VOLCANOES CLIL: lesson 2 5D 2016/17

Transcription

1 VOLCANOES CLIL: lesson 2 5D 2016/17

2 Recap and Warmer Try to remember at least 5 words you looked for during the last lesson. What did your classmates assert about Earth science? Do you realise how much in your life depends on raw materials? Try to use the handouts to give an example.

3 CLIL: learning objectives Describe the structure of a generic volcano Describe the types of volcanoes Describe the different types of volcanic eruptions Describe how volcanoes form Describe where volcanoes occur and why Describe the main active volcanoes in the world Describe the main active Italian volcanoes

4 Volcano: what s exactly? Volcano A vent (we d call it a rupture) in Earth's surface through which molten rock and gases escape. The term also refers to deposits of ash and lava that accumulate around this vent ssary-v.shtml 125/parts-of-a-volcano/

5 Students at work! Match the numbers with the correct name and definition (you could also draw the volcano). There re more definitions than numbers on the picture, try to collocate them. Make a list of the verbs describing the actions. You have fiftteen minutes.

6 Scaffolding Social constructivist learning in essence focuses on interactive, mediated and student-led learning. This kind of scenario requires social interaction between learners and teachers and scaffolded (supported) learning by someone or something more expert teacher, other learners or resources

7 There are many types of volcanic structures And they are a consequence of the magma and lava composition.

8 FISSURE VOLCANO 1. Fissure vent: few meters wide, many kilometers long. 2. Fluid basaltic lava or, it s the same, mafic lava (even if not always). 3. Absence of esplosive activity. 4. Basaltic plateaux. (there s no mountain). SHIELD VOLCANO 1. Central vent 2. Fluid basaltic lava 3. Gentle lava eruptions with little explosive activity and spectacular fire fountains 4. Broad, low-angled cone

9 STRATOVULCANO or COMPOSITE CONE 1. Central vent 2. Lava composition is highly variable; alternating mafic to felsic lavas. 3. Eruptions change widely because it is related to magma composition (viscosity and gases) and to the amount of water. They can be from slightly to highly explosive. 4. Gentle lower slopes, but steep upper slopes; concave upward; small summit crater.

10 Students at work! 2 Find out the parameters that define magma types. Make a list of the unknown words. Solve the crosswords (homework). You have fifteen minutes.

11 MAGMA TYPES They are defined by composition, viscosity, temperature and gas content. COMPOSITION: silica content (SiO 2 ), or better the sulfate ion content, define magma types: from mafic magmas (relatively low silica and high Fe and Mg contents) to felsic magmas, (relatively high silica and low Fe and Mg contents). VISCOSITY: the ability of a substance to resist flow. It depends on temperature and silica content: decreases when temperature grows, increases when silica content grows. GAS CONTENT: size and distribution of the bubbles influence the viscosity and highly cause the explosiveness of the eruption.

12 From MAGMA TYPES to ERUPTION STYLES SiO 2 MAGMA TYPE TEMPERATU RE (centigrade) VISCOSITY GAS CONTENT ERUPTION STYLE ~50% mafic ~1100 low low nonexplosive ~60% intermediate ~1000 intermediate intermediate intermediate ~70% felsic ~800 high high explosive

13 ERUPTION STYLES Eruption styles from nonexplosive to explosive Fissure eruptions or Icelandic eruptions Hawaiian eruptions Strombolian eruptions Vulcanian eruptions Plinian eruptions Hydrovulcanic eruptions

14 Fissure eruptions or Icelandic eruptions They are generated from sites along a linear fracture. Regional fracture systems can appear where the Earth's crust is broken and pulled apart by tensional forces (divergent plate margins). Because Iceland is the subaerial extension of the Mid-Atlantic Ridge, it is one of the world's most active sites for basaltic fissure eruptions. As fluid, gas-poor basaltic magma rises up through a fissure, it is extruded at the surface as a wall of incandescent, liquidto-plastic fragments known as a curtain of fire. It generates a basaltic plateaux.

15 Hawaiian eruptions Hawaiian eruptions are the calmest of the eruption types. They are characterized by the effusive emission of highly fluid basalt lavas with low gas contents. The relative volume of ejected pyroclastic material is less than that of all other eruption types. The hallmark of Hawaiian eruptions is steady lava fountaining and the production of thin lava flows that eventually build up into large, broad shield volcanoes. Eruptions are common in central vents near the summit of shield volcanoes, and along fissures radiating outward from the summit area. Lava advances downslope away from their source vents in lava channels and lava tubes.

16 Explosive eruptions Explosive eruptions occur where cooler, more viscous magmas try to reach the surface. Magma surface can harden, making a cap so dissolved gases cannot escape easily, so pressure may build up until gas explosions blast rock and lava fragments into the air. The greater the explosivity the greater the amount of fragmentation. Individual eruptive fragments are called pyroclasts ("fire fragments") while Tephra (Greek, for ash) is a generic term for any airborne pyroclastic accumulation. Whereas tephra is unconsolidated, a pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock type. Lava flows are much more thick and sticky so do not flow downhill as easily. These eruptions build up more steeply-sloping.

17 Vulcanian eruption 1. Magma is an basaltic-intermediate type. 2. The «cap» isn t thick so strombolian activity is characterized by short-lived, explosive outbursts of pasty lava ejected a few tens or hundreds of meters into the air 3. The eruptions are quite frequent, even steady, and not excessively dangeruos. Strombolian eruption 1. Magma is felsic-intermediate type. 2. The cap is more thick ; the high viscosity of these magmas makes it difficult for the vesiculating gases to escape. This leads to the build up of high gas pressure and explosive eruptions. 3. Lava flows are quite absent, while tephra and pyroclasts are present. Plinian or vesuvian eruption 1. Magma is felsic type. 2. Plinian eruptions generate large eruptive columns that are powered upward by the thrust of expanding gases, with exit velocities of several hundred meters per second. Some reach heights of ~45 km. 3. Lahar and pyroclastic flows

18 atch?v=kznwnpntb6k Lahars or volcanic mudflow These lethal mixtures of water and tephra have the consistency of wet concrete, yet they can flow down the slopes of volcanoes or down river valleys at rapid speeds, similar to fast-moving streams of water. Lahars can vary from hot to cold, they are generated by the basal melting of glacial ice or of large quantities of snow and ice, eventually by pouring rain. Pyroclastic flows They are the most deadly of all volcanic phenomena. It is a mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice. It moves like a snow avalanche, except that they are fiercely hot, contain toxic gases, and moving at hurricaneforce speeds, often over 100 km/hour.

19 Students at work 3! Prepare at least three questions for your classmates. Be ready to answer them. Write a short test (200 words) about one of the following eruption: Vesuvio, 79 d.c. Nevado del Ruiz, 1985 Mont st. Helens, 1980 Pinatubo, 1991 Krakatoa, 1883 Pelée, 1902 You have fifteen minutes! A wee joke!

20 Hydrovolcanic eruption are generated by the interaction of magma with either groundwater or surface water. The high explosivity is a hallmark of hydrovolcanic activity: as the water is heated, it flashes to steam and expands explosively, thus fragmenting the magma into exceptionally fine-grained ash. Note the radial cloud emanating from the base of the eruptive column. This phenomenon is a base surge, a characteristic of these eruptions, derived from the gravitational collapse of the "wet" eruptive column, which is denser than those associated with "dry" eruptions. Base surge deposits are wedge-shaped, with their thickest end near the vent.

21 Distribution of volcanoes

22 Distribution of volcanoes Volcanoes are not randomly distributed on the surface of the earth, rather they are found in certain well-defined belts. Ring of fire or Circum - Pacific belt

23 1. The Circum-Pacific belt: This is the most important belt of volcanoes. It extends through the Andes of South America, Central America, Mexico, to the Mountains of Western United States, up to the Aleutian Islands, then to Japan, the Philippines, New Guinea, the Solomon Islands, New Caledonia and New Zealand. This belt has 80 active volcanoes and meets the mid-continental belt in the East Indies. In Alaska there is a Valley of Ten Thousand Smokes. 3. The Mid- Atlantic belt: As the name indicates, this belt includes the volcanoes of the Mid-Atlantic Ridge. The volcanoes associated with the Atlantic Ocean are located either on swells or ridges rising from the sea floor, or on or near the edge of the continent where it slopes abruptly into the deep oceanic basins. Volcanoes in this belt are generally of fissure-eruption type. 2. The Mid-Continental Belt: This belt has various volcanoes of the Alpine mountain chain (exinct), Mediterranean Sea, Volcanoes of the Aegean Sea. Mt. Ararat, Elburz and Hindukush are also included in this belt. It is interesting that there are several volcanic free zones found along the Alps and the Himalayas. The Rift Valleys of Africa have volcanoes such as Kilimanjaro, Elgon. 4. Hot spots: these are volcanoes that are not found on plate boundaries. they normally create a chain of volcanoes e.g. the islands of Hawaii.

24 Italian volcanoes Extinct volcanoes. Volcanoes which last erupted over 10,000 years ago are defined as extinct. These include the Amiata, Vulsini, Cimini, Vico, Sabatini, Pontine Islands, Roccamonfina and Vulture volcanoes Dormant volcanoes.whereas dormant volcanoes have erupted during the last 10,000 years but are currently in a period of dormancy. According to a more precise definition, volcanoes having a current period of dormancy shorter than the longest period of dormancy registered previously are considered dormant. Here we have: Colli Albani, Phlegraen Fields, Ischia, Vesuvius, Salina, Lipari, Vulcano, Ferdinandea Island and Pantelleria. Amongst these, Vesuvius, Vulcano and Phlegraen Fields, have a very low eruptive frequency and their conduits are now obstructed. Not all the dormant volcanoes present the same risk level, both for the hazard of expected phenomena as well as for the differing extent of population under exposure. Furthermore some have a secondary vulcanism phenomena (degassing from the ground, fumaroles, etc.) which may well cause situations of risk.