~Synopsis of Plate Tectonic Theory~
And Its Implications for Earth History.....
Plate tectonics is the theory that the outer rigid layer of the earth (the lithosphere) is divided into "plates" that move across the earth's surface relative to each other, like slabs of ice on a lake (click picture below for a larger version).
The movement of the plates is driven by convection cells in the mantle. Most geologic activity takes place at plate boundaries, including most earthquakes, volcanos, production of igneous rocks, major metamorphism, and mountain building processes. Interplate regions are less active.
There are three kinds of plate boundaries:
- Divergent boundaries are where plates separate from each other, and magma oozes up from the mantle into the crack (a fissure volcano) making the ocean basin wider. This is known as sea floor spreading.
- Convergent boundaries are where plates come together, but to do so one of the plates must dive below the surface into the mantle along a subduction zone. Convergent boundaries produce mountain chains of very large, explosive volcanos (composite type).
- Transform boundaries are where plates slide past each other, ideally with little or no vertical movement. Most transform boundaries are below sea level and so not easy to see. The San Andreas fault in California is a transform boundary.
~Plate Tectonics and the Earth's Evolution~
Plate tectonic processes have led to the evolution of the earth. When the earth originated it contained no continents, and consisted of only a few kinds of igneous rock - roughly the composition of the moon. But through plate tectonic processes the earth has evolved first to form volcanic island chains (volcanic arcs scattered across a single world wide ocean), and then for these to enlarge to form the large continental masses we live on today.
These evolutionary processes take place because igneous rocks evolve at convergent and divergent plate boundaries. At both places magma is generated, and through processes of fractional melting (partial melting), whole new rocks are created including those that form continental masses.
A second significant idea derived from plate tectonic theory is that as the continents have grown through time they have alternately come together to form supercontinents, only to fragment again to form smaller isolated continents.
The last supercontinent, Pangaea, formed about 300 million years ago when isolated continents collided. Only this one large continent existed, balanced by one large ocean, Panthalassa. During the fragmentation stage (beginning about 230 million years ago and still going on) the present Atlantic ocean opened up, and all the continents scattered across the globe.
Prior to Pangaea, a supercontinent called Rodinia existed approximately 600 million years ago. It also formed from the accumulation of isolated continents, only to fragment shortly after its formation. There is indication of other supercontinents proceeding Rodinia as well.
The present day Atlantic ocean is getting wider because of sea floor spreading, and the Pacific ocean is getting smaller along subduction zones (convergent plate boundaries) under North and South America, and Japan as western North America and Asia move closer together. Sometime in the future the Pacific ocean will close completely and Asia and North America will collide to form another supercontinent.
~The Supercontinent Cycle~
During each supercontinent cycle, a supercontinent and a single super-ocean exist, followed by the fragmentation of the supercontinent to form smaller continents. These supercontinent fragments collide to repeat the cycle. This has been going on for 4 billion years, requiring about half a billion years for each cycle.
This supercontinent cycle< is a cycle with a directional and irreversible evolutionary trend, as shown in the figure below, and
does not simply go round and round, for with each cycle, new continental crust is generated, and the continents get bigger. Actual continental movements are quite complex, however, because continents do not just move east and west- they also move north and south, as well as various combinations of these movements. Continents do not always fragment and collide the same way with each cycle.
The earth, which began with only oceans, has evolved its continents beginning 4 billion years ago with small volcanic chains (arcs), created by convection cells and subduction zones, . . .
that evolved into proto-continents through many individual subduction zones, . . .
that evolved into micro-continents when the proto-continents collided, . . .
that evolved into the major continents today (such as North and South America, Siberia, Australia), that continue to move through supercontinent cycles (e.g. Pangaea and Rodinia)
Contributed by Lynn Fichter