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Milankovitch Cycles, Sea Level Rise, & Tsunamis Orbital Variations: Milankovitch Cycles -eccentricity variation in the shape of Earth’s orbit -obliquity axial tilt -precession wobble Correlated with climate predicted by deep sea sediments Quaternary ice ages Predicts a cooling period Needs land mass near the poles to support ice sheets No human influence Variations in Earth’s Orbit-Eccentricity Variations in Earth’s axial tilt-Obliquity 3% difference between aphelion and perihelion 20% - 30% maximum Decrease tilt = decrease temperature variation between winter and summer T = ~100,000 years T = ~41,000 years Variations in Earth’s axial wobble-Precession Controls where in the orbit seasons occur Sea-Floor Sediments Organisms at the surface Balance between ocean surface waters and the atmosphere As climate changes so does the composition of the surface organisms Recorded in sediments as the organisms die T = 23,000 - 26,000 years 1 Rates of Sedimentation • Ocean Basins: slow, pelagic sediments – 0.5 – 1.0 cm/ 1000 years – Average Accumulation 500 – 600 m – Thickness depends on age – Oldest sea floor is 200 million years Biogenous Biogenous Siliceous (SiO2) diatoms, radiolaria (photosynthetic) ocean is under-saturated dissolves quicker in warm water Calcareous (CaCO3) cocolithophorids, pteropods, foraminifera dominant pelagic sediment Lysocline = calcium carbonate dissolves CCD = dissolution equals accumulation 4500m Diatoms = cool regions Radiolaria = warmer regions Oxygen Isotope Analysis from Ice Cores Oxygen Isotope Analysis from Ice Cores Isotope = One of two or more atoms having the same atomic number (number of protons) but different mass numbers (protons + neutrons) Precipitation & glacial ice are enriched in 18O 16O Ocean 18O ice ages 16O increases warmer periods ratios Water forms with either 16O 16O Oceans are enriched in 18O evaporates easier cocolithophorids, pteropods, foraminifera = record ratios in their shells (CaCO3) 2 Oxygen Isotope Analysis Temperature Variations 18O more easily evaporated during warm periods Ice cores record the warm periods Pockets of air within the crystal lattice yield gasses (CO2 and CH4), pollen, ash, pollutants Link between CO2 and CH4 concentrations and temperature changes 14 Tsunami (Harbor Wave) Tsunamis Seismic Disturbing force Restoring force Amount of energy in ocean surface Type of wave Gravity disruption Wind landslides Surface tension Gravity Tide Tsunami Seiche Wind wave Capillary wave (ripple) 2412 hr.hr. 100,000 sec 10,000 sec 1,000 sec (1 1/4 days) (3 hr) (17 min) 100 sec 10 sec 1 sec 1/10 sec 1/100 sec 1 10 100 Period (time, in seconds for two successive wave crests to Frequency (waves pass a fixed point) per second) • • Displacement of a large volume of water Shallow water waves Long wavelength Long period (few minutes to over an hr) http://www.seed.slb.com/en/scictr/watch/living_planet/tsunami.htm Satellite images of a coastal village in Banda Aceh, Indonesia, before and after the December 26, 2004 tsunami. 3 Plates Rigid Slabs of Rock Seven major plates – Pacific, African, Eurasian, North American, Antarctic, South American, Australian Minor plates – Nazca, Indian, Arabian, Philippine, Caribbean, Cocos, Scotia, Juan de Fuca 20 Density is a key concept for understanding the structure of Earth – differences in density lead to stratification (layers). Waves associated with earthquakes: P waves – Primary, compressional, arrive first, pass through solid, liquid and gas Density measures the mass per unit volume of a substance. S waves – Secondary, ‘side-to-side’ or shear waves, arrive second, cannot pass through liquid, pass through solid Density = _Mass_ Volume Water has a density of 1 g/cm3 Granite Rock is 2.7 g/cm3 Seismograph P S Mantle 21 Earth’s Crust: cold, brittle Focus 0 10 P 20 Minutes 30 S Core 40 50 Surface waves 22 Lithosphere & Asthenosphere: More detailed description of Earth’s layered structure according to mechanical behavior of rocks, which ranges from very rigid to deformable thin layer, 0.4% of Earth’s mass and 1% of its volume Continental Crust: 1. lithosphere: rigid surface (100 – 200 km) shell that includes upper mantle and crust (here is where ‘plate tectonics’ work), cool layer •Primarily granitic type rock (Na, K, Al, SiO2) •40 km thick on average •Relatively light, 2.7 g/cm3 Oceanic Crust •Primarily basaltic (Fe, Mg, Ca, low SiO2) •7 km thick •Relatively dense, 2.9 g/cm3 cool, solid crust and upper (rigid) mantle “float” and move over hotter, deformable lower mantle 23 2. asthenosphere: layer below lithosphere, part of the mantle, weak and deformable (ductile, deforms as plates move), partial melting of material happens here, hotter layer (200 – 400 km) 24 4 Evidence for Seafloor Spreading Synthesis of Continental Drift and Seafloor Spreading --> Theory of Plate Tectonics Main points of theory (Wilson, 1965): • • • • Earthquake epicenters Heat flow Ocean Sediments Radiometric dating of rocks of ocean and continental crust • Magnetism Earth’s outer layer is divided into lithospheric plate Earth’s plates float on the asthenosphere Plate movement is powered by convection currents in the asthenosphere seafloor spreading, and the downward pull of a descending plate’s leading edge. Hess and Dietz in 1960 proposed a model to explain features of ocean floor and of continental motion powered by heat mantle convection 25 26 Divergent plate boundary marked by mid-ocean ridge Transform fault (spreading center) Oceanic lithosphere Subduction fueling volcanoes Asia Convergent plate boundary marked by trench Asthenosphere Model of Mantle Convection spreading centers – where new sea floor and oceanic lithosphere form subduction zones – where old oceanic lithosphere descends Africa Descending plate pulled down by gravity Philippine Trench Mantle upwelling Superplume Outer core Mariana Trench Mantle Mid-Atlantic Ridge Inner core Hot South America Cold Possible convection cells Rapid convection at hot spots Peru–Chile Trench Hawaii East Pacific Rise 27 Improved Mapping, WWII Age and thickness of sea floor sediment – heat flow Earthquake Epicenters Shallow epicenters – crustal movement (less than 100 km) 29 28 Mid-deep epicenters subduction (greater than 100 km) 30 5 The Earth’s Magnetic Field • Rocks record the direction of magnetic field (Magnetite) • Magnetic field direction changes through geologic time – polar reversals recorded in rocks * 560 °C = rock solidifies (Curie Point) * Captures magnetic signature • Particles of Magnetite align with the direction of Earth’s magnetic field at the time of rock formation 31 The patterns of paleomagnetism support plate tectonic theory. The molten rocks at the spreading center take on the polarity of the planet while they are cooling. When Earth’s polarity reverses, the polarity of newly formed rock changes. (a) When scientists conducted a magnetic survey of a spreading center, the Mid-Atlantic Ridge, they found bands of weaker and stronger magnetic fields frozen in the rocks. (b) The molten rocks forming at the spreading center take on the polarity of the planet when they are cooling and then move slowly in both directions from the center. When Earth’s magnetic field reverses, the polarity of new-formed rocks changes, creating symmetrical 32 bands of opposite polarity As plates float on the deformable aesthenosphere, they interact among each other. The result of these interactions is the existence of 3 types of boundaries: (a) Divergent: plates move away from each other, examples: * Divergent oceanic crust: the Mid-Atlantic Ridge * Divergent continental crust: the Rift Valley of East Africa (b) Convergent: plates move toward each other. Three possible combinations: continent-ocean, ocean-ocean, continent-continent (c) Transform: neither (a) nor (b), plates slide past one another – transform faults. * Example: San Andreas fault 33 Oceans are created along divergent boundaries 34 Mid Atlantic Ridge South Indian Ridge Recall that seafloor spreading was an idea proposed in 1960 to explain the features of the ocean floor. It explained the development of the seafloor at the Mid-Atlantic Ridge. Convection currents in the mantle were proposed as the force that caused the ocean to grow and the continents to move. The breakdown of Pangea showing spreading centers and mid-ocean ridges 2 kinds of plate divergences 35 36 6 Modern divergence East African Rift System East African Rift System 37 38 Supercontinent Pangaea Pannotia/Gondwanaland Rodinia Nuna Wilson Cycles Formation (Ma) 350 650 900 1800 39 Island Arcs Form, Continents Collide, and Crust Recycles at Convergent Plate Boundaries Breakup (Ma) 250 550 760 1500 Mode Ref. Atlantic (4) Pacific (4, 5) Pacific (4, 6, 39) Atlantic? (4) 40 Convergent Plate Boundaries • Continent – Ocean Convergent Plate Boundaries - Regions where plates are pushing together can be further classified as: • Oceanic crust toward continental crust the west coast of South America. • Oceanic crust toward oceanic crust occurring in the northern Pacific. • Continental crust toward continental crust – one example is the Himalayas. • Ocean – Ocean 3 kinds of plate convergences 41 • Continent – Continent 42 7 • Continent – Ocean • Mount St. Helens Continent – Ocean West Coast of South America 43 Island Arcs Form, Continents Collide, and Crust Recycles at Convergent Plate Boundaries The formation of an island arc along a trench as two oceanic plates converge. The volcanic islands form as masses of magma reach the seafloor. The Japanese islands were formed in this way. 44 Convergent Plate Boundaries Ocean-Ocean Aleutian Islands, Alaska Motion of the plates: Mechanisms – not fully understood Rates: average 5 cm/year Mid-Atlantic Ridge = 2.5 – 3.0 cm/yr East-Pacific Rise = 8.0 – 13.0 cm/yr 45 46 Plate Movement above Mantle Plumes and Hot Spots Provides Evidence of Plate Tectonics Ocean – Ocean Caribbean Islands (See also Figure 3.33 on page 89 of textbook) 47 Formation of a volcanic island chain as an oceanic plate moves over a stationary mantle plume and hot spot. In this example, showing the formation of the Hawai’ian Islands, Loihi is such a newly forming island. 48 8 Chapter 3: Summary Keep in mind that the important points in this chapter are: 1. 2. 3. 4. 5. 6. 7. 8. Internal Layers: inner core, outer core, mantle, crust (continental and oceanic). P and S waves – used to study Earth’s layered structure Lithosphere and Asthenosphere – defined according to mechanical behavior of rocks Isostasy – pressure balance between overlying crust and astheosphere deformation Continental drift – plates/continents moving about surface; deduced from definitive evidence: ridges, rise, trench system, seafloor spreading, spreading centers, subduction zones Evidence of crustal motion: earthquakes epicenter, heat flow, radiometric dating, magnetism Plate Tectonics – 7-8 major plates, 3 types of plate boundaries Convergent Plate Boundaries – ocean-continent, ocean-ocean, continent-continent 49 Chapter 3: Key Concepts Some seismic waves–energy associated with earthquakes–can pass through Earth. Analysis of how these waves are changed, and the time required for their passage, has told researchers much about conditions inside Earth. Earth is composed of concentric spherical layers, with the least dense layer on the outside and the most dense as the core. The lithosphere, the outermost solid shell that includes the crust, floats on the hot, deformable asthenosphere. The mantle is the largest of the layers. Large regions of Earth’s continents are held above sea level by isostatic equilibrium, a process analogous to a ship floating in water. Plate motion is driven by slow convection (heat-generated) currents flowing in the mantle. Most of the heat that drives the plates is generated by the decay of radioactive elements within Earth. 50 Chapter 3: Key Concepts Plate tectonics theory suggests that Earth’s surface is not a static arrangement of continents and ocean, but a dynamic mosaic of jostling segments called lithospheric plates. The plates have collided, moved apart, and slipped past one another since Earth’s crust first solidified. The confirmation of plate tectonics rests on diverse scientific studies from many disciplines. Among the most convincing is the study of paleomagnetism, the orientation of Earth’s magnetic field frozen into rock as it solidifies. Most of the large-scale features seen at Earth’s surface may be explained by the interactions of plate tectonics. Plate tectonics also explains why our ancient planet has surprisingly young seafloors, the oldest of which is only as old as the dinosaurs – that is, about 1/23 of the age of Earth. 51 9