Plate tectonics theory and mechanisms

Plate Tectonics Theory

The story of Plate Tectonics is the story of continents drifting from place to place, breaking apart, colliding, and grinding against each other (Story pp). It is also the story of terrestrial mountain ranges rising up while being pushed together, of oceans opening and closing, of undersea mountain chains girdling the planet like seams on a baseball, and of violent earthquakes and fiery volcanoes (Story pp). Plate Tectonics describes the intricate design of a complex, living planet in a state of dynamic flux (Story pp).

Examination of the globe usually results in the observation that most of the continents seem to fit together like a puzzle (Plate pp). For example, the west African coastline appears to snuggle into the eat coast of South America and the Caribbean sea, and a similar fit appears across the Pacific (Plate pp). The fit is even more evident when the submerged continental shelves are compared rather than the coastlines (Plate pp).

In 1912, Alfred Wegener, 1880-1930, noticed this same thing and proposed that the continents were once compressed into a single proto-continent that he called Pangaea, meaning "all lands," and over time they have drifted apart into their current distribution (Plate pp). Wegener believed that Pangaea was intact until about 300 million years ago during the late Carboniferous period, when it started to break up and drift apart (Plate pp). However, his hypothesis lacked a geological mechanism to explain how the continents could drift across the earth's surface as he proposed (Plate pp). Wegener's inability to provide an adequate explanation of the forces responsible for continental drift and the prevailing belief that the earth was solid and immovable resulted in the scientific dismissal of his theories (Plate pp).

In 1929, Arthur Holmes elaborated on one of Wegener's many hypotheses with the idea that the mantle undergoes thermal convection, and this repeated heating and cooling results in a current that may be enough to cause continents to move, yet his idea received little attention until the 1960's (Plate pp). By then, "greater understanding of the ocean floor and the discoveries of features like mid-oceanic ridges, geomagnetic anomalies parallel to the mid-oceanic ridges, and the association of island arcs and ocean trenches occurring together and near the continental margins," suggested convection might be at work (Plate pp).

These discoveries and more led Harry Hess and R. Deitz to publish similar hypotheses based on mantle convection currents, known as "sea floor spreading," and although the idea was basically the same as that proposed by Holmes over three decades earlier, now there was much more evidence to further develop and support the idea (Plate pp).

Today, scientists have a fairly good understanding of how the plates move and how such movements relate to earthquake activity (Understanding pp). The majority of movement occurs along narrow zones between plates where the results of plate-tectonic forces are most evident (Understanding pp). The four type of plate boundaries are:

Divergent boundaries - where new crust is generated as the plates pull away from each other.

Convergent boundaries - where crust is destroyed as one plate dives under another.

Transform boundaries - where crust is neither produced nor destroyed as the plates slide horizontally past each other.

Plate boundary zones - broad belts in which boundaries are not well defined and the effects of plate interaction are unclear

(Understanding pp).

The consequences of plate movement are easy to recognize around Krafla Volcano, in the northeastern part of Iceland, where existing ground cracks have widened and new one appear every few months (Understanding pp). From 1975 to 1984, numerous episodes of surface cracking, called rifting, took place along the Krafla fissure zone, some accompanied by volcanic activity in which the ground would gradually rise 1-2 m before abruptly dropping, signaling an impending eruption (Understanding pp). During this same time frame, the displacements caused by rifting totaled approximately 7 m (Understanding pp).

The size of the Earth has not significantly changed during the past 600 million years, and most likely, not since shortly after its formation some 4.6 billion years ago (Understanding pp). The Earth's unchanging size implies that the crust must be destroyed at about the same rate as it is being created, just as Harry Hess surmised (Understanding pp). Such destruction or recycling of the crust takes place along convergent boundaries where plates are moving toward each other, and at times one plate sinks, or is subducted, under another (Understanding pp). The location where sinking of a plate occurs is called a subduction zone (Understanding pp).

The zone between two plate sliding horizontally past one another is called a transform-fault boundary, or simply a transform boundary, a concept originated with Canadian geophysicist J. Tuzo Wilson, who proposed that these large faults or fracture zones connect two spreading centers, called divergent plate boundaries, or, less commonly, trenches, called convergent plate boundaries (Understanding pp). The majority of transform faults are found on the ocean floor and commonly offset the active spreading ridges, producing zig-zag plate margins, and are generally defined by shallow earthquakes (Understanding pp).

A few occur on land, such as the San Andreas fault zone in California, which connect the East pacific Rise, a boundary to the south, with the South Gorda- Juan de Fuca-Explorer Ridge, another divergent boundary to the north (Understanding pp). The Blanco, Mendocino, Murray, and Molokai fracture zones are but a few of the numerous fracture zones, or transform faults, that scar the ocean floor and offset ridges (Understanding pp). The San Andreas is one of the few transform faults that is exposed on land (Understanding pp).

In some regions, the boundaries are not well defined due to the plate-movement deformation that occurs there extends over a broad belt, called a plate-boundary zone (Understanding pp). One of these zones marks the Mediterranean-Alpine region between the Eurasian and African Plates, "within which several smaller fragments of plates, called micro-plates, have been recognized (Understanding pp). Due to the fact that plate-boundary zones "involve at least two large plates and one or more micro-plates caught up between them, they tend to have complicated geological structures and earthquake patterns" (Understanding pp).