what are three characteristic features you would expect to find looking at a volcano?

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

Volcanic Landforms Volcanic landforms are controlled past the geological processes that form them and human action on them after they have formed. Thus, a given volcanic landform volition be characteristic of the types of material it is fabricated of, which in turn depends on the prior eruptive beliefs of the volcano. Although afterwards processes tin modify the original landform, we should be able to find clues in the modified course that pb u.s. to conclusions nigh the original formation procedure.  Here we discuss the major volcanic landforms and how they are formed, and in some cases, afterwards modified.  Most of this material will 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 (most   10o).

• Shield volcanoes are composed nearly entirely of relatively thin lava flows congenital up over a central vent.

• Nigh shields were formed past depression viscosity basaltic magma that flows easily down slope away class the summit vent.

• The depression viscosity of the magma allows the lava to travel downwards slope on a gentle slope, but equally it cools and its viscosity increases, its thickness builds upward on the lower slopes giving a somewhat steeper lower slope.

• Most shield volcanoes have a roughly circular or oval shape in map view.

• Very little pyroclastic material is found within a shield volcano, except near the eruptive vents, where pocket-size amounts of pyroclastic textile accrue equally a result of fire fountaining events.

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

Vents for most shield volcanoes are central vents, which are round vents nearly the summit.  Hawaiian shield volcanoes too have flank vents, which radiate from the pinnacle 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 (likewise called Composite Volcanoes)

• Have steeper slopes than shield volcanoes, with slopes of 6 to 10^o depression on the flanks to  xxx^o nearly the top.

The steep slope almost the summit is due partly to thick, brusque sticky lava flows that do not travel far downward slope from the  vent.

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

• Stratovolcanoes show inter-layering of lava flows and pyroclastic cloth, which is why they are sometimes chosen blended volcanoes.  Pyroclastic material can make upwards over fifty% of the book 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 unremarkably more explosive than shield volcanoes.

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

• Long periods of quiet (times of inactivity) lasting for hundreds to thousands of years, make this type of volcano particularly dangerous, since many times they accept shown no historic activity, and people are reluctant to listen warnings about possible eruptions.

Cinder Cones (besides called Tephra Cones)

• Cinder cones are small volume cones consisting predominantly of tephra that result from strombolian eruptions.  They usually consist of basaltic to andesitic material.
• They are actually fall deposits that are congenital surrounding the eruptive vent.
• Slopes of the cones are controlled by the angle of repose (bending of stable slope for loose unconsolidated fabric) and are usually betwixt about 25 and 35^o.

They evidence an internal layered construction due to varying intensities of the explosions that deposit dissimilar sizes of pyroclastics.

• On young cones, a depression at the pinnacle of the cone, called a crater, is evident, and represents the area 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 near the base of the cone during the later stages of eruption.
• Cinder and tephra cones usually occur effectually height vents and flank vents of stratovolcanoes.
• An first-class case of cinder cone is Par�cutin Volcano in Mexico. This volcano was born in a farmers corn field in 1943 and erupted for the next 9 years.  Lava flows erupted from the base of operations of the cone eventually covered two towns.
• Cinder cones frequently occur in groups, where tens to hundreds of cones are found in one area.

Maars Maars result from phreatic or phreatomagmatic activeness, wherein magma heats up groundwater, pressure builds equally the water to turns to steam, and so the water and preexisting rock (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 eventually plummet back into the crater, the vent is filled with loose material, and, if the crater still is deeper than the h2o table, the crater fills with water to form a lake, the lake level coinciding with the water tabular array.

Volcanic Domes (also called 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, but instead piles up over the vent.
  

Blocks of almost solid lava interruption off the outer surface of the dome and curl down its flanks to form a breccia effectually the margins of domes. The surface of volcanic domes are by and large very rough, with numerous spines that take been pushed upwards by the magma from below.

Nearly dome eruptions are preceded by explosive eruptions of more gas rich magma, producing a tephra cone into which the dome is extruded. Volcanic domes can be extremely unsafe. because they form unstable slopes that may collapse to betrayal gas-rich sticky magma to atmospheric force per unit area.   This tin can effect in lateral blasts or Pelean type pyroclastic flow (nu�eastward ardentes) eruptions.

Craters and Calderas

• Craters are round depressions, normally less than i 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 equally a upshot of collapse of a volcanic construction.  The collapse results from evacuation of the underlying magma sleeping room.
  

• In shield volcanoes, similar 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 formation of a caldera results from rapid evacuation of the underlying magma chamber by voluminous explosive eruptions that form extensive autumn deposits and pyroclastic flows. Calderas are ofttimes enclosed depressions that collect rain water and snow melt, and thus lakes frequently grade within a caldera. Crater Lake Caldera in southern Oregon is an 8 km diameter 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 as far away every bit 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 now forms an island called Wizard Island.

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.

The Yellowstone caldera is an important instance, as it illustrates the amount of repose time that might be expected from big rhyolitic systems, and the devastating consequence caldera forming eruptions can accept on widespread areas. Yellowstone Caldera which occupies most of Yellowstone National Park, is actually the tertiary caldera to course in the area within the past two million years.  The iii calderas formed at 2.0 meg years ago, ane.3 meg years ago, and the latest at 600,000 years ago.  Thus the repose time is on the average nigh 650,000 years.

• Tephra fall deposits from the latest eruption are constitute in Louisiana and into the Gulf of Mexico, and covered much of the Western function of the U.s.a..

• The eruption 600,000 years agone produced about 1000 km³ of rhyolite (in comparing, the eruption of Mt. St. Helens in May of 1980 produced only 0.75 km³.

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

Resurgent Domes

• Later the formation of a caldera by collapse, magma is sometimes re-injected into the area below the caldera.  This can upshot in uplift of one or more areas within the caldera to form a resurgent dome.  Two such resurgent domes formed in the Yellowstone caldera, every bit shown above.
  

• If magma leaks back to the surface during this resurgent doming, so eruptions of minor volcanic domes tin occur in the area of the resurgent domes.

Geysers, Fumaroles and Hot Springs
A fumarole is vent where gases, either from a magma trunk at depth, or steam from heated groundwater, emerges at the surface of the Globe.  Since nigh magmatic gas is H2O vapor, and since heated groundwater volition produce HiiO vapor, fumaroles will only exist visible if the water 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 downward and is heated past a body of magma or hot rock.  A hot spring results if this hot water can find its fashion back to the surface, usually along fault 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.

  Bacteria forming microbial mats under the h2o are responsible for the coloration frequently seen in hot springs. Different species, with dissimilar 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 h2o.  When the steam pressure builds so that information technology is higher than the pressure level of the overlying water in the system, the steam will motility 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 past the time it takes for the steam pressure to build in the underlying plumbing system.

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 gradient and build upward plateaus.

• The only historic case occurred in Iceland in 1783, where the Laki basalt erupted from a 32 km long fissure and covered an expanse of 588 km² with 12 km³ of lava.  As a effect 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 one thousand thousand years 12 million years ago.  One of the basalt flows, the Roza catamenia, was erupted over a period of a few weeks traveled almost 300 km and has a volume of virtually 1500 km3.

Volcanoes and Plate Tectonics

Global Distribution of Volcanoes

In the give-and-take we had last lecture about how magmas class, we pointed out that since the upper parts of the Earth are solid, special conditions are necessary to form magmas. These special weather condition do not be 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 master settings.

Along divergent plate boundaries, such as Oceanic Ridges or spreading centers. In areas of continental extension (that may become divergent plate boundaries in the time to come). Along converging plate boundaries where subduction is occurring. And, in areas called "hot spots" that are unremarkably located in the interior of plates, abroad from the plate margins.
Diverging Plate Margins Active volcanism is currently taking place along all of oceanic ridges, but almost of this volcanism is submarine volcanism and does not generally pose a threat to humans.
1 of the simply places where an oceanic ridge reaches above sea level is at Iceland, along the Mid-Atlantic Ridge.  Here, well-nigh eruptions are basaltic in nature, but, many are explosive strombolian types or explosive phreatic or phreatomagmatic types.  As seen in the map to the correct, the Mid-Atlantic ridge runs direct through Iceland

Volcanism likewise 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 split.  The extensional deformation occurs because the underlying mantle is rising from below and stretching the overlying continental chaff. Upwelling curtain may cook to produce magmas, which then rise to the surface, oftentimes along normal faults produced past the extensional deformation.  Basaltic and rhyolitic volcanism is common in these areas.  In the same area, the crust has rifted apart along the Red Ocean, and the Gulf of Aden to form new oceanic ridges.  This may also be the fate of the East African Rift Valley at some time in the future.

• Other areas where extensional deformation is occurring within the chaff 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 Margins
All around the Pacific Ocean, is a zone oft referred to as the Pacific Ring of Burn down, where most of the globe'due south most agile and most unsafe volcanoes occur.  The Ring of Fire occurs because most of the margins of the Pacific ocean coincide with converging margins along which subduction is occurring

The convergent boundary along the coasts of South America, Central America, United mexican states, 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 Primal American Volcanic Belt, the Mexican Volcanic Chugalug, 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, along 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 alter to andesitic and rhyolitic magma.
  
• Because these magmas are oftentimes gas rich and have all have relatively high viscosity, eruptions in these areas tend to be violent, with mutual Strombolian, Vulcanian, Plinian and Pelean eruptions.
  
• Volcanic landforms tend to exist cinder cones, stratovolcanoes, volcanic domes, and calderas.
• Repose periods betwixt eruptions tend to be hundreds to thousands of years, thus giving people living nigh 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 usually associated with what is called a hot spot.  Hot spots appear to result from plumes of hot drape cloth upwelling toward the surface, independent of the convection cells though to cause plate motion.  Hot spots tend to be fixed in position, with the plates moving over the top.  As the rise plume of hot mantle moves up it begins to melt to produce magmas.  These magmas then rising to the surface producing a volcano.  Just, as the plate carrying the volcano moves away from the position over the hot spot, volcanism ceases and new volcano forms in the position at present over the hot spot. This tends to produce bondage of volcanoes or seamounts (erstwhile volcanic islands that have eroded beneath sea level).

Volcanism resulting from hotspots occurs in both the Atlantic and Pacific ocean, but are more evident on the sea floor of the Pacific Bounding main, because the plates here movement at college velocity than those under the Atlantic Ocean.  A hot spot trace shows up as a linear chain of islands and seamounts, many of which can be seen in the Pacific Sea. The Hawaiian Ridge is i such hot spot trace.  Here the Large Island of Hawaii is currently over the hot spot, the other Hawaiian islands still stand above body of water level, but volcanism has ceased.  Northwest of the Hawaiian Islands, the volcanoes accept 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 concatenation has resulted from a modify in the direction of plate move over the hot spot.  Note that when the Emperor Seamount concatenation was produced, the plate must have been moving in a more northerly direction.  The age of the volcanic rocks at the bend is about 50 million years.

Yellowstone appears to be over a continental hot spot that has produced a concatenation of volcanoes equally the North American Plate moves southwestward over the hot spot.  (see effigy 6.38 in your text)

Examples of questions on this textile that could be asked on an exam Ascertain the following and country 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 exist extremely dangerous? Compare and contrast geysers, hot springs, and fumaroles. What kind of volcanic landforms would you expect to notice in each of the following tectonic settings (a) diverging plate purlieus, (b) converging plate boundary, (c) hot spot. Give examples of volcanoes that occur at (a) hot spots, (b) diverging plate boundaries, and (c) converging plate boundaries.

References Return to EENS 3050 Homepage

yirawalasuat1964.blogspot.com

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

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