What Lies Beneath Pre Solar Nebula—4.6 Billion yrs ago Protoplanetary Disk—50 million years later Formation of the moon Return of the Volatiles Comets and Asteroids -lingering remnants of planetary formation Oceans and Basalt • Oceans cover 71% of Earth’s surface – 5 km deep on average, up to 11 km in trench If the ocean floor is so deep, how do we study it? Sampling Methods • Collecting sediment/rock directly – Dredge • Large net dragged along ocean floor – Sediment Core • Weighty hollow pipe dropped to ocean floor—sediment sample – Drilling • Cylindrical cores of sediment/rock Remote Sensing • Remote “sampling” – Magnetometers • Instrument that measures a magnetic field • Magnetic stripes – Echo sounders • Sound signal from a ship—deeper stuff=longer time to return to ship Deep-sea Submersibles Features of the Ocean Floor Mid-Ocean Ridge System Continuous, submarine mtn chain 80,000 km long rising an average of 23 km above the surrounding sea floor Rift valley Transform Faults How Oceans Form Life on the Seafloor? • Black smokers – Hot water dissolves materials as it passes through rock – Black color: finegrained metals that precipitate in “cool” ocean water – 400°C sulfurous H2O – Chemosynthesis • Bacteria feed on H2S Hydrothermal vents at a mid-ocean ridge. Ocean Trenches, Island Arcs • Accreted Terranes • Do the continents grow? Seamounts and Guyots • Basaltic seafloor features – Seamount: Submarine mountain, ≥1 km above sea floor created by a hot spot – Guyot: a flat-topped seamount cut by waves Wh Atoll Is It Made Of? • Atoll: Circular coral reef forming a ring of islands around a lagoon – Lagoon: shallow, enclosed water basin – Living coral keep up with rate of sinking “Young” Oceans, Thin Sediments • Why aren’t the seafloors as old as the continents? • Structure of the ocean floor Passive Continental Margins • Contin. Shelf – Sedimentation and Isostasy • Contin. Slope – Thinner cont. crust • Contin. Rise – Apron of debris Carbonate Platforms • In warm areas lacking terrigenous sediment – Thriving reefbuilding organisms – Thick limestone beds accumulate Active Continental Margins • Note thickness of shelf – Why is it only this size?