The sea floor


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The sea floor

  1. 1. The Water Planet
  2. 2. A. The Geography of the Ocean Basins The oceans cover 71% of the planet and regulate its climate and atmosphereThere are four ocean basins Pacific – the deepest and largest Atlantic Indian Arctic – smallest and shallowestConnected to the main ocean basins are shallow seas e.g. Mediterranean Sea, Gulf of Mexico, South China Sea They all connect to form a world ocean where seawater, materials, and organisms can move about
  3. 3. B. The Structure of the EarthIn the early molten Earth, lighter materials floated toward the surface They cooled to form the crust The atmosphere and oceans then formed Earth is the right distance from the sun for liquid water, and life, to exist1. Internal Structure The dense core is mostly iron Solid inner core and liquid outer core The swirling motions produce the Earth’s magnetic field The mantle is outside the core and under the crust Near molten rock slowly flows like a liquid The crust is the outer layer, comparatively thin Like a skin floating on the mantle
  4. 4. 2. Continental and Oceanic Crusts There are differences in the crust that make up sea floors and continentsa. Ocean crust Made of basalt – a dark mineral More dense Thinner Younger rock; 200 mil yearsb. Continental crust Made of granite – lighter color Less dense Thicker Older rock; 3.8 bil years So continental crust floats high on the mantle and ocean crust floats lower That’s why ocean crust is covered by water
  5. 5. The Origin and Structure of the Ocean Basins The Earth is a world of constant transformation, where even the continents moveA. Early Evidence of Continental Drift 400 years ago Sir Francis Bacon noted the continental coasts of the Atlantic fit together like pieces of a puzzle Later suggested the Americas might have been once joined to Europe and Africa Geologic formations and fossils matched from opposing sides Alfred Wegner gave hypothesis of Continental Drift in 1912 Suggested that all the continents had once been a supercontinent, named Pangea Started breaking up ~180 mil years ago
  6. 6. B. The Theory of Plate Tectonics Could not explain how the continents moved The Theory of Plate Tectonics explains it all Continents do drift slowly around the world
  7. 7. Discovery of the Mid-Ocean Ridge After WWII sonar allowed detailed maps of the sea floor They discovered the mid-ocean ridge system A chain of submarine volcanic mountains that encircle the globe, like seams on a baseball The largest geological feature on Earth Some of the mountains rise above sea level to form islands, e.g. Iceland The mid-Atlantic ridge runs down the center of the Atlantic Ocean and follows the curve of the opposing coastlines Sonar also discovered deep trenches
  8. 8. Significance of the Mid-Ocean Ridge Why are they there? How were they formed? Lots of seismic and volcanic activity around the ridges and trenches Rock near the ridge is young and gets older moving away from the ridge There is little sediment near the ridge, but it gets thicker moving away Found symmetric magnetic bands on either side of the ridge which alternate normal and reversed magnetism
  9. 9. Creation of the Sea Floor Huge pieces of oceanic crust are separating at the mid-ocean ridges Creating cracks called rifts Magma from the mantle rises through the rift forming the ridge The sea floor moves away from the ridge This continuous process is called sea-floor spreading New sea floor is created This explains why rocks are older and sediment is thicker as you move away from the ridge This also explains the magnetic stripes found in the sea floor
  10. 10. Sea-Floor Spreading and Plate Tectonics The crust and part of the upper mantle form the lithosphere 100 km (60 mi) thick, rigid It’s broken into plates May be ocean crust, continent crust, or both The plates float on a fluid layer of the upper mantle called the asthenosphere. At mid-ocean ridges the plates move apart If the plate has continental crust it carries the continent with it Spread 2-18 cm/year This explains continental drift
  11. 11.  As new lithosphere is created, old lithosphere is destroyed somewhere else Some plate boundaries are trenches where one plate sinks below the other back down into the mantle and melts Called subduction Trenches are also called subduction zones The plates colliding can be ocean - continent ¨ Ocean plates always sinks below ¨ Produces earthquakes and volcanic mountain ranges; e.g. Sierra Nevada The plates colliding can be ocean - ocean ¨ Earthquakes and volcanic island arcs; e.g. Aleutian Islands The plates colliding can be cont - cont ¨ Neither plate sinks, instead they buckle ¨ Producing huge mountain ranges; e.g. Himalayas
  12. 12. A third boundary type is shear boundary or transform fault The plates slide past each other Causes earthquakes; e.g. San Andreas Fault Two forces move the plates Slab-Pull theory - the sinking plate pulls the rest behind it Convection theory – the swirling mantle moves the plate
  13. 13. C. Geologic History of the EarthContinental Drift and the Changing Oceans 200 mil years ago all the continents were joined in Pangea It was surrounded by a single ocean called Panthalassa 180 mil years ago a rift formed splitting it into two large continents Laurasia – North America and Eurasia Gondwana – South America, Africa, Antarctica, India, and Australia The plates are still moving today Atlantic ocean is growing, Pacific is shrinking
  14. 14. The Record in the Sediments Two types of marine sediments: Lithogenous – from the weathering of rock on land Biogenous – from skeletons and shells of marine organisms ¨ Mostly composed of calcium carbonate or silica Microfossils tell what organisms lived and past ocean temperatures Climate and Changes in Sea Level The Earth alternates between interglacial (warm) period and ice age (cold) periods Sea level falls during ice ages because water is trapped in glaciers on the continents
  15. 15. The Geological Provinces of the Ocean Two main regions of the sea floor Continental margins – the submerged edge of the continents Deep-sea floorA. Continental Margins Boundaries between the continental and ocean crust Consists of shelf, slope and rise The Continental Shelf The shallowest part Only 8% of the sea floor, but biologically rich and diverse Large submarine canyons can be found here Ends at the shelf break, where it steeply slopes down
  16. 16.  The Continental Slope The edge of the continent Slopes down from the shelf break to the deep-sea floor The Continental Rise Sediment accumulates on the sea floor at the base of the slope Active and Passive Margins Active margin – the subducting plate creates a trench Narrow shelf, steep slope, and little or no rise Steep, rocky shorelines Passive margin – no plate boundary Wide shelf, gradual slope, and thick rise
  17. 17.  Deep-Ocean Basins 10,000-16,000 ft Abyssal plain - flat region of the sea floor Seamounts – submarine volcanoes Guyots – flat-topped seamounts Both were once islands, but now covered with water Trenches – the deepest part of the ocean Mariana Trench is 36,163 ft deep
  18. 18.  The Mid-Ocean Ridge and Hydrothermal Vents At the center of the ridge, where the plates pull apart, is a central rift valley Water seeps down through cracks, gets heated by the mantle, then emerges through hydrothermal vents 350oC (660oF) Dissolved minerals from the mantle, like sulfides, are brought up Black smokers form when minerals solidify around a vent Marine life, including chemosynthesizers, exist around hydrothermal vents