Photo of a volcano errupting.

I have always been fascinated about volcanoes, and have even been to a couple, so have been looking forward to this section!!!

First of all- Volcano – the term came from a little volcanic island in the Mediterranean off Sicily called “Vulcano” – Vulcano was thought, in roman times, to be the chimney of the forge of Vulcan- the Roman god of fire and blacksmith of the gods.  Hot lava, clouds and dust from Vulcano was thought to be from Vulcan beating out thunderbolts for Jupiter, king of gods, and weapons for Mars, the god of war…  from http://www.crystalinks.com/volcanomyth.html  :O)  I can see why the Romans would think that! Particularly if they had been drinking wine boiled in lead vats…

The modern day definitions of volcano/ volcanism, from Britannica concise encyclopedia:

volcano, Vent in the crust of the Earth from which molten rock, hot rock fragments, ash, gas, and steam issue.

volcanism, also spelled vulcanism, any of various processes and phenomena associated with the surficial discharge of molten rock, pyroclastic fragments, or hot water and steam, including volcanoes, geysers, and fumaroles.

So what the hec are pyroclastic fragments??? turns out it is the ash and rocky fragments resulting from the massive explosion from expanding gases during an eruption.  The more violent the eruption, the more pyroclastic fragments produced.  Pyroclastic fragments that have been airborn are also known as tephra – if the pyroclastic fragments are amalgamated into a larger form, they form pyroclastic rock.  Hydroclastic fragments are fragments produced through the explosion of lava as it hits water or ice.  The explosion is caused by the rapid cooling of the lava which allows the dissolved gases to escape quickly as the lava solidifies, causing it to shatter.

Lava is a term used to describe magma or molten rock as it erupts through a vent and after it is cooled and solidified.

hmmm I can see a word list or glossary is going to have to be written for or by students – maybe as part of a webquest.

I have looked at quite a few websites on volcanoes and have found TONNES of pretty pictures – very little that relate them to plate tectonics though, which is what we need to know….

http://www.geology.sdsu.edu/how_volcanoes_work/  seems to be the best site encountered thus far – with a bit of stuff on the U.S. Geological Survey website to back up the authenticity of their information!!!

The map above shows a picture of the earths plates – showing a clear representation of the offsets in the mid-ocean plate boundaries (i.e. the plate boundaries are neither uniform or straight).  The map also shows, with red dots, the positioning of the approx 550 active volcanoes of the world (although most of these are dormant and have been for many years…).  As you can see by this picture, almost all the terrestrial volcanic activity occurs along plate lines – the few that occur mid-plate are around designated “hot spots”.

In the section on plate tectonics there was a review of the 3 distinct plate boundary types

1. Divergent boundaries – plates move apart and new crust is formed

2. Convergent boundaries – plates come together crust is destroyed

3. Transform boundaries – plates slide past one another

(The USGS mention at this point that some plate boundaries are uncertain)

Volcanoes are abundant at both divergent and convergent plate boundaries – but not transform plate boundaries.  Spreading center volcanism occurs at divergent plate boundaries and subduction zone volcanism occurs at convergent plate boundaries.  Volcanic eruptions that occur mid plate are classified as intra-plate volcanism.

Here is a FANTASTIC picture from Encyclopedia Britannica that shows the three environments of volcanism from http://www.britannica.com/EBchecked/topic/632078/volcanism

The only potential problem I see with the picture is that it kind of looks as if the core is coming up through channels to the surface – it is not actually the molten core that is coming out… If it was – then the oceanic crust would be made from iron!!!

It is still an excellent picture and one well- worth showing if a better option does not come up.  So back to the info on the How Volcanoes work website… A very good website!!!

Spreading center volcanism:

Spreading center volcanism is the most productive volcanism and occurs mostly in oceans. Sea floor spreading, described in the Plate Tectonics section of this blog, is an example of spreading center volcanism.  Spreading center volcanism occurs at divergent plates which are pulled apart as a result of convection currents in the mantle.  As the plates pull apart a vent or “rift” is created between the plate boundaries and is filled with rising magma/lava (mix of molten rock, volatile compounds and solids) from the mantle/ asthenosphere.  The process of sea floor spreading is continuous, although different parts of the diverging fault may spread at different rates.  On average, sea floor spreading in the Atlantic ocean is increasing the ocean’s size by around 2cm per year.  Ridges form at divergent boundaries due to the continual release of lava.  Ridges are areas of built-up basalt/ crust that can be as large as a mountain – in-fact some of the largest mountain ranges in the world are actually underwater near divergent fault lines. As the crust is pulled apart at divergent plate boundaries, rifts can also develop away from the central vent/rift due to tension.  These rifts can spill magma and can also build up over time into decent sized underwater basalt mountains/volcano islands.  Generally with these rift volcanoes, basalt magma will pond just beneath the crust and every now and then a tap, jerk or knock from plate movement will allow the magma to surface as lava.  Spreading center eruptions are usually fissure eruptions (fissure volcanoes) – i.e. there is no exciting explosion – magma just pours out of the cracks/ rifts/ vents in the lithosphere and these rifts can be very small or up to a few kilometers long.  Underwater spreading center volcanism often produces bulbous shapes known as pillow basalt – the basalt-rich lava from volcanic activity is cooled rapidly in the deep water and sets into pillow shaped mounds.

Underwater spreading center volcanism produces thermal springs that allow heat stable bacteria to flourish – the life connection to underwater volcanoes is represented beautifully by David Attenborough in this video

Pillow Basalt from rift volcano

Iceland lies on the mid Atlantic ridge at the divergent boundary between the Eurasian  and North American plates.  Iceland is an example of an island built up as a result of spreading center volcanism.  Around 1/3 of Iceland is volcanically active and, being on top of a divergent plate boundary, it is growing in size every year.  Eventually, it is thought that Iceland will break in two as the ridge gets bigger and eventually allows the inflow of sea through the country.

The volcanoes associated with continental divergent plate boundaries are normally shield volcanoes – called this because they look like big shields rather than forming very tall mountainous volcanoes like the ones you see in movies (picture from Encyclopedia Britannica).  These shield volcanoes take their shape from the usually non-explosive slow lava flow.

A picture of a Shield volcano in Iceland

Subduction zone volcanism:

Unlike the usually quiet spreading center volcanism, Subduction zone volcanism produces more violent episodes.  The most active volcanic region on earth, the ring of fire, is laden with subduction zone volcanoes.  The ring of fire occurs at the converging boundaries of the Pacific Plate.

So as a refresher of convergent boundaries – this is when two plate boundaries are being forced together and one boundary usually gets pushed underneath the other – i.e. the lithosphere of one plate ends up subducting under the other.  In the ring of fire, the oceanic crust always subducts under continental crust or other oceanic crust (usually just near to continental crust), which leaves a massive oceanic trench.  Oceanic trenches resulting from convergent plate boundaries are the deepest points on earth and can be up to 11km deep (e.g. the Mariana trench).

As the subducting slab is pushed further under the ascending plate, it encounters higher pressures and more extreme temperatures.  The high water content of the descending oceanic crust, along with the high carbon dioxide content, actually serves to reduce the melting point of the various metals and minerals in the descending rocky lithosphere.  As the rocky lithosphere begins to melt it becomes less dense and rises. Eventually this magma makes its way to the surface and over years and eruptions can build up to form large volcanoes – usually occurring  in a linear belt parallel to the oceanic trench. When the convergent plate boundaries occur in the ocean the belt of volcanoes is known as an island arc and when the boundaries are at the edge of a continent it produces a terrestrial volcanic arc.

The Aleutian island chain is an example of an island arc – it consists of a chain of around 300 island volcanoes in the Pacific ocean (part of the ring of fire) just off the coast of Alaska.  An example of a volcanic arc are the Cascade volcanoes of the USA – these run up the coast from California through Oregon, Washington and British Columbia, and include the famous Mount St Helens volcano in Washington that had a severe eruption in 1980.

Island arc formed by oceanic-oceanic subduction  Volcanic arc formed by oceanic-continental subduction
 Island arc formed by
oceanic-oceanic subduction
 Volcanic arc formed by
oceanic-continental subduction

Volcanoes produced by subduction zone volcanism are usually stratovolcanoes and are made up of alternating layers of igneous rock (cooled solidified lava) and tephra (solid materials/ash emitted through eruption). The molten magma composite from the subducted lithosphere rises as it becomes more liquid/ less dense, and pools in a magma chamber under or within the stratovolcano. Because the pressure is relatively low in the chamber, the gasses dissolved in the magma are allowed to escape, causing a buildup of gases (including CO2, SO2, Cl2, H2O).  Magma and gas will build up to the point where the pressure behind the volcanic cone is too high and there will be a sudden and explosive eruption to blow of the cone and allow the escape of the gas, tephra and some lava.

Usually, stratovolcanoes do not involve as extensive lava flow as shield volcanoes.  Sometimes vents appear in the side of volcanoes due to weaknesses – these may serve to allow gas to escape but are also the origin of formation of parasitic cones which can also blow off during an eruption.

Here is a lovely diagram of a strata volcano  

The most famous volcanoes, including Vesuvius, are stratovolcanoes.

Intra-plate volcanism:  

It makes sense why you would have volcanic activity at plate boundaries – but how about Hawaii??? Hawaii is in the middle of the Pacific Ocean, far from any plate boundary.  The current most excepted theory is that the Hawaiian volcanoes, and several other intra-plate volcanoes, occur at “hot spots“.  Hot spots are sub-lithosphere intense plumes of magma that occur as part of convection currents.  As mentioned previously, convection currents occur because the magma in the mantle close to the earth’s core is extremely hot – as a result it is less viscous and dense than the cooler harder magma above it.  Magma from the mantle next to the core will rise as a result of the lower density, and the the more dense magma above it will sink toward the core due to gravity.  It is thought, in some areas, the plumes of intensely hot molten magma which have risen to the asthenosphere can be up to 300km in diameter (hence the term hot spot!!!).  The intense heat of the hot spot makes the lithosphere very fragile and prone to cracks/ vents.  As a vent opens up it causes an outpour of lava – and over time these outpours can build a volcanic island.   Hot spots stay in a constant location, while the lithosphere is pulled in a particular direction slowly as a result of tectonic plate activity.  As a result of this “continental drift” it is common to find a chain of volcanoes at a hot spot – with the youngest and most active volcano being directly over the hot spot and older inactive volcanoes, that used to lie over the hot spot, lying nearby.  An example of this is the Hawaiian-Emperor seamount chain (Hawaiian islands).   The main island of Hawaii is the newest and most volcanically active, as you move north west you will see progressively older extinct volcanic islands which used to lie over the hot spot (millions of years ago…).   A new submarine volcano is forming to the southeast of the main island of Hawaii – it is though that this will eventually become the main volcanic island while the Hawaiian “Big Island” volcanoes will become extinct.

This picture comes from the USGS (United States Geological Survey) website.

Shield volcanoes are the predominant type of volcano occurring at hots pots – they have a tendency not to produce explosive eruptions, although they occasionally will.  Hawaii is made up of 5 separate shield volcanoes, one of which, Kilauea, has been in a constant state of eruption since 1983, making it the most active volcano on earth.

The spots in red are considered to be major Hot Spots – the yellow and green spots are considered to be less significant hot spots – AND WOW!!! LOOOK WE ARE ON THE MAP!!! looks to be something going on in Victoria… I was thinking of doing earthquake and volcanic activity in Australia in the Elaborate phase of a unit so students could apply their knowledge about volcanoes and earthquakes to our country (make it more real for them!!!)- and we have our own hotspot!!! (all be it a mild one… must read more about that…)   Oooh oooh – have found a great website that talks about the history of volcanoes and earthquakes in oz – as well as what is sill active http://home.iprimus.com.au/foo7/volcmap.html.

The details of the volcanism types would probably best be covered in the explain section of a plate tectonics unit… Need to know a bit more about volcanoes though… have a few holes in my volcano knowledge…

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