Speed of the Continental Plates

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Bibliographic Entry Result
(w/surrounding text)
Standardized
Result
Read, H. H. & Janet Watson. Introduction to Geology. New York: Halsted, 1975: 13-15 "… which the advancing plates move and have been calculated on this basis by Le Pichon at values of 5–10 cm per year." 5–10 cm/yr
"Continental Drift." Encyclopedia Britannica. 15th ed. 1993. "… subsequent plate movements averaging about 2 cm (0.8 inch) per year …." 2 cm/yr
"Plate Tectonics." Encarta. CD-ROM. Redmond, WA: Microsoft, 1995. "In the North Atlantic, the rate of movement is only about 1 cm (about 0.4 in) per year, while in the Pacific it amounts to more than 4 cm (almost 2 in) annually." 1–4 cm/yr
Hamilton, Rosanna L. Earth's Interior and Plate Tectonics. 1995. "Earth's lithosphere is divided into eight large plates with about two dozen smaller ones that are drifting above the mantle at the rate of 5 to 10 cm/yr." 5–10 cm/yr
Park, R. G. Geological Structures and Moving Plates. New York: Chapman & Hall, 1988: 70. [see below] 1–10 cm/yr
Sleep, Norman H. & Sean C. Solomon. "Some Simple Physical Models for Absolute Plate Motions." Journal of Geophysical Research. 1974: 2557-2567. [see below] [see below]
Tamaki, Kensaku. Absolute Plate Motion Calculator. University of Tokyo: Ocean Research Institute. [see below] [see below]

Plate tectonics is a relatively new theory in the field of geology. It states that the lithosphere of the earth is divided into a small number of plates which float on and travel independently over the athenosphere, which lies over the mantle. Much of the earth's seismic activities occurs at the boundaries of these plates. It is a relatively slow movement, driven by thermal convection currents and other geological activity originating deep within the earth's mantle. This theory of plate tectonics replaced the previous one of continental drift, where it was thought that just the continents themselves drifted over the earth's surface. There are basically eight large plates -- African, Antarctic, Eurasian, Indian-Australian, Nazca, North American, Pacific and South American; and various smaller ones -- Anatoliah, Arabian, Caribbean, Cocos, Philippines, Somali, and Juan de Fuca (to name a few).

Most of my research turned up theories and not hard experimental data and those measurements that I was able to obtain were averages or estimations. The numbers found in each source did not exactly agree with each other but their ranges were reasonably similar. The majority of the research showed that the plates moved at the average rates between approximately 0.60 cm/yr to 10 cm/yr. Some sources stated that in the North Atlantic, the rate of movement is only about 1 cm (about 0.4 in) per year, while in the Pacific it amounts to more than 4 cm (almost 2 in) annually, while two others said the plates, in general, traveled from 5 to 10 cm/yr.

However, the velocities, in some sources, of the plates are measured as absolute and/or relative motion. I found the absolute velocities to some of the plates from one source. They are as follows:

* Stated velocities are author's judgment.
Unable to locate original text of the graph.
Plate Absolute Velocity (cm/yr)*
Antarctic ~2.05
African ~2.15
Arabian ~4.65
Caribbean ~2.45
Cocos ~8.55
Eurasian ~0.95
Indian ~6.00
Nazca ~7.55
North American ~1.15
Pacific ~8.10
Philippine ~6.35
South American ~1.45

I obtained the relative velocities of the plates by combining the data from two sources. With some help, I discovered a great calculation program on the Internet (see bibliography) specific for this purpose. I used the coordinates from another source (see bibliography) that calculated the relative rotational motions of the plates to the Pacific plate. The relative velocities of some of the plates relative to the Pacific plate using this method are as follows:

* Velocities are relative to the Pacific plate.
Original table also had direction.

† Velocities are zero since these plates are not in contact with the Pacific plate.
Plate Relative Velocity (cm/yr)*
Antarctic 1.13
African 0†
Arabian 0†
Caribbean 0†
Cocos 0.20
Eurasian 0.52
Indian 0†
Nazca 1.33
North American 0.68

The relative and absolute velocities can be calculated using the Internet program but the coordinates are crucial, for very different results may be obtained if the coordinates are not correct. Due to various activity on different points on and below the earth's surface, the movement of one plate is not uniform. One plate can have many velocities depending upon the location. However, due to my lack of knowledge in this topic and the conflicting numbers of scientists, my values may be off. I suppose there's no agreeable data because there is probably no real efficient method of measuring all the tectonic plates on earth. There are many factors involved with plate tectonics; they don't just move in one path, but they individually take many different paths. They interact with each other and affect the motions by applying various types of stress and pressure, etc. at different points on the earth. Also, the earth is constantly changing and so there is no exact number to reflect the velocities of the plate tectonics.

Zhen Shao Huang -- 1997

Bibliographic Entry Result
(w/surrounding text)
Standardized
Result
Kanamori, Hiroo & Emily E. Brodsky. "The Physics of Earthquakes." Physics Today. (June 2001): 34. "The relative plate motion determined from these data is about 2-7 cm/year, which translates into a strain accumulation rate of approximately 3 × 107/y along plate boundaries" 2–7 cm/yr

Editor's Supplement -- 2001

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