GLY 155 Introduction to Physical Geology, W. Altermann

Transcription

1 Eyjafjallokull Volcano Shield Volcano on Iceland Phreatomagmatic eruption 1

2 Eyjafjallokull Volcano Shield Volcano on Iceland Phreatomagmatic eruption Eyjafjallokull Volcano Shield Volcano on Iceland Phreatomagmatic eruption 2

3 Historical eruptions n = 5350 Volcanism & Volcanic edifices Subduction zones Intra plates Rift zones Eruptions: Explosive: strato (composite) volcanoes Effusive: shield volcanoes and fissures Explosivity index [E] E = E = percentual Prozentualer content Anteil der of Pyroclastics Effusive: effusiv: E E % Phreato-magmatic: Explosions in contact to Explosive: explosiv: E E % external water (ground-, lake-, ocean-, melt water) Magmatic: Explosion and fragmentation of lava through juvenile volatiles Three types of volcanic eruptions are differentiated accordingly to their historical places of eruption: Hawaiian: as a rule low viscosity basaltic magmas, up to 500 m high lava fountains and/or extensive Lava flows. Strombolian: SiO 2 -richer and thus higher viscosity, basaltic magmas, rich in volatiles with large gas accumulations in the roof of the magma chamber. Such accumulations can explode violently at sudden pressure release. Magma fragments are ejected in ballistic trajectories out of the volcanic vent and are piled up to a scoria cone around the volcanoes. Eruption column can rich 10 km height, and lapilli und ash are accumulated from these columns and clouds to pyroclastic deposits. Plinian (Vesuvian): SiO 2 -rich, differentiated and volatile rich magmas with up to 65 km high eruptive columns and laterally extensive tephra-sheaths. 3

4 The form of Volcanic edifices is ruled by the Explosivity Index E (or VEI) Shield volcanoes, form with flat conical shape, made up of low viscosity basaltic lavas flows. Usually not very high, but can rich up to c m a.m.s.l.) and covering very large areas, like Hawaii volcanoes. Strato volcanoes, complex, hundreds to thousands of metres high (>6000m a.m.s.l.), terrestrial volcanic form, with relatively steep flanks (up to c. 33º). Built of intercalated: Lava, primary volcaniclastic sediments (pyroclastic flows) secondary (reworked) volcaniclastic sediments (lahars) Strato volcanoes occur in areas of pronounced, intermediate to SiO 2 -rich magmatism. The activity can stretch over several millions of years and produce a complicated edifice of subvolcanic intrusions, dikes and sills below the volcanic cone. Subdivision of volcanoes according to the duration of volcanic activity: Monogenetic volcanoes, activity of few days, weeks or few years simple volcanic edifice (e.g. scoria cone). Scoria cone, few tens of metres to few hundreds of metres high, terrestrial volcanoes formed through strombolian eruptions. In the central part of the scoria cone (vent facies) partly welded large scoria agglomerates. In the distal vent facies deposition of finer, not welded pyroclastics (tuffs). Scoria is a volcanic rock containing many holes or vesicles. It is most generally dark in color (generally dark brown, black or purplish red), and basaltic or andesitic in composition. Scoria is relatively low in mass as a result of its macroscopic vesicles, but in contrast to pumice, scoria has a specific density greater than 1, and sinks in water. Complex volcano, long-lived (up to several million years), complexly architectured volcanic edifices, occurring over differentiated magma chambers (e.g. strato volcano) 4

5 Parasitary craters Fissure zone Shield volcanoes Main Crater Caldera Lava layers Hawaii Magma chamber Eruptive styles 5

6 Strato volcanoes Main Crater Radial fissure Ring fissure Secondary crater Tephra Lava Non-volcanic basement Magma chamber 6

7 Shield volcanoes Active volcanoes above hot spot Strato volcanoes Oceanocean plates collision Ocean-continent plates collision Shield volcano, Island Strato volcano Mt. Shasta 7

8 Basaltic (phreatic) eruptions Haymey, Island Basaltic flow: Haymey, Island 8

9 Sequence of events: formation of a fissure eruption Fissure-eruption (effusive) 9

10 Fissure eruption: Laki fissure, Iceland Plateau-Basalts (Flood basalts) Originate above Hot Spots and at at divergent plate boundaries, at at long lasting basaltic magmatism 10

11 Flood basalts, Isle of Skye Press & Siever, 1991 Columbia-River-Plateau 11

12 Flood basalts: Columbia Plateau, Washington Flood basalts (plateau basalts) Deccan-Trapps (India) Kaoko-Veld (Namibia) Paranà-Basin(South-America) Lesotho Mountains (Lesotho) 12

13 Submarine Volcanoes Most submarine eruptions are fissure eruptions at great depth Central feeder vent Lava flow depth basaltic lava flows as forming the Hawaii islands Press & Siever, 1995 Pillow lava Older intrusions Slumped flank Slope talus Oceanic sediments Basaltic ocean crust Most sea mounds occur at spreading zones Pillow-Lava, round, pillow-formed lava, typical of subaquatic low effusion rate eruptions. 13

14 Pillow basalts, Pacific Archean pillow basalts, Barberton Greenstone Belt, RSA Magmatic Eruptions: Hawaiian Eruptions: as a rule low viscosity basaltic magmas, up to 500m high lava fountains and/or extensive lava flows and lava lakes. Magma not fragmented and deposited within few km area of the vent continuous eruptions Explosive degassing of the magma chamber is however possible after which formation of lava lakes and large lava flows occur Typical form: scoria cone (d = 250 to 2500m; height = d/5) and often fissure eruptions with large lava flows 14

15 Magmatic Eruptions: Strombolian Eruptions - slightly SiO 2 -richer, more viscose basaltic magmas, with gas eruptions within the magma feeder dike. Magma becomes fragmented and large fragments ejected in ballistic trajectories, and form scoria cones around the vent. Eruption columns up to 10 km high with large amounts of lapilli and ash, producing thick and but localised tephra deposits. Discrete explosions repeating within seconds, hours or few days. Stromboli - type locality Typical form - scoria cones Lava fragments are typically smaller than in Hawaiian scoria cones (up to cm diameter max.) Strombolian activity is often just a short episode in a life time of a volcano. Scoria is a volcanic rock containing many holes or vesicles. It is most generally dark in color (generally dark brown, black or purplish red), and basaltic or andesitic in composition. Scoria is relatively low in mass because of its macroscopic vesicles, but in contrast to pumice, it has a specific density greater than 1, and sinks in water. Scoria cones aligned along a volcanic fissure, Island (stratovolcano cones) 15

16 Pyroclastics: explosive eruption - glowing hot rock fragments at different sizes. The eruption column carries hot gases and ashes into the stratosphere (above 45 km height). Tephra: accumulation of loose pyroclastics Volcanic Ash (<2mm) Lapilli (<64mm) Volcanic Bombs (>64mm) Tuff: hardened, solidified volcanic ash and lapilli content: solidified lava fragments and xenoliths, country rocks Pumice or scoria: glassy lava fragments with gas-filled pores and bubbles. SiO 2 -rich melts concentrate at the roof of the magma chamber, foam and solidify to pumice or scoria. Pumice has a specific density <1 (often andesitic or felsic) and floats on water. Magmatic Eruptions: Plinian Eruptions - highly explosive, SiO 2 -rich differentiated and volatiles rich magmas. Eruption columns reaching 65 km height and producing laterally widespread tephra - sheaths. Strongest, most energetic eruptions, producing hundreds of, km 3 of ash. Often rhyolitic magmas, strongly fragmented and distributed over vast areas. Eruptions mostly after long periods of quiescence (hundreds of years). Eruption duration of only few days to weeks. Gas-column acceleration up to 4.5 km height at 2100km/h Convective turbulent clouds, building a mushroom shaped column and pyroclastic flows. Ash, magma fragments (scoria) and pumice. examples: Krakatau, Mt. St. Helens, Mt. Pinatubo (subduction zones) 16

17 Blocks and Bombs Pyroclastic breccia Lapilli rock Ash Subdivision of pyroclastic rocks according to the vol. % of ash, lapilli and bombs Vitric tuff Lithic tuff Crystal tuff Fragments Crystals Subdivision of tuffs according to their glass content: Crystal tuffs and lithic tuffs can have welded particles and thus form welded tuffs. 17

18 Pyroclastic flows: horizontally directed flows, from the eruption cloud, that can be transported for up to 100 km distance at velocities of up to 700 km/h. Ignimbrites: unconsolidated or welded, lithified, pumice-rich deposits of pyroclastic flows (pumice flows, ash flows) Block-and-ash flows: special type of pyroclastic flow (ash and lava blocks), formed due to collapse or eruption of a lava dome. Surges: shock pressure waves preceding pyroclastic flows (ground surges) or due to magma-water interaction (base surges) Pyroclastic flows are devastating, catastrophic events caused by Laterally directed eruptions Collapse of eruption columns Collapse of volcanic domes or mega domes Carcote Tephra, Chile 18

19 Tuffs in Ankara Eruption cloud Trass Secondary ash cloud/ phoenix cloud 19

20 Wind direction Gas clouds carrying fine glass shards Ascending turbulent ash columns Collapsing eruption column Explosive clouds of gas & vapour & fine ash Explosive clouds of vapour and fine ash, secondary clouds pyroclastic flow Sea Wind Umbrella equal density Convective rise Accellerating gas column Convective air uptake Fragmentation Zone Vesuvius-Eruption 79 AC Plinius the elder, died on the second day of the eruption Gas explosions in the magma Eruption column of Plininan (and Strombolian) eruptions 20

21 Vesuvius - Crater Pompei Vesuvius Eruption 79 AC. 21

22 The largest volcanic eruptions of the last 200 years Height of the eruption column in km Pichler, H.,

23 Mt. Pinatubo, pyroclastic cloud, andesitic to rhyolithic eruption Village destroyed by a glowing pyroclastic flow, Java, Galunggung eruption,

24 Village destroyed by a glowing pyroclastic flow, Java, Galunggung eruption, 1983 Village destroyed by a glowing pyroclastic flow, Java, Galunggung eruption,

25 Lahar = sedimentary mass flow, of cold or hot volcanic material mixed with water. Frequent occurrence between volcanic eruptions Lahars, Indonesia 25

26 Thought questions for this chapter Why are eruptions of stratovolcanoes generally more explosive than those of shield volcanoes? While on a field trip, you come across a volcanic formation that resembles a field of sandbags. The individual ellipsoid forms have a smooth, glassy surface texture. What type of lava is this, and what information does this give you about its history? 26