| Passive and active plate margins | | | | The Mid Atlantic Ridge is a classical example of |
| Plate margins can be classified as either passive or | | | | this. Destruction of the oceanic lithosphere occurs |
| active. Active plate margins can be further divided | | | | in the subduction zones. The subducted plate |
| into constructive or destructive. Let us focus on | | | | descends into the hot mantle and is destroyed as |
| the passive plate margins first. Perhaps a better | | | | it melts. The coast of Japan offers an example of |
| term for this is conservative plate margins where | | | | this. |
| the plates simply slide alongside each other. | | | | Why is there no ocean floor older than 160Ma. |
| There is no destruction of the ocean floor | | | | How is it possible to date ocean crust? |
| involved here. | | | | There is no ocean floor older than 160Ma for |
| The San Andreas Fault provides us with a good | | | | several main reasons: a) The process of |
| example of this kind of margin. The term | | | | subduction destroys the ocean floor. This has |
| "passive" may be a slight misnomer in that there | | | | already been discussed. As new ocean floor is |
| is earthquake activity and the formation of | | | | formed it pushes the floor on either side away |
| transform faults and fracture zones. However, as | | | | and this may eventually enter a subduction zone |
| stated there is no destruction or formation of | | | | and be destroyed. b) A related process is the |
| ocean floor involved here. The active plate | | | | fact that all oceans seem to go through a life |
| margins can be divided into a) constructive plate | | | | cycle of approximately 200 million years . The life |
| margins and b) destructive plate margins. a) This | | | | cycle starts with embryonic ocean formation such |
| is where there is a separation of the plates. | | | | as we see in Africa's Rift Valley, advances |
| Magma rises and can create features such as the | | | | through young, mature and declining phases ( the |
| Mid Atlantic Ridge. b) Destructive plate margins | | | | Pacific destructive plate margins illustrate this |
| are areas where subduction is occurring. The | | | | phase for example) and finally enters the terminal |
| heavier oceanic plate slides under the lighter | | | | phase, such as we see in the Mediterranean Sea. |
| continental plates and the ocean floor is being | | | | A relict scar could also be considered part of this |
| consumed essentially in the Earth's mantle. For | | | | life story of ocean birth and death. Since the |
| example the Pacific Ocean is surrounded by the | | | | whole process takes approximately 200 million |
| "Ring of Fire" subduction zone. | | | | years it may explain why there is no ocean floor |
| Ocean lithosphere, its formation and eventual | | | | older than 160Ma as in the embryonic phase and |
| destruction. | | | | relict phase we are not really dealing with oceans |
| The Ocean lithosphere is approximately 100km | | | | in fact. It is possible to date the ocean crust as |
| thick ( therefore significantly thinner than the | | | | the plates move apart and spread over the |
| continental lithosphere) and this refers to the | | | | abyssal plain as they take on the polarity of the |
| crust and the upper part of the mantle. The | | | | Earth's magnetic field. This work was described by |
| lithosphere is composed mainly of peridotite. The | | | | Matthews and Vine. Also generally speaking the |
| upper part of the lithosphere is the crust which is | | | | older the ocean crust the further away from the |
| made up mainly of lighter granitic rock. The | | | | spreading ridges it will be. The denser material also |
| oceanic crust is thinner and denser than the | | | | sinks further away from the surface of the sea . |
| continental crust and made up mainly of basaltic | | | | Given the age/depth relation the age of the |
| rock. The entire lithosphere (oceanic and | | | | ocean crust can also be estimated. |
| continental) sits on top of the viscous lower layer | | | | The main features of an ocean basin |
| called the asthenosphere which forms part of the | | | | The main features of ocean basins are: 1) A Mid |
| upper mantle. The lithosphere is composed of 7 | | | | Ocean Ridge 2) The abyssal plain on either side of |
| major plates and 6 minor ones. New oceanic | | | | this ridge 3) Constructive plate margins 4) |
| lithosphere , or at least the oceanic crust, is | | | | Destructive plate margins with a deep ocean |
| formed at constructive plate boundaries. At sea | | | | trench, 5) Pelagic sediments 6) Sea Mounts and |
| floor spreading ridges the asthenosphere wells up | | | | submarine volcanoes. 7) Oceanic Islands e.g. Mona |
| and cools and forms the oceanic floor on either | | | | Loa 8) Guyots Not all of the above necessarily |
| side of the boundary. | | | | occur in one ocean basin. |