EAS/BIOEE 154 Lecture 5 Introduction to Oceanography Collisions, Plumes & Sedimentary Processes Subduction, Water, and Volcanism Hydrothermal activity at mid-ocean ridges hydrates oceanic crust. Oceanic crust dehydrates during subduction as a consequence of increasing temperature and pressure. Water released causes the overlying mantle to melt. Subduction zone volcanoes erupt much more explosively than most other volcanoes. Release of dissolved water drives these explosions. Hot Spots and Oceanic Islands Some volcanoes, like the Hawaiian ones, occur in the interior of plates; something, other than plate tectonics, must be causing them. These volcanoes almost always occur in linear chains The Mantle Plume Theory Plumes are convection currents rising from the deep mantle Carry heat from the deep Earth to the surface Rising plumes are initially all-solid, then begin to partially melt in the upper 100 to 200 km. As plates move over the plume, a chain of volcanoes is created. Mantle Plume Initiation When the start, plumes are thought to have large heads When the plume head reaches the surface, it melts, producing massive outpours of lavas, producing oceanic plateaus and flood basalts. Atolls and their formation Atolls are (roughly) circular coral reefs with an empty lagoon in the center. Darwin’s Theory of Atoll formation Fringing reef forms around oceanic volcano Volcano get smaller because of erosion and thermal contraction as it cools Fringing reef becomes barrier reef and, when the volcano sinks beneath the surface, an atoll Sediments: History Book of the Planet Examples of what marine sediments record: Movement of plates Ocean currents and their changes Changing Climates Mountain building and continent collision Evolution of Life Asteroid Impacts Example: The K-T boundary impact and mass extinction Dinosaurs had reigned the Earth for 150 million years (Ma), then suddenly disappeared 65 Ma ago. What happened? The Record Preserved in Marine Sediments . The Cretaceous-Tertiary Boundary marked in many regions by a boundary clay - fossils, etc. absent. – layer found in drill cores EAS/BIOEE 154 Lecture 5 throughout the world . Iridium (Ir) Anomalies in the Boundary Clay . Soot (from mass forest fires) . “tektites” – impact melts . shocked quartz . Eventually, an impact crater of this age was found at Chicxulub, Yucatan Peninsula Sedimentary Cycle Erosion begins with Weathering Mechanical - Action of wind, water, freeze & thaw cycles that break rock. Chemical - Chemical reaction between water and rock. Chemical Weathering Produces two products: Clay minerals and ions These ultimately becomes the sediments of the oceans Transport Agents Wind Water Ice Effect of Transport Sorting: Particle size of particles become more uniform Rounding: Individual particles become more rounded, as rough edges are broken off. Some Study Questions What causes melting to occur in subduction zones? Why do subduction zone volcanoes, such as Pinatubo, Krakatau, or Mt. St. Helens, erupt so much more explosively than other volcanoes? Explain the difference between the volcanic “island arcs” that form along subduction zones and “island chains” that form in the middle of ocean basins. Give one example of an island arc, and one example of an island chain. Explain how mantle plumes produce island chains. Why are mantle plumes thought to initial with large ‘heads’? Darwin explain the origin of atolls in terms of volcanoes sinking beneath sea-level. At the time, he could not explain why volcanoes sink. Can you? What effect did the collision between India and Asia have on sediments in the Indian Ocean? Why were high concentrations of Ir in marine sediments seen as evidence of an impact 65 million years ago? .