2 VOLCANOES The 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.
3 So, 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.
5 As 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 margins Or where they are moving away from each other – constructive margins There are a few exceptions – called hot spots – where the magma bursts through a particularly thin bit of oceanic crust, for example in Hawii.
7 Distribution of some of the worlds major volcanoes Distribution of some of the worlds major volcanoes Ring of Fire
8 How 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!
11 Shield Volcanoes Broad, slightly domed-shaped Made of layers of frozen lava May be 100s of km across and have a large crater Produced by mild eruptions of large volumes of lava Non-explosive eruptions Mauna Loa on Hawaii is a good example
12 Cinder Cone Volcanoes Made of loose grainy cinders and no lava Steep slope angle Rather small size about 1 km across with a small crater Frequently occur in groups Moderately explosive Example: Paricutin (Mexico)
13 Composite Volcanoes Most are located adjacent to the Pacific Ocean (e.g., Fujiyama, Mt. St. Helens) Large, classic-shaped volcano (1000s of ft. high & several miles wide at base) with a small crater Composed of layers of lava flows with layer of cinder and ash in between May have very explosive eruptions
21 How 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)
22 Types of Lava Volcanic 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 are – its viscosity (its stickiness), and –the amount of gases dissolved in the liquid rock.
23 Viscosity Is 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.
24 Viscosity The 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.
25 Viscosity Other 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.
26 Why 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 runny The 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 out But if you have a lot gas in sticky larva then you get enormous and very dangerous explosions
30 Volcanoes and climate Explosive eruptions emit huge quantities of gases and fine-grained debris into the atmosphere which filter out and reflect a portion of the incoming solar radiation Examples of volcanoes affecting climate Mount Tambora, Indonesia – 1815 Krakatau, Indonesia – 1883 Mount Pinatubo, Philippines - 1991
31 VOLCANIC ACTIVITY ACTIVE - 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
33 Homework Choose 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 cone Find out 4 other things about it
34 Suggestions for your other 4 things A picture of your volcano Geographic setting: –Is it on an island or in a mountain range? –Is it near towns or cities or is it isolated When 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?