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