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GEOG 3251 summer ‘10 Adina Racoviteanu Objectives • Understand how Earth is dynamic and how moving plates form ocean basins, mountain ranges, islands, volcanoes, and earthquakes • Identify the three general categories of plate boundaries recognized by scientists: convergent, divergent, and transform • Understand how the theory of plate tectonics was developed and supported Why is the Earth so restless? • ground shakes violently • volcanoes erupt with explosive force • mountain ranges rise to incredible heights Catastrophism world view Until 1700s: common belief: • Biblical Flood shaped Earth's surface • All earthly changes were sudden and caused by a series of catastrophes. Uniformitarian principle • 1785 : James Hutton, a Scottish geologist proposes new world view: The present is the key to the past. • geologic forces and processes acting on the Earth today are the same as those that have acted in the geologic past. EARTH’S LAYERS • Three layers: – the core, – the mantle and – the crust • Chemical & mechanical differences Like boiled egg THE CORE • Composition: mostly iron and nickel • Temperature: very hot, even after 4.5 billion years of cooling. • divided into two layers: – a solid inner core – liquid outer core. CORE GENERATES CURRENTS • Core is so hot it radiates natural heat to the upper layers. • convection currents generated. •The convection currents cause the movement of the tectonic plates. MANTLE • Composition: mafic minerals - rich in the elements iron (Fe) magnesium (Mg), silicon, and oxygen • Dense, hot layer of semi-solid rock • Upper part of the mantle is cooler and more rigid than the deep mantle CRUST • Composition: oxygen and silicon with lesser amounts of aluminum, iron, magnesium, calcium, potassium, and sodium. • rigid and very thin LITHOSPHERE • Greek (lithos = stone) rigid outermost layer made of crust and uppermost mantle • broken up into the moving plates that contain the world's continents and oceans ASTHENOSPHERE • Greek (asthenos = weak) = part of the mantle that flows, a characteristic called plastic behavior. • The flow of the asthenosphere is part of mantle convection, which plays an important role in moving lithospheric plates. CRUST/MANTLE AGAIN • lithosphere – – – – hard ~100 km thick crust floats on top continental crust 20 to 70 km thick – oceanic crust ~ 8 km thick • asthenosphere – soft – ~3000 km thick – “fluid-like” Crust: 2 types • Oceanic crust is made of relatively dense rock called basalt • Continental crust is made of lower density rocks, such as andesite and granite. Oceanic vs.continental crust Oceanic Continental basaltic rocks granitic rocks "mafic” Mg, Fe “felsic” (Fe-Si, Al) denser less dense thinner thicker EARTH STRUCTURE SUMMARY • • The Earth is sphere with a diameter of about 12,700Kilometres. Temperature and pressure rise with depth. • CORE: – – – – temperature is believed to be an incredible 5000-6000°c. Inner core: solid FE and NI Outer core: liquid Fe and Ni Complex convection currents give rise to a dynamo effect which is responsible for the Earth's magnetic field. • MANTLE: – It's made of solid rock and behaves like an extremely viscous liquid - (This is the tricky part... the mantle is a solid which flows????) • CRUST: – very thin (average 20Km) – OCEANIC CRUST: thinner (10 kilometres) – CONTINENTAL CRUST: thicker (35 kilometres on average). • The continental crust in the Himalayas is some 75 kilometres deep. THEORIES OF MOUNTAIN ORIGIN • CONTINENTAL DRIFT – Alfred Wegener (1880 – 1930) • PLATE TECTONICS – Harry Hess and G. Mason (1962) VIDEO • PLATE TECTONICS INTRO CONTINENTAL DRIFT FOSSIL EVIDENCE • Fossils of the same species found on several different continents. • Wegener’s idea: species dispersed when the continents were connected and later carried to their present positions as the continents drifted. For example, Glossopteris, a fern, was found on the continents of South America, Africa, India, and Australia. TORN NEWSPAPER IDEA • Rock sequences in South America, Africa, India, Antarctica, and Australia show remarkable similarities. • Wegener’s idea: the same three layers occur at each of these localities. SHIFTING CLIMATE SHIFTING CLIMATE ZONES • Wegener’s interpretation: • climate zones remained stationary and the continents drifted to different locations. • The drift of the continents caused the apparent movement of the climate zones. ARGUMENTS AGAINST • What mechanism moves the continents?? Wegener’s idea: Earth's spin caused the continents to move, plowing through the oceanic plate and producing mountains on their leading edges. Continental Drift Evidence Summary • • • • Puzzle idea: continents fit together Torn newspaper: matching geology Fossils: same fossil on different continent Paleoclimate SEA-FLOOR SPREADING In the late 1950's, scientists mapped the present-day magnetic field generated by rocks on the floor of the Pacific Ocean. SEA FLOOR SPREADING HO When mapped, the anomalies produce a zebra-striped pattern of parallel positive and negative bands. The pattern was centered along, and symmetrical to, the mid-ocean ridge Computer-generated detailed topographic map of a segment of the Mid-Oceanic Ridge. "Warm" colors (yellow to red) indicate the ridge rising above the seafloor, and the "cool" colors (green to blue) represent lower elevations. New discoveries: Sea-floor spreading • 1962: Hess (1962): New ocean floor is formed at the rift of mid-ocean ridges. VIDEO SEA FLOOR SPREADING SEA FLOOR SPREADING HO SEA FLOOR SPREADING • So, continents are no longer thought to plow through oceanic crust • Continents are part of plates that move on the soft, plastic asthenosphere. • driving force: convection currents If new oceanic lithosphere is created at mid-ocean ridges, where does it go? Convection cells in the mantle help carry the lithosphere away from the ridge. The lithosphere arrives at the edge of a continent, where it is subducted or sinks into the asthenosphere. PLATE TECTONICS SUMMARY • Plate = large, rigid slab of solid rock • Tectonics (Greek) = to build • PLATE TECTONICS: states that the Earth's outermost layer is fragmented into a dozen or more large and small plates that are moving relative to one another as they ride atop hotter, more mobile material.