Plate Tectonics

If sea floor spreading only produces basaltic (oceanic) rock, where did the continents come from? Knowledge of the processes involved is somewhat limited, but formation of the early continents resulted from subduction at oceanic-oceanic convergent margins. When plates subduct, a process known as partial melting occurs. Partial melting of mafic rock results in the production of magma that is more felsic in composition; that is, it has a composition intermediate between basalt and granite. In addition, weathering of mafic rock at the earth's surface also produces sediments with a more felsic composition. When these sediments subduct, they yield magma of felsic composition via partial melting.

Repeated episodes of subduction and partial melting, followed by volcanic eruption, produced lavas of increasingly felsic composition. Finally, this cycle formed volcanic island arcs that were too buoyant to be subducted and became a permanent part of Earth's surface. When sea floor spreading pushes one of these buoyant volcanic island arcs toward a subduction zone, rather than subducting, it welds, or accretes, onto the side of the volcanic island arc forming on the other side of the trench. Over time, these microcontinents, through accretion, formed larger continental masses.

Continents "float" on the plastic material making up the mantle like a block of wood floats on water. As erosion occurs, sediments are carried from mountains and higher elevations out to sea, where they accumulate on the continental shelf, forming wedges of sediment. Such accretionary wedges can extend far out to sea, depending on the size and shape of the continental shelf. As erosion moves sediments from the interior of the continent to the edges, the continent gets thinner but its surface area becomes larger. If conditions remain stable, accretionary wedges can go on accumulating for a very long time, reaching hundreds of miles out into the ocean. Sometimes, the wedge becomes so thick it rises above sea level to become dry land.

Continents have either passive or active margins. Passive margins are found where the continent's edge is on the same plate as the adjacent ocean, and it is along passive margins that accretionary wedges form. Active margins are found where the continent and the bordering oceanic crust are on separate plates. In these situations, a subduction zone is usually present. In general, the continents bordering the Atlantic Ocean have passive margins, while those surrounding the Pacific Ocean, which has a very active MOR, have active margins.