Saturday, May 7, 2011

Plate Accretion

The present geography of Planet Earth is just the latest snapshot in its 4.6 billion year evolution. The relative size and position of continents and oceans have undergone constant change; while this process continues today, we are only aware of the occasional earthquakes and volcanic eruptions that result from the underlying tectonic forces. Our life span is too short to appreciate any visible change in the surface geography; indeed, only subtle changes have occured since the birth of our species.

However, over the eons of geologic time, oceans have opened and closed and, in concert, land masses have rifted apart and cemented together. In areas where oceanic crust and continental crust collide, the more dense basalt of the oceanic crust dips (subducts) below the continental crust, disappearing into an ocean trench and recycled into Earth's mantle. As it decends, this ocean crust begins to melt and a chain of volcanoes, known as a volcanic island arc, develops along the outer edge of the overlying plate. In some regions, larger land masses, having rifted from other continents, arrive with the ocean crust that surrounded them; as the latter subducts, the land mass, having a lower density, collides with its new parent continent. The upper layer of the ocean crust, scraped away as its plate slides beneath the overlying continental plate, may also acccumulate along the coast, creating a complex geology of oceanic sediments and exotic terranes. Finally, all of these new land masses (island arcs, exotic terranes, oceanic sediments) may close off the oceanic trench, forcing the subduction zone further out from the expanding continent.

Accreted segments make up large sections of Earth's continents; in North America, most of New England and the Southeastern Piedmont accumulated in pieces during the Paleozoic Era while most of the Continent west of the Rocky Mountains is a puzzle of terranes added throughout the Mesozoic Era and Tertiary Period. Today, new rift zones are spreading and terranes are inching toward their future homes; though invisible to the untrained eye, these tectonic processes continue to shape the surface of our planet.