2VOLCANOESThe word "volcano" comes from the little island of Vulcano in the Mediterranean Sea off Sicily (Italy).Centuries ago, the people living in this area believed that Vulcano was the chimney of the god Vulcan ( he was the blacksmith of the Roman gods -- he made things out of metals).They thought that the hot lava pieces and clouds of dust erupting from Vulcano came from Vulcan's furnace as he made thunderbolts for Jupiter, king of the gods, and weapons for Mars, the god of war.In Polynesia the people attributed eruptive activity to the beautiful but wrathful Pele, Goddess of Volcanoes, whenever she was angry or spiteful.Today we know that volcanic eruptions are not super-natural but can be studied and interpreted by scientists.
3So, What is a volcano?Volcanoes are mountains, but they are very different from other mountains; volcanoes are built by the accumulation of their own eruptive products -- lava, bombs (hardened lava blobs), ashflows, and tephra (airborne ash and dust).A volcano is most commonly a cone shaped hill or mountain built around a vent that connects with pockets of molten rock (magma) below the surface of the Earth.The term volcano also refers to the opening or vent through which the molten rock and associated gases are expelled.
5As you heard last week… Volcanoes generally form along plate margins Either where the plates are approaching each other and one is sliding under the other – destructive marginsOr where they are moving away from each other – constructive marginsThere are a few exceptions – called hot spots – where the magma bursts through a particularly thin bit of oceanic crust, for example in Hawii.
7Distribution of some of the world’s major volcanoes Ring of Fire
8How do they form? This one is forming on a destructive plate As the oceanic crust slips beneath the continental plate, some of the magma gets trapped above.This is force out through any crack it can find in the continental plate.Violà – you have a volcano!
11Shield Volcanoes Broad, slightly domed-shaped Made of layers of frozen lavaMay be 100s of km across and have a large craterProduced by mild eruptions of large volumes of lavaNon-explosive eruptionsMauna Loa on Hawaii is a good example
12Cinder Cone Volcanoes Moderately explosive Example: Paricutin (Mexico) Made of loose grainy cinders and no lavaSteep slope angleRather small size about 1 km across with a small craterFrequently occur in groupsModerately explosiveExample: Paricutin (Mexico)
13Composite VolcanoesMost are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens)Large, classic-shaped volcano (1000’s of ft. high & several miles wide at base) with a small craterComposed of layers of lava flows with layer of cinder and ash in betweenMay have very explosive eruptions
20E: Popocatepetl, Mexico Which type is this?E: Popocatepetl, Mexico
21How did you do? A = composite (Mount Rainier, WA) B = cinder cone (Paricutin, Mexico)C = shield (Mauna Loa, Hawaii)D = cinder cone (Papua New Guinea)E = composite (Popocatepetl, Mexico)
22Types of LavaVolcanic eruptions may be quiet outflows of lava which are so peaceful that one can stand close enough to toss in pebbles or so explosively violent that they blow mountains apart and blast everything within a hundred miles to smithereens (not recommended for close observation).Both the shapes of volcanoes and the violence of volcanic eruptions depend on the same rather mundane thing: the physical properties of erupting lavas.The two most important properties of lava areits viscosity (its stickiness), andthe amount of gases dissolved in the liquid rock.
23ViscosityIs a term that describes the fluidity or "runniness" of the lava.Some lavas are very "runny," not quite like water, but more like warm honey or hot wax.When these lavas erupt, they flow for large distances before cooling enough to turn solid.You can imagine what kind of volcanic mountain you could make with runny lavas by thinking about (or carefully doing) pouring hot wax on a large sheet of paper.
24ViscosityThe wax spreads out into a large, flat layer. Let it cool and harden and then pour another layer. The second "flow" of wax will partly pond on the first and partly run off onto the paper to form another flat layer.If you keep pouring more and more "flows," you will get a large, but almost flat pile of wax.Therefore, many small eruptions of runny lavas form large, almost flat mountains like shield volcanoes.Eruptions of huge amounts of really runny lavas form flood basalt type volcanoes.
25ViscosityOther lavas are very "pasty" like soft clay and hardly flow at all. Imagine piling lumps of soft ice cream on a big, flat dish--you would end up with a tall, narrow pile. Likewise, pasty lavas form tall, steep-sided mountains like composite volcanoes.
26Why is some lava sticky and some not? The viscosity of lava depends on several things.The amount of silica . More than 80% and it is VERY sticky. Get as low as 50% and it is quite runnyThe amount of dissolved gas in it. If there is a lot of gas in a runny lava, you might get a fountain of cinder flying outBut if you have a lot gas in sticky larva then you get enormous and very dangerous explosions
27Two types of lava Pahoehoe and Aa Pahoehoe lavas are thin. They flow smoothly and are often formed by small volumes of hot, fluidbasalt. The higher the volume of lava emitted the faster the current. Pahoehoe flows move forwards in tongues or lobes and are characterized by a glassy, plastic skin. They may embrace obstacles at a rate of about 50m an hour. When the pahoehoe lava flow cools, it often solidifies to a smooth surface.Aa flows are emitted from the vent at high rates ranging to 50km an hour, often with much lava fountaining. They are characteristic of viscous magmas. Aa flows are animated with sporadic bursts of energy. They may push down houses, walls and forests. However, the hallmark of aa lava flows is the very rough surface it produces when it cools and solidifies.
28Other volcanic landforms CalderasSteep-walled depressions at the summitSize generally exceeds 1 km in diameterExample Santorini in Greece
30Volcanoes and climateExplosive eruptions emit huge quantities of gases and fine-grained debris into the atmosphere which filter out and reflect a portion of the incoming solar radiationExamples of volcanoes affecting climateMount Tambora, Indonesia – 1815Krakatau, Indonesia – 1883Mount Pinatubo, Philippines
31VOLCANIC ACTIVITYACTIVE - A volcano that has erupted in the recent past and is expected to erupt again. Any names?DORMANT - A “sleeping” volcano. A volcano that has not erupted in the recent past, but could erupt again. One in Yellowstone has not erupted fro 640,000 years BUT there are still earthquakes and steam emissions so it could!EXTINCT - A volcano that is not expected to erupt again. This is because there is no longer a source of magma. Castle rock in Edinburgh is an example
33HomeworkChoose an active volcano (or at least one that has erupted in the last 20 years or so)Find out its name and where it is – I will send you a world map to move a X to the right spot.Find out which sort of volcano it is – shield, composite or cinder coneFind out 4 other things about it
34Suggestions for your other 4 things A picture of your volcanoGeographic setting:Is it on an island or in a mountain range?Is it near towns or cities or is it isolatedWhen did your volcano last erupt?what do you know about it?Was the eruption hazardous to people? What happened?If the volcano erupts again, what are the possible hazards? Does it pose a threat now?What are 2 cool facts about your volcano?