(6) Plate Tectonics

1. Introduction to Geophysics and Planetary Physics Geophysics 97 (6) Plate Tectonics

2. Geophysics 98 Plattentektonik – Plattengrenzen Eurasian Plate NorthAmerican Plate Eurasian Plate African Plate Pacific Plate SouthAmerican Plate Indo-Australian Plate Antarctic Plate The most important lithosphericplates and their boundaries (Press & Siever).

3. Geophysics 99 Plate Tectonics – Moho & Co Schematic representation of the convection in the Earth’s mantle. Source: P & S, 1994 (all). ContinentalCrust OceanicCrust Lithosphere (rigid) Mantle Upper Mantle Asthenosphere (plastic) Core 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. Mid-Ocean Ridge Ocean Sediments 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. OceanicCrust Hypsographische Kurve der Erdoberfläche Kumulative Verteilung der Flächenanteile bestimmter Höhenstufen (bezogen auf den Meeresspiegel). Gabbro Moho Magma Chamber Peridotite Mantle

4. Geophysics 100 Mid-Ocean Ridges Source: NOAA The mid-ocean ridges form a world-encompassing mountain range with 60000 km length.

5. Geophysics 101 Black Smokers & Co 3 m long Tube Worms are part of a symbiotic community, which is based on Chemosynthesis by sulfur bacteria. New oceanic crust is built in the form of pillow lava. „Black Smokers“ (Hydrothermal Vents)– Sulfur and heavy metals precipitate from sea water, which has been heated up to 400°C.

6. Geophysics 102 Plate Tectonics – Schematic D & D, 1996 At mid-ocean ridges new (basaltic) oceanic crust is formed, which then descents in Subduction Zones to become part of the mantle again.

7. Geophysics 103 Paleomagnetism 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 Geographic Pole Geomagnetic Pole Mid-Ocean Ridge Gilbert (reverse) Gauss (normal) Matuyama (reverse) Brunhes (normal) normal revers After P & S, 1994 Thermoremanence: When magma cools below the Curie Point, the ambient magnetic field (and its direction) is “frozen in”.

8. Geophysics 104 Seafloor-Age The Seafloor-Spreadingcan 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

9. Geophysics 105 Subduction 1 Volcanic-Magmatic Zone Divergent Plate Boundary Convergent Plate Boundary Deep-sea trench Mid-ocean Ridge Lithosphere Mountain Building Deep-sea trench ContinentalCrust Oceanic Crust Ocean–Continent Collision Asthenosphere Deep-sea trench Island Arc Earthquake Foci MagmaFormation Divergent and convergent plate boundaries. After P & S, 1994. Ocean–Ocean Collision

10. Geophysics 106 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.

11. Geophysics 107 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).

12. Geophysics 108 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).

13. Geophysics 109 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.

14. Geophysics 110 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.

15. Geophysics 111 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.

16. Geophysics 112 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

17. Geophysics 113 The Ocean Floor Transform Fault Axis of the East Pacific Ridge Transform Faults are very common (but usually nit visible) on the ocean floor near the mid-ocean ridges. Map by Heezen & Tharp, 1997.

18. Geophysics 114 Building of an Ocean Rift Valley Oceanic Crust Continental Shelf Continental Slope Continental Crust Continental Rise Plate Boundary Mid-Ocean Ridge Passive Margin Passive Margin Formation of a Rift Valley and building of a new ocean with passive continental margins (like the “young“ Atlantic). After P & S, 1994.

19. Geophysics 115 Plate Tectonics – Earth Hitory Drifting continents, formation and breakup of Pangaea.

20. Geophysics 116 Hot Spots und Mantle Plumes Active Volcano Guyot Hot Spot – Ascending Magma Hot Spots and volcanic Island Chains Movement of the Lithosphere-Plate

21. Geophysics 117 Island Chains The Hawaiian Islands are the textbook example for an Island Chain, formed by a Hot Spot. Active Volcanoes can cur-rently only be found on „Big Island“ and Maui – and on Loihi, which is still 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.

22. Geophysics 118 Hot Spots in the Pacific Ocean Emperor Seamounts Hawaii Islands Hawaii Hot Spot Galapagos Tuamoto Archipelago Pitcairn Easter Island Source: SdW Cook Islands 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).

23. Geophysics 119 From Volcano to Atoll 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.

24. Geophysics 120 From Volcano to Atoll An once again with the German terms. The explanation for the formation of atolls was found by Charles Darwin. Source: P & S, 1994

25. Geophysics 121 Hot Spots und their Traces SOurce: NASA Volcanic Islands– Galapagos (top left) Subsiding Volcanoes with fringing and barrier reefs and Atolls–Tahiti and Bora Bora, French Polynesia (left) Atolls with completely submerged volcanoes – Maldives (above).

26. Geophysics 122 Hot Spots und their Traces Hawaii – a textbook „Hot Spot“, currently under Hawaii – „Big Island“, but also creating a new island – Loihi(currently –969 m, as of 2010). Walvis Hot Spot – actively involved in forming the South Atlantic, currently under Tristan da Cunha, Traces: Rio Grande Rise in the West, Walvis Ridgein the East. Source: SdW

27. Geophysics 123 Flood Basalts Source: SdW Enormous Flood Basalts (Large Igneous Provinces) are deposited when the „Mushroom Head“ of a Mantle Plume penetrates the crust (fortunately not in historic times).

28. Geophysics 124 Deccan Flood Basalts Deccan Traps, India.

29. Geophysics 125 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.

30. Geophysics 126 Hot Spots and Shield Volcanoes Source: NASA Maun Loa in Hawaii (above, in the background; right: satellite image) 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 9000 m from the sea floor. Olympus Mons on Mars (far right), the tallest mountain in the solar system is even 22 km higher than its surrounding..

31. Geophysics 127 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 glacierEyjafjallajökull – 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.

32. Geophysics 128 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.