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(6) Plate Tectonics Geophysics 109 Introduction to Geophysics and Planetary Physics.

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Presentation on theme: "(6) Plate Tectonics Geophysics 109 Introduction to Geophysics and Planetary Physics."— Presentation transcript:

1 (6) Plate Tectonics Geophysics 109 Introduction to Geophysics and Planetary Physics

2 Geophysics 110 The most important lithospheric plates and their boundaries (Press & Siever). Pacific Plate Eurasian Plate African Plate North American Plate South American Plate Indo- Australian Plate Antarctic Plate Eurasian Plate Plate Tectonics – Plate Boundaries

3 Plate Tectonics – Moho & Co Hypsographische Kurve der Erdoberfläche Kumulative Verteilung der Flächenanteile bestimmter Höhenstufen (bezogen auf den Meeresspiegel). The Lithosphere is rigid and brittle, it comprises the Earth’s Crust and the uppermost layer of the Mantle, floating on the plastic, partly molten part of the mantle, which is called Asthenosphere. Schematic representation of the convection in the Earth’s mantle. Source: P & S, 1994 (all). At Mid-Ocean Ridges new oceanic crust is continuously built in the form of Sheeted Dikes und Pillow Lava. The upper part of the magma chamber solidifies as Gabbro. The Mohorovičič-Discontinuity (short form: Moho) is the boundary between crust and mantle. Asthenosphere (plastic) Lithosphere (rigid) Upper Mantle Continental Crust Oceanic Crust Mantle Core Mantle Ocean Sediments Mid-Ocean Ridge Magma Chamber Gabbro Peridotite Oceanic Crust Moho Geophysics 111

4 Mid-Ocean Ridges The mid-ocean ridges form a world-encompassing mountain range with 60 000 km length. Source: NOAA Geophysics 112

5 In Iceland (Thingvellir) the Mid-Atlantic Ridge can be observed above sea level. The Eurasian Plate is left of the Rift, the North-American Plate is to the right (UF). Mid-Ocean Ridges Geophysics 113

6 Black Smokers & Co „Black Smokers“ (Hydrothermal Vents) – Sulfur and heavy metals precipitate from sea water, which has been heated up to over 400 °C. New oceanic crust is built in the form of pillow lava. 3 m long Tube Worms are part of a symbiotic community, which is based on Chemosynthesis by sulfur bacteria. Geophysics 114

7 Black Smokers & Co Hydrothermal Vents – like the Sully Vent in the NE Pacific (Univ. of Washington), can host enormous masses of Anomuran Crabs (Kiwa sp., A.D. Rogers), deep-sea shrimps (Rimicaris Hybisae, C. German) – or the “Yeti Lobster” (Kiwa Hirsuta, A. Fifis). Geophysics 115

8 Plate Tectonics – Schematic At mid-ocean ridges new (basaltic) oceanic crust is formed, which then descents in Subduction Zones to become part of the mantle again. D & D, 1996 Geophysics 116

9 Paleomagnetism Thermoremanence: When magma cools below the Curie Point, the ambient magnetic field (and its direction) is “frozen in”. Schematic representation of the field lines of the geomagnetic field (Dipole). The axis is tilted 11° with respect to the rotation axis: I = Inclination  m = magnetic latitude Mid-Ocean Ridge Geographic Pole Geomagnetic Pole normal revers Brunhes (normal) Gauss (normal) Matuyama (reverse) Gilbert (reverse) After P & S, 1994 Geophysics 117

10 Seafloor-Age The Seafloor-Spreading can be illustrated with Isochrones, connecting points with the same age on the ocean floor. The colored bands correspond to geologic times, during which the ocean floor formed. Oceanic crust is considerably younger than continental crust. There is virtually no sea- floor older than 200 Million Years. Source: P & S, 1994 Geophysics 118

11 Subduction 1 Ocean–Continent Collision Ocean–Ocean Collision Divergent and convergent plate boundaries. After P & S, 1994. Oceanic Crust Asthenosphere Continental Crust Magma Formation Deep-sea trench Lithosphere Mid-ocean Ridge Divergent Plate Boundary Convergent Plate Boundary Mountain Building Earthquake Foci Deep-sea trenchIsland Arc Deep-sea trench Volcanic- Magmatic Zone Geophysics 119

12 Island Arcs – Java The Indonesian islands Sumatra, Java, Bali, Lombok … are a prime example for an Island Arc. Volcanoes line up – here we can see Semeru (back) and Bromo (left). Picture: D. Walton. Geophysics 120

13 Island Arcs – Aleutians The Aleutians are another “textbook example” for island arcs. Here the Cleveland Volcano erupts, as seen from the International Space Station – ISS (J. N. Williams). Geophysics 121

14 Island Arcs The Alaska-Peninsula (left: NASA) can be considered as a part of the Aleutians which is connected to the continent. Japan is also a prime example for an island arc (right: ESA). Geophysics 122

15 Mountain Building The (snow covered) Andes (left: NASA) are a prime example for mounting building by Ocean – Continent Collision. The Himalaya is the result of (still ongoing) Continent – Continent Collision. Geophysics 123

16 Subduction 2 Continent – Continent Collision: This process usually starts with the subduction of oceanic crust, which was present between the converging continents. Sediments on the shrinking oceanic plate become part of the emerging mountain range, parts of the oceanic crusts, which are incorporated into the continental crust are called Ophiolites. The result is termed Tectonic Melange. When the continents actually start colliding only the mantle material can be further subducted, the crust roughly doubles in thickness. From P & S, 1994. Geophysics 124

17 Terranes Microcontinents, island arcs and island chains that are present on the subducted oceanic, will be incorporated into the continent as Terranes. The geology of the Pacific Cordillera in North America is particularly complicated since about half of the Northern “Paleo-Pacific” has been subducted underneath. Pictures: P & S, 1994. Geophysics 125

18 Transform Faults Transform Faults are characterized by lateral movement of crustal blocks. The most famous example is the San Andreas Fault in California. Left: the region called „Elkhorn Scarp“ (Elk  Elch). The lateral movement is nicely illustrated (above) by the offset in the course of Wallace Creek (currently ~100 m). Aerial photographs: David Lynch Geophysics 126

19 The Ocean Floor Transform Faults are very common (but usually nit visible) on the ocean floor near the mid-ocean ridges. Map by Heezen & Tharp, 1997. Axis of the East Pacific Ridge Transform Fault Geophysics 127

20 Ocean Forming Formation of a Rift Valley and building of a new ocean with passive continental margins (like the “young“ Atlantic). After P & S, 1994. Plate Boundary Mid-Ocean Ridge Passive Margin Oceanic Crust Continental Crust Continental Shelf Continental Slope Continental Rise Rift Valley Geophysics 128

21 In the Afar Triangle we find fresh cracks in the Earth crust (left: T. Wright, right: E. Baker). The region is below sea level (Danakil-Depression) but does not have a connection to the Read Sea (yet). Geophysics 129 Ocean Forming

22 Plate Tectonics – Earth History Drifting continents, formation and breakup of Pangaea. Geophysics 130

23 The emergence of the Réunion-Hotspot was responsible for the fast movement of India during the Tertiary (Credit: R.D. Müller, Nature 2011). Geophysics 131 Plate Tectonics – Earth History

24 It could also have been like this … (Creadit: C. Heiner & S. Brune). Geophysics 132 Plate Tectonics – Alternatives

25 Hot Spots and Mantle Plumes Guyot Active Volcano Movement of the Lithosphere-Plate Hot Spots and volcanic Island Chains Hot Spot – Ascending Magma Geophysics 133

26 Island Chains The Hawaiian Islands are the textbook example for an Island Chain, formed by a Hot Spot. Active Volcanoes can currently only be found on „Big Island“ and Maui – and on Loihi, which is still well below sea level. Kauai is already heavily eroded, hosting the „Grand Canyon of the Pacific”. Beyond the Island of Midway only Seamounts are left. Source: T&T, 1999. Geophysics 134

27 Hot Spots in the Pacific Ocean Hawaii Hot Spot Pitcairn Easter Island Cook Islands Galapagos Hot Spots in the Pacific and volcanic island chains, formed by the movement of the pacific plate over the (more or less stationary) hot spots (“Textbook example” = Hawaii). Hawaii Islands Emperor Seamounts Tuamoto Archipelago Source: SdW Geophysics 135

28 Geophysics 136 If a Hot Spot Island is formed in the right climate region, it will soon be surrounded by a fringing coral reef, which will be transformed into a barrier-reef when the volcano starts submerging. The final state is an Atoll with a central Lagoon. Source: T & T, 1999. From Volcano to Atoll

29 Geophysics 137 An once again with the German terms. The explanation for the formation of atolls was found by Charles Darwin. Source: P & S, 1994

30 Hot Spots und their Traces Volcanic Islands – Galapagos (top left) Subsiding Volcanoes with fringing and barrier reefs and Atolls: Tahaa+Raiatea, Bora Bora, and Tupai, French Polynesia (left) Atolls with completely submerged volcanoes – Maldives (above). SOurce: NASA Geophysics 138

31 Hawaii – a textbook „Hot Spot“, currently under Hawaii – „Big Island“, but also creating a new island – Loihi (currently –969 m, as of 2014). Walvis Hot Spot – actively involved in forming the South Atlantic, currently under Tristan da Cunha, Traces: Rio Grande Rise in the West, Walvis Ridge in the East. Source: SdW Geophysics 139 Hot Spots und their Traces

32 Flood Basalts Enormous Flood Basalts (Large Igneous Provinces) are deposited when the „Mushroom Head“ of a Mantle Plume penetrates the crust (fortunately not in historic times). Source: SdW Geophysics 140

33 Deccan Flood Basalts Deccan Traps, India. Geophysics 141

34 Fissure Eruptions at Hot Spots Fissure eruption, Piton de la Fournaise (La Réunion) Fissure eruptions in Hawaii: Puu Oo (top right) Kilauea (left and right) as (very modest) analogs for the formation of flood basalts in the past. Geophysics 142

35 Hot Spots and Shield Volcanoes Geophysics 143 Maun Loa in Hawaii (above, in the background; right: satellite image, NASA) is the largest Shield Volcano on Earth, nourished by a Hot Spot. Together with the slightly higher (but less voluminous) Mauna Kea (above, foreground), it rises more than 9 000 m from the sea floor.

36 Even bigger is Olympus Mons on Mars – it rises more than 22 km above its surrounding (NASA). Quelle: NASA Geophysics 144 Mighty Shield Volcanoes

37 Hot Spot - Volcanism A Hot Spot can also be found under Iceland – right on the mid- atlantic ridge. Because of the basaltic lava involved, eruptions are usually quiet – like the fissure eruption from Eyjafjalla in March 2010 (Reuters). In April 2010, however, the eruption happened underneath the glacier Eyjafjallajökull – and the combination lava + water is very explosive – the ascending magma was fragmented into tiny particles – volcanic ash. (Picture: Marco Fulle). Charge separation in the eruption column can lead to spectacular lightnings. Geophysics 145

38 Hot Spot – Volcanism Eyjafjallajökull was clearly responsible for the ash cloud, creating havoc in European air traffic (left, NASA) – but not for the spectacular aurora above (Reuters) – and also not for the modest weather in Austria in May 2010. Only considerably larger eruptions can have an impact on. Geophysics 146


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