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Midterm Exam I September 26, 2:10 HW714 Questions from the Concept Check (CC) boxes in textbook: ¾ Chapters 1, 2, 3 and 4 ¾ 60 multiple choice questions ¾ this exam constitutes 22% (only) of your total (overall) grade Chapter 1: 2,3,4, 8,11,14,15 - Read about historical developments in oceanography in USA. Chapter 2: 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 and 14. Please Note: 1. I will not be in class for this exam. The exam will be administered by Ms Dana Reimer and her assistant, both are experienced instructors. You should not need them to answer questions beyond clarifications of text at the most. Grades will be available at our next class, October 3rd. 2. Remember to bring pencils!!! (No. 2) and to follow the instructions about writing your names on both exam and answer sheets, as well as to ‘bubble’ your names (no IDs!) on the appropriate place of the answer sheet (on the back). 1 Chapter 3: 1, 2, 4, 5, 7, 8, 9, 10, 11, 12, 13, 16, 17, 18, 19, 20, 21, 23, 24 and 25. Read the textbook discussion of how the theory of plate tectonics developed over time.: Chapter 4: 2, 3, 4, 6, 7, 8, 9, 10, 11, 13, 15, 17 and 20. 2 Relative amount of water in the different reservoirs on Earth’s surface Mid-Atlantic Ridge The average depth of the ocean is 4 ½ times as great as average land elevation (about 840 m or ½ mile). Note the extent of the Pacific Ocean, Earth’ Earth’s most prominent single feature. Ocean’s deepest spot Earth’s highest mountain Pacific Ocean Basin Earth’s largest feature 3 4 1 North Pole Understanding the Ocean Began with Voyaging for Trade and Exploration ¾ Voyaging on water was important to many early civilizations. The 60° N Latitude 30° N Egyptians, Cretans, and Phoenicians were all skilled sailors. ¾ Cartographers, Cartographers, or chart makers, recorded information about locations Latitu de and landmarks and currents. Charts are detailed graphic representations of water and waterwater-related information. 0 ° Equat or ¾ The Library at Alexandria, Alexandria, in Egypt, was founded in the third century B.C. This library stored information on every area of human endeavor. endeavor. ¾ Eratosthenes of Cyrene was the second librarian at Alexandria. He 30° S South Pole was the first to calculate the circumference of Earth. He also invented invented a system of longitude and latitude. Determining Longitudes ¾ The principles of celestial navigation were invented at the Library at Alexandria. 5 6 Formation of the Earth Longitude and Time • Big Bang 13*109 years ago The clock is set to noon when the Sun is at its zenith above the prime meridian • • • ¾ Formation of elementary particles ¾ Gravitational formation of dense regions stars 12*109 years ago First Earth Water 4.6*109 years ago 3.9*109 years ago 109 = 1 billion year Every 15° of longitude correspond to 1 hour 7 • Sources of Water: ¾Mantle rocks ¾Outer space • Earth’s shape – Oblate Spheroid • average radius ~ 6400 km 8 2 Earth, Ocean and Atmosphere accumulated in layers sorted by density The planet grew by the aggregation of particles. Meteors and asteroids bombarded the surface, heating the new planet and adding to its growing mass. At the time, Earth was composed of a homogeneous mixture of materials. How did water and water vapor form on early Earth? ¾ The Sun stripped away Earth’ Earth’s first atmosphere ¾ Gases, including water vapor, released by the process of outgassing, outgassing, replaced the first atmosphere. ¾ Water vapor in the atmosphere condensed into clouds. ¾ After millions of years, the clouds cooled enough for water droplets to form. ¾ Hot rain fell and boiled back into the clouds. ¾ Eventually, the surface cooled enough for water to collect in basins. Earth lost volume because of gravitational compression. High temperatures in the interior turned the inner Earth into a semisolid mass; dense iron (red drops) fell toward the center to form the core, while less dense silicates move outward. Friction generated by this movement heated Earth even more. The result of density stratification: an inner and outer core, a mantle, and the crust. 9 The evolution of our atmosphere Age and Time 1 billion = 1,000,000,000 or 109 Earth is 4.6 * 109 years old Oceans are 4.2 * 109 years old Oldest rocks date from 3.8 * 109 years ago First evidence of life dates from 3.6 * 109 years ago 1 million = 1,000,000 or 106 Ocean and atmosphere reach the state we know today 800 * 106 years ago 100 Concentration of Atmospheric Gases (%) Methane, ammonia 75 Atmosphere unknown initial rise of O2 2.7 b. y. ago – but conclusive evidence is from 2.3 b. y. ago Nitrogen 50 Water 25 Carbon dioxide 0 4.5 Oxygen 3 4 Time (billions of years ago) 2 Early atmosphere quite different from today’s 10 1 Fig. 2-11,11p. 49 12 3 Evidence for layered Earth - Seismic wave: Chapter 3 P waves = Primary Waves • Geologic Structure of Earth • Continental Drift – Seafloor Spreading • Mantle Convection - Plate Tectonics S waves = Secondary Waves Density measures the mass per unit volume of a substance. 13 Lithosphere & Asthenosphere:: More detailed description of Earth’s layered structure according to mechanical behavior of rocks, which ranges from very rigid to deformable 1. lithosphere: rigid surface shell that includes upper mantle and crust (here is where ‘plate tectonics’ work), cool layer 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 (100 – 200 km) (200 – 400 km) 15 The layers: 1. Core: 3500 km thick, average density 13 g/cm3, 30% of Earth’s mass and 16% of its volume ¾Inner core: primarily iron & nickel ¾Outer core: liquid (partially melted) 3. Mantle: Magnesium and iron, less dense, solid but can flow;70% Earth’s mass & 80% of its volume, 2866 km thick, @ Temp of 100-3200°C 4. Crust – rigid, thin outer layer; 0.4% of Earth’s mass and 1% of its volume 14 Earth’ Earth’s outer layer is the Crust: cool, rigid (brittle), thin surface layer – rocks on crust side are chemically different than rocks on mantle side – separation between crust and mantle is called Mohorovičić discontinuity (the Moho) Moho) – two types: 1. Continental Crust: * 40km thick on average * Relatively light 2. Oceanic Crust * 7km thick * Relatively dense Buoyancy: depends on the mass and density of the object and of the liquid in which object floats Isostasy: state of gravitational equilibrium between the lithosphere and asthenosphere such that the tectonic plates “float” at an elevation that depends on their thickness and density. 16 4 Model of Mantle Convection Continental Drift – Seafloor Spreading Evidence from spreading centers – where new sea floor and oceanic lithosphere form subduction zones – where old oceanic lithosphere descends 1. Earthquake epicenters 2. Heat flow *Review Figure* 3. Radiometric dating of rocks of ocean and continental crust *Review Figure* 4. Magnetism ¾ ¾ ¾ 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 Æ mantle convection 17 Earthquake Epicenters Improved Mapping, WWII 18 As plates float on the deformable aesthenosphere, aesthenosphere, they interact among each other. The result of these interactions is the existence of 3 types of boundaries: Shallow epicenters – crustal movement (less than 100 km) Mid-deep epicenters – subduction (greater than 100 km) Plates Æ Rigid Slabs of Rock 19 (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 20 5 Symmetrical pattern around Mid-Ocean Ridges Oceans are created along divergent boundaries 2 kinds of plate divergences 3 kinds of plate convergences 9 9 9 9 Magnetic field direction changes through geologic time and these polar reversals recorded in rocks Motion of the plates: plates: Mechanisms – not fully understood Rates: average 5 cm/year MidMid-Atlantic Ridge = 2.5 – 3.0 cm/yr EastEast-Pacific Rise = 8.0 – 13.0 cm/yr Many discoveries contribute to the theory of plate tectonics but the most compelling evidence comes from The Earth’s Magnetic Field At the Curie Point, 560 °C, rock solidifies and captures magnetic 22 signature 21 Plate Movement above Mantle Plumes and Hot Spots Provides Evidence of Plate Tectonics Chapter 4 Continental Margins and Ocean Basins Bathymetry of Sea Floor Continental Margins and Ocean Basins Submarine Canyons Hydrothermal Vents Trenches Volcanic island chains form as an oceanic plate moves over a stationary mantle plume and hot spot. Example: Hawaiian Islands. 23 24 6 LIDAR: Light Imaging Detection And Ranging Bathymetry: a map of the ocean floor Vw ~= 1500 m/s (Pres, Temp, Salinity) Use SOUND (how?) to map ocean floor Va ~= 344 m/s (Pres, Temp, other) Echo sounding Multibeam Systems Satellite Altimetry Satellite altimetry and use light (how?) to map ocean floor Echo Sounders Bounce Sound off the Seabed Vw ~= 1500 m/s 25 26 Using satellite measurements to map the ocean floor Write three sentences to convey the information summarized in this chart Analysis of Gravitational Anomalies (how?) is used to obtain the bathymetry of ocean floorfloor-ridges, shelves 27 28 7 An active margin Describe this picture and define all items Continental margin Peru–Chile Plate Trench boundary Nazca Plate Submarine canyon profile (cut through continental shelf) Sediment Continental shelf Continental slope Oceanic ridge Continental rise Sediment Continental crust (granitic) Continental crust (granitic) Oceanic crust (basaltic) Asthenosphere Plate movement A passive margin Andes Plate boundary Mountains Broad South America continental shelf Atlantic Ocean South American Deep basin Plate Afr Pacific Ocean Continental margin Deep-ocean basin Narrow continental shelf ican Plate Plate movement Subduction zone (deep and shallow earthquakes) Plate movement Mid-Atlantic Ridge (spreading centers, shallow earthquakes) • Atlantic = Passive Margin; little/no geologic activity Oceanic crust (basaltic) • Pacific = Active Margin; geologic activity Describe this picture and define all items 29 30 The ocean floor can be classified as Continental Margins (a) Continental Margins – the submerged outer edge of a continent (b) Ocean Basin – the deep seafloor beyond the continental margin There are two types of continental Margins (a) passive or trailing margins: margin of continent that moves away from spreading center – Atlantic-style margins (also Artic Ocean, Antarctica and Indian Ocean). Very little volcanic or earthquake activity is associated with passive margins. (b) active or leading margins: plate boundary located along a continental margin – ocean trenches where there is subduction of oceanic lithosphere – narrow, steep, with volcanic mountains (West Coast of the Americas). Active margins are the site of volcanic and earthquake activity. 31 • Region where continental crust meets oceanic crust • Continental Shelf • Shelf Break • Continental Slope • Continental Rise 32 8 Anatomy of a passive margin Continental Margins Continental shelvesÆ shelvesÆ Gently sloping (~0.5 degrees) Depositional environments Average width 65 km (40 miles) Average depth 130 m (430 feet) Narrow(Wide) along Active(Pasive) margins (~ 140 m) Shelf Break Æ Continental Slope Æ Extends from break to ocean basin Steep (3 – 6 degrees, As high as 25 deg. Little/no deposition deep sediments accumulate here, thickness varies Edge of the continental shelf Change in slope Continental Rise Æ Base of the continental slope slope 0.5 – 1 degree Depositional environment 33 Submarine Canyons Form at the Junction between Continental Shelf and Continental Slope 34 Turbidity Currents - Fast moving avalanches of mud and sand scour slopes; form turbidite deposits; 90 km/hr (56 mi/hr) These are features of some continental margins. They cut into the continental shelf and slope, often terminating on the deep-sea floor in a fan-shaped wedge of sediment. 35 36 9 Features of the Sea floor Basins Æ sections of the abyssal plain separated by continental margins, ridges, and rises. Oceanic Ridges Hydrothermal Vents Abyssal Plains and Abyssal Hills Seamounts and Guyots Trenches and Island Arcs Seafloor: Seafloor: 4000 – 6000 m water depth, 30% of the Earth’ Earth’s surface Abyssal Plain: vast, flat plain extending from the base of the continental slope. Ocean Basins: sections of the abyssal plain separated by continental margins, ridges, and rises. 37 Sea floor features 38 FlatFlat-topped seamounts eroded by wave action are called guyots. guyots. Seamounts are volcanic projections from the ocean floor that do not rise above sea level. FlatFlat-topped seamounts eroded by wave action are called guyots Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges. 39 Abyssal hills are flat areas of sedimentsediment-covered ocean floor found between the continental margins and oceanic ridges. Abyssal hills are small, extinct volcanoes or rock intrusions near the oceanic ridges. ridges. 40 10 MidMid-Ocean Ridges and Rises Hydrothermal vents are sites where superheated water containing dissolved minerals and gases escapes through fissures, or vents. Cool water (blue arrows) is heated as it descends toward the hot magma magma chamber, leaching sulfur, iron, copper, zinc, and other materials materials from the surrounding rocks. The heated water (red arrows) returning to to the surface carries these elements upward, discharging them at hydrothermal springs on the seafloor. An oceanic ridge is a mountainous chain of young, basaltic rock at an active spreading center of an ocean. 41 Major ocean trenches Trenches are arcarc-shaped depressions in the ocean floor caused by the subduction of a converging ocean plate. Most trenches are around the edges of the active Pacific. Trenches Trenches are the deepest places in Earth’ Earth’s crust, 3 to 6 kilometers (1.9 to 3.7 miles) deeper than the adjacent basin floor. The ocean’ ocean’s greatest depth is the Mariana Trench where the depth reaches 11,022 meters (36,163 miles) below sea level. 43 level. 42 Chapter 4 - Summary • Bathymetric devices used to study seabed features include multibeam echo sounder systems and satellites that use sensitive radar for altimetry. • Seafloor features result from a combination of tectonic activity and the processes of erosion and deposition. • Near shore, the features of the ocean floor are similar to those of the adjacent continents because they share the same granitic basement. The transition to basalt marks the edge of the continent and divides ocean floors into two major provinces, The submerged outer edge of a continent is called the continental margin. The deepdeep-sea floor beyond the continental margin is called the ocean basin • Features of the continental margins include continental shelves, continental slopes, submarine canyons, and continental rises. • Features of the deepdeep-ocean basins include oceanic ridges, hydrothermal vents, abyssal plains and hills, seamounts, guyots, guyots, trenches, and island arcs. 44 11