What Properties of the Source Magma Lead to the Formation of a Shield Volcano?

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 class them and act on them afterwards they accept formed. Thus, a given volcanic landform will be feature of the types of material it is made of, which in turn depends on the prior eruptive beliefs of the volcano. Although later processes tin alter the original landform, we should exist able to detect clues in the modified form that lead us to conclusions nearly the original formation process.  Here we discuss the major volcanic landforms and how they are formed, and in some cases, afterward modified.  Most of this material volition be discussed with reference to slides shown in form that illustrate the essential features of each volcanic landform.

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

• Shield volcanoes are equanimous virtually entirely of relatively thin lava flows built upwards over a central vent.

• Most shields were formed past low viscosity basaltic magma that flows easily down slope abroad grade 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 upwardly on the lower slopes giving a somewhat steeper lower slope.

• Most shield volcanoes take a roughly round or oval shape in map view.

• Very little pyroclastic cloth is found within a shield volcano, except well-nigh the eruptive vents, where modest amounts of pyroclastic material accumulate equally a result of fire fountaining events.

• Shield volcanoes thus course by relatively non-explosive eruptions of depression viscosity basaltic magma.

Vents for nigh shield volcanoes are central vents, which are circular vents near the superlative.  Hawaiian shield volcanoes also have flank vents, which radiate from the pinnacle and have the form of en-echelon fractures or fissures, chosen 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 (too chosen Composite Volcanoes)

• Take steeper slopes than shield volcanoes, with slopes of 6 to 10^o low on the flanks to  30^o near the elevation.

The steep slope well-nigh the meridian is due partly to thick, curt viscous lava flows that do not travel far down slope from the  vent.

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

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

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

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

• Stratovolcanoes sometimes accept a crater at the summit that is formed by explosive ejection of fabric from a key vent.  Sometimes the craters take been filled in by lava flows or lava domes, sometimes they are filled with glacial water ice, and less unremarkably  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 historic activeness, and people are reluctant to heed warnings nearly possible eruptions.

Cinder Cones (also chosen Tephra Cones)

• Cinder cones are minor book cones consisting predominantly of tephra that consequence from strombolian eruptions.  They unremarkably consist of basaltic to andesitic material.
• They are actually autumn deposits that are congenital surrounding the eruptive vent.
• Slopes of the cones are controlled past the angle of repose (angle of stable slope for loose unconsolidated material) and are usually between about 25 and 35^o.

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

• On young cones, a depression at the meridian of the cone, called a crater, is axiomatic, and represents the expanse above 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 usually emitted from vents on the flank or virtually the base of operations of the cone during the after stages of eruption.
• Cinder and tephra cones usually occur around top vents and flank vents of stratovolcanoes.
• An fantabulous example of cinder cone is Par�cutin Volcano in Mexico. This volcano was built-in in a farmers corn field in 1943 and erupted for the next ix years.  Lava flows erupted from the base of the cone eventually covered two towns.
• Cinder cones often occur in groups, where tens to hundreds of cones are institute in one area.

Maars Maars effect from phreatic or phreatomagmatic activity, wherein magma heats up groundwater, pressure builds equally the water to turns to steam, and then the water and preexisting rock (and some new magma if the eruption is phreatomagmatic) are blasted out of the footing 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 grade a lake, the lake level coinciding with the h2o table.

Volcanic Domes (also chosen Lava 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, merely instead piles up over the vent.
  

Blocks of nearly solid lava break off the outer surface of the dome and gyre down its flanks to class a breccia around the margins of domes. The surface of volcanic domes are generally very rough, with numerous spines that take been pushed up by the magma from below.

Nigh dome eruptions are preceded by explosive eruptions of more than gas rich magma, producing a tephra cone into which the dome is extruded. Volcanic domes tin be extremely dangerous. because they form unstable slopes that may plummet to expose gas-rich viscid magma to atmospheric pressure.   This tin effect in lateral blasts or Pelean type pyroclastic period (nu�e ardentes) eruptions.

Craters and Calderas

• Craters are circular depressions, unremarkably less than 1 km in diameter, that grade as a effect of explosions that emit gases and tephra.

• Calderas are much larger depressions, circular to elliptical in shape, with diameters ranging from 1 km to fifty km.  Calderas grade as a issue of collapse of a volcanic structure.  The collapse results from evacuation of the underlying magma sleeping accommodation.
  

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

In stratovolcanoes the collapse and germination of a caldera results from rapid evacuation of the underlying magma chamber by voluminous explosive eruptions that form extensive fall deposits and pyroclastic flows. Calderas are often enclosed depressions that collect pelting water and snow melt, and thus lakes often form inside a caldera. Crater Lake Caldera in southern Oregon is an viii km bore caldera containing a lake  The caldera formed virtually 6800 years ago as a result of the eruption of about 75 km3 of rhyolite magma in the form of tephra, constitute equally far away as Canada, accompanied by pyroclastic flows that left thick deposits of tuff on the flanks of the volcano.  Subsequent eruptions have built a cinder cone on the floor of the caldera, which at present forms an island called Sorcerer Island.

Larger calderas have formed within the by 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 exist expected from large rhyolitic systems, and the devastating outcome caldera forming eruptions tin have on widespread areas. Yellowstone Caldera which occupies nearly of Yellowstone National Park, is actually the third caldera to form in the area within the past 2 one thousand thousand years.  The 3 calderas formed at 2.0 million years ago, i.3 meg years ago, and the latest at 600,000 years ago.  Thus the repose time is on the average 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 U.s.a..

• The eruption 600,000 years agone produced about grand km³ of rhyolite (in comparison, the eruption of Mt. St. Helens in May of 1980 produced just 0.75 km³.

• Magma all the same underlies Yellowstone caldera, every bit evidenced by the large number of hot springs and geysers in the area.

Resurgent Domes

• After the formation of a caldera by collapse, magma is sometimes re-injected into the expanse beneath the caldera.  This can outcome in uplift of one or more areas within the caldera to form a resurgent dome.  Ii such resurgent domes formed in the Yellowstone caldera, as shown higher up.
  

• If magma leaks back to the surface during this resurgent doming, so 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 torso at depth, or steam from heated groundwater, emerges at the surface of the World.  Since near magmatic gas is H2O vapor, and since heated groundwater volition produce H2O vapor, fumaroles volition simply be visible if the water condenses.  (H2O vapor is invisible, unless aerosol of liquid water take condensed).

• Hot springs or thermal springs are areas where hot water comes to the surface of the Earth.  Cool groundwater moves downwards and is heated past a body of magma or hot stone.  A hot jump results if this hot water tin can find its way back to the surface, ordinarily along mistake zones.

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

  Leaner forming microbial mats under the water are responsible for the coloration ofttimes seen in hot springs. Different species, with unlike colors thrive at different temperatures.

• A geyser results if the hot spring has a plumbing organization that allows for the accumulation of steam from the boiling water.  When the steam force per unit area builds so that information technology is college than the force per unit area of the overlying water in the system, the steam will motion 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 fourth dimension betwixt eruptions is controlled by the fourth dimension it takes for the steam pressure to build in the underlying plumbing arrangement.

Plateau Basalts or Flood Basalts

• Plateau or Flood basalts are extremely large book outpourings of low viscosity basaltic magma from fissure vents.  The basalts spread huge areas of relatively low slope and build upwards plateaus.

• The only historic example occurred in Iceland in 1783, where the Laki basalt erupted from a 32 km long cleft and covered an area of 588 kmⁱⁱ with 12 km³ of lava.  Every bit a result of this eruption, homes were destroyed, livestock were killed, and crops were destroyed, resulting in a famine that killed 9336 people.

In Oregon and Washington of the northwestern U.S., the Columbia River Basalts represent a series of lava flows all erupted within about 1 million years 12 million years ago.  1 of the basalt flows, the Roza flow, was erupted over a period of a few weeks traveled near 300 km and has a book of about 1500 km3.

Volcanoes and Plate Tectonics

Global Distribution of Volcanoes

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

Forth divergent plate boundaries, such as Oceanic Ridges or spreading centers. In areas of continental extension (that may become divergent plate boundaries in the future). Forth converging plate boundaries where subduction is occurring. And, in areas chosen "hot spots" that are usually 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.
I of the just places where an oceanic ridge reaches above sea level is at Iceland, along the Mid-Atlantic Ridge.  Here, most eruptions are basaltic in nature, just, many are explosive strombolian types or explosive phreatic or phreatomagmatic types.  Every bit seen in the map to the right, the Mid-Atlantic ridge runs directly through Republic of iceland

Volcanism likewise occurs in continental areas that are undergoing episodes of extensional deformation.  A classic example is the Due east African Rift Valley, where the African plate is existence split.  The extensional deformation occurs because the underlying mantle is rising from beneath and stretching the overlying continental crust. Upwelling pall may cook to produce magmas, which so rise to the surface, often forth normal faults produced by the extensional deformation.  Basaltic and rhyolitic volcanism is common in these areas.  In the aforementioned area, the crust has rifted autonomously along the Red Ocean, and the Gulf of Aden to form new oceanic ridges.  This may also be the fate of the Eastward African Rift Valley at some fourth dimension in the future.

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

Converging Plate Margins
All effectually the Pacific Sea, is a zone oft referred to as the Pacific Ring of Burn down, where most of the world's most active and most dangerous volcanoes occur.  The Band of Burn occurs because most of the margins of the Pacific bounding main coincide with converging margins along which subduction is occurring

The convergent boundary along the coasts of Southward America, Central America, Mexico, the northwestern U.S. (Northern California, Oregon, & Washington), western Canada,   and eastern Alaska, are boundaries along which oceanic lithosphere is being subducted below continental lithosphere.  This has resulted in the formation of continental volcanic arcs that form the Andes Mountains, the Cardinal American Volcanic Belt, the Mexican Volcanic Belt, the Pour 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, along the northern and western margins of the Pacific Sea are zones where oceanic lithosphere is being subducted beneath oceanic lithosphere.  These are all island arcs.

• Basaltic magmas generated past flux melting of the drapery overlying the subduction zone.
• Through magmatic differentiation, basaltic magmas change to andesitic and rhyolitic magma.
  
• Because these magmas are frequently gas rich and have all have relatively high viscosity, eruptions in these areas tend to be violent, 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 exist hundreds to thousands of years, thus giving people living well-nigh these volcanoes a false sense of security.

Hot Spots Volcanism too occurs in areas that are not associated with plate boundaries, in the interior of plates.  These are about ordinarily associated with what is called a hot spot.  Hot spots appear to outcome from plumes of hot mantle material upwelling toward the surface, independent of the convection cells though to crusade plate movement.  Hot spots tend to be stock-still in position, with the plates moving over the superlative.  Every bit the rising plume of hot mantle moves upward it begins to melt to produce magmas.  These magmas and then ascension to the surface producing a volcano.  Simply, as the plate conveying the volcano moves away 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 bondage 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, just are more than evident on the sea floor of the Pacific Ocean, considering the plates here move at higher velocity than those under the Atlantic Ocean.  A hot spot trace shows up as a linear chain of islands and seamounts, many of which tin can exist seen in the Pacific Sea. The Hawaiian Ridge is 1 such hot spot trace.  Hither the Big Island of Hawaii is currently over the hot spot, the other Hawaiian islands even so stand up above sea level, but volcanism has ceased.  Northwest of the Hawaiian Islands, the volcanoes take eroded and are at present seamounts. The ages of volcanic rocks increase forth the Hawaiian Ridge to the northwest of Hawaii. The prominent bend observed where the Hawaiian Ridge intersects the Emperor Seamount concatenation has resulted from a change in the direction of plate motion over the hot spot.  Annotation that when the Emperor Seamount chain was produced, the plate must accept been moving in a more than northerly direction.  The age of the volcanic rocks at the bend is well-nigh 50 million years.

Yellowstone appears to be over a continental hot spot that has produced a chain of volcanoes equally the North American Plate moves southwestward over the hot spot.  (see effigy half dozen.38 in your text)

Examples of questions on this material that could be asked on an exam 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 form?  Requite several examples. What is the difference betwixt 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 lot await to find in each of the following tectonic settings (a) diverging plate boundary, (b) converging plate purlieus, (c) hot spot. Give examples of volcanoes that occur at (a) hot spots, (b) diverging plate boundaries, and (c) converging plate boundaries.

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Source: https://www.tulane.edu/~sanelson/Natural_Disasters/volclandforms.htm

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