The Mechanism of Plate Motion
It is clear by now that there is movement deep in the earth as evidenced by earthquakes and volcanoes. The question that comes to mind then is “What is moving in the earth’s interior? Why and how is it moving?” These questions prompted a series of scientific geologic studies, and in the 1960’s a plate tectonic theory (plate motion) was formulated (Weil 2006). “In 1965, Tuzo Wilson introduced the term plate for the broken pieces of the Earth’s lithosphere…in 1967, Jason Morgan proposed that the Earth’s surface consists of 12 rigid plates that move relative to each other…in 1967, Xavier Le Pichon published a synthesis showing the location and type of plate boundaries and their direction of movement” (Hawaiian Natural History Association, “The Birth of Plate Tectonics”, 2005).
In order to understand plate motion, it is first necessary to know the physical composition of the earth’s interior, for these elements are all involved in plate motion. Basic knowledge in science revealed that the interior of the earth is made up of layers of mantle and crust. The ones that are in the surface of the earth are the oceanic crust and the continental crust. The oceanic crust is what we call the “ocean floor” and the continental crust is what made up the “continental masses” or lands that we know so well (refer to figure1) (“Theory” 1). From them we have the oceanic plates and the continental plates. These two “crusts” along with the “rigid” upper mantle is what we call the lithosphere. Beneath the lithosphere is the athenosphere, which is made of “plastic rocks under great pressure” (“Theory” 1). In other words, athenosphere area is a very hot place.
Although it is now clear with the geologists regarding the existence and movement of the plates, they are not still sure which exactly causes plate motion (Weil 2006). They are puzzled between two possibilities:
1. Mantle convection currents causes plate motion (Weil 2006)
2. Surface boundary and plate forces causes plate motion (Weil 2006)
Between these two possibilities the geologists are primarily interested in proving or disproving the arguments that plates move only as a response to the athenosphere mantle convection currents , or that the plates themselves are the ones who causes the movement ( due to surrounding forces ) and consequently affecting the athenosphere mantle below(Weil 2006).
Let us now consider the mechanism of plate motion in detail by discussing the two possibilities. In the first possibility, convection (release of heat by boiling) currents in the athenosphere are thought to be the cause of plate motion (“Theory” 2). Most geologists consider this phenomenon as the most likely cause of plate motions. As stated earlier, the area in the athenosphere is very hot and the molten rocks inside become less dense so that it rises up towards the edge of the athenosphere. As it rises, it left a void behind it so that the surrounding molten rocks move to fill the void. But when the other molten rocks fill the void, it also left its own corresponding void so that the other rocks or elements on the side move to fill the avoid and so on until the first molten rocks that move up does the filling ( for it then cools and sink back) completing a circular process known as “convection cell”. The convection cell movement creates convection currents that naturally drive the plates in motion for it sits on top of the lithosphere rigid mantle crust that is just above the athenosphere ( Strickler 1997).
The next possibility involves surface boundary and plate forces. These forces are thought to be strong enough so as to make it able to move a wide variety of plate sizes. These forces may move the plates slowly usually with observable effect ranging from tens of millions years or the force may cause a sudden movement causing earthquakes. Identified plate forces are “ridge push, slab pull, trench suction, collisional resistance and basal drag” (see figure 2) (Weil 2006).
Ridge push may exhibit itself as either a boundary or a body force. The body force is horizontal force acting on the ocean floor as a direct effect of the “cooling and thickening of the oceanic lithosphere with age” (Weil 2006). On the other hand, Bott (1993) stated that the Ridge Push boundary force, is caused by the “gravity wedging” effect when hot, “buoyant” mantle from below the ridge crest rises up to cause horizontal pressure( qtd in Weil 2006 ). In this case, the effect of the force is limited at the edge of the lithospheric plate, felt only in the area covering the length of the ridge (Weil 2006).
The Slab Pull forces are found in the subduction (depression) zone. Subduction zones are created when the more dense oceanic crust collides with the less dense continental crust so that the former sinks forming a subducting slab below. According to Capple and Tullis (1977), the slab pull force is the force that pulls the slab deeper and is “dependent on the angle, temperature, age and volume of the subducting slab…” (qtd in Weil 2006). Wilson (1993) said that it is believed that slab pull is a very strong boundary force (qtd in Weil 2006).
The Collisional Resistance may be said to be the negative counterpart of slab pull. Whenever a subducting slab exists, there is a corresponding “resistive force” exerted by the viscous, more ductile upper mantle. According to Ziegler (1992) the sum of the two opposing forces (slab pull and resistive) equals the Net Slab force exerting at the” colliding margin”. Richards (1992) revealed that in recent studies , however, the slab itself balance the slab force so that the slab force do not actually contribute to plate movements (qtd in Weil 2006).
Trench Suction forces occur in the trenches created by the subduction zones and it is more often referred to as the net trenchward pull (qtd Forsyth and Uyeda (1975) and Chase (978) in Weil 2006). Along with the formation of subduction zone is the “small-scale convection in the mantle wedge”, in the shallow subsurface, resulting to trench suction.
Basal Shear Traction or Basal Drag forces plays a significant role in plate motion for they are the ones who gave an indication whether plate motions are “active” or “passive”. Basal drag is created as a resulting resistance or dragging force when the two surface layers of the upper mantle and the lithosphere meet. Geologists consider the effect of basal drag to be small but when it involves big surface plates, it can create a big total resistance (Weil 2006).
Whichever causes the plate motion, there are three sure ways of plate movement as dictated by the corresponding plate boundaries. Plates either “move away from, toward, or slide past each other”. Geologists respectively identify these movements as “divergent, convergent, and transform plate boundaries” (Hawaiian Natural History Association, “Types of Plate Motion”, 2005). In the case of divergent plate boundary (see figure 3), oceanic or continental plates move away from each other. Prime example of this is the mid-Atlantic Ridge, near the middle of the Atlantic Ocean (“Theory” 1). On the other hand, there are three types of convergent boundaries (see figure 4), depending on which of the lithospheric plates collides. The first type is when denser oceanic plates collide with the continental plates resulting in the formation of subduction zones and volcano. Example of this is the subducting oceanic Nazca plate in the South American continent. The next type is when two continental plates collide resulting to the formation of mountains. The third type is when two oceanic plates collide resulting to the subducting of a more dense oceanic plate, which then eventually leads to the formation of volcanic islands (“Theory” 1). In the last plate movement, transform plate boundaries (see figure 5), the prime example is the San Andreas Fault in California. This fault was created when the “Pacific Plate slides past the North American Plate” (Hawaiian Natural History Association, “Types of Plate Motion”, 2005).
The issue of which of the two possible mechanisms is responsible for plate motion is what the geologists are trying to find out for sure at present. Scientific geologic studies are still ongoing to prove or disprove any of the two possibilities.
Figure 1 . (Taken from http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part13.html )
Figure 2: Basic schematic of different Plate Driving Forces (Taken from http://www.umich.edu/~gs265/tecpaper.htm)
Figure 3 (Taken from http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part13.html)
Figure 4. (Taken from http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part13.html)
Figure 5. (Taken from http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part13.html)
Hawaii Natural History Association. “The Birth of Plate Tectonics”. A Teacher’s Guide to the Geology of Hawaii Volcanoes National Park. North Dakota and Oregon Space Grant Consortia. 2005. Accessed February 19, 2008 ;http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part11.html;
Hawaii Natural History Association. “Types of Plate Motion”. A Teacher’s Guide to the Geology of Hawaii Volcanoes National Park. North Dakota and Oregon Space Grant Consortia. 2005. Accessed February 19, 2008 <http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part13.html>
Strickler, Mike. “What causes the plates to move?” University of Oregon. April 1997. Accessed February 19, 2008 from http://jersey.uoregon.edu/~mstrick/AskGeoMan/geoQuerry35.html
__________. “Theory of Plate Tectonics”. Accessed February 19, 2008 <bin.lps.org/manila/lnesci/Ch4.2Notes.pdf>
Weil, Arlo Brandon. “Plate Driving Forces and Tectonic Stress”. University of Michigan. 2006. Accessed February 19, 2008 <http://www.umich.edu/~gs265/tecpaper.htm>