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Transcript
Plate Tectonics A Scientific Revolution Unfolds Why? Why is Mount Everest over 29000 feet high? Plate Tectonics The unifying concept of the Earth sciences The outer portion of the Earth is made up of about 20 distinct “plates” (~ 100 km thick), which move relative to each other Plate tectonics explains: Volcanoes Earthquakes Sea floor spreading Mountain chains Occurrence of same fossils on different continents Plate Tectonics Integrates evidence from many branches of science First suggested based on evidence from cartography, geology and paleontology Fully embraced after evidence from geophysical measurements However, the proof of plate tectonics was a long rambling road Atlantic Coastline 300-400 years ago (after accurate maps became available) it was noticed that the coastline on both sides of the Atlantic could fit together like a jigsaw puzzle Atlantic Coastline The fit was even better when you took the continental slope into consideration In 1858, Antonio Snider-Pellegrini, an American published a sketch showing this fit Gondwanaland In the 1890s, the Austrian Geologist Eduard Suess suggested that the present day southern continents once been formed in a super continent which he called Gondwanaland The name comes from the Gondwana region in India Gondwanaland Gondwanaland included Antarctica, South America, Africa, Madagascar, Australia-New Guinea, New Zealand and Indian (and today we include Arabia) Gondwanaland Unfortunately, Suess did not have any evidence other than the jigsaw puzzle fit He could not explain how Gondwanaland split up into the modern continents However, he got to be on a stamp Continental Drift Proposed by the prominent geologist Alfred Wegner in 1912 based on his observation of drifting sheets of ice The concept is that large-scale horizontal movements of the Earth’s continents are responsible for the major topographical features such as mountains and ocean basins Called his super-continent “Pangaea” Wegner spent the next two decades, until his death, promoting continental drift Rejection of Continental Drift Talk about an idea whose time had not arrived! Most scientist ridiculed Alfred Wegener’s idea Not even the public believed it possible No proof that continents move No “driving mechanism” that could move the continents. “Why should the continents move?” Accumulating Evidence Fossil record Earth’s crust and mantle Magnetism and the Earth’s magnetic field Paleomagnetism Magnetic reversals The topography of the seafloor Age of the seafloor Seafloor spreading Polar wandering Fossil Record The fossil record had revealed that the geology and paleontology matched on opposite sides of the Atlantic Ocean Fossil Record In fact, there are matching fossil records that span across all of the continents Without continental drift, this is hard to explain Crust and Mantle The Earth’s crust and uppermost mantle are solid, somewhat brittle and elastic This outmost solid layer is called the lithosphere It is thinnest under the oceans, averaging 50 kilometers (km) thick The lithosphere under the continents can be up to 100 kilometers thick Crust and Mantle The layer below the lithosphere is called the asthenosphere and it extends to a depth of 300 kilometers The rock is not rigid and can flow plastically due to the high temperature and moderate pressures The existence of the plastic asthenosphere made plate motion more feasible Magnetism Most iron-bearing minerals are at least weakly magnetic Each magnetic mineral has a Curie temperature, the temperature below which it remains magnetic Above the Curie temperature the mineral is not magnetic The Curie temperature varies from mineral to mineral, but it is always below the melting temperature of the mineral Earth’s Magnetic Field The Earth has a magnetic field This why a compass points to the north The simple presence of iron in the Earth’s core is not enough to account for the Earth’s magnetic field The high temperatures in the Earth’s core are far above the Curie temperature for any magnetic mineral Earth’s Magnetic Field It is believed that the Earth’s magnetic field originates in a layer called the outer core The outer core is a metallic fluid consisting mainly of iron This metallic fluid is in motion and the convection currents act like a giant dynamo, converting mechanical energy into magnetic energy Paleomagnetism A hot magma is not magnetic As a magma cools and solidifies, the iron-bearing minerals (such as ferromagnesian silicates) crystallize Eventually, the minerals cool below the Curie temperature and the iron-bearing minerals become magnetic Paleomagnetism Like tiny compass needles, these magnetic minerals align themselves parallel to the lines of force of the Earth’s magnetic field This remnant magnetism, which is also called paleomagnetism, points to the north pole like a sign post But... Magnetic Reversals About a century ago, a sequence of lava flows were found in France where some of the flows had the north and south poles reversed Therefore, the north pole and south pole must have repeatedly swapped positions Magnetic Reversals These magnetic reversals have occurred though out the history of the Earth They occur on an irregular basis ranging in time from tens of thousands of years to millions of years Magnetic Striping on Seafloor In the 1950s, the Atlantic seafloor was found to consist of alternating stripes of normal and reversely magnetized rocks Topography of the Atlantic Also, in the 1950s, it was discovered that an underwater mountain range ran north-south in the middle of the Atlantic Ocean The Mid-Atlantic Ridge rises as high as 2 kilometers above the abyssal plain Atlantic Ocean Sea Floor In the 1960s, samples were collected from the Atlantic seafloor using special ships with drill rigs The rocks of the Atlantic seafloor were discovered to be basalt Basalt contains radioactive isotopes (such as U235) which can be dated Atlantic Ocean Sea Floor It was discovered that the youngest rocks of the Atlantic Ocean seafloor are found along the mid-oceanic ridge And that farther you move away from the ridge, the older the rocks become on either side of the ridge The oldest rocks are along the continental boundaries Plates that Move In 1965 Wilson proposed the concept that the crust of the Earth is a mosaic of interacting plates – hence “plate tectonics” These plates move relative to each other The continents ride on these plates Geologic features, such as mountains, volcanoes and earthquakes occur along the plate boundaries Plates that Move Plates Move Slowly Plates move at different speeds: Measured using GPS and magnetic data Slow spreading: 30 mm/yr Fast spreading:100 mm/yr Or about the thickness of a fingernail in one day Plates Move Slowly Plate movement is best described as chaotic Plates that Move The red dots show that most major earthquakes occur along plate boundaries Plates that Move The black triangles show that most volcanoes occur along plate boundaries When Plates Rub and Collide 3 Types of Plate Boundaries In 1965, the Canadian Geologist, J. Tuzo Wilson, proposes that tectonic plates interact in three different ways along their boundaries 1. Transform 2. Divergent 3. Convergent Divergent Plate Boundaries Usually start within continents Can grow to become ocean basin Divergent Plate Boundaries A. Rifting occurs where tensional forces thin the crust, magma ascends and volcanoes form B. The crust is pulled apart, forming a valley Divergent Plate Boundaries C. A long, narrow lake or sea forms D. Eventually, an expansive ocean basin and ridge are created Divergent Plate Boundaries Can see the spreading on the island of Iceland Continental Rifts Two examples are the East African and Rio Grande Rift Valleys This can be the beginning of ocean formation (although it may not get that far) Rifting often begins at a triple junction Continental Rifts Note that uprise and decompression of the underlying asthenosphere results in magma formation The crust responds by brittle fracture Continental Rifts Early rift sediments are downfaulted into the developing rift (graben) Erosion takes place on the sides of the rift valley East African Rift Zone Perhaps the most interesting and spectacular plate tectonic rift zone on the land surface Ol Doinyo Lengai Ruwenzori East African Rift Zone The East African Rift has a north-south orientation with the African Plate moving to the west and the Somali Subplate moving to the east East African Rift Zone A triple junction, where three plates are pulling away from one another: the Arabian Plate, and the two parts of the African Plate (the Nubian and the Somalian) splitting along the East African Rift Zone Rio Grande Rift The Rio Grande Rift Valley extends north from Mexico, near El Paso, Texas through New Mexico into central Colorado The rift is now essentially quiet tectonically, but significant deformation and faulting with major offsets was responsible for the formation of the rift during the past 30 million years Rio Grande Rift The geology has been described as a "taffy-like" thinning of the lower crust, with upper crust faulting in many places, to produce the rift valley Rio Grande Rift “Two features of the region most clearly exemplify the Southwest's geologic-climatic-demographic circumstances. These features constitute two "natural geographic corridors" or focal areas concerning the relationship between people and the Southwests physical environment: Arizona's Mogollon Rim Corridor and New Mexico's Rio Grande Rift Corridor” Is the Earth Expanding? NO! The Earth is maintaining a constant diameter New crust is created at mid-oceanic ridges The old crust sinks back into the Earth at subduction zones along convergent plate boundaries Convergent Boundaries There are three types of convergent plate boundaries Oceanic–Oceanic Japanese Islands Oceanic–Continent Andes Mountains Continent–Continent Himalaya Mountains We will look at each type Oceanic - Oceanic Under the ocean, one plate slides underneath another at a subduction zone Oceanic - Oceanic A deep trench forms along the contact Oceanic - Oceanic Magma wells up along the subduction zone creating volcanoes, which can form volcanic island arcs such as the Japanese Islands Oceanic - Continent An oceanic plate slides under a continental plate at a subduction zone Oceanic - Continent The relative densities of the oceanic crust versus the continental crust are important: The continental crust is lighter and has an average density of 2.8 g/cm3 The oceanic crust is heavier and has an average density 3.2 g/cm3 Therefore, the heavier oceanic crust sinks beneath the more buoyant, lighter continental crust Oceanic - Continent A deep trench forms along the subduction zone Oceanic - Continent The lighter continental crust is pushed up and forms a mountain range Oceanic - Continent Magma wells up along the subduction zone creating volcanoes, which adds to the size of the mountains and creates a continental volcanic arc Andes Mountains The Andes Mountains stretch over 5500 miles along the Pacific side of South America Pacific Ring of Fire Continent - Continent Both continental plates are lighter than the heavier, underlying asthenosphere So, when two continental plates collide, they are too buoyant to be subducted Instead, the continental crust is deformed and uplifted, creating mountains such as Mount Everest Why? Because the continent of India collided with the continent of Asia Continent - Continent Transform Plate Boundaries A transform fault plate boundary occurs when two plate slide past each other in opposite directions Transform Plate Boundaries The most famous example is the San Andreas Fault Zone in California The portion of California in blue is heading northwest to Alaska This is the most studied fault zone in the world San Andreas Fault Earthquakes Each red dot marks an earthquake Notice how many have occurred in California They are not kidding when they talk about the “BIG ONE” San Andreas Fault Zone Transform Plate Boundaries Hot Spot Volcanism All of the mountains in this map of the floor of the Pacific Ocean are volcanoes Note that there are several long chains of volcanoes But there are also many individual volcanoes None of these volcanoes are formed along plate boundaries Hot Spot Volcanism These isolated areas of volcanic activity are not associated with plate boundaries These volcanoes are found both on continents and out in the ocean Hot Spot Volcanism A plume of hot mantle material rises up to the bottom of the lithosphere Melting occurs and magma is created These are called “hot spots” They can last for many millions of years Hot Spot Volcanism The magma works its way to the surface over the hot spot and a volcano is formed Out in the ocean, the volcano may or may not reach the surface and form an island In fact, few oceanic volcanoes reach the surface Hot Spot Volcanism As the moving plate carries the volcano away from the hot spot, the volcano dies out With time, another volcano may form This cycle may repeat many times, forming a long chain of under sea volcanoes and islands Hot Spot Volcanism The Cortes Bank Seamount, 100 miles offshore of San Diego, is a 17-mile long remnant of a undersea volcanic mountain range that rises to within 3 feet of the surface Hot Spot Volcanism Fisherman and scuba divers knew about it, but it was not on any nautical maps The U.S. Navy found it the hard way in 1985 with the aircraft carrier Enterprise Hot Spot Volcanism The Hawaiian Islands are the classic example of hot spot volcanism Hot Spot Volcanism Every island in the Hawaiian Island chain was created by volcanic eruptions that occurred as the Pacific Oceanic Plate crossed over a hot spot Hot Spot Volcanism In fact, the 7 main Hawaiian Islands are just the southern most portion of two great undersea mountain ranges, composed of hundreds of volcanoes, that runs for 6000 kilometers (4000 miles) across the floor of the Pacific Ocean Hot Spot Volcanism The plate has moved to the northwest for more than 65 million years The big island of Hawaii is now over the hot spot Hot Spot Volcanism The volcanoes that formed the Hawaiian islands are classified as shield volcanoes Hot Spot Volcanism Shield volcanoes are characterized by broad, gentle slopes built up from repeated basaltic lava flows Hot Spot Volcanism Not all eruption create big islands Molokini represents one eruption that is slowly being reclaimed by the sea Hot Spot Volcanism Loihi Volcano, the youngest volcano of the Hawaiian Island Chain, lies about 20 km off the south coast of the Big Island Currently it rises 3500 meters above the surrounding sea floor and its summit is about 1000 meters beneath the surface What Causes the Plates to Move A driving force for plate tectonics has not been definitely identified Each plate weighs trillions upon trillions of metric tons Thought to be convection of the mantle Friction at base of the lithosphere transfers energy from the asthenosphere to the lithosphere Convection may have overturned asthenosphere 4–6 times Convection in the Mantle Hot mantle material rises at the spreading midoceanic ridges Convection in the Mantle Some of the magma erupts, but most spreads out under the lithosphere and drags the crust along Convection in the Mantle Eventually, the slowly cooling material sinks back into the mantle Convection in the Mantle An alternate hypothesis is that the convection process may involve two convection layers Convection in the Mantle Or perhaps the convection process behaves in a chaotic manner Moving Continents It is interesting to speculate on how the Earth looked as the continents were moved around...