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Geology Plate tectonics – the dynamic Earth The rock cycle – from a tectonic view Earth’s structure and geophysics Earthquakes Plate tectonics – the dynamic Earth Plate tectonics is one of the most important scientific revolution of the 20th century. It provides an explanation for practically all observations on the surface of the Earth, and processes originating below the surface. The Old Theories 1. The “Shrinking Apple” After being hot planet Earth was cooled and shrank 2. The concept of Geosyncline: In the 19th century: sedimentary rocks accumulate in basins, magmatic rocks uplifted by internal processes created mountain ranges. 1. “The moving continents” - Alfred Wegener proposed “Continental Drift” in 1915 he noticed that continents fitted together like pieces of a puzzle. The theory was not accepted at the time - scientists couldn’t imagine horizontal movement of the crust and no one knew what would drive such motion. 2. The continents don’t only fit together – There is an overlap of fossil assemblages, structures, and rock types up to the time in geologic history when the plates split apart. 3. Glacial deposits from the same period in the past can be found in different latitudes in the Southern Hemisphere. 4. The mountain belts, volcanic activity and earthquakes are localized along narrow belts such as “The Ring of Fire” 5. Sea floor maps reveal very interesting and strange data: A. The depth of most of the sea floor is 4 km B. There is more then 70,000 km of mountain belts (ridges) in the oceans. Most of them are built from volcanic rocks. C. There are very deep trenches along the margins of the pacific ocean, near Asia and South America. 6. There are magnetic stripes along the Mid-Atlantic Ridge. Symmetry can be found by measuring the magnetic data. Actual Data (South of Iceland) Magnetic ``Stripes'' on Seafloor Tectonics plate boundaries Divergent Plate Boundaries 1. Mid-Ocean Ridges (spreading centers) 2. Continental rift 1.Rifting: Our Rift Valley 2. Embryonic ocean: The Red Sea 3. Young Ocean: The Atlantic Ocean Mid-Atlantic Ridge (color indicates seafloor age) Generation of sea floor magnetic anomalies Divergent Plate Boundaries = Plate Tectonic Forces Plate #1 Plate #2 faulting • Extension, spreading, rifting, tension, thinning of the lithosphere, normal faulting Convergent Plate Boundaries Subduction Zones: Oceanic-Continental Subduction Zones: Oceanic vs. Oceanic Pachapaqui Mining Area, Andes, Peru Machu Picchu, Andes, Peru Mt. Fuji, Japan Convergent Plate Boundaries Continental Collision Zones: Continent-Continent Himalayas and Tibet Because both are buoyant, neither continental plate subducts * broad areas of high elevation (large mountain ranges and high plateaus) result. * No volcanism. Convergent Plate Boundaries = Plate Tectonic Forces Plate #1 • • Plate #2 Convergence, compression, mountain building, thickening of the lithosphere, thrust faulting Types: – Ocean-ocean (subduction; island arc; volcanoes) – Ocean-continent (subduction; long mountain chains; volcanoes; strongest earthquakes) – Continent-continent (continental collision zone; wide, high mountainous region; no volcanism) סדרו לפי הסדר ותארו את התהליך .אילו סוגי גבול לוח מתוארים? Tectonics plate boundaries Transform Plate Boundaries Mid-Atlantic Ridge (ocean floor age) Divergent Transform Divergent http://www.ngdc.noaa.gov/mgg/image/images.html#crustage Transform Plate Boundaries = Plate Tectonic Forces Plate #2 Plate #1 • • • Shear, no change in lithospheric thickness, strike-slip faulting No volcanism, no significant change in topography Shallow faulting Tectonics - plate configuration Tectonics – what drives the motion: What causes the plates to move? Mantle convection produces lateral forces at the boundary between the mantle and the crust Ridge push Trench rollback Slab pull Hawaii Islands – Hot spot Oceanic plate move above hot spot Mantle Plume As the plate moves, each island is dragged away from the heat source and a new volcano forms. Tectonics – the continent cycle and the rock cycle: Tectonics – the continent cycle: .1 .2 .5 :דוגמאות :מחזור ווילסון Earth’s structure: The Earth Interior’s mantle convection Heat transfer from the core to the mantle produces slow convection of the mantle material (in the order of centimetres per year) 1. Volcanoes are found at which type of plate boundary? a) b) c) d) e) 2. Convergent (continent-continent) Transform Subduction zone Divergent Both c and d Seafloor magnetic anomalies a) b) c) d) e) documents the history of magnetic polarity reversals on Earth are associated with island arc formation form at ocean-ocean convergence zones are caused by sediment accumulating atop the seafloor trend perpendicular to mid-ocean ridges עוצמת השדה המגנטי גדולה יותר ליד הקטבים, כוח המשיכה קטן יותר במשווה כי כדה"א פחוס Earthquakes Basic Terminology • • • • Fault: discontinuity in the Earth’s crust; the two sides of the fault may move relative to one another (analogous to fracture or crack) Earthquake: abrupt motion across a fault Focus (a.k.a. hypocenter): point within the Earth where the fault rupture starts Epicenter: point of the Earth’s surface directly above the focus רעידות אדמה וסיכונים סיסמיים Earthquakes: Types of faults • Three types: Normal, thrust and strike-slip How do plate motions cause earthquakes? Earthquakes: Seismology • Seismic waves: Vp= ~6 km/sec Vs= ~3 km/sec Fast P-wave Surface waves Tp Ts S-wave Earthquakes: Seismology • Measurement of seismic waves from earthquake Fast P-wave Surface waves Time between arrivals determines the distance from the earthquake hypocenter. Tp DT Ts S-wave Why is this so important? Earthquakes: Seismology • Measurement of seismic waves from earthquake Fast P-wave Surface waves Time between arrivals determines the distance from the earthquake hypocenter. Tp DT Ts Why is this so important? S-wave X=VT DT=Ts-Tp ; DT=X/Vs–X/Vp = …= X(Vp-Vs)/VpVs x Vp Vs t ; X=DTVpVs/(Vp-Vs) Earthquakes: Seismology • Measurement of seismic waves from earthquake Fast P-wave Surface waves Time between arrivals determines the distance from the earthquake hypocenter. Vp= ~6 km/sec Vs= ~3 km/sec DT S-wave An easy way to calculate: X=VT ; X= Xp=Xs X = Vp *Tp = Vs * (Tp+DT) = Vs * Tp + Vs * DT Tp= Vs * DT / (Vp-Vs) Here: Tp = 3km/sec * 300sec / 3km/sec = 300sec Where Earthquakes Occur Intraplate Earthquake Shallow earthquakes (most damaging) – Tension at mid-ocean ridges; Lateral slip on transform faults Deep earthquakes - Thrusting; Down-dip compression of subducting plates The very active Mediterranean X=VT ; X= Xp=Xs Vp *Tp = Vs * (Tp+DT) ….. Tp=? ….. X=? .א Timeearthquae= TimeP_arrival – Tp .ב T = Timeearthquae+4000/Vs .ג Waves – seismic frequency and wavelength : Wavelength – Velocity Frequency – Period - אורך גל מהירות תדירות זמן מחזור t l V f T X=VT f=1/T T=1/f [Hz]=[1/sec] l=VT = V/f T T t T Waves – seismic frequency and wavelength : l Wavelength – אורך גל V Velocity מהירות f Frequency – תדירות f=1/T T Period זמן מחזור T=1/f X=VT [Hz]=[1/sec] l=VT = V/f Waves – Tsunami: X=VT בהצלחה