Plate tectonics' explanatory power:

The Galapagos Islands are the visible manifestation of an oceanic hot spot.

Indeed, being near the spreading center between the Cocos and Nazca plates (which has, itself, drifted to the north over the hot spot) their history is somewhat like that of Iceland. The Galapagos hot spot is responsible for both the Cocos and Carnegie Ridges.


What makes plates move? In a nutshell, they are moved by convection in the mantle. This is the process by which material circulates through a region that is unevenly heated. In a tea kettle, for instance:

The tops of convection cells (units of convective circulation) can often be seen in cups of tea or black coffee. Condensing water vapor marks to top of rising columns of warm water. Dark lines separating them marks the location os sinking cooler water.)

In the Earth, convection in ductilely deforming solid rock brings heat from the interior to the surface, powering the movement of plates. Exactly how big the convection cells are and how quickly their material moves is enigmatic.

Even though we don't see this convection directly, we know it happens from thermodynamics: There are two modes of heat transfer:

Suppose the Earth had cooled from conduction only. Thermodynamic calculations show that a given parcel of heat could only have moved only 400 km in 5 g.y. by conduction alone. That means that below 400 km, everything must be molten. That is isn't shows that convection must be at work, also.

It is manifested at the surface by two subsidiary mechanisms. To understand how they work, you must keep this fact firmly in mind:


Hotter solids expand. Colder solids contract.


  • Ridge-push: Sea floor spreading centers, being made of hotter material, tend to stand above the general level of the sea floor. (cuz hotter solids expand) As a result, they create a topographic gradient and tend to want to flow "downhill" across the asthenosphere. In so doing, they pull apart, reducing pressure at the ridge and encouraging hot mantle rocks to melt and erupt as new sea floor.


  • Slab pull: As oceanic crust moves away from the spreading center, it becomes cooler, more dense (cuz colder solids contract), and less bouyant, ready to sink at the slightest provocation. When it encounters a slightly more bouyant plate, it is forced downward and subducts. At a certain threshold of pressure, a minerological change occurs in which basalt (a volcanic igneous rock and the primary constituent of the oceanic crust) is transformed to eclogite, a much denser and more compact material. At this point, it REALLY starts to sink, pulling the parts still on the surface with it.
    Lubrication: The entire process is speeded up by the partial melting that occurs around the slab when the injection of water lowers the melting point. This literally lubricates the subducting plate.

    The net effect is that between rudge push and slab pull, plates readily flow downhill under their own power and actually help drive convection. Currently, we just don't know the relative importance of these two mechanisms, but they are not mutually exclusive.


    Morals of plate tectonics:

    There are some proverbial truths to start thinking about. for plate tectonics to occur, certain conditions must be met:

    An important aside: The continuous melting and volcanism occurring at convergent plate boundaries is the "refinery" at which the lighter materials of continental crust are separated from the mantle and oceanic crust and concentrated in continental masses.


    Consider these comparisons.

    Comparison with other terrestrial planets: