PowerLecture A Microsoft® PowerPoint® Link Tool for academic.cengage/com/earthsci Essentials of Physical Geology 5 th Edition Reed Wicander | James S. Monroe

Chapter 9 The Seafloor

Introduction  The seafloor makes up the largest part of the Earth’s surface. Fig. 9.2, p. 225

Exploring the Oceans Early Exploration - Scientific study of the ocean basins began in the late 1700’s.  Scientist discovered that the ocean was not flat, but had a varied topography like the land.  Today, research ships investigate the sea floor by drilling, echo sounding, and seismic profiling. Fig. 9.4 a-b, p. 226

Exploring the Oceans  How Are Oceans Explored Today?  Echo sounding  In echo sounding, ocean depth is calculated  a) from the speed of sound in water and  b) the time required for sound waves to travel from their source at the surface to the seafloor and back to a detector on the surface. Fig. 9.3, p. 226

Exploring the Oceans  How Are Oceans Explored Today?  Seismic Profiling  In seismic profiling, the energy generated at the source reflects back to the surface where it is recorded by hydrophones floating in the water. Fig. 9.3, p. 226

Exploring the Oceans  How Are Oceans Explored Today?  Research vessels and submersibles  Drill the ocean floor  Collect samples Fig. 9.4 a-b, p. 226

 Although the oceanic crust has never been completely penetrated by drilling, drill core samples, and direct observations from submersibles have enabled scientists to determine its structure and composition.  Ophiolites, fragments of oceanic crust now on land, reveal what an ideal oceanic crust would consist of.  Peridotite, often metamorphosed to serpentinite, at the base  Overlain by layered gabbro, then massive gabbro.  Overlain by sheeted basaltic dikes, then pillow basalts  Finally covered with a thin veneer of deep sea sediments. Oceanic Crust – Its Structure and Composition Fig. 9.6 p. 227

The Continental Margins  Continental margins lying below sea level separate the continents from the ocean basins.  The continental margins include the continental shelf, continental slope, and in some places a continental rise. Fig. 9.7, p. 228

The Continental Margins  Continental Shelf  Continental shelves are the gently sloping portions of the continents existing at relatively shallow depths, and varying in width from ten's of meters to more than 1,000 kilometers. Fig. 9.7, p. 228

The Continental Margins  Continental Slope and Rise  The continental slope starts at an average depth of 135 m and is located where the slope of the sea floor increases from less than 1° to several degrees. Fig. 9.7, p. 228 Fig. 9.8 a, p. 229

The Continental Margins Submarine Canyons, Turbidity Currents, and Submarine Fans  Submarine canyons are typical features of the continental slope, but commonly extend into the shelf.  Some may have formed by stream erosion during the Pleistocene when sea level was lower, but others, not associated with landward streams, were probably eroded by turbidity currents. Fig. 9.8 a, p. 229

The Continental Margins  Submarine Canyons, Turbidity Currents, and Submarine Fans  The continental rise is typified by the overlapping deposits of submarine fans composed of turbidity current-transported sediments. Fig. 9.8a and b, p. 229

The Continental Margins  Types of Continental Margins  Active Continental Margins  A narrow shelf and a slope descending directly into an ocean trench characterize active continental margins. These margins are also characterized by volcanism and earthquakes. Fig. 9.9a, p. 230

The Continental Margins  Types of Continental Margins  Passive continental margins  Passive continental margins lack volcanism and have little seismic activity.  These margins usually have a wide continental shelf, and a slope which merges with a continental rise. Abyssal plains commonly extend seaward of the continental rise. Fig. 9.9b, p. 230

What Features Are Found in The Deep-Ocean Basins?  The seafloor is at an average depth of 3.8 kilometers. It is has been explored at great depths with submersibles.  Out of the photic zone – completely dark  Temperatures near freezing  Pressure 200 – 1000 atmospheres Fig. 9.10, p. 231

What Features Are Found in The Deep-Ocean Basins?  Abyssal Plains  The abyssal plains are the flattest place on Earth!  Deep sea sediments form a thin veneer over the surface  Common in the Atlantic Ocean Fig. 9.10, p. 231 Fig. 9.11, p. 231

What Features Are Found in The Deep-Ocean Basins?  Oceanic trenches are the surface expressions of subduction zones.  They are long, narrow, features which reach the greatest oceanic depths  Characterized by low heat flow and frequent seismic activity. Fig. 9.10, p. 231

What Features Are Found in The Deep-Ocean Basins?  Oceanic ridges  Oceanic ridges are long, continuous submarine mountain ranges composed of volcanic rock which has been extruded on the sea floor. Fig. 9.10, p. 231  Oceanic ridges nearly encircle the world but are offset at intervals by large fracture zones.

What Features Are Found in The Deep-Ocean Basins?  Submarine Hydrothermal Vents  These vents on the seafloor at or very near spreading ridges are where circulating water is heated to over 400°C and discharged into seawater.  As the hot water circulates through the oceanic crust it dissolves out metals that discharge from plumes, called black smokers, on the seafloor, creating rich mineral deposits Fig b-c, p. 233

What Features Are Found in The Deep-Ocean Basins?  Seafloor Fractures - also called transform faults  Offset the oceanic ridges at right angles to the ridge axis.  Hundreds of kilometers long  Buried beneath ocean sediments further from the ridges.  Sites of shallow earthquake activity Fig. 9.13, p. 234

What Features Are Found in The Deep-Ocean Basins?  Seamounts, Guyots and Aseismic Ridges  Seamounts and guyots are common on the sea floor.  Seamounts are remnants of extinct oceanic volcanoes. Many rise more than a kilometer from the ocean floor. They form as the oceanic crust moves over a hot mantle plume.  Guyots have the same origin but have flat tops created when the ocean volcano sinks to sea level. Once the top is near sea level, erosion by waves flattens the top. Fig. 9.14, p. 235

What Features Are Found in The Deep-Ocean Basins?  Seamounts, Guyots and Aseismic Ridges  Aseismic ridges are long, narrow ridges and broad plateaus that rise off the seafloor.  Lack seismic activity  Commonly form near hot-spot volcanoes, consisting of seamounts and guyots extending away from spreading ridges. Fig. 9.14, p. 235

Sedimentation and Sediments on the Deep Seafloor  Deep sea sediments are fine grained  Sediments of the deep sea consist largely of the  carbonate and siliceous skeletons of microscopic organisms (ooze)  pelagic clays which are derived from continents and oceanic islands. Mostly wind-blown dust and volcanic ash. Fig. 9.16b-d, p. 238

Sedimentation and Sediments on the Deep Seafloor  Sediments on the Deep Sea Floor Fig. 9.16, p. 238

Reefs  Reefs are wave resistant structures built by the skeletons of corals and other marine organisms.  Fringing, barrier, and atoll reefs form sequentially as the volcano sinks below sea level.  Reefs will grow rapidly to stay in the photic zone Geo-inSight 1., p. 236

Geo-inSight 2-3., 6., p Barrier Reef

Geo-inSight 4-5.,, p. 237 Atoll reef  Atolls form as the extinct volcano sinks below sea level. Many of these are found in tropical seas of the eastern Pacific where reefs fringe the volcano. As the guyot sinks reef growth continues, creating a circular reef with a central lagoon.

Resources from the Oceans The Exclusive Economic Zone  The United States claims all rights to the natural resources within 200 nautical miles of its coastline, including petroleum, natural gas, gravel and various metals. Fig. 9.17, p. 239 Fig. 9.18, p. 239

Resources from the Oceans  Methane hydrate - energy for the future?  Single methane molecules bound up in ice crystals are found in huge quantities in sediments of the continental margins.  The amount of carbon in methane hydrate exceeds that in all coal, oil, and natural gas, so if a technology is developed to recover it economically, it may become an energy source.  Problem: may contribute to global warming

Resources from the Oceans  Other minerals  Manganese nodules – contain manganese plus copper, nickel and cobalt  Massive sulfides – formed at hydrothermal vents – contain iron, copper, zinc and other metals  Phosphorite – found in shallow water, used for fertilizer Fig. 9.15, p. 235 Fig. 9.12c, p. 233

End of Chapter 9