GY 111 Lecture Notes D Haywick 2008 09 1 GY 111 Lecture Note Series Volcanoes and volcanic land forms Lecture Goals A Volcanic land forms B Volcanic eruptions rock types case studies C Measurements of activity Reference Press et al 2004 Chapters 5 and 6 Grotzinger et al 2007 chapters 2 4 and 12 A Volcanic landforms Volcanoes form some of the most impressive land forms on the planet but they aren t always classic cone shaped features Volcanoes that are dead or extinct see part C below very quickly lose their cone shape and eventually will completely erode away Before that happens though they can form near vertical volcanic necks These features are the remnants of the volcanic pipe see sketch 1 below and had originally been filled by magma When all volcanic activity stopped the volcano went extinct the magma cooled and crystallized usually to a porphyry This material is usually much harder and more resistant to erosion than the lava flows ash that composed the flanks of the volcano As a result the pipes remain long after the volcano has vanished Another prominent feature that hangs around long after a volcano is gone are dikes Near the center of a volcano surrounding the pipe dikes often occur in swarms that radiate from the pipe almost like the spokes in a bicycle wheel Not surprisingly they are called radial dikes Think that all volcanic land forms are positive topographic features i e they are hills Think again When volcanoes collapse or blow up they may leave behind a deep depression called a caldera see image at lower right If the collapse occurred as the result of a powerful explosion e g Krakatoa the caldera is said to be an explosive caldera If the collapse occurred more passively as a result of an emptied magma chamber the ground simply caves into a large hole below the Earth s surface the resulting hole is said to be a collapse caldera Either way calderas can be big miles across If the depression is later filled by water it s called a crater lake a caldera lake would be a better term see image on next page The last landform that we will discuss today is a positive topographic feature that is composed of multiple volcanic cones When oceanic lithosphere subducts below oceanic lithosphere a convergent plate boundary such as this GY 111 Lecture Notes D Haywick 2008 09 2 marks the boundary between the Pacific Plate and the North American Plate west of Alaska a series of volcanoes pops up along the edge of the subduction zone on the overriding plate see sketch 2 below Eventually the volcanoes reach sea level and form a linear or curved string of islands They are called island arcs and along with trenches mark areas of subduction on the sea floor Island arcs can be thin and sporadic e g the Aleutian Islands of Alaska or quite massive e g Japan however either way they are characterized by volcanoes that are usually explosive see case studies below B Volcanic eruptions 1 rock types Last time we discussed some of the basic properties of volcanoes we discussed the different types of volcanoes cinder cone composite stratovolcano shield as well as fissure eruptions and very briefly discussed how lava magma viscosity affects volcano shapes and sizes We didn t really have a chance to fully discuss the consequences of volcanic eruptions and how geologists assess the risk of living by volcanoes That was an oversight that we will fix today 1 Shield volcanoes fissure eruptions These eruptions generally produced large quantities of very fluid lava some of which can travel over 100 km before cooling to rock There are dangers with this type of eruption They have been known to cut off roads bury villages and set large forests on fire But provided that you are not an idiot and there is not some catastrophic event see below it is pretty hard to get killed by a lava flow from a shield volcano or fissure eruption As you will learn in an upcoming lecture probably in the lab this week the type of igneous rock that is generally erupted from shield volcanoes is basalt Basalt lava flows come in two distinct types based upon the surface texture that results upon cooling Lava that forms a smooth upper surface and a ropy texture is called pahoehoe Lava that forms a jagged upper surface is called aa Both terms are Hawaiian Pahoehoe lava Aa lava GY 111 Lecture Notes D Haywick 2008 09 3 2 Composite volcanic eruptions Note this lecture significant overlaps lecture 11 pyroclastic eruptions These eruptions can be killers The reason is that they can be explosive in nature All that viscous magma can really plug up the old plumbing and when the pressure finally exceeds the strength of the rock holding it back BOOM Explosive eruptions like this produce voluminous amounts of ash pulverized rock gas and rocks The most powerful of these eruptions are called Plinian and were named after Pliny the Younger who was the first to write an account of just such an eruption see below Composite volcanoes produce a mixed bag of igneous rocks but volcanic ash pumice tuff andesite and rhyolite pictured right are the most common extrusive varieties Volcanic eruptions 2 case studies time limitations may not permit discussion of all of these a Shield volcano 1 Kilauea Hawaii This is a true story and it involves your humble instructor Remember how I said earlier that only idiots get killed by lava eruptions from shield volcanoes Well you are being taught by an idiot who almost got himself killed during just such as eruption I was in Hawaii with Glenn Sebastian then the Chair of my department in 1997 to attend a conference on geoscience education believe it or not I try to retrain myself in education as often as possible As I mentioned in class Kilauea has been erupting for over a decade and it was producing a fair amount of pahoehoe lava when we were in Hawaii A group of us led my your humble instructor decided to walk out to see the lava spill into the ocean As you can imagine the mixture of hot lava and cool ocean water gives rise to quite a spectacular reaction The only problem was that in order to get to the best spot we had to actually walk across the active lava flow This isn t normally a problem if you are walking a long way from the volcanic vent The surface of the lava cools to rock and it can usually support several people We were several miles away from the vent so we knew that we d be okay And no our Nikes did not melt on the way There were several pretty cool things to see along the way Lava flows had crossed the road in