An Introduction to Plate Tectonics
Earth history is easier to understand once you have two important forms of large-scale thinking. The first of these is to think in terms of geologic time, measured in millions and billions of years.
While time of this magnitude is difficult for humans to comprehend, without it one cannot grasp how our globe has come to have its present characteristics. With the availability of immense amounts of time, the history of the Earth can be seen to result from the operation of normal forces and processes seen to be operating today.
Even small, incremental changes, such as the few feet of movement that accompanied the recent earthquake in Turkey, can have major consequences if there is sufficient time.
Websites on Geologic Time:
The University of California Museum of Paleontology, Berkeley
Web Geological Time Machine
United States Geological Survey's
Explanation of Geologic Time
|THE GEOLOGIC TIME SCALE|
|Eon||Era||Period||Epoch||Lower Age Boundary (millions of years ago)|
It also helps to be able to think about Earth forces operating on a global scale; this is summed up by the theory called Plate Tectonics. An informative review of this theory can be found at the United States Geological Survey site called This Dynamic Earth. The essence of this theory is that heat from radioactivity causes the Earth's interior to flow slowly, resulting in the lateral (sideways) movement of thin rock slabs (called plates) over the Earth's surface. While once considered revolutionary, plate tectonics theory is now so solidly supported by evidence of many kinds that it is second nature for geologists to explain local histories in terms of such things as the collision of two plates or the movement of a continent from one climate zone to another. How fast do plates move? About 2 to 5 centimeters per year (1 to 2 inches per year), about the same speed that your fingernails grow.
We know, then, that the outermost part of Earth consists of a series of large slabs (tectonic plates; lithospheric plates) that move slowly over the globe, powered by flow in the interior mantle.
At some locations tectonic plates (lithosphere) move away from each other (diverge), and in the rift thus opened molten magma wells up from the mantle below to make new plate material. At opposite locations around the Earth, those same moving tectonic plates crash into each other (converge), raising up mountain systems, such as the Himalayas or the Alps. Continents are those parts of the plates that consist of lighter, older rocks that ride passively, like rafts, on the moving plates.
The forward, leading edge of a continent is the site of volcanoes, earthquakes, and developing mountains; because these are the conditions presently around the margin of the Pacific, such continental margins are said to be of Pacific-type.
The trailing edge of a continent lies within a plate, so it has no volcanoes, earthquakes, or young mountains and typically looks like a low-relief, gentle coastal plain where thick sequences of sedimentary rock accumulate over millions of years. As these are the conditions on those parts of the present continents flanking the Atlantic Ocean, they are said to be Atlantic-type continental margins. At the present time, the basic pattern of plate movement is away from the center of the Atlantic Ocean, and toward the Pacific Ocean.
You can see these two types of continental margin on the global relief map of Earth (below). Around the margin of the Pacific Ocean, where plates are converging, the continents are mountainous, and the region has many volcanoes and earthquakes. Those parts of continents bordering the Atlantic Ocean, on the other hand, are much flatter, with little happening other than the deposition of sediment. The Atlantic Ocean basin is progressively widening, while the Pacific Ocean Basin is becoming smaller as the tectonic plates move over it.
But every half billion years or so, the flow directions in the underlying mantle change, and the pattern of tectonic plate motion reverses. The super-continent that formed under the old regime of movement now begins to be split apart as smaller continent segments move away from each other. New ocean basins form in the widening rifts between the separating plates, and the former ocean basins that surrounded the super-continent begin to close. That is the situation today (see diagram below), as broken-apart continental fragments of the former super-continent Pangaea are moving away from each other.
When plate movement directions change, margins of continents can undergo drastic transformations. A continental margin that had been within a plate and passive (Atlantic-type) when the ocean basin was widening will be converted to an active (Pacific-type) continental margin when motion directions reverse and the adjacent plates now converge (and vise versa). The history of plate movement shows that those parts of continents that border ocean basins alternate repeatedly between two states: (1) a trailing edge margin of Atlantic-type that is tectonically quiet and where large quantities of sediment accumulate to great thicknesses, and (2) a leading edge margin of Pacific-type that is tectonically very active, with large volcanoes, many earthquakes, and high mountains. Another way of looking at this history is that it shows the cyclically repeated opening and closing of ocean basins, with a full cycle lasting something like one half billion years. This cycle is commonly called the Wilson Cycle, in honor of J. T. Wilson, who first got the idea.
The features we refer to as mountain systems (orogens) tend to have a consistent pattern of development. First, kilometers-thick sequences of sedimentary deposits accumulate over a couple of hundred million years onto passive Atlantic-type continental margins. Then, when mantle flow patterns change and plate movement directions reverse, the continental margin changes to Pacific-type, and those thick sedimentary deposits are compressed, deformed and intruded by molten magma from below.
|Introduction||Plate Tectonics||Episode 1||Episode 2||End of Episode 2||Episode 3||Episode 4|
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