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Plate Tectonics – plate margins; dynamics; effects Jarðsaga 1 -Saga Lífs og Lands – Ólafur Ingólfsson Excellent web pages: http://earth.leeds.ac.uk/dynamicearth/index.htm http://pubs.usgs.gov/publications/text/dynamic.html Earth´s inner structure What are the continental plates? The Earth's rocky outer crust is not a solid shell; it is broken up into huge, thick plates that drift atop the soft, underlying mantle. Over long periods of time, the plates drift across the surface of the globe, and they also change in size as their margins are added to, crushed together, or pushed back into the Earth's mantle. The plates are generally between 80-400 km thick. Plate margins and motions Divergent= constructive; convergent= destructive; transform= conservative Constructive plate boundaries ...where new lithosphere is created - by sea floor spreading. Divergent boundaries occur along spreading centres where plates are moving apart and new crust is created by magma pushing up from the mantle. Sea Floor Spreading Types of transform fault Atlantic Mid-Ocean Ridge – the best known constructive plate margins Submerged MOR The mid ocean ridges extend for some 70,000 km along the floor of the ocean. Note the sediment blankets away from the ridge Pillow basalt along the midAtlantic ridge. Submerged open fissures Black smokers – cradles of life...? Black smokers are vents of mineral laden waters circulating through the hot rocks in the rift valley. This warm water provides a unique habitat for exotic life that subsists without sunlight. Mid-Oceanic rift reflected in the geological history of Iceland The Icelandic Hotspot Idealized cross section of a mid ocean ridge Crustal thickness in Iceland Hotspots in the world A hotspot is an area where there has been a continous volcanism for a very long time. They occur both on diverging plate boundaries and inside plates. Were thought to be the major force driving plate tectonics, but that view is not prevailing today. The Hawaiian hotspot East African Rift Valley East Africa may be the site of the Earth's next major ocean. • Continental rifting occurs when divergent plate margins develop in continents. • They typically have normal faults, shallow earthquakes, and basalt and rhyolitic magmatism What drives the plates? • Until the 1990s, prevailing explanations about what drives plate tectonics have emphasized mantle convection. • Some geologists argue that the intrusion of magma into the spreading ridge provides an additional force (called "ridge push") to propel and maintain plate movement. • Most scientists now favor the notion that forces associated with subduction are more important than seafloor spreading. Convergent Plate Boundaries • The size of the Earth has not changed significantly since shortly after its formation 4.6 billion years ago. •The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created. • Such destruction of crust takes place along convergent boundaries where plates are moving toward each other, and sometimes one plate sinks (is subducted) under another. • The location where sinking of a plate occurs is called a subduction zone. Ocean-Continent convergence ta Be (A l ph a Su bd uc tio n) b Su n tio c du Oceanic-Continental Plate Margins Off the coast of South America the oceanic Nazca Plate is pushing into and being subducted under the continental part of the South American Plate. The overriding South American Plate is being lifted up, creating the towering Andes mountains, the backbone of the continent. Strong, destructive earthquakes are common in this region. The convergence of the Nazca and South American Plates has deformed and pushed up limestone strata to form towering peaks of the Andes Andean margin Andean volcanism Ruiz, in Columbia (5,321 m), capped by snow and ice. The 1985 eruption of Ruiz produced mudflows which reached the town of Armero and killed more than 23,000 people Mt Villaricha in Chile. 55 eruptions since 1558 Some Andean volcanoes Colima (4100 m) is the most active volcano in Mexico. Eruptions often explosive, producing tephra as well as avalances. Most recent eruption was in 1994 Arenal, in Costa Rica, was regarded as a dormant stratovolcano. That changed in July of 1968. An explosive eruption produced hot avalanches and ejected blocks that devastated the west flank of the volcano and killed 78 people. Arenal has been continuously active since then. Strombolian eruptions occur at intervals of several minutes to hours. The Ring of Fire... Oceanic-oceanic margins: Island Arches Japanese Volcanoes Sakurajima is a stratovolcano on the the island of Kyushu, and one of the most active volcanoes on Earth. Since it's first recorded eruption in A.D. 708, Sakurajima has been in nearly constant eruption. Aso (1,592 m) Aso has produced more explosive eruptions than any other volcano in the world: It is a caldera about 20 km in diameter. The first documented eruption in Japan was at Aso 553. Since then it has erupted 167 times, most recently in 1993. Sakurajima (1117 m) Subduction-zone earthquakes - Often very violent and destructive - - The destruction of Tokyo On 1 September 1923, one of the worst earthquakes in world history hit the Kanto plain and destroyed Tokyo, Yoko-hama and the surroundings. About 140,000 people fell victim to this earthquake and the fires caused by it. The destruction... The destruction was of the same order as the effects of the American nuclear bombs at Hiroshima and Nagasaki in 1945... The Kobe-earthquake in 1995: Indonesia – a subduction zone Indonesia: a line-up of volcanoes Some of Earth´s most violent volcanoes are in Indonesia The 1815 eruption of Tambora was the largest eruption in historic time. About 150 cubic kilometers of ash were erupted. Ash fell as far as 1,300 km from the volcano. An estimated 92,000 people were killed by the eruption. The eruption of Krakatau in 1883 produced one of the largest explosions recorded on Earth. It destroyed much of Krakatau, leaving only a remnant, killing about 30,000 people. Since 1927, small eruptions have been frequent and have constructed a new island, Anak Krakatau Other subduction zones also have violent volcanoes Mt. Pelee in the West Indies (1337 m) is famous for the May 8, 1902 eruption which killed 29,000 people and destroyed the city of St. Pierre. Mt Pelee is at the plate boundary where the N American plate subducts the Caribbean Plate. The destruction was caused by a nuee ardente, a type of pyroclastic flow that consists of hot solid particles and gases Continental-continental convergence The Himalayan mountain range demonstrates one of the most spectacular consequences of plate tectonics. When two continents meet head-on, neither is subducted because the continental rocks are relatively light and, like two colliding icebergs, resist downward motion. Instead, the crust tends to buckle and be pushed upward or sideways. The collision of India into Asia 50 million years ago caused the Eurasian Plate to crumple up and override the Indian Plate. After the collision, the slow continuous convergence of the two plates over millions of years pushed up the Himalayas and the Tibetan Plateau to their present heights. Most of this growth occurred during the past 10 million years. Continent-Continent Convergent plates Where Continental plate meets continental plate, granite is produced due to enormous heat and pressure. Increasing metamorphic change occurs with increasing temperatures and pressures. If the melting point is reached, the change is no longer metamorphic, and igneous rocks are formed. Granite • Granite is a coarse-grained igneous rock composed mostly of light-colored, light-density, nonferromagnesian minerals. The earth's continental areas are dominated by granite and by rocks with the same mineral composition of granite. Formation of the Himalayas The initial mountain building process started 70-60 MY when the two land plates of India and Eurasia began to collide with each other. As a result, the shallow seabeds of the Tethys Sea rapidly folded and raised into longitudinal ridges and valleys. A second major phase occurred about 25 MY, and the last phase started about 800 TY. The former sea-bed was elevated to form a great mountain range. Visual Evidences of Himalayan Formation Very precise measurements by GPS have shown that the Indian plate is continuously moving north at the rate of about 2 cm/yr. Because of this reason the Himalayas are rising at the rate of about 5 mm/yr. The thrusting is demonstrated by the strongly folded longitudinal ridges and consistent directions of thrust, folding and overthrust Mt Everest The fossil-bearing marine strata in the folded and upthrusted mountains and ridges show the uplift. The top of Mt Everest is of stratifed, fossiliferous marine sediments. Mt Everest is still rising, at the rate of 8.2 mm/year. Active tectonics Because the Himalayas are still rising, they are tectonically unstable (large earthquakes frequent) and geomorphologically unmature and very active The Mediterranean area Another region where a continental plate meets another continental plate in collision is the Mediterrean area. There, the African plate and the Eurasian plate collide. This causes the frequent earthquakes in e.g. Greece and Turkey. Volcanism in Italy and Greece is related to complex plate boundaries... Santorini and Hellenic volcanism: The eruption of Santorini in Greece in 1,650 B.C. was one of the largest in the last 10 ka. About 30 cubic km of ash was erupted in a huge plinian column, causing the volcano to collapse, producing a c aldera. Ash fell over a large area in the eastern Mediterranean and Turkey. The eruption probably caused the end of the Minoan civilization on the island of Crete Santorini has had eleven eruptions since 197 B.C. The most recent eruption at Santorini was in 1950 50 m of Minoan tephra Stromboli, Italy Stromboli is one of the Aeolian Islands of Italy. The island is about 2 km in diameter and reaches 900 m above sea level. It rises 3,000 m above the floor of the Tyrrhenian Sea. It has been in nearly continuous eruption for about 2,000-5,000 years. Vesuvius, Italy (1,281 m) Vesuvius is famous for the catastrophic eruption in 79 A.D. that buried the towns of Herculaneum and Pompeii. It has erupted 30 times since, last time in 1943-1944 Transform boundaries The zone between two plates sliding horizontally past one another Most transform faults are found on the ocean floor. A few occur on land, for example the San Andreas fault zone in California. The San Andreas fault zone is about 1,300 km long. It slices through two thirds of the length of California. Along it, the Pacific Plate has been grinding horizontally past the North American Plate for 10 million years, at an average rate of about 5 cm/yr. Land on the west side of the fault zone (the Pacific Plate) is moving in a north-westerly direction relative to the land on the east side of the fault zone (the North American Plate). The 1906 San Fransisco earthquake The 1906 earthquake in San Francisco is one of the most famous earthquakes in history although not the largest recorded nor the most killed, it was one of the first major earthquakes to be comprehensively studied, reported and photographed. 7 m of displacements The main epicentre of the earthquake was near San Francisco, but a length of some 430 km of the San Andreas fault had given way with about 7 m large horizontal displacements 50km NW of San Francisco. What is the rate of continental drift? http://cddisa.gsfc.nasa.gov/926/slrtecto.html Rate of continental drift measured by GPS and satellites Rates usually between 2050 mm/yr. Highest rate measured on Easter Island, 150 mm/yr The effects of continental drift and plate tectonics on climate and environment • Changes in the global configuration of the Continental landmasses alter Ocean currents. Ocean currents largely control heat transfer between high and low latitudes. • Continental drift controls periods of ice age climate, like the present. The effects of continental drift and plate tectonics on climate and environment... • Mountain-building episodes alter planetary waves and atmospheric circulation patterns. • Build-up of high mountains and plateaux change runoff and watersheds, albedo, vegetation, animal habitats etc • Erosion and wethering of high mountains radically changes lowland and nearshore environments The effects of continental drift and plate tectonics on climate and environment... • Volcanism associated with plate tectonics affects the chemical composition of Earth´s atmosphere and thus directly and indirectly affects life, environments and climate... The effects of continental drift and plate tectonics on climate and environment... Plate tectonics and continental drift create and destroy sea-way passages and land connections, hindering or enhancing migrations of plants and animals Examples: Break-up of Gondwana, closing of the Panama sea-way, isolation of Antarctica and Australia, opening of the Bering Strait... Bering Strait Plate tectonics are largely responsible for Earth as we know it Witout Plate Tectonics, Earth would be an area of peneplain continents, much impoverished in environments that sustain biological diversity Plate tectonics have affected Earth´s environments for most part of its 4,6 BY history 4.0-3.2 BY: Development of the Lithosphere begins. 3.2-2.6 BY: Development of continental plates and cratons 2.4-2.0 BY: First break-up of continents 2.0-1.6 BY: Major crustal growth 1.6-1.4 BY: First Supercontinent forms 1.0-0.5 BY: Two Supercontinents 0.5 BY to Today 1. Precambrian Supercontinents break up 2. Pangaea Supercontinent forms 3. Pangea breaks up, first into Gondwana and Laurasia, then into the various tectonic plates that are recognized today. The future... Continental drift and plate tectonics will continue to shape and reshape the Earth´s surface... In the long run, plate tectonics continue to control Earth´s environments... References, web-pages etc • Stanley, Earth System History, chapters 8 and 9. • This Dynamic Earth - The Story of Plate Tectonics: http://pubs.usgs.gov/publications/text/dynamic.html#anchor10790904 • Stórskemmtileg “on-line” kennslubók, “The Physical Environment” með góðum texta, myndum og kvikmyndum: http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/title_page.html Kvikmynd um landreks- og plötukenninguna og þau gögn sem liggja að baki: http://www.scctv.net/annenberg/Earth_Revealed_05.asx • http://www.scotese.com/earth.htm • http://www.ucmp.berkeley.edu/geology/tectonics.html • http://volcano.und.nodak.edu/vw.html • http://en.wikipedia.org/wiki/Eduard_Suess • http://www.geology.utoronto.ca/glg105h/lecture16/CDsummarylast.htm • http://www.seismo.unr.edu/ftp/pub/louie/class/100/plate-tectonics.html • http://www.uh.edu/~jbutler/anon/quakes.htm