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Lecture 13

EPSC201 Lecture 13 Notes.doc

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Earth & Planetary Sciences
Course Code
EPSC 201
Anthony Williams- Jones

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EPSC201 - Lecture 13 Notes Volcanism is associated with spreading ridges. Iceland is a good example. Volcanoes are also associated wit hot spots, such as Hawaii. Volcanoes are forming from magma that is subjected from decompression melting. Upwelling of mantle (plume) causes melting, which in turn causes a volcano. The magma has a very low viscosity, meaning it flows easily. These volcanoes are termed shield volcanoes, which have very subtle hills because the lava flows far out from the crater. The lava is made of basalt in these types of volcanoes. At low silica content, the lava is made of isolated tetrahedrons. This flows easier then lavas made of complex silicate mixtures. Some magmas, in a rift environment, have high al- kali content (sodium and potassium). This is com- pared to ridge magma. There is also carbonate lava in the rift environment. Intrusions is magma that did not make it to the surface, while extru- sions are lava that did make it through to the sur- face. Africa contains rift volcanoes, so one would expect to see alkalia and carbonate content. Subduction environment produces magmas of intermediate compositions such as rhyolites and andeosite, which has medium amount of silica. The type of volcanoes in Iceland have a high sili- ca content. Basalt is associated with hot spots, and low viscosity. Explosive activity is only seen in high silica environments, which cause more viscous lavas. The laval is more solid, and not able to release pressure as easily. Stratovolcanoes – carbonate lavas (baking soda – bicarbonates), these magmas have high silica content, which increases its explosive activity. Chimney Shape – windows into the magma chamber. The crust is formed right on the top. Pahoehoe is basaltic lava that has a smooth, billowy, undulating, or ropy surface. These surface features are due to the movement of very fluid lava under a congealing surface crust. Some lavas at high temperatures have a similar vis- cosity compared to water. Venus has a surface temperature of 500 deg. Celsius, but there is flow patterns observed on the surface (oxbows and meanders). The Russians landed a probe and analyzed rocks before the probe got destroyed. All the rocks are very alkali, so there could be an igneous province similar to the carbonitites found on earth, which are very low viscos- ity and could flow to create this flow patterns on Venus. The valleys on venus are caused by natrocarbonatite, a type of lava. As lava cools it forms the hydrated complex. This can completely change the mineralogy and ap- pearance. Stratovolcanoes are layered volcanoes. Each time they erupt, a new layer is formed. The fined grained ash travels further then the bigger chunks, that just roll down, and don’t travel as far. On the side of the volcano, there is hot gaseous eruptions that form plums. IfIf there is an ice cap on the volcano, they become much more dangerous. The pressure builds up and eventually explodes at much higher pressure. The ice can melt and cause water to run down the volcano, forming ravines and dangerous water flows. These carry debris away from the volcano and cause massive damage. Most towns are near rivers, and this increase in water flow (at a speed of 60 km/h) can be extremely dangerous. If there is an active volcano, there must be a warning system in place. There was a volcano in Columbia that had a huge ice cap. In 1985 there was a huge eruption. The ice cap was around 80 meters thick. The preceding year had many little tremors and mini eruptions. There was a team of Italian volcanologists who were measuring activity just a few months before it went off. They placed lots of seismographs around it, and measured its activi- ties. There can be up to hundreds of mini earthquakes per hour. The key point here is that volca- noes give warning signs. The team of Italians spoke to the villages and tried to warn them about the lavas. The villagers didn’t heed the warning. The Italians even tried to contact the prime min- ister, but nobody listened. Then one night, there was a medium size eruption. This sent the lavas, mud flows and debris flows towards the villages. One side of the ice cap got melted more than the other, so one direction got devastated. About 6 hours later, the lahar was about 3 meters high and reached the village. It destroyed the entire village got destroyed, and about 20000 peo- ple died and about 4000 survived. Highly viscous magma is able to push its way up to the surface easier, and will causes seismic activity. Fuji is one of the most monitored volcanoes in the world. Note its big ice cap, which has the potential to cause lavas. Fortunately the Japa- nese know all about this, and have many warn systems in place. Geologists occasionally get caught in volcanic eruptions. Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. There is a lot of
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