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Introduction to Geophysics and Planetary Physics (6) Plate Tectonics Geophysics 114 Geophysics 115 Plate Tectonics – Plate Boundaries Eurasian Plate North American Plate Eurasian Plate African Plate Pacific Plate South American Plate IndoAustralian Plate Antarctic Plate The most important lithospheric plates and their boundaries (Press & Siever). Geophysics 116 Plate Tectonics – Moho & Co Lithosphere (rigid) Asthenosphere (plastic) Schematic representation of the convection in the Earth’s mantle. Source: P & S, 1994 (all). Continental Crust Oceanic Crust Mantle Upper Mantle 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. 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. Ocean Mid-Ocean Ridge Sediments Oceanic Crust Gabbro Hypsographische Kurve derChamber Erdoberfläche Moho Magma Peridotite Kumulative Verteilung der Flächenanteile bestimmter Mantle Höhenstufen (bezogen auf den Meeresspiegel). Geophysics 117 Mid-Ocean Ridges Source: NOAA The mid-ocean ridges form a world-encompassing mountain range with 60000 km length. Geophysics 118 Mid-Ocean Ridges 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) . Geophysics 119 Black Smokers & Co „Black Smokers“ (Hydrothermal Vents) – Sulfur and heavy metals precipitate from sea water, which has been heated up to over 400 °C. 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. Geophysics 120 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). 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. Geophysics 121 Geophysics 122 Paleomagnetism Geographic Pole Mid-Ocean Ridge Geomagnetic Pole Gilbert (reverse) Schematic representation of the field lines of the geomagnetic field (Dipole). The axis is tilted 11° with respect to the rotation axis: tan I 2 tan m Gauss (normal) I = Inclination m= magnetic latitude Matuyama (reverse) Brunhes (normal) After P & S, 1994 normal revers Thermoremanence: When magma cools below the Curie Point, the ambient magnetic field (and its direction) is “frozen in”. Geophysics 123 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 seafloor older than 200 Million Years. Source: P & S, 1994 Geophysics 124 Subduction 1 Divergent Plate Boundary Convergent Plate Boundary Deep-sea trench Mid-ocean Ridge Lithosphere VolcanicMagmatic Zone Mountain Building Deep-sea trench Continental Crust Oceanic Crust Ocean–Continent Collision Asthenosphere Earthquake Foci Magma Formation Divergent and convergent plate boundaries. After P & S, 1994. Deep-sea trench Island Arc Ocean–Ocean Collision Island Arcs – Java Geophysics 125 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. Island Arcs – Aleutians Geophysics 126 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 127 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 128 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 129 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 130 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 131 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 132 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. Geophysics 133 Ocean Forming Rift Valley Oceanic Crust Continental Shelf Continental Slope Continental Crust Passive Margin Continental Rise Plate Boundary Mid-Ocean Ridge 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. Geophysics 134 Ocean Forming 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). Plate Tectonics – Earth History Drifting continents, formation and breakup of Pangaea. Geophysics 135 Plate Tectonics – Earth History Geophysics 136 The emergence of the Réunion-Hotspot was responsible for the fast movement of India during the Tertiary (Credit: R.D. Müller, Nature 2011). Plate Tectonics – Alternatives It could also have been like this … (Creadit: C. Heiner & S. Brune). Geophysics 137 Geophysics 138 Hot Spots and Mantle Plumes Active Volcano Guyot Hot Spot – Ascending Magma Hot Spots and volcanic Island Chains Movement of the Lithosphere-Plate Geophysics 139 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 140 Hot Spots in the Pacific Ocean Emperor Seamounts Hawaii Islands Hawaii Hot Spot Galapagos Tuamoto Archipelago Pitcairn Easter Island Cook Islands Source: SdW 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). Geophysics 141 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. Geophysics 142 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 Geophysics 143 Hot Spots und their Traces SOurce: NASA 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). Geophysics 144 Hot Spots und their Traces Bikini – Atoll (NASA). Geophysics 145 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 2014). Source: SdW 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. Geophysics 146 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). Geophysics 147 Deccan Flood Basalts Deccan Traps, India. Geophysics 148 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 149 Hot Spots and Shield Volcanoes 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 9000 m from the sea floor. Geophysics 150 Mighty Shield Volcanoes Quelle: NASA Even bigger is Olympus Mons on Mars – it rises more than 22 km above its surrounding (NASA). Geophysics 151 Hot Spot - Volcanism A Hot Spot can also be found under Iceland – right on the midatlantic 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. Hot Spot – Volcanism Geophysics 152 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.
