Learning Objectives

1. The ocean formed from the condensation of gasses released by degassing of the earth as it cooled. Salt content was added from the gasses and chemical weathering (Box 18.1).

2. Continental shelves are shallow, submarine platforms of variable widths inclined very gently seaward covered by young sediments that become finer offshore. Wide shelves may have gravel at their outer edges that was deposited during lower Pleistocene sea levels. Continental slopes are steeply inclined toward ocean depths.

3. Submarine canyons are erosional features that cross continental shelves and slopes caused by a combination of down-canyon sand movement, bottom currents, river erosion during low sea level, and turbidity currents. Abyssal fans are formed of land-derived sediment moved down the submarine canyon by turbidity currents. Turbidity currents were responsible for the 1929 Grand Banks cable breaks, and they produce graded bedding.

4. A passive continental margin includes a continental shelf, slope and rise, and lacks earthquakes, volcanoes, and young mountain belts. The continental rise is a wedge of sediments deposited by turbidity currents (deposits exhibit graded bedding) moving down the continental slope and contour currents (not graded and wedge shaped) moving along the slope. Abyssal plains are exceedingly flat, covered by graded sediments deposited by turbidity currents that cover irregularities on the sea floor.

5. Active continental margins lack continental rises and abyssal plains, but possess oceanic trenches associated with Benioff zones, volcanoes, low heat flow, and negative gravity anomalies.

6. Mid-oceanic ridges have rift valleys along their crests, except for the Pacific Ocean ridge. Other geologic activity includes shallow-focus earthquakes, high heat flow, basaltic eruptions, hydrothermal activity, and exotic organisms.

7. Fracture zones cross and may offset the mid-oceanic ridges. They produce shallow-focus earthquakes and along segments called transform faults.

8. Conical seamounts are extinct volcanoes, sometimes forming islands. Guyots are flat-topped seamounts, cut by wave activity, that have subsided to their present depths. Aseismic ridges link guyots and seamounts on the sea floor.

9. Reefs are wave-resistant ridges formed by coral and other calcareous organisms. Three types include: fringing - attached directly to shore, barrier - separated from land by a lagoon, and atoll - circular reefs that rim lagoons that form around subsiding volcanoes.

10. Terrigenous sediments on the sea floor were derived from land and deposited by turbidity and/or contour currents. Pelagic sediments are clays and fine-grained skeletons of microorganisms that settle through the ocean water. Pelagic sediments are absent from mid-oceanic ridges.

11. Oceanic crust is divided into three layers: 1 - marine sediment, 2 - pillow basalt overlying sheeted dikes, 3- thought to be sill-like gabbro, but unsampled by deep drilling at the moment. Ophiolites found in mountain chains exhibit a similar three-layered sequence. They are interpreted as sea floor from marginal ocean basins, but are not typical ocean floor.

12. Rocks of the sea floor are younger than 200 million years, in contrast to continental crust that is 3-4 billion years old.