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Differences in Formative Process
Oceanic plates are formed by divergent plate boundaries. These zones, located along mid-ocean ridges, represent areas where the new oceanic crust is being created. As lava flows from these volcanic ridges, it quickly cools, forming extrusive igneous rock. Continental plates are formed primarily by convergent plate boundaries. These zones represent areas where oceanic plates are being forced under the continental plates. As they are subducted, they melt to form magma. This magma cools over millions of years, producing intrusive igneous rock and new continental crust.
Differences in Composition
Oceanic plates are mafic in nature, comprised of basalt rock. This oceanic rock is high in iron, magnesium and calcium content. More than half of this basalt rock is comprised of the minerals pyroxene and olivine. In contrast, continental plates are felsic in nature, comprised of granite rock. This continental rock is primarily comprised of aluminum and sodium, plus the minerals feldspar and quartz. Continental crust also contains 10 times more potassium than is found in oceanic rock.
Differences in Density
Because of their heavy ferromagnesian elements, oceanic plates are much denser than continental plates. The average density of ocean plates is approximately 200 pounds per cubic foot, while continental crust averages approximately 168 pounds per cubic foot. This difference in relative density is what causes oceanic plates to subduct beneath the more buoyant continental plates. This also allows the denser oceanic plates to sink further into the fluid asthenosphere, causing them to lie below sea level. In contrast, the more buoyant continental plates float higher, resulting in dry land.
Differences in Age
The age of oceanic and continental plates is radically different. This is the result of the tectonic process. Oceanic plates are continually being renewed at divergent boundaries and recycled in subductive zones. As a result, the oldest oceanic rocks are less than 200 million years old. In contrast, continental plates take a long time to create but are rarely destroyed. As a result, continental rock can be as old as 3.8 billion to 4 billion years old.
Differences in Range and Thickness
Oceanic plates cover approximately 71 percent of Earth's surface, while continental plates cover 29 percent. While oceanic plates cover far more area, they are much thinner than continental crust. Oceanic plates average only five miles in thickness, compared to an average of 25 miles for continental plates. The combination of their respective area and average thickness means that there is actually twice as much continental rock as oceanic rock.