Formation and Destruction Cycle of Oceanic Lithosphere
Continental rifting: The birth of a new ocean basin
• A new ocean basin begins with a the formation of a continental rift (example: the African Rift valleys, Figure 5.29).
• The Red Sea is an example of a rift valley that has lengthened and deepened into a narrow linear sea
• If spreading continues the Red Sea will grow wider and develop an oceanic ridge similar to the Atlantic Ocean.
Subduction: The destruction of oceanic lithosphere
• Oceanic lithosphere subducts because its overall density is greater than the underlying mantle
• Subduction of older, colder lithosphere results in STEEP descending angles of the sinking oceanic lithosphere.
• Younger, warmer oceanic lithosphere is more buoyant and angles of descent are SHALLOW.
• Research indicates that parts, or even entire oceanic basins, have been destroyed along subduction zones.
Destruction of oceanic lithosphere adds new material to continental crust
As oceanic lithosphere sinks back into the asthenosphere it carries large quantities of seawater and sediment with it. As it sinks, the increased heat and pressure forces water and gases out of the rock. This combination of trapped water and gases allows some of the material melt. The resulting magma that forms is depleted in iron and magnesium, but enriched in aluminum and silica (felsic in composition). This is because mafic minerals have both higher melting temperatures and higher density than felsic minerals.
As a result a natural refining process occurs... mafic material sinks back into the mantle whereas the molten felsic material (along with trapped water and gases) separate and work their way back to the surface. This results in the formation of volcanoes in the region above where subducting lithospheric slabs are sinking. This felsic material is less dense and becomes incorporated into new continental crust. Over long periods of time, enough felsic material accumulates to build continents, a process that may take billions of years.