Plate tectonics
Plate tectonics (from the Greek for "builder", "tekton") is a theory of geology underlying the phenomenon of continental drift. The theory of plate tectonics presupposes that the earth's outermost surface (crust) is made up of two sub-layers, the rigid lithosphere and the semi-molten asthenosphere. Beneath the crust is the upper mantle, a layer made up of molten rock (magma).
The rigid stone making up the lithosphere "floats" on the fluid-like asthenosphere, and, in areas where magma wells up from the mantle below the crust, may develop fractures. This results in the lithosphere being separated into contiguous masses of solid earth and rock known as tectonic plates. Due to slow currents in the asthenosphere's material, the plates floating atop this material undergo slow motions in different directions. These floating plates regularly jostle one another, with interactions along these boundaries being responsible for earthquakes,volcanoes, mountains, and [[oceanic trench|oceanic trenches].
Tectonic plates are roughly divided into two types: continental and oceanic plates. The distinction is based on the density of the materials making the plates; oceanic plates are denser than continental plates; as a result, the oceanic plates generally lie below sea level, while the continental plates project above sea level.
The same currents that move tectonic plates with respect to one another also tend to bring molten material closer to the surface at points where a circulation cell is drawing the material upward. In minor cases, this can result in a hot spot, where the material of the plate is eaten away from below, leaving openings for volcanoes such as those producing the islands of Hawaii. In major cases, the rising magma continuously pushes itself into the midst of the plate, eventually cooling to form new plate material. Thus, some tectonic plates are being widened by the addition of new material, via upwelling of magma from below. Currently the Atlantic oceanic plate is in the process of expanding, fuelled by a continuous input of magma along the Mid Atlantic Ridge. The region of Iceland, which straddles this ridge, is thus gaining territory at a rate of a few centimeters per century.
When an opening fault zone underlies a continent, a structure known as a rift forms with a depression over the fault zone. Volcanic mountain ranges often lie parallel to and close to the rift. The rift will typically form either a rift valley filled with a line of shallow lakes or a narrow sea such as the Sea of Cortez or the Red Sea. Abrupt changes in direction in rifting occur at points where two rifts join. There is typically a third, failed, rift called an aulacogen present at the junction. The best known currently active rift is the great rift valley of eastern Africa. Rift Valleys mark regions where continents will fracture and widening seas will eventually intrude.
Plate boundaries are described as:
- Transform boundaries or transform-fault boundaries occur where plates slide, perhaps more accurately grind, past each other. One of the best known and highly visible transform boundary is the San Andreas fault in California. It is the release of energy when such a boundary suddenly slips that is the cause of some types of earthquakes.
- Divergent boundaries are typified by the rifts of the oceanic ridge system and, on land and running through nearby sea, the famous Rift Valley of Africa. Along these rifts new surface is being formed and spreading occurs.
- Convergent boundaries are typified by the oceanic trenches where an oceanic plate is sliding under a continental or oceanic plate in a process termed subduction. Almost invariably, on the continental side of such trenches, there will be a distinctive mountain range. The Andes are an example. The collision of convergent continental plates typically creates high mountain ranges with the Himalayas a classic example.
- The least distinct type, the plate boundary zone tends to be an area with two major plates and one or more small plates fragments. The activity in these zones is not nearly as clear cut as in the other types and earthquake types may be more highly varied.
Stresses created by plate movements are a major cause of Geologic faults. Not all faults can be directly traced to current plates and many occur well inside continental plates and may have origins not directly tied to the plate motions. It follows that earthquakes are found largely in the plate boundaries and as a result of plate activity. (See: USGS map)
The most massive fault system is created by the divergent boundaries in the oceanic ridge system. Spreading is not uniform. Where spreading rates of adjacent ridge areas are different massive transform faults occur. These are the Fracture Zones, many bearing names, that are a major source of submarine earthquakes. A sea floor map will show a rather strange pattern of blocky structures that are separated by linear features perpendicular to the ridge axis.
If one views the sea floor between the fracture zones as conveyor belts carrying the ridge on each side of the rift away from the spreading center the action becomes clear. Crest depths of the old ridges, parallel to the current spreading center, will be older and deeper. It is here that one of the key pieces of evidence forcing acceptance of sea floor spreading was found. Airborne geomagnetic surveys showed a strange pattern of symmetrical magnetic reversals on opposite sides of ridge centers. The pattern was far too regular to be coincidental. The widths of the opposing bands were too closely matched. Scientists had been studying polar reversals and the link was made. The magnetic banding directly reflected the polar reversals. This has been confirmed by dates for the rock within the bands. In reality the banding furnishes a map in time and space of both spreading rate and polar reversals.
Plate boundaries are the home of the majority of the world's active volcanoes with the Pacific Plate's "Ring of Fire" being most active and famous.
Impact on Earth Sciences
The acceptance of continental drift, sea floor spreading--all elements of plate tectonics-- can be compared to the Copernican revolution in astronomy (See: Nicolaus Copernicus). Within a matter of only several years geophysics in general and geology in specific were revolutionized. The parallel is striking.
Pre Copernican astronomy had been highly descriptive and predictions could be made. At that level it worked. The problem came in "how"--they knew a planet would be in a position. How did it get there when the Earth was stationary? Observations were fit into a horribly complex system of motions and spherical layers to explain their motions about the stationary Earth. Then came acceptance of the fact that our planet is among those orbiting the sun.
Before acceptance of plate tectonics geology in particular was trapped in the same box. The revolution was much more sudden. What had been rejected for decades by any respectable scientific journal was eagerly awaited within a few short years in the 1960-1970 period. Any geological description before this had been highly descriptive. All the rocks were described and assorted reasons, sometimes in excruciating detail, were given for why they were where they are. The descriptions are still valid. The reasons today sound much like pre Copernican astronomy.
One simply has to read the pre plate descriptions of why the Alps or Himalayas exist to see the difference. What force pushed obvious sea floor rock thousands of meters above the sea's level? Why are there "convex and the concave margins" of the Alpine chain? Complexity that boils down to technical jargon without much clarity as to "why" is the answer.
With plate tectonics answers quickly fell into place or a path to the answer became clear. Collisions of converging plates had the force to lift sea floor into thin atmospheres. What could possibly cause those trenches oddly placed just off island arcs or continents and why were there so many volcanoes on the land side? Subduction at converging plates had that power and created the raw material and disruption for the volcanic activity.
Mysteries were no longer mysteries. Forests of complex and obtuse answers were swept away. Why were there striking parallels in the geology of parts of Africa and South America? Why did Africa and South America look strangely like two pieces that should fit to anyone having done a jigsaw puzzle? Look at some pre tectonics explanations for complexity. For simplicity and one that explained a great deal more look at plate tectonics. A great rift valley, like the one now on the other side of Africa, had split into the Atlantic--and was still at work.
We have inherited some of the old terminology, but the underlying concept is as radical and simple as "The Earth moves" was in astronomy.
See: Alfred Wegener, List of Tectonic Plate Interactions, obduction, subduction
- This Dynamic Earth provides an excellent overview of the subject.