~A Plate Tectonic Rock Cycle~
Does the Earth cycle, Or Has It Evolved Cyclically?
The earth is an open system and it dissipates energy. The energy comes from the Earth's molten interior, which has kept it tectonically active for 4.5 billion years. Energy from the interior has driven the Earth's physical/chemical evolution and has been ultimately responsible for creating all the rocks, continents, mountains, foreland basins, and all other land forms. All of these rock and tectonic features on Earth reflect the underlying principle not only of geology but the universe: "minerals and rocks are stable only under the conditions at which they form; change the conditions and the rocks change too".
The simplest model we have of the earth is the rock cycle (right). It summarizes the core concept of geology: all rocks are related to each other, and can be transformed one to the other. The cycle is the most theoretically abstract description of these rock relationships. It incorporates or is expandable to all rock processes, but does not necessarily specify or justify them. It also suggests the pathways by which one rock can transform into another, but does not explicate the necessary conditions under which these transformation take place.
The problem with the traditional rock cycle is that it implies that rocks just cycle endlessly from one to the other. The drawing above expresses this cyclical nature. It also expresses the view of the 19th century Uniformitarian school of thinking, captured best by James Hutton when he said the earth has "no vestige of a beginning, and no prospect of an end." He envisioned earth processes going round and round but never getting anywhere, never evolving.
Earth processes, however, do move forward with time, as indicated by the Wilson cycle. The direction we see in the earth's evolution shows up in a number of ways. These include the increasing diversification of rocks with time, the increasing size of the continental masses (increasing volume of Granite), and the changing tectonic provinces. A model other than the basic rock cycle is necessary to interpret how the earth operates and progresses. The Tectonic Rock Cycle is such a model (below) which shows a complete summary of the processes that lead to the evolution of the physical earth.
The Tectonic Rock cycle
Observe on the major pathways of the figure that one begins with the parent rock (komatiite Suite), goes through the tholeiite »calc-alkaline »alkaline suites, through the sedimentary processes (yellow box), through the Barrovian metamorphism (Greenschist »Amphibolite »granulite), and twists back toward the calcalkaline and alkaline suites.
The path fdoes not cycle back on itself completely. Instead, it begins with a mafic/ultramafic parent rock and through fractionation squeezes out a rock whose composition gets ever lower on Bowen's Reaction Series. Even sedimentary processes fractionate the rocks chemistry. Observe that the sedimentary rocks in the yellow box when metamorphosed to the melting state result in an igneous rock low on the reaction series.
Thus, the tectonic rock is operating like the drawings below right. The basic rock cycle on the left just goes round and round without getting anywhere. The tectonic rock cycles (Wilson cycles) on the right not only go round they also go ahead a little each cycle. That is, each round of the Wilson increases the diversity of rocks on the earth, and increases the volume of felsic igneous rocks.
The Earth is not just a rock cycle, it is an evolutionary rock cycle. So, to answer the question, Does the Earth cycle, Or Has It Evolved cyclically? We conclude that it evolves cyclically through Wilson Cycles, each cycle, adding a little more felsic igneous rock to the planet, and not incidently increasing the size of the continents.
Contributed by Lynn Fichter