When Low Viscosity Lava Flows Beneath the Surface It Travels Though __________________ .

EENS 3050	Natural Disasters
Tulane University	Prof. Stephen A. Nelson
Volcanic Landforms, Volcanoes and Plate Tectonics

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 exist characteristic of the types of textile it is made of, which in turn depends on the prior eruptive behavior of the volcano. Although later processes can change the original landform, we should be able to find clues in the modified form that lead us to conclusions most the original formation process.  Here we discuss the major volcanic landforms and how they are formed, and in some cases, subsequently modified.  Most of this material will be discussed with reference to slides shown in class that illustrate the essential features of each volcanic landform.

Shield Volcanoes A shield volcano is characterized past gentle upper slopes (most 5o) and somewhat steeper lower slopes (almost   10o).

• Shield volcanoes are composed almost entirely of relatively thin lava flows built upward over a central vent.

• Nearly shields were formed by low viscosity basaltic magma that flows hands down slope away form the summit vent.

• The low viscosity of the magma allows the lava to travel down slope on a gentle slope, but as it cools and its viscosity increases, its thickness builds up on the lower slopes giving a somewhat steeper lower slope.

• Nigh shield volcanoes accept a roughly circular or oval shape in map view.

• Very little pyroclastic fabric is found within a shield volcano, except near the eruptive vents, where small amounts of pyroclastic textile accrue as a result of fire fountaining events.

• Shield volcanoes thus class by relatively non-explosive eruptions of low viscosity basaltic magma.

Vents for most shield volcanoes are cardinal 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.

Stratovolcanoes (also called Composite Volcanoes)

• Have steeper slopes than shield volcanoes, with slopes of 6 to x^o depression on the flanks to  30^o near the pinnacle.

The steep slope about the summit is due partly to thick, short viscous lava flows that practise not travel far down slope from the  vent.

• The gentler slopes near the base are due to accumulations of material eroded from the volcano and to the accumulation of pyroclastic fabric.

• Stratovolcanoes show inter-layering of lava flows and pyroclastic material, which is why they are sometimes called blended volcanoes.  Pyroclastic material can make upwards over 50% of the volume of a stratovolcano.

• Lavas and pyroclastics are usually andesitic to rhyolitic in composition.

• Due to the college viscosity of magmas erupted from these volcanoes, they are usually more explosive than shield volcanoes.

• Stratovolcanoes sometimes have a crater at the elevation that is formed by explosive ejection of fabric from a key vent.  Sometimes the craters have been filled in by lava flows or lava domes, sometimes they are filled with glacial water ice, and less commonly  they are filled with h2o.

• 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 take shown no celebrated activity, and people are reluctant to heed warnings most possible eruptions.

Cinder Cones (also called Tephra Cones)

• Cinder cones are pocket-sized volume cones consisting predominantly of tephra that issue from strombolian eruptions.  They unremarkably consist of basaltic to andesitic material.
• They are actually fall deposits that are built surrounding the eruptive vent.
• Slopes of the cones are controlled by the angle of quiet (angle of stable gradient for loose unconsolidated material) and are usually between nearly 25 and 35^o.

They show an internal layered structure due to varying intensities of the explosions that deposit different sizes of pyroclastics.

• On young cones, a depression at the tiptop of the cone, called a crater, is axiomatic, and represents the area in a higher place the vent from which material was explosively ejected.  Craters are usually eroded away on older cones.
• If lava flows are emitted from tephra cones, they are commonly emitted from vents on the flank or near the base of the cone during the subsequently stages of eruption.
• Cinder and tephra cones commonly occur around summit vents and flank vents of stratovolcanoes.
• An excellent example of cinder cone is Par�cutin Volcano in Mexico. This volcano was born in a farmers corn field in 1943 and erupted for the adjacent nine years.  Lava flows erupted from the base of operations of the cone eventually covered two towns.
• Cinder cones oft occur in groups, where tens to hundreds of cones are constitute in ane surface area.

Maars Maars result from phreatic or phreatomagmatic action, wherein magma heats up groundwater, pressure builds as the water to turns to steam, and and then the water and preexisting stone (and some new magma if the eruption is phreatomagmatic) are blasted out of the ground to form a tephra cone with gentle slopes.

Parts of the crater walls somewhen collapse back into the crater, the vent is filled with loose material, and, if the crater still is deeper than the water tabular array, the crater fills with water to form a lake, the lake level congruent with the water table.

Volcanic Domes (also chosen Lava Domes)

• Volcanic Domes outcome from the extrusion of highly viscous, gas poor andesitic and rhyolitic lava.  Since the viscosity is and so high, the lava does not flow away from the vent, but instead piles up over the vent.
  

Blocks of virtually solid lava intermission 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 more often than not very crude, with numerous spines that have been pushed up by the magma from below.

Near dome eruptions are preceded past explosive eruptions of more than gas rich magma, producing a tephra cone into which the dome is extruded. Volcanic domes can be extremely dangerous. because they grade unstable slopes that may plummet to expose gas-rich sticky magma to atmospheric pressure.   This can event in lateral blasts or Pelean blazon pyroclastic menses (nu�e ardentes) eruptions.

Craters and Calderas

• Craters are circular depressions, usually less than 1 km in diameter, that form equally a consequence of explosions that emit gases and tephra.

• Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from i km to l km.  Calderas class as a event of plummet of a volcanic construction.  The plummet results from evacuation of the underlying magma sleeping room.
  

• In shield volcanoes, like in Hawaii, the evacuation of the magma chamber is a dull drawn out processes, wherein  magma is withdrawn to erupt on from the rift zones on the flanks.

In stratovolcanoes the collapse and formation of a caldera results from rapid evacuation of the underlying magma bedchamber by voluminous explosive eruptions that class all-encompassing fall deposits and pyroclastic flows. Calderas are oftentimes enclosed depressions that collect rain water and snow melt, and thus lakes often course inside a caldera. Crater Lake Caldera in southern Oregon is an 8 km bore caldera containing a lake  The caldera formed about 6800 years ago as a consequence of the eruption of nigh 75 kmthree of rhyolite magma in the class of tephra, plant as far away every bit Canada, accompanied past pyroclastic flows that left thick deposits of tuff on the flanks of the volcano.  Subsequent eruptions accept built a cinder cone on the flooring of the caldera, which at present forms an island called Sorcerer Island.

Larger calderas take formed inside 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.

The Yellowstone caldera is an important example, as it illustrates the amount of repose time that might be expected from large rhyolitic systems, and the devastating effect caldera forming eruptions can have on widespread areas. Yellowstone Caldera which occupies most of Yellowstone National Park, is actually the third caldera to class in the area within the past 2 million years.  The three calderas formed at two.0 meg years ago, one.iii 1000000 years ago, and the latest at 600,000 years agone.  Thus the repose fourth dimension is on the boilerplate about 650,000 years.

• Tephra autumn deposits from the latest eruption are found in Louisiana and into the Gulf of Mexico, and covered much of the Western part of the Us.

• The eruption 600,000 years ago produced virtually grand km³ of rhyolite (in comparing, the eruption of Mt. St. Helens in May of 1980 produced only 0.75 km³.

• Magma still underlies Yellowstone caldera, equally evidenced by the large number of hot springs and geysers in the expanse.

Resurgent Domes

• Later the formation of a caldera past plummet, magma is sometimes re-injected into the surface area below the caldera.  This tin can effect in uplift of i or more areas within the caldera to class a resurgent dome.  Two such resurgent domes formed in the Yellowstone caldera, as shown above.
  

• If magma leaks back to the surface during this resurgent doming, then eruptions of small volcanic domes can occur in the area of the resurgent domes.

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 HiiO vapor, and since heated groundwater will produce H2O vapor, fumaroles will only be visible if the h2o condenses.  (HtwoO 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 Globe.  Absurd groundwater moves downwards and is heated past a body of magma or hot stone.  A hot bound results if this hot water tin can discover its way dorsum to the surface, ordinarily along fault zones.

  Minerals dissolved in the high temperature water are often precipitated when the h2o cools at the surface. This produces spectacular deposits of travertine (chemically precipitated calcite, or siliceous sinter.

  Bacteria forming microbial mats under the water are responsible for the coloration oft seen in hot springs. Unlike species, with different colors thrive at different temperatures.

• A geyser results if the hot spring has a plumbing organisation that allows for the accumulation of steam from the boiling water.  When the steam pressure builds and so that information technology is higher than the force per unit area of the overlying h2o in the system, the steam will move rapidly toward the surface, causing the eruption of the overlying water.   Some geysers, like Old True-blue in Yellowstone Park, erupt at regular intervals.   The time between eruptions is controlled by the time it takes for the steam pressure level to build in the underlying plumbing system.

Plateau Basalts or Overflowing Basalts

• Plateau or Flood basalts are extremely large volume outpourings of low viscosity basaltic magma from scissure vents.  The basalts spread huge areas of relatively depression gradient and build up plateaus.

• The simply historic example occurred in Iceland in 1783, where the Laki basalt erupted from a 32 km long crack and covered an expanse of 588 km² with 12 kmⁱⁱⁱ of lava.  As a issue of this eruption, homes were destroyed, livestock were killed, and crops were destroyed, resulting in a dearth that killed 9336 people.

In Oregon and Washington of the northwestern U.Due south., the Columbia River Basalts represent a series of lava flows all erupted inside nearly 1 meg years 12 one thousand thousand years agone.  One of the basalt flows, the Roza flow, was erupted over a period of a few weeks traveled about 300 km and has a volume of about 1500 km3.

Volcanoes and Plate Tectonics

Global Distribution of Volcanoes

In the word we had last lecture about how magmas form, nosotros pointed out that since the upper parts of the Globe are solid, special weather are necessary to form magmas. These special conditions do not exist everywhere below the surface, and thus volcanism does not occur everywhere.  If nosotros look at the global distribution of volcanoes we see that volcanism occurs four principal settings.

Along divergent plate boundaries, such equally Oceanic Ridges or spreading centers. In areas of continental extension (that may go divergent plate boundaries in the future). Along converging plate boundaries where subduction is occurring. And, in areas called "hot spots" that are commonly located in the interior of plates, away from the plate margins.
Diverging Plate Margins Active volcanism is currently taking place along all of oceanic ridges, but virtually of this volcanism is submarine volcanism and does not by and large pose a threat to humans.
One of the simply places where an oceanic ridge reaches to a higher place sea level is at Iceland, along the Mid-Atlantic Ridge.  Here, most eruptions are basaltic in nature, but, many are explosive strombolian types or explosive phreatic or phreatomagmatic types.  Every bit seen in the map to the correct, the Mid-Atlantic ridge runs directly through Iceland

Volcanism also occurs in continental areas that are undergoing episodes of extensional deformation.  A classic example is the E African Rift Valley, where the African plate is being divide.  The extensional deformation occurs because the underlying mantle is ascension from below and stretching the overlying continental crust. Upwelling curtain may melt to produce magmas, which then rise to the surface, often along normal faults produced by the extensional deformation.  Basaltic and rhyolitic volcanism is common in these areas.  In the aforementioned area, the crust has rifted apart along the Red Sea, and the Gulf of Aden to form new oceanic ridges.  This may also exist the fate of the E African Rift Valley at some time in the future.

• Other areas where extensional deformation is occurring inside the crust is Basin and Range Province of the western U.Due south. (eastern California, Nevada, Utah, Idaho, western Wyoming and Arizona) and the Rio Grande Rift, New Mexico.  These are also areas of recent basaltic and rhyolitic volcanism.

Converging Plate Marginsouth
All around the Pacific Ocean, is a zone often referred to every bit the Pacific Ring of Burn down, where most of the world'due south most active and most dangerous volcanoes occur.  The Ring of Fire occurs considering most of the margins of the Pacific ocean coincide with converging margins along which subduction is occurring

The convergent purlieus forth the coasts of South America, Central America, Mexico, the northwestern U.S. (Northern California, Oregon, & Washington), western Canada,   and eastern Alaska, are boundaries forth which oceanic lithosphere is being subducted beneath continental lithosphere.  This has resulted in the formation of continental volcanic arcs that form the Andes Mountains, the Central American Volcanic Belt, the Mexican Volcanic Belt, the Cascade Range, and the Alaskan volcanic arc.

The Aleutian Islands (west of Alaska), the Kurile-Kamchatka Arc, Japan, Philippine Islands, and Marianas Islands, New Zealand, and the Indonesian Islands, forth the northern and western margins of the Pacific Ocean are zones where oceanic lithosphere is beingness subducted beneath oceanic lithosphere.  These are all island arcs.

• Basaltic magmas generated by flux melting of the mantle overlying the subduction zone.
• Through magmatic differentiation, basaltic magmas change to andesitic and rhyolitic magma.
  
• Because these magmas are often gas rich and have all take relatively loftier viscosity, eruptions in these areas tend to be vehement, with common Strombolian, Vulcanian, Plinian and Pelean eruptions.
  
• 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.

Hot Spots Volcanism also occurs in areas that are not associated with plate boundaries, in the interior of plates.  These are almost commonly associated with what is called a hot spot.  Hot spots appear to result from plumes of hot mantle material upwelling toward the surface, independent of the convection cells though to cause plate movement.  Hot spots tend to be fixed in position, with the plates moving over the top.  As the rising plumage of hot drapery moves upward it begins to melt to produce magmas.  These magmas then ascent to the surface producing a volcano.  But, as the plate conveying the volcano moves abroad from the position over the hot spot, volcanism ceases and new volcano forms in the position now over the hot spot. This tends to produce chains of volcanoes or seamounts (former volcanic islands that have eroded beneath sea level).

Volcanism resulting from hotspots occurs in both the Atlantic and Pacific ocean, just are more than evident on the sea floor of the Pacific Bounding main, because the plates here motility at higher velocity than those under the Atlantic Ocean.  A hot spot trace shows up equally a linear concatenation of islands and seamounts, many of which tin be seen in the Pacific Ocean. The Hawaiian Ridge is one such hot spot trace.  Here the Big Isle of Hawaii is currently over the hot spot, the other Hawaiian islands still stand up in a higher place bounding main level, only volcanism has ceased.  Northwest of the Hawaiian Islands, the volcanoes have eroded and are now seamounts. The ages of volcanic rocks increase along the Hawaiian Ridge to the northwest of Hawaii. The prominent bend observed where the Hawaiian Ridge intersects the Emperor Seamount chain has resulted from a change in the direction of plate motion over the hot spot.  Note that when the Emperor Seamount chain was produced, the plate must accept been moving in a more northerly direction.  The age of the volcanic rocks at the curve is about 50 million years.

Yellowstone appears to exist over a continental hot spot that has produced a chain of volcanoes as the N American Plate moves southwestward over the hot spot.  (see figure half-dozen.38 in your text)

Examples of questions on this material that could be asked on an examination Define the following and state what kind of magma characteristically erupts from each: (a) shield volcano, (b) stratovolcano, (c) cinder cone, (d) maar, (3) lava dome. What is a caldera and how do calderas course?  Give several examples. What is the difference between a lava dome and a resurgent dome? Why are volcanic domes considered to be extremely dangerous? Compare and contrast geysers, hot springs, and fumaroles. What kind of volcanic landforms would you await to find in each of the post-obit tectonic settings (a) diverging plate boundary, (b) converging plate boundary, (c) hot spot. Requite examples of volcanoes that occur at (a) hot spots, (b) diverging plate boundaries, and (c) converging plate boundaries.

References Render to EENS 3050 Homepage

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