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• 1 media/Big Intro Volcanoe picture.jpg 2016-06-01T12:04:54+00:00 Volcanism 104 image_header 2016-07-21T12:47:46+00:00 Introduction
  
  ​Volcanoes are large features on the Earth's surface that are integrally related to plate tectonics.  They form all along the oceanic ridge system and above zones of subduction and also above "hotspots" at the base of the lithosphere that are well away from plate boundaries.  Below these hotspots there is an upward flow of hot material in the mantle (called a "mantle plume"), rising  to a point beneath the Earth's crust; hotspots create melting of the overlying lithosphere and crust, creating magma that feeds volcanoes at the Earth's surface.  There are currently over 60 hotspots that have been identified world wide (45 are shown in map), many of which lie along oceanic ridges but many are unrelated to plate boundaries.
  
  A volcano is a mound of material that is extruded to the Earth's surface from a vent that is connected to a magma chamber within the crust or lithosphere by way of a feeder conduit. 
  
  
  Magma is molten rock within the Earth and when it reaches the surface, it is called "lava".  The temperature at which rocks will melt ranges from 700 to 1300 degrees Celsius, depending on the composition of the rocks that are melting and, particularly, on the amount of water that is present; water reduces the temperature at which rocks will melt.   The feeder conduit delivers molten material, fragments of cooled and hardened material along with gases to the vent.  In some cases the feeder conduit will bifurcate and supply material to multiple vents at the surface.
  
  Volcanic deposits
  
  There are two major classes of volcanic deposits: lava  and pyroclastic material.  Lava dominates volcanoes that undergo effusive eruptions (characterized by flowing molten rock that subsequently hardens to form lava deposits).  Pyroclastic material is produced by explosive eruptions that generate largely solid fragments ranging from huge boulders to fine dust.  As we will see later in this section, the style of eruption also determines the overall morphology of volcanoes.
  
  The magma that feeds an effusively erupting volcano has a composition that is similar to basaltic rocks that dominate the sea floor and has a relatively small proportion of silica dioxide (SiO₂).  Silica dioxide-poor magmas tend to have a relatively low viscosity, they readily flow through the vent to the surface of a volcano and then over the land surface, away from the vent, until they cool and solidify to form lava deposits. The volcano that is produced is built from these lava flows.
  
  The magma that feeds explosively erupting volcanoes has an overall composition that is similar to granitic rocks that dominate the continents and is relatively rich in SiO₂.  Due to the high concentration of SiO₂ the magma is very viscous and tends to plug the vent, inhibiting the steady flow of magma to the surface.  Also because of the high viscosity of the magma gases readily become trapped (they cannot escape from the thick magma) and as pressure builds in a plugged vent these gases become compressed, storing energy that, once released, will add to the total energy released when an explosive eruption takes place.  The volcano that forms is built from the pyroclastic material produced by repeated explosive eruptions.
  
  Lava
  
  Lava flows at the surface of a volcano can be up to 1200 degrees C and can flow readily down the slopes of a volcano, away from the vent.  The photo below shows hot lava spewing out of a vent like a hose.
  
  
  Once at the surface the lava can flow for kilometers as it cools, becomes more viscous and eventually solidifies, adding to the volume of rock making up the volcano.  The you-tube video linked below shows lava flowing over the surface of an effusive volcano.
  
  
  
  Pyroclastic material
  
  The broken up rock that makes up pyroclastic material is also called tephra.  This material includes not only broken up volcanic rock from within the vent but, particularly in the case of powerful explosive eruptions, also includes rocks that make up the entire volcanic landform that is destroyed by the explosion.
  
  Tephra is subdivided into subclasses based on the size of the particles.
  
  Ash includes all tephra that is finer than 2mm in diameter.  This is a very wide range of sizes from coarse sand down to very fine dust that can be carried high up into the upper atmosphere where it can be distributed world wide by wind.
  
  Lapilli  is tephra that ranges in size from 2 mm to 64 mm in diameter and an example is shown in the following photograph.
  
  
  
  A particle of tephra that is larger than 64 mm in diameter is referred to as a block if it is angular in shape with sharp, broken edges whereas if it is rounded it is referred to as a bomb.  Bombs are typically ejected from a volcano as soft, hot magma whereas blocks are hard and brittle. 
  
  Classification of volcanoes
  
  There are four main classes of volcano, each differing in terms of their overall morphology that is determined by the behaviour of the volcano during eruption.
  
  Shield Volcanoes
  
  Effusive volcanic eruptions of basaltic lavas produce a volcano that has relatively low surface slopes that dip away from a single vent or series of vents.  These volcanoes are made up almost exclusively of the deposits of lava flows.  The photo below shows Mauna Kea (a Hawaiian volcano) in the distance.  The shape of these volcanoes is like an inverted shield (i.e., concave down) like the shields used by Vikings (Season 6 of which was amazing!).  The slopes on shield volcanoes are normally around 15 degrees, far shallower than other constructive volcanoes.
  
  
  Shield volcanoes can undergo eruptions that continue without stopping for decades, such as Kilauea (Hawaii) erupting since 1983.  They have the potential to generate massive piles of basaltic lava and, therefore, huge shield volcanoes.  For example, Mauna Loa (the largest of 5 volcanoes that make up the island of Hawaii) has been intermittently active and growing upward from the sea floor for the past 1 million years or so, reaching sea level about 400,000 years ago.  It has amassed a total volume of about 75,000 cubic kilometers over its history.
  
  
  Cinder Cones
  
  Volcanoes that are are built largely by the accumulation of basaltic pyroclastic material produced by explosive eruptions are called Cinder Cones.  The inverted cone shape has steeper slopes than a shield volcano, typical slopes range from 35 to 40 degrees from the horizontal.  Lava Butte (Oregon) is a cinder cone that was last active about 7,000 years ago found in Newberry Volcano National Monument. The photo of Lava Butte (below) shows the classic shape of this type of explosive volcano.
  
   Cinder cones are the most common class of volcano on Earth.  They are normally much smaller than the other types of volcanoes.  They are most often associated with shield volcanoes where they develop from vents that are distant from the main vent and deliver cooler, more viscous magma to the surface.  Eruptions are commonly of tephra that is thrown out of the vent, accumulating the cone-shaped mound all around the vent.  After an initial pyroclastic phase, it is also common for cinder cones to erupt lava as the eruption progresses.  The lava oozes out the side, rather than from the top crater, such as at Lava Butte, where lava emerged from the base of the volcano and flowed to the lower left in the image above.
  
  Stratovolcanoes (a.k.a. composite volcanoes)
  
  Stratovolcanoes are made up of both lava deposits and pyroclastic deposits (which is why they are sometimes called "composite volcanoes"). They undergo a "constructive" phase that is characterized by a mix of effusive eruptions of lava, along with some small explosive eruptions, that build large, rather steep-sided volcanic mountains.  Mount Shasta (a part of the Cascade Range in the Rocky Mountains) is a spectacular stratovolcano that most recently erupted in 1786.
  
  
  
  Stratovolcanoes are much more dangerous than shield volcanoes and cinder cones.  They normally experience relatively long periods of eruption, often combined with long periods of dormancy, followed by a relatively brief destructive phase that is punctuated by an explosive eruption that destroys much of what had been previously constructed.  The danger lies in the fact that during the dormant phase, the level of risk to humans who might inhabit the area around such a volcano is small.  However, once the volcano passes into the destructive phase the level of risk increases exponentially.  The explosive eruption includes an eruption column (a pillar of ash and smoke) that rises 10s of kilometres about the volcano.  In many cases repeated explosive eruptions take place and they may culminate in a final massive eruption that destroys much of the volcano.  The video, below, shows the eruption of Mount St. Helens in May of 1980.
  
  
  
  Supervolcanoes
  
  Supervolcanoes include the most dangerous and destructive of all volcanoes.  These are defined as any volcano that ejects one thousand cubic kilometers or more material during an eruption (called a super-eruption).  These include effusive volcanoes that erupt large volumes of largely basaltic lava over relatively long periods of time and explosive eruptions of tremendous magnitude (erupting over 1000 km³ of largely tephra over days to months).  The most spectacular of the explosive supervolcanoes are those with silicon-rich magmas.  Fortunately, no such supervolcanoes have erupted during historic times.  The largest historic eruption was that of the Indonesian volcano Tambora that erupted in 1815, ejecting over 150 km³, an order of magnitude smaller than even the smallest supervolcano. The largest eruption of the last 70,000 years took place on New Zealand's North Island 26,500 years ago and ejected 1,170 km³ of pyroclastic material.  The largest known super-eruption took place 27.8 million years ago in the San Juan volcanic field of Colorado; the total volume of ejected material is estimated to have been 5,000 km³.   The magnitude of super-eruptions make them the most dangerous natural phenomena that is derived from the Earth (only exceeded by major impacts by bodies from beyond the Earth).  However, the risk that this class of volcano poses is mitigated by the fact that they are also the rarest and least frequently erupting volcanoes.  On average, an explosive supervolcano erupts with a frequency of about once every 50,000 years.  
  
  The best known active supervolcano is Yellowstone Caldera that is the major tourist attraction of Yellowstone National Park (note that its cartoon equivalent - Jellystone Park – has no caldera so relax, Yogi, Booboo and others are not immediately threatened by a supervolcano).  The park sits on a giant caldera about 80 km long and 50 km wide which is the scar left by three super-eruptions that took place over the past 2 million years. The first of the three super-eruptions ejected about 2,500 km³ of material.  Yellowstone Park was not known to be a sleeping supervolcano until the 1970s and that is one the reasons that these are particularly dangerous volcanoes; they do not have the characteristics of the more common classes of volcanoes so they are hard to recognize as potentially explosive landforms.
  
  The problem in recognizing supervolcanoes is that once they have exploded they preserve a caldera but little else of the pre-eruption surface features of a volcano.  These large calderas hide in plain sight awaiting discovery prior to some future eruption.
  
  Volcanoes and plate tectonics
  
  As noted in the introduction, volcanoes are closely linked to plate tectonics and related processes.  For example, the largest features of the Earth's surface, the oceanic ridges, are made up of basaltic volcanoes.  Where they reach the surface, they form largely shield volcanoes and associated cinder cones.  As we saw in the section on plate tectonic processes, many volcanoes are formed along the margin of over-riding plates of convergent plate boundaries.  The volcanoes that form in this tectonic setting are commonly stratovolcanoes (along with many smaller are cinder cones).  Where oceanic crust subducts beneath oceanic crust, these stratovolcanoes form island arcs along the over-riding plate, running parallel to the oceanic trench.  Where oceanic crust subducts beneath continental crust the stratovolcanoes form in the mountain belts that parallel the trench on the over-riding plate.  The margin of the Pacific ocean basin is often called the "ring of fire" (NOT the one that Johnny Cash sang about) because the Pacific is surrounded by oceanic trenches and their associated volcanic arcs.
  
  
  
  The class of volcano that forms over hotspots depends on the type of crust that is involved.  When a hotspot is beneath oceanic crust the volcanoes that form are basaltic shield volcanoes.  A particularly good example of hotspot volcanoes formed on oceanic crust make are those that make up the Hawaiian Islands.  The following map shows the chain of islands that make up the Hawaiian Islands in the Pacific Ocean.  The chain extends from the island of Hawaii in the southeast to Kure in the northwest.  The age of the islands increases from southeast to northwest.  The island of Hawaii is about 500,000 years old but still very active; it is made up of five volcanoes, three of which continue to be active (not including Loihi which is a recently formed submarine volcano that is 24 km to the southeast of Hawaii that won't grow to the ocean surface to form a new island for another 250,000 years). The island of Nihoa is about 7 million years old and the most northwestern island of Kure is about 30 million years old and both of these islands are made up of now extinct volcanoes.  This island chain formed because the Pacific Plate is moving towards the northwest (as shown by the red arrow in the figure), over a hotspot that has been relatively stationary over many millions of years.  The island of Hawaii is currently immediately above the hotspot and is receiving lava to its very active volcanoes.  The island of Nihoa formed when its location on the Pacific Plate was over the hotspot 7 million years ago, but was subsequently pushed off the hotspot as the plate moved further to the northwest while the hotspot remained in place.  Nihoa is now well away form the source of magma so that it is no longer volcanically active.  The island chain formed in this manner as plate movement pushed successive volcanic islands off the hotspot while the plates moved towards the northwest.
  
  
  
  The figure below shows the formation of islands of the Hawaiian chain forming over the past 5 million years or so as the plate moved over the hotspot that is currently beneath Hawaii.  Note that if the plates did not move over the hotspot (i.e., if plate tectonics didn't operate on Earth) rather than a chain of volcanic islands we would have one massive volcanic island that grew in place over the 30 million years that it took for the island chain that we see in the map.
  
  
  
  When hotspots lie beneath continental crust, the magma that is generated has a granitic composition and is particularly viscous, gaseous and explosive. Like basaltic hotspot volcanoes, because of plate tectonics the crustal plates move over time whereas the hotspots are more-or-less stationary.  As a result, the location of the caldera on the crust that is produced by the supervolcanic eruption has moved over time as the crust moved relative to the relatively stationary hotspot.  The map below shows the changing position of the Yellowstone caldera over the past 12.5 million years.  Each caldera is left behind by supervolcanic eruptions.  The southwestern movement of the plate produces a chain of calderas, each representing the past location of the hotspot with respect to the moving crust (this chain is called a "hotspot track" in the figure).  The calderas along the track become younger towards the northeast.