Introduction to volcano characteristics and activity Lesson 1

What determines volcanic activity? Volcanic activity is an eruption of magmatic material onto the Earths surface. The types of volcanic activity is often determined by it’s geographical and tectonic plate setting. Most volcanic eruptions occur on a divergent or convergent plate boundary (many around the Pacific Ring of Fire).. Pacific Ring of Fire African Rift valley Chilean Volcanoes Siebert L, Simkin T (2002-). Volcanoes of the World: an Illustrated Catalog of Holocene Volcanoes and their Eruptions. Smithsonian Institution, Global Volcanism Program Digital Information Series, GVP-3, (http://www.volcano.si.edu/world/).

Volcanoes at divergent plate boundaries 1) Convecting magma from the Earth’s asthenosphere undergoes depressurisation as it nears the crust, causing it to become more molten. 2) This rising magmatic material applies pressure causing deformation of the Earth’s crust, there by allowing magma to rise to the surface through cracks. 3) Due to the relatively thin crust found at oceanic divergent plate boundaries magma cools only slightly so that when it reaches the surface it erupts between 1200-1000 degrees Celsius, resulting in a low viscosity. 4) Due to lack of gas bubble formation in the magma the eruptions are often low in explosivity, leading to fire fountain eruptions or low viscosity basaltic lava flows from the rift valley source. 5) Over time eruptions may build up to form ocean islands. Source: www.indiana.edu

Volcanoes at convergent plate boundaries 1) Volcanism occurs at two types of convergent plate boundary: i) oceanic and oceanic ii) oceanic and continental 2) These boundaries are often associated with more explosive volcanism and a wider range of volcanic products. 3) The subducting oceanic plate undergoes an increase in heat and pressure thereby causing its upper layer to melt. 4) This created magmatic material starts meets the overlying crust (either oceanic or continental) and applies pressure. It forces its way through cracks in the crust. 5) On route to the surface it start to cool, becoming more viscous and also melts other surrounding rock, adding water vapour. These create gas which build in the magma. 6) Magma may become stored on route to the surface in magma chambers as it reaches similar density to the surrounding rock. 7) If magma takes a long time to reach the surface it may become pressured by trapped gas therefore leading to explosive eruptions.

Source; http://www.earthlyissues.com/volcano.htm Types of volcanic cone Source; http://www.earthlyissues.com/volcano.htm The type of cone is often a reflection of the material erupted. Fissure and shield which have low slope angularity are associated with divergent plate boundaries where less viscous lava is more common. There are often volcanic products associated with explosions such as ash. Ash-cinder, and composite (stratovolcano) are often composed of tephra which has been erupted in explosive events therefore are more common at convergent plate boundaries. These may be associated with ash, lapilli, bombs, pyroclastic flows and viscous lava. Where ash/lava mixes with water/snow lahars (mudflows can form).

Images of volcanic cones This is a volcano rift (fissure) at Laki in Iceland which famously erupted in 1783. This eruption was associated with a divergent plate boundary. It covered the surrounding area in tephra and large viscous lava flows. None of the volcanic slopes are steep in gradient and few of the volcanic cones reach great heights (altitude). Source: Wiki commons

Images of volcanic cones Mt St Helens before and after the 1980 eruption Source: Wiki commons (both images) This volcano found near a convergent plate boundary produced extensive ashfall and pyroclastic blasts, destroying vast areas of forest around the volcano. The blast was so strong the volcanic eruption removed the top of the mountain in the blast. Tephra mixing with ash caused extensive lahar damage.

Volcanic products Fluid lava flows (pahoehoe) often associated with low viscosity eruptions Source: Wiki commons AS pahoehoe lava flow cools it often becomes slower moving and more blocky Source: Wiki commons

Volcanic products Volcanic material erupted in explosive activity is often referred to as tephra or pyroclastic material. This is classified by size Volcanic ash is any particle below 2mm, lapilli between 2-64mm and volcanic bombs above 64mm.

Volcanic products This image shows a pyroclastic flow from the Mt Mayon volcano in the Phillipines. These are superheated (and fast travelling) clouds of pyroclastic material, formed by collapse of the volcanic column or collapse of accumulated viscous lava. They are deadly and few have survived a direct hit by one. Volcanic mudflows, also more commonly known as lahars, their Indonesian name where they are common. Lined to steep sided cones, they form where ash mixes with rainfall or melts snow to create fast-moving mudflows. This image is a lahar from the Indonesian volcano Galunggung on the island of Java.

Volcanic eruption styles The style of volcanic eruption is linked to the level of explosivity. This can range from the least explosive Icelandic to the most explosive Plinian. Source: http://media.web.britannica.com Eruption characteristic Description Icelandic Low viscosity and highly effusive. May have water therefore phreatic. Hawaiian Effusive, minor explosivity, with fire fountain and low viscosity lava flow. Strombolian Eruption with gas bubbles so eject tephra (ash & lapilli). Vulcanian Higher gas build up than Strombolian, highly viscous lava. A series of short-lived explosions, with tephra including bombs. Pelean Explosive eruptions with range of tephra with nuee ardent (glowing cloud) pyroclastic flows Plinian Highly explosive, large eruption column (up to 45km), range of tephra and pyroclastic fallout, including flows.

Classifying volcanic explosivity Volcanic explosivity is measured using the VEI scale (Volcano Explosivity Index) which is a measurement of the volume of erupted material. Each volcano does not have a characteristic mark on the scale. Instead each individual eruption is given its own point on the scale. Hawaiian and Icelandic eruptions are commonly found near the bottom of the VEI scale, while supervolcanic eruptions are up to VEI 8, e.g. the Yellowstone eruptions.