IV) Structure of a volcano A) Volcanic activity begins when a fissure develops in crust as magma moves forcefully toward surface B) Path usually localized into circular conduit/pipe C) Pipe terminates at surface at vent D) Successive eruptions of lava, pyroclastic material separated by inactivity builds a volcano E) Crater: steep-walled depression at summit of many volcanoes 1) Build upward as ejected fragments collect around vent 2) Some volcanoes have multiple summit craters F) Calderas: very large, more or less circular depressions 1) Large collapse structures 2) May or may not form in association with volcano G) Early stages of volcanic discharges occur from central summit vent 1) Older may be emitted from fissures at flank or base 2) Parasitic cone: small cone produced by continued activity from flank eruptions 3) Fumaroles: vents that emit only gases V) Types of volcanoes: Form of a particular volcano determined by composition of contributing magma A) Shield volcanoes 1) Produced by accumulation of fluid basaltic lavas flows averaging a few meters thick, little pyroclastic material 2) Shape of a broad, slightly domed structure that resembles a warrior's shield, Most are modest in size 3) Most have grown from deep ocean floor to build islands or seamounts a) Hawaiian chain b) Iceland c) Galapagos 4) Some occur in continents--in East Africa 5) Young shields emit very fluid lava from central vent and have sides with gentle slopes (1-5 degrees) 6) Mature shields have steeper slopes in middle sections a) Summits and flanks comparatively flat b) Lava discharged from summit vents and from rift zones along slopes i) Most are fluid pahoehoe typechange into aa type ii) Once eruption is established a large amount of lava flows through welldeveloped lava tubes--can travel much farther c) Large steep-walled caldera occupies summit i) Form when roof of volcano collapses as magma from central reservoirs migrates to flanks
ii) Feeds fissure eruptions d) Activity is more sporadic and pyroclastic ejections more likely e) Lavas increase in viscosity--steeped slope of summit area which becomes capped with clusters of cinder cones 7) Kilauea, Hawaii--most active volcano in world a) Before each eruption it inflates as magma migrates upward and accumulates in central reservoir b) Earthquakes warn of impending activity c) Most activity in last 50years along flanksEast Rift Zone d) When activity became localized a cinder and spatter cone (Puu Oo) was built e) New vent opened up in summer 1986smooth-surfaced pahoehoe lava formed a lava lake i) Often lava escaped through tunnels to feed pahoehoe flows that moved down SE flank of volcano toward sea ii) Occasionally overflowed B) Cinder cones (scoria cones): built from ejected lava fragments that look like cinders/clinkers as they begin to harden while in flight 1) Usually product of relatively gas-rich basaltic magma 2) Consist of rounded to irregular fragments that are scoria a) Occasionally generate ash and pumice fragments with eruption of silica-rich magma b) Mostly extrude look pyroclastic material sometimes lava (from vents near base) 3) Have a high angle of repose--Young cones steep sided with slopes 30-40 degrees 4) Have large, deep craters in relation to overall size 5) Relatively symmetrical but many are elongated and higher one size that was downwind during eruptions 6) Usually product of single eruptive episode a) Once event ends the pipe connecting vent to magma chamber solidifies b) Volcano never erupts again c) This is why there are small 7) Paricutin, Mexico volcanic episode a) Two weeks prior to first eruption--earth tremors occurred b) Sulfurous gases began billowing from small depression in cornfield c) Hot glowing rock fragments ejected from vent--throwing some as high as 6000m i) Larger fragments fell near crater ii) Some rolled down slope iii) Finer ash fell over much of area--burning and covering the village d) First lava flow from fissure opened just north of cone--after a few months of flow began to emerge from base i) Clinkery aa flow covered the village of San Juan Parangaricutiro ii) After nine years of intermittent pyroclastic explosions and continuous discharge of lava activity ceased C) Composite cones (stratovolcanoes) 1) Most located in Ring of Fire--narrow zone that rims Pacific a) Includes a chain of continental volcanoes on west coast of North/South America b) Most active--located along curved belts of volcanic islands adjacent to deepocean trenches in northern and western Pacific c) Stretches from Aleutians to Japan and Philippines, ends on North Island of New Zealand 2) Large, nearly symmetrical structure composed of both lava and pyroclastic deposits a) Conical shape, with steep summit area and gradually sloping flanks b) Product of gas-rich magma with an andesitic composition
c) Generate thick viscous lavas that travel short distances d) May generate explosive eruptions that eject lots of pyroclastic materials e) Steep summit area with gradual sloping flanks 3) Growth of composite cone a) Pyroclastic material and lava emitted from central vent b) As it matures, lavas tend to flow from fissures along flanks of cone c) Activity may alternate with explosive eruptions that eject pyroclastic material from summit activity or occur simultaneously d) Growth of cone due to viscous lavas and pyroclastic ejecta i) Coarse fragments accumulate near summit crater--their source (contribute to steep slopes) ii) Finer ejecta deposited as thin layer over large area--flatten flank of cone iii) Earlier lavas flow greater distances from vent than later lavas--contributes to wide flank e) Most have a complex history i) Mounds of volcanic debris surround many cones--provide evidence that large section of volcano slid downslope--massive landslide ii) Some have horseshoe-shaped depressions at summits as result of explosive eruptions 4) Proposed eruption on island of Santorini contributed to collapse of Minoan civilization around Crete--lost continent of Atlantis a) Plato: Atlantis swallowed by sea in single day and night b) Eruption generated tall, billowing eruption column i) Huge quantities of pyroclastic materials ii) Ash and pumice rained from plume for days blanketing surrounding area c) Summit of Santorini collapsed creating a caldera d) Once majestic island consists of five small islands e) Eruption and collapse of Santorini generated tsunamis 5) Eruption of Vesuvius (AD79) a) Entombed Pompeii with layer of pumice 3m thick b) Eruption most likely began as steam discharges in morning i) Fine ash and pumice fragments formed a tall eruptive cloud ii) Debris from cloud showered Pompeii Rate of 12-15cm/hr Roofs of Pompeii collapsed iii) Surge of hot dust and gas swept down flanks c) Most died from suffocated of ash-laden gases VI) Other volcanic hazards A) Pyroclastic flow--flows of gases infused with incandescent ash and larger rock fragments 1) Nuee ardentes: "glowing avalanches"--capable of flowing down steep volcanic slopes at speeds up to 200km/hr a) Ground hugging portion is rich in particulate matter--suspended by jets of buoyant gases passing upward through the flow b) Air overtaken and trapped by advancing flow may be heated sufficiently to provide buoyancy to particulate matter of nuee ardente 2) Pull of ground is force that causes heavier-than-air flows to sweep downslope a) Results when powerful eruption blasts pyroclastic material laterally from side of volcano b) Probably form from collapse of tall eruption columns that form over volcano during explosive event 3) Mount Pelee--destroyed St. Pierre a) Mantled by only a thin layer of volcanic debris
b) Masonry walls knocked over like dominoes B) Lahars: mudflow generated by large composite cones 1) Occur when volcanic debris becomes saturated water with and moves down steep volcanic slopes following gullies and stream valleys a) May be triggered when large volumes of ice/snow melt during eruption b) Heavy rainfall saturates weathered volcanic deposits c) Can occur when volcano is not even erupting 2) Mt. St. Helens erupted in 1980 forming several lahars a) Raced down valleys of Toutle river b) Destroyed or severely damaged nearly all homes and bridges VII) Other volcanic landforms A) Calderas: large collapse depressions having a more or less circular form 1) Diameters exceed 1km 2) Formed by: a) Collapse of summit of a large composite volcano following an explosive eruption of silica-rich pumice and ash b) Collapse of top of shield volcano caused by subterranean drainage from central magma chamber c) Collapse of a large area independent of any preexisting volcanic structures caused by discharge of colossal volumes of silica-rich pumice and ash along ring fractures 3) Crater Lake-type calderas a) Crater lake formed when composite cone violently extruded 50-70km3 of pyroclastics b) With loss of support the summit of the cone collapsed c) Rainwater filled the caldera d) Later volcanic activity built a small cinder cone in the lake--Wizard Island 4) Hawaiian-type calderas a) Hawaii's active shield volcanoes both have large calderas at their summits b) Formed by gradual subsidence of the summit as magma slowly drained laterally from central magma chamber to rift zone--producing flank eruptions 5) Yellowstone-type calderas a) 630,000years ago in Yellowstone--1000km3 of pyroclastic material erupted producing a caldera b) Event produced showers of ash as far away as Gulf of Mexico c) Evidence of this activity are the many hot springs and geysers d) Depressions so large and poorly defined that many remained undetected until high-quality aerial images were available e) Formation of a large Yellowstone-type caldera forms: i) When silica-rich magma body is emplaced near surface up warping the overlying rocks ii) Ring fractures develop in roof providing a pathway to surface for highly viscous gas-rich magma iii) Initiates an explosive eruption ejecting high volumes of pyroclastic materials Destroys most living things in its path Hot fragments of ash/pumice fuse together forming welded tuff iv) With loss of support roof of magma chamber collapse creating caldera f) Slow upheaval (resurgence) of floor of caldera following eruptive phase i) Structures consist of large, somewhat circular depression containing a central elevated region g) Calderas of this type are largest volcanic structures on earth B) Fissure eruptions: extrude very fluid basaltic lava
1) Greatest volume of volcanic material is extruded from fractures in crust (fissures) 2) Flood basalts: long narrow cracks may emit a low-viscosity basaltic lava blanketing a wide areas 3) Mid-Atlantic Ridge--regularly experiences fissure eruptions a) Generated over 20separate vents b) Initially extruded sulfurous gases and ash deposits that built several small cinder cones c) Followed by huge outpourings of fluid basaltic lava C) Lava domes: steep-sided dome-shaped mass of congealed lava 1) Created by silica rich lavas that hardly flow 2) Typify the late stages of activity of mature, chiefly andesitic composite cones a) Form in the summit crater and as parasitic structures on flanks of these cones following an explosive eruption of gas-rich magma b) Exemplified by volcanic dome that continues to "grow" at Mt. St. Helens 3) Most form in association with preexisting composite cones or shield volcanoes but some form independently--Mono Craters, CA D) Volcanic pipes: short conduits that connect magma chambers to surface 1) When they extend to great depths--ultramafic magmas migrate up and produce rocks that are samples of mantle 2) Best known are diamond-bearing structures of South Africa E) Volcanic neck 1) Volcanoes on land are being lowered by erosion and weathering a) Cinder cones are easily eroded b/c they are composed of unconsolidated materials b) But rock occupying the pipe is often more resistant may remain standing about surrounding terrain after cone is gone 2) Shiprock, New Mexico has many exposed volcanic pipes (necks) VIII)Intrusive igneous activity A) Plutons: structures that result from emplacement of igneous material at depth 1) Can only be studied after uplifting and erosion have exposed them 2) Great variety of shapes and sizes a) Tabular: table-top shape b) Massive c) Discordant: cut across existing structures d) Concordant: form parallel to features like sedimentary strata 3) Closely associated with volcanic activity B) Dikes: tabular discordant bodies produced when magma is injected into fractures 1) Force exerted by displaced lava can separate walls of fracture further 2) Once crystallized these sheet like structures have thicknesses from <1cm->km 3) Often found in groups that served as vertically oriented pathways followed by molten rock that fed ancient lava flows 4) Parent pluton usually not observable 5) Some dikes are found radiating from an eroded volcanic neck a) Active ascent of magma is thought to generate fissures in volcanic cone out of which lava flowed b) Weather more slowly than surrounding rock--have appearance of a wall C) Sills and laccoliths: concordant plutons that form when magma is intruded in nearsurface environments 1) Sills: tabular plutons formed when magma is injected into sedimentary bedding surfaces a) Horizontal sills most common b) Relatively uniform thickness and large aerial extent
c) Likely products of very fluid lava i) Magmas with low silica content ii) Composed of the rock basalt d) Emplacement requires that overlying sedimentary rock be lifted to a height equal to thickness of sill i) Only form at shallow depths where pressure exerted by weight of overlying rocks is low ii) Although intruded b/t layers of sedimentary rock they can be locally discordant e) Closely resemble buried lava flow--both are: i) Tabular ii) Often exhibit columnar joints Form as igneous rocks cool and develop shrinkage fractures produce elongated pillar like columns iii) Often have aphanetic textures--emplaced magma cools quickly f) Form when magma has been forcefully intruded b/t sedimentary layers i) Fragments of overlying rock can be found only in sills ii) "Baked" zones in rock above/below are trademarks 2) Laccoliths a) Formed when magma is intruded b/t sedimentary layers in a near surface environment i) More viscous magma ii) Less fluid--collects as a lens-shaped mass that arches overlying strata upward b) Can occasionally be detected b/c of dome-shaped bulge it creates near surface D) Batholiths: largest intrusive igneous bodies 1) Occur in linear structures 2) Very thick--possibly extending dozens of km. into crust 3) Must have a surface exposure greater than 100km2 to be a batholiths--smaller ones are stocks 4) Consist of rock types having more granitic compositions--diorite also found 5) Large ones have shown to consist of large numbers of distinct plutons that were intruded over millions of years 6) May compose core of mountain systems a) Uplifting and erosion have removed surrounding rock b) Exposing resistant igneous body c) Rocks that make up these were generated near top of a magma chamber 7) In shield areas--roots of former mounts--lower portions of batholiths are exposed 8) Emplacement a) When magma body nears surface it encounters relatively cool, brittle rock that resists deformation b) Stoping: continued upward movement when fractures develop in overlying host rock that all magma to rise and dislodge blocks of rock c) Once incorporated into magma body rocks will melt and alter composition of magma d) Magma will cool sufficiently so upward movement is stopped e) Evidence: xenoliths--unmelted remnants of host rock found in igneous bodies that have been exhumed by erosion IX) Plate tectonics and igneous activity A) First group: Most volcanoes are at margins of ocean basins 1) Especially ring of fire 2) Mainly composite cones
3) Volatile-rich magma with intermediate composition B) Second group: emit very fluid basaltic lavas and are confined to deep ocean basins 1) Many active submarine volcanoes 2) Small seamounts that lay along axis of ridge 3) Pressures are so great seawater does not boil explosively even in contact with lavas C) Third group: volcanoes irregularly distributed in interiors of continents 1) None in Australia nor eastern 2/3 of North/South America 2) Africa has many potentially active ones 3) Most diverse D) Plate motions provides the mechanisms by which mantle rocks melt to generate magma E) Volcanic activity rarely occurs at transform plate boundaries F) Convergent plate boundaries: 1) Oceanic-oceanic a) What happens: i) Slabs of oceanic crust are bent as they descend into mantle generating trench ii) Increase in pressure/temp. drives volatiles from crust iii) Mobile fluids migrate upward into piece of mantle b/t subducting slab and overriding one iv) Water rich fluids--reduce melting point of mantle rock--enough to trigger melting v) Partial melting of rock generates basaltic magma--slowly migrates upward b) Results in development of volcanic arc i) Develop roughly parallel to associated trench ii) Can be constructed on oceanic or continental lithosphere iii) Grow large enough for tops to rise above surface into island arcs iv) Includes Aleutians, Tonga, and Marianas c) Island arc volcanism: i) Early: eruption of fluid basalts that build numerous shield like structures on ocean floor ii) Cone building activity w/massive basaltic intrusions and magma added on underside of crust thickens the arc crust iii) Mature volcanic arcs are underlain by a comparatively thick crust which impedes upward movement of mantle basalts iv) Provides time for magmatic differentiation heavy iron-rich minerals crystallize and settle out--leaving silica rich melt Volatiles concentrate in more silica-rich magma v) So magmas are more viscous and more explosive eruptions 2) Oceanic-Continental a) Produces a continental volcanic arc b) Continental crust is much thicker and has higher silica content than oceanic crust i) there is a highly evolved mantle-derived magma ii) May change from dry, fluid basaltic magma to viscous andesitic/rhyolitic magma 3) Pacific basin is bordered by convergent plates which explains Ring of Fire 4) Includes Mt. Hood, Mt. Rainier, Mt Shasta G) Divergent plate boundaries 1) Greatest volume of magma produced along ridge system in association w/spreading 2) What happens" a) Below ridge axis where lithospheric plates are being pulled apart the solid but mobile mantle responds by rising upward to fill in
b) Decompression melting: melting w/o addition of heat (most common process that mantle rocks melt) c) Produces basaltic magma which rises d) Collects in reservoirs below ridge crest i) About 10% will erupt along fissures--Temporarily welds plates together ii) Some bulbous pillow lavas create seamounts iii) Some produce fluid flows which create more subdued topography 3) East African Rift: continental rift a) Magma generated by decompression heating b) Has produced large composite cones c) Form when mantle-derived basalts evolve into volatile rich andesitic magma as they migrate up through silica rich rocks in continent H) Intraplate igneous activity 1) Includes Canary Islands, Hawaiian islands, and Yellowstone 2) Occurs where a mantle plume ascends toward surface a) Mass of hotter than normal mantle material b) Appear to form at core-mantle boundary c) Has a bulbous head that draws out a narrow stalk beneath it as it rises d) Near top of mantle decompression heating generates basaltic magma that can eventually trigger surface volcanism 3) Results in a hot spot--a localized volcanic region a) Land surface often elevatedbuoyed up by a plum of warm low-density material b) Mantle beneath hot spots is 100-150oc hotter than normal 4) Where plume has existed for long periods of time a chain of volcanic structures can form as plate moves over hot spot 5) Probably responsible for basaltic lava that create large basalt plateaus a) These plumes have large head connected to a long-narrow tail rising from coremantle boundary b) As much of 10-20% of mantle material making up plume rapidly melts which triggers a burst of volcanism which produces plateau c) Released large amounts of CO2 which may have altered Cretaceous period climate 6) Extending away from most large flood basalt provinces is a chain of volcanic structures which terminates over an active hot spot marking current position of tail of plume X) Volcanic eruptions can change climate A) Suspended volcanic material will filter out a portion of incoming solar radiation which drops temp. in lowest layer of atmosphere B) Mt. St. Helens--significant local and regional effects for short period 1) Cooling was slight--<0.1oc 2) Long-term lowering of temp. was negligible C) El Chichon eruption in 1982--had greater cooling effect that St. Helens 1) Less explosive but emitted lots more sulfur dioxide gas than Mt. St. Helens 2) Produced a dense cloud of tiny sulfuric-acid particles--aerosols a) Take several years to settle out b) Lower temp. b/c they reflect solar radiation back to space D) Mt. Pinatubo eruption in 1991--Lower global temp. by 0.5oc b/c of haze of tiny aerosols E) Overall, impact on climate of single volcanic eruption is relatively small and short lived