So I am now pretty cluey on all the tectonic/ volcano connection stuff – super interesting – but there is other stuff about volcanoes I don’t know – like how they are classified and what actually comes out of them??? Although I do know that the stuff that comes out of volcanoes at hot spots and divergent faults is usually slow moving basalt type stuff…
So in the previous section I have mentioned fissure volcanoes, rift volcanoes, shield volcanoes and stratovolcanoes – are their more kinds of volcanoes?
A study of 10 different internet sites and 2 text books tells me that there is really not a clear classification system for volcanoes since almost all the websites have said totally different things – even the extremely reputable university based websites and the US geological survey say different things about classification… So I have decided to tackle this section based on what Geoscience Australia use as there classification but to also mention other “types” mentioned elsewhere. The discrepancy in listed types of volcanoes is going to effect the way this stuff is taught… if this is being taught by a webquest, or a research type activity – it will be important that the information students are exposed to are consistent… i.e. probably need to tell them what links to look at and print out other resources for them that show consistent names…
Geoscience Australia can be found here http://www.ga.gov.au/hazards/volcano/volcano-basics/what.html.
Geoscience Australia classify the 3 kinds of volcanoes as Shield Volcanoes, Composite or Stratovolcanoes and Caldera volcanoes. Other websites also mention Scoria or Cinder cone volcanoes, Fissure volcanoes and Fumaroles.
A volcanoes appearance is going to be a result of what comes out of it and how it comes out.
What I know so far…. What comes out of a volcano is known as lava… it is derived from the earths mantle where it is known as magma until it reaches the surface. Gases also come out of volcanoes – these gases were previously dissolved into the magma. I know that volcanoes in hots pots (e.g. Hawaii) and divergent plates (e.g. Iceland/ Oceanic Ridge) are usually Fissure volcanoes and Shield volcanoes – are usually non-explosive and involve large amounts of mainly basaltic liquid magma. I know Volcanoes at converging plates are usually explosive and the lava is more viscous and contains a mix of molten material from the mantle and molten rock/ minerals and water from the earth’s crust. I know that Stratovolcanoes are common in these areas and they are built from layers of lava and tephra – which are fragments/ particulate matter resulting from the explosion – and that the explosion is more severe due to the nature of the magma pooling under the volcano.
Shield Volcanoes (and fissure volcanoes) and Basalt Lava:
As mentioned, lava from Shield volcanoes (and fissure volcanoes) is usually very hot and runny. This is because it is made from Basalt, otherwise called Mafic lava. Basalt has a high content of iron and magnesium and a low content of aluminium and silica compared with other types of lava. Aluminium, silica, calcium and many other dissolved minerals (but most significantly silica) will polymerize in hot magma and the polymerization alters the flow capacity making the lava much more viscous. With the lowest levels of silica and very hot temperatures of up to 1200°C, basalt lava has the lowest viscosity of all lava types. Because of the shallow slopes of shield volcanoes and the fact that the lava is still quite a lot more viscous than water, humans can still usually outrun lava flow from shield and fissure eruptions.
Shield volcanoes (and fissure volcanoes) generally erupt gently and are non-explosive because the low viscosity of the magma allows the various gases dissolved in the magma to escape easily when the magma reaches lower pressure areas at the earth’s crust. So what gases are we talking about??? Well the majority is water vapor (around 60%) and carbon dioxide (10-40%) – particularly at subduction zone volcanic activity which usually involves molten oceanic crust being added to the magma from the mantle. Subduction zone volcanoes usually also have lots of chlorine gas (from salt water NaCl.H2O). Other gases dissolved in magma include nitrogen, sulphur dioxide (SO2), hydrogen sulphide (H2S), argon, helium, neon, methane, carbon monoxide. Small amounts of hydrogen chloride, hydrogen flouride, hydrogen bromide, nitrogen oxide, sulphur hexaflouride and organic compounds such as the highly toxic methylmercury, halocarbons (like CFCs) and halogen oxyradicals. (from H. Sigurdsson et al. (2000) Encyclopedia of Volcanoes, San Diego, Academic Press). So as a divergence… one thing that strikes me as hugely obvious about these gases… many greenhouse gases and contributors of acid rain!!! Also lots of pretty toxic and stinky polluting gases in there like H2S, rotten egg gas… which is my experience of volcanoes… they reek of rotten eggs…
My favourite volcano, Kilauea volcano in Hawaii that has been erupting constantly since 1983, lets out heaps of sulphur dioxide gas (a major contributor of acid rain)
Kilauea volcano, Hawaii
So from our previous notes – Fissure volcanoes and Shield volcanoes are going to have a similar composition of magma and will have a similar eruption pattern. The eruption pattern from these volcanoes is normally termed “Hawaiian” – a Hawaiian eruption means lava will typically spurt a little way into the air and then run down a mountain or pool into a lake (like the picture of Kilauea above).
Stratovolcanoes and andesite/ felsic lava:
Stratovolcanoes, as mentioned, are usually explosive and made up of layers of lava and tephra – were tephra is particulate matter resulting from the explosive eruption. The lava from Stratovolcanoes (also called composite volcanoes) is composed of a mixture of the iron/ magnesium rich mantle, and a variety of minerals/ molten rock/ water from the lithosphere which has subducted at the convergent plate. The lava from Stratovolcanoes is generally richer in silica, calcium and other minerals as apposed to basaltic lava – as a result, there is some polymerization of minerals and a higher viscosity to basalt lava. Lava from stratovolcanoes is usually either andesite lava or felsic lava (rhyolite/ dactite). Felsic lava is composed of rhyolite or dactite and has the most silica and therefore the most polymerization and highest viscosity. Andesite has a lower level of silica and polymerization but is still far more viscous than basalt. Lava flowing from stratovolcanoes is generally up to 950 °C. Because andesitic and felsic lava is quite viscous and sticky, it is more difficult for gas dissolved in the pooling magma to escape as it reaches the lower pressures at the earth’s crust and begins to cool. As a result, rapidly expanding bubbles build up in the magma and extreme pressure builds up inside the magma chamber/ conduit/ pipe. It is thought that the bubbles expand up to 1000 times their normal size as they pool in the magma chamber – the building pressure eventually causes the magma to explode (called a magmatic eruption), blasting it into tiny pieces of volcanic ash, and explodes the weakest point of the volcano off, usually at the summit – to allow gas, rising lava and ash to escape. The resulting thick sticky lava flow from this kind of eruption usually creates steep sided volcanoes. Felsic lava, being more viscous, produces a higher level of explosivity than andesite volcanoes and are much more dangerous to humans as a result.
The kinds of eruptions that occur at stratovolcanoes are varied – with more than one kind commonly occuring at the same volcano at subsequent eruptions or even through a series of continuous eruptions. The kinds of eruptions are usually named after volcanoes where the pattern was first recorded – such as:
Strombolian Eruptions - Named after Stromboli, one of the stratovolcanoes making up the Aeolian island arc (as mentioned in the plate tectonic section). Strombolian eruptions are generally low level mildly explosive reactions but generally involve the ejection of glowing cinders, lapilli (tephra) and lava bombs for tens of meters into the sky. Cinder cones are normally produced at the vent of the volcano due to the style of eruption. Cinders and lapilli (pyroclastic particles) predominate over volcanic ash.
Vulcanian Eruptions - Named after Vulcano, another Aeolian island arc stratovolcano. This kind of eruption is much more dramatic and involves eruption of a dense cloud of ash-laden gas exploding from the crater and rising high above the peak – steaming ash commonly causes a whitish cloud near the upper level of the cone. An extensive amount of pyroclastic rock is released from vulcanian eruptions in explosions described to sound like cannons being fired. These eruptions are explosive because of the silicon-richer andesite magma.
Vesuvian/ Plinian Eruptions – Named after the famous stratovolcano, Mount Vesuvius, near Naples in Italy (famous for burying the town of Pompeii in 79AD). Vesuvian eruptions are also called Plinian eruptions (from Pliny the younger/elder who described the 79AD eruptions) and are the most explosive and dangerous, with magma usually being of the highly viscous, silica rich, rhyolitic type. Plinian eruptions involve massive explosions which send a column of gas, pyroclastic fragments and ash high into the stratosphere to form a cauliflower-shaped cloud high above the volcano.
- In a “Vesuvian” eruption, as typified by the eruption of Mount Vesuvius in Italy in A.D. 79, great quantities of ash-laden gas are violently discharged to form cauliflower-shaped cloud high above the volcano. Krakatoa near Java is another volcano which has had this kind of eruption in the past, along with Mt St Helens in Washington which erupted in 1980. The explosions during plinian eruptions often create massive calderas - cauldron like volcanic features caused by the massive collapsing of land following emptying of the magma chamber. The other feature of plinian eruptions is that they can often result in a massive reduction of size of a volcano because of the amount of summit exploded off the volcano during the eruption. It is also common for this type of eruption to lead to deadly pyroclastic flows – A pyroclastic flow is a fast-moving current of extremely hot gas (which can reach temperatures of about 1,000 °C (1,830 °F)) and rock (tephra), which travel away from a volcano at speeds generally as great as 700 km/h. The flows normally hug the ground and travel downhill, or spread laterally under gravity.
“The Eruption of Vesuvius as seen from Naples, October 1822″ from V. Day & Son,
Phreatic Eruptions – Phreatic comes from the Greek phrear meaning well or spring – this is a type of eruption that tend to occur either on its own or preceding an alternate type of eruption – it is an eruption driven by explosive expanding steam from cold ground water, ice or surface water coming in contact with intensely hot volcanic rock or magma. The distinguishing feature with this kind of explosions is that they only blast out fragments of pre-existing solid rock from the volcanic conduit – they do not erupt magma. An enormous amount of steam, ash and rock – and often poisonous gas such as H2S are emitted in a large plume from the volcano and the explosion often leaves a large crater called a maar. A maar is a massive low level crater that is characteristically filled with water. Phreatic activity can be quite violent – there were several phreatic eruptions at Mt St Helens in Washington before the major Plinian eruption in 1980. Kilauea in Hawaii (my new favourite shield volcano) also commonly experiences phreatic eruptions. It is thought that the eruption that obliterated Krakatoa in 1883 could well have been a phreatic eruption – it emitted the loudest noise ever recorded in history!!! ( info from U.S. Geological Survey http://www.usgs.gov/science/science.php?term=1209&type=feature)
Pelean Eruptions – In These are otherwise called glowing cloud eruptions and are similar to plinian eruptions in that they usually involve andesite or more silicon rich and viscous rhyolitic magma. The distinguishing features are the glowing avalanche of hot volcanic ash/ significant pyroclastic flow that can move downhill up to 160km per hour and the formation of lava domes. Lava domes are domes shaped mounds protruding from the crater of a volcano caused by very slow moving viscous lava – they are dynamic and can grow, shrink, erode and solidify. Lava domes can be up to several hundred meters high and can grow quite quickly. Lava domes can be particularly dangerous because they shatter during an eruption. Mt St Helens in Washington has had a few pelean eruptions since the large plinian eruption in 1980. Mt St Helens now has several domes which occasionally erupt – one of which slowly grew from 2004-2008 without eruption – it is now deemed to be stable.
Pelean eruptions have been known to occur with shield volcanoes too – notably they have been known to occur in Iceland, a region of diverging plate boundaries.
So… lots of different kinds of eruptions and it seems those at stratovolcanoes are typically much more dangerous than those at dome volcanoes.
The other thing mentioned in my research on stratovolcanic activity are lahars. Lahars are mudflows which are composed of volcanic material/ tephra mixed with water- commonly from either heavy rainfall during an eruption (example – they have occurred when hurricane activity has occurred at the same time as a volcanic eruption) or from melted ice. The mixture forms a cement like mud which is fluid when moving but solid when stationary and can bulldose anything in its path. Lahars are commonly the main cause of death from volcanoes – they can flow quite quickly (up to 100km per hour) and can be massive – engulfing entire towns quite quickly. A notable event occurred in Nevado del Ruiz, a stratovolcano in Columbia which erupted in 1985 to produce a lahar which killed 23,000 people and buried an entire town under 5 meters of mud.
Ok so this is an odd one!!! Some websites say these are the scariest most horrific of volcanoes and have the potential for mass devastation – that is because at some stage a stratovolcano has erupted to such extreme that it has part emptied the massive magma chamber and the volcano has caved in to leave a large caldera 1.6km + wide. It seems that this type of volcano has been promoted as the life changing earth changing kind because if it exploded again with the same force it would have devastating effects similar to the original eruption that created it. Some caldera volcanoes are designated as Supervolcanoes (possibly this may have come from a Discovery Chanel doco…). Caledra Volcanoes can occur on hot spots or converging plates. The commonly used example of a supervolcano caldera is Lake Toba in Indonesia (on converging plates…) and the largest volcanic lake in the world. Some paleontologists believe the volcano erupted around 75,000 years ago and it caused mass devastation - plunging the earth into a 10 year winter and wiping out all but around 5-10,000 humans. in 1883, Krakatoa, an island/volcano in Indonesia not far from Lake Toba erupted with a massive explosion that obliterated 3/4 of the island and caused a tsunami that wiped out islands and killed 37,000 people. Krakatoa is now a submarine (underwater) caldera but has a little volcano (Anak Krakatoa) growing from it at a rate of 6 meters per year). Another commonly referenced caldera volcano is Yellowstone park in the USA – it is a massive caldera on a hot spot which has recently been moving slowly and has a history of erupting every 6000 years or so (and is just about due to do it again!!!). Yellowstone is considered, at least by the discovery channel, to be the next most likely place for a supervolcanic eruption that could wipe us out, change the weather and cause mass destruction to the planet… The lava from Caldera volcanoes is usually rhyolitic and cool at temperatures less than 800C.
I mentioned the amount of greenhouse gases that come from volcanoes previously – Caldera volcanoes have the potential to trigger significant widespread climate change due to the excessive amount of particulate matter, which can change weather patterns, and the excessive amount of greenhouse gas emitted from a mega eruption. Extreme acid rain is another potential ecological hazard of super eruptions.
Now what have I missed out??? Many websites like the award winning How Volcanoes Work website http://www.geology.sdsu.edu/how_volcanoes_work/index.html also mention scoria cones, also called cinder cones, and mud volcanoes
Scoria volcanoes/ Scoria cones/ Cinder cones:
Scoria volcanoes are usually termed cinder cones or scoria cones rather than volcanoes. They are the littlest of volcanoes – they are made up of basalt tephra piled into a cone. They can occur on their own or are commonly found as parasitic cones on the side of other volcanoes. Scoria cones occur as a result of strombolian eruptions which explode loads of basalt tephra into the sky which then settles back to form a tephra cone (as mentioned above). They are distinct in that they have straight sides and large summit craters and are also distinguished by the fact they normally only ever erupt once (most, but not all, fit into this catagory.
Here is a scoria cone which is a parasitic cone on the Hawaiian shield volcano Mauna Kea
So last of all fumaroles….
These are not usually classified as volcanoes as such but are openings in the earth’s crust (a crack, fissure or hole), generally around or on volcanoes, where volcanic gases are erupted out. As a result of the gases, fumaroles tend to emit clouds of steam and gas and are often covered in pretty yellow sulphur crystals from the large amount of sulphur gases emitted from most of them. A fumarole can be considered to be just like a hot spring but where all the water has been converted to steam before it reaches the surface of the earth’s crust.
So hot springs…. a hot spring is a spring produced by the emergence of geothermically heated ground water. They are not necessarily but are often found around volcanic areas and hot spots – they can also be man made. In Australia there are geothermal hot springs in the Mornington Peninsula in Victoria. Geothermal electricity involves a similar process to producing a geothermal hot spring – it involves tunneling deep into the earth’s crust and pumping down water to turn into steam- then using the steam to turn a turbine (like many other forms of electricity generation).
Geysers are a type of hot spring that involve ground water coming in content with magma which causes massive steam and water vents that spurt up into the sky. Geysers are often taken over as electricity generating sites if they are consistent – otherwise they are turned into tourist sites since they are pretty spectacular. Related to hotsprings and geysers are bubbling mud pools – this is similar to a hot spring but involves mud which has commonly been produced by mixing tephra from volcanic activity with water.
Rotorua on the north island of New Zealand is an amazing but incredibly stinky place I have been too… well worth a visit. Rotarua is an area of extensive geothermal activity situated in a 22km caldera (now I know what that is!!!) – created by a major volcanic explosion 240,000 years ago that exploded enough tephra to completely cover 4000 square kilometers. There are many lava domes inside the caldera and it is a place of excessive geothermal activity – containing hot springs, geysers, bubbling sulphury mud pools and fumaroles. Rotarua looks amazing but it does stink of H2S and there are lots of areas of pretty yellow sulphurous rock around the place. A couple of lovely piccies from Rotarua http://www.rotoruanz.com/resources/images/geothermal showing geothermal activity.
So the only questions I still have about volcanoes… why do people live near them? well I have a fair idea it is because of the nutrient and mineral rich soils generated as a result of volcanic activity. They are also pretty places.
A good website I found http://www.geography-site.co.uk/pages/physical/earth/volcanoes/why%20people%20live%20near%20volcanoes.html goes through each of the reasons people live near volcanoes… and there are 500 million people living close to or on top of volcanoes… MINERALS… lots of minerals, come from the mantle during volcanic eruptions and volcanic rocks can be rich sources of all minerals including silver, gold, copper, lead and zinc… all valuable and are mined extensively. GEOTHERMAL ENERGY…. a very cheap energy source to harness if you live near a volcano – in Iceland and New Zealand, geothermal energy is commonly harnessed to power their turbines and generate electricity. FERTILE SOILS… usually a volcanic area has to have been around thousands of years before the rocks weather enough to make fertile soil – when this happens though the soil is extremely fertile. The Darling Downs region in Queensland is an example of red fertile volcanic soil from a long extinct and eroded shield volcano. TOURISM… odd I know to want to rock up to a volcano which may erupt at any time… or indeed is erupting… but they are fascination places and I have been to two of them myself so can vouch for that curiosity. My dad had the same curiosity but nearly came undone by Bagana, a volcano in Bougainville, he and his pilot flew into it to take a look (out of curiosity) and less than 24 hours later it erupted (and has been pretty much erupting consistently since!!!). Why people live near volcanoes is probably a discussion to have at the beginning of a class to get those brains connecting volcanoes to ecology…
So teaching details about volcanoes… I can see perhaps this being organised into a jigsaw activity in the explain phase, although I would do some hands on activities to familiarize students with where volcanoes occur, how many are active and an opportunity to see the “ring of fire” and see how volcanoes tend to occur in chains (Google Earth would be good for this)… and perhaps different types of lava? in the explore phase to provide students with some scaffolding prior to - in the explore phase.
THE Volcanic Explosivity Index… (probably also for the explore phase, elaborating on the concept that different lava types cause different eruptions)
This is a measure of the explosivity of a volcanic eruption – it looks at ejecta volume and plume.
from Newhall, Christopher G.; Self, Steve (1982). “The volcanic explosivity index (VEI): An estimate of explosive magnitude for historical volcanism”. Journal of Geophysical Research 87 (C2): 1231–1238.
||Occurrences in last 10,000 years*
||< 10,000 m³
||< 100 m
||> 10,000 m³
||> 1,000,000 m³
||> 10,000,000 m³
||Cordón Caulle (1921)
||> 0.1 km³
||≥ 10 yrs
||> 1 km³
||> 25 km
||≥ 50 yrs
||Mount St. Helens (1980)
||> 10 km³
||> 25 km
||≥ 100 yrs
||> 100 km³
||> 25 km
||≥ 1000 yrs
||5 (+2 suspected)
||> 1,000 km³
||> 25 km
||≥ 10,000 yrs
||Taupo (26,500 BP)
Australia’s volcanic history!!!! Yes I have mentioned volcanoes in Australia before – no volcano has erupted here for thousands of years and all our volcanoes are considered to be extinct on the main land – there is a possible designated hotspot at Lord Howe island which has volcanic activity as well as a possible minor one just under Victoria. Like much of the earth in the past, Australia had a good deal of volcanic activity. Having never been located on a plate boundary, all of our volcanic activity has been “hot spot” volcanic activity and we have many remnants of shield volcanoes and cinder cones all over the country. As well as the Romsey Australia site previously mentioned, I took a look at the fantastic resources on the Geoscience Australia website http://www.ga.gov.au/image_cache/GA10095.pdf which takes students through the geological history of Australia from the PANGEA days in the cambrian period on… including where major volcanic activity was occurring and what minerals were deposited in extensive amounts and what life was present in the region. There are fantastic pictures of all of these extinct volcanic regions http://www.volcanolive.com/australia.html here… An example of an extinct volcano familiar to all of us Gold Coasters is the Tweed Volcano formed 23 million years ago when this point of eastern Australia passed over a hot spot. The Tweed volcano was an extensive shield volcano and has eroded extensively in the 23 million years leaving some impressive large remnants including Mount Warning, Lamington Plateau, Springbrook Plateaus and Tamborine mountain. These features sit in a Caldera but the caldera wasn’t formed by a supervolcano – just an eroded extinct shield volcano. The volcanic rocks we climb on and surf near at Burleigh heads are formed from the basalt lava flows from the Tweed volcano and the entire volcanic area is extremely fertile, supporting an extremely biodiverse range of plant and animal species and intermittent rain forest environments. Also close to home are the Glasshouse Mountains – these are actually massive rhyolite plugs/ cores from eroded away volcanoes that were active 26 million years ago.
As mentioned – I was thinking volcanic and earthquake activity in Australia would be good to do in the elaborate phase of a plate tectonics unit, so students can use their new knowledge to predict our volcano and earthquake activity.
I think maybe the super exciting volcanic eruptions (sorry… devastating but fascinating is perhaps a more appropriate way to describe them… ) could also be something well placed in the elaborate phase. Once students understand eruption types and terminology they would be better placed in understanding why the events were so devastating.