Marine Geology

A. Mid-ocean ridges

1. New ocean crust forms at mid-ocean ridges

2. New ocean crust has very little sediment cover.

3. In time the ocean crust moves away from the mid-ocean ridges

B.  Deep-sea trenches

1.  Ocean crust is destroyed at deep-sea trenches through subduction

<>          2.  Subduction is the process where ocean crust gets shoved into the mantle.

C. Fracture zones cut mid-ocean ridges perpendicularly

1. Fracture zones are usually strike-slip faults, sometimes called transform faults.

2. They form because of variable stress on different parts of the mid-ocean ridge; i.e., different segments of the mid-ocean ridge may spread at different rates.

D. Abyssal plains are the flat ocean floors situated between the mid-ocean ridges and the plate boundaries (edge of the continent).

1. The abyssal plain lies at a lower depth than the mid-ocean ridges because they have cooled over time as the crust moved away from the ridge. (heat usually causes expansion, cooling usually causes a slight reduction in size)

2. The older the ocean crust, the more opportunity that sediment has had to accumulate on it.

E. Seamounts are mountains that rise from the ocean floor because of volcanic activity.

1. Often seamounts rise above sea level to form islands

2. When volcanic activity around a seamount ceases, it begins to erode. If erosion continues long enough, the island will completely erode away, and an undersea, flat-topped seamount called a guyot will result.

3. Seamounts usually form above hot spots or magma plumes, like Hawaii.

4. As ocean crust travels over a hot spot, it produces successive seamounts that form an island chain, or aseismic ridge. They are called aseismic ridges because they don't produce much earthquake activity.

A. Passive margins (like the east coast of North America) have very little seismic activity.

1. Continental shelves are submarine platforms that form at the edge of the continent. Geologically, continental shelves are part of the continental crust and not the ocean crust.

2. At the edge of the continental shelf, the very edge of the continent drops down to the oceanic crust. This drop off is the continental slope.

3. The continental rise is the wedge of sediment that is shed off the continent and continental shelf onto the oceanic crust.

B. Active margins (like the west coast of South America and North America) have quite a lot of seismic activity.

1. Active margins usually have oceanic trenches running parallel to the continent.

2. There is usually a narrower continental shelf than is found in passive margins, and little or no continental rise.

A. Submarine canyons are valleys that cut across the continental shelf to the continental slope.

1. Rivers flowing from the continent may form currents on the shelf that play a role in eroding sediment to form the canyon.

2. Submarine fans form at the base of the continental rise where most submarine canyons end.

B. Turbidity currents are massive, submarine landslides that generate strong currents. They occur often in submarine canyons or at the continental slope. They are sudden and catastrophic events.

1. They are often triggered by earthquakes (see Grand Banks incident, pg. 401).

2. Turbidity currents can be massive in scale, travelling up to 60 km/hr, travelling up to 700 km from the source

C. Reefs are wave-resistant marine communities that build complex and extensive structures in clear, shallow marine water.

1. Fringing reefs attach directly to the dry land

2. Barrier reefs form offshore, separated from the coast by a calm lagoon. Barrier reefs lie parallel to the coast.

3. Atolls are circular reefs that usually build up on guyots (volcanic islands that have eroded to below sea level). The book discusses how fringing reefs around a volcanic islands evolve to barrier reefs and atolls (see pg. 408).