Transcript Ocean Floor
Exploring the inner and outer shells of earth Earth consists of a series of concentric layers or spheres which differ in chemistry and physical properties. Chemical Layers Physical Layers Physical state is determined by the combined effects of pressure and temperature. • Increasing pressure raises the melting point of a material. • Increasing temperature provides additional energy to the atoms and molecules of matter allowing them to move farther apart, eventually causing the material to melt. • Both pressure and temperature increase toward the center of the Earth, but at variable rates. How do we learn about the inner structure of the planet? Seismology is the study of elastic waves that travel through the earth Two main wave types: Compression waves (Pwaves): travel by squeezing and expanding medium they travel through. They can travel through both solids and liquids (e.g., sound waves). Shear waves (S-waves): travel by shearing medium they pass through. S-waves can travel only through solids since particles need to be bonded to each other to propagate wave (similar to a bull whip) Earthquake are an incredible source of seismic waves • Seismic waves travel along there quickest route, generally through, the planetary interior to the seismic stations, changing speed every time material properties change. Reconstructing the internal structure of the planet • Combining information from many seismic waves we can get a detailed internal structure of the earth. • Crust & upper mantle: large increases in seismic velocities and density • Mantle: Gradual velocity and density increase • Outer core: dramatic density increase and no S-waves • Inner core: Jump in density and P-wave velocity, S-waves return Other planets? Seismic Velocity and Density solid solid, more dense Liquid Iron solid Iron Earthquakes produce waves in the ocean Sea level: RISE FALL Courtesy: K. Satake, unpublished World Seismicity (1898-2003) Mw ≥ 6.0 Mw ≥ 7.7 Many large earthquakes along subduction zonesevents Most “Great” earthquakesoccur are subduction mega-thrust The Physiography of the North Atlantic Ocean Floor continental margins deep ocean basins midoceanic ridges Type of continental margins Continental margins are the submerged edges of the continents and consist of massive wedges of sediment eroded from the land and deposited along the continental edge. The Continental Margin can be divided into three parts: the Continental shelf, the Continental slope, and the Continental rise. Passive Continental Margin Transport of sediments from the shelf to the continental rise • Deep Ocean Province is between the continental margins and the midoceanic ridge and includes a variety of features from mountainous to flat plains: Abyssal plains, Abyssal hills, Seamounts, and Deep sea trenches. Deep Ocean Basin • Midoceanic Ridge Province consists of a continuous submarine mountain range that covers about one third of the ocean floor and extends for about 60,000 km around the Earth. Midocean Ridge Hydrothermal Vents 03_13a WHY DO LAND AND OCEAN EXIST? Geologic Differences between 2-3 Continents and Ocean Basins Continents and ocean basins differ in composition, elevation and physiographic features. • Elevation of Earth’s surface displays a bimodal distribution with about 29% above sea level and much of the remainder at a depth of 4 to 5 kilometers below sea level. • Continental crust is mainly composed of granite, a light colored, lower density, igneous rock rich in aluminum, silicon and oxygen. • Oceanic crust is composed of basalt, a dark colored, higher density, volcanic rock rich in silicon, oxygen and magnesium. Geologic Differences between Continents and Ocean Basins WHY DO LAND AND OCEAN EXIST? OCEANIC CRUST = THIN AND DENSER CONTINENTAL CRUST = THICK AND LITHER • Continents are thick (30 to 40 km), have low density and rise high above the supporting mantle rocks. • Sea floor is thin (4 to 10 km), has greater density and does not rise as high above the mantle. Oceanic Crust Versus Continental Crust altimetry Satellites in orbit around the planet use radar altimetry to measure the height of the sea level (accuracy of 2 cm). http://www.ecco-group.org/animations_iter21/TP_ps21.mpeg Model of the shape of the Earth geoid: The equipotential surface of the Earth's gravity field which best fits, in a least squares sense, global mean sea level (MSL) http://www.esri.com/news/arcuser/0703/geoid1of3.html Why ocean bathymetry? Ocean Circulation and Climate Heat Transport affected by topography! Ocean Circulation and Climate Ocean Currents take different paths! Ocean Circulation and Climate Deep ocean mixing and pathways Ocean Circulation and Climate Mixing energy and dissipation of tides! Social impacts: Tsunami Social impacts: Gas and oil extraction Evidence of Global Warming in the Climate System: Glacier melting • Kilimanjaro: ice caps are 80% gone since early 1900’s • All glaciers in tropics are melting rapidly • Impacts: water supply, power generation, tourism, local climate and ecology Arctic - the most sensitive ecosystem? • Arctic sea ice has shrunk by 1 million sq km and thinned from 3.1m to 1.8m average – More freshwater, reduced ability to travel over ice – All summer ice gone in this century • Ecological consequences huge! Arctic warming • Sea ice melting – Key feedback! (animation) Reduces albedo (reflectivity) of earth, allowing more radiation to be absorbed http://www.gsfc.nasa.gov/topstory/2003/1023esuice.html • Warming temperature – Melts permafrost – Ice sheet stability? Key sea level question! • Rate of warming 8x faster in last 20 years than in last 100 1979 2003 Arctic ecosystems impacts • Reduced ice: less algal production under ice = undermines base of food chain • Seal pups emerge just when ice is melting - earlier melt means they are exposed before ready to thrive • Caribou need ice to island-hop; they are falling through as ice thins • Polar bears hunt on ice in winter, retreat to land in summer. Less ice forces them onto land earlier Higher sea level… • As water warms, it expands (“thermal expansion”). • Glaciers are melting • Observation: 3mm/yr in past few decades • Prediction: ~0.5m rise by the end of this century, 2-4m in 500 years This will have a major impact on •Developed coastal regions •Low-lying island nations •Intensity of coastal flooding during storm surges •Coastal ecosystems (e.g. mangroves, estuaries) More intense storms… Hurricanes get their energy and staying power from warm water in the tropical oceans. As waters get warmer, we expect that hurricanes will become more intense. Significant change not yet observed. Oceanography and Climate - millenial timescales Vertical circulation of the ocean MOVIE: The Day After Tomorrow Oceanography and Climate Climate changes are defined to occur on different space and time scales. What makes it interesting is that processes happening on different temporal and spatial scales are not separable and interact with each other COMPLEXITY and NONLINEARITY WHY? The ocean has a very long memory. We will learn about this, and about the mechnism of interaction between ocean-atmosphere-landecosystems.