The lithosphere is the Earth’s rigid outermost layer, encompassing the crust and the uppermost, solid portion of the mantle. This outer shell is broken into large tectonic plates that move slowly across the planet’s surface, a process known as plate tectonics. The nature and behavior of these plates are determined by whether they are capped by continental or oceanic material. These two types of lithosphere differ fundamentally in their physical size, chemical composition, and geological lifespan. Their contrasting properties explain many major geological features, including the formation of mountains, the depth of the oceans, and the locations of earthquakes and volcanoes.
Differences in Physical Dimensions
The continental lithosphere is characterized by substantial and highly variable thickness. It typically ranges from 100 to 200 kilometers in depth, sometimes thicker beneath ancient continental interiors. The crustal part of this lithosphere is also thick, averaging about 35 kilometers but reaching up to 70 kilometers beneath major mountain ranges.
In contrast, the oceanic lithosphere is significantly thinner and more uniform. Its total thickness averages about 100 kilometers. The oceanic crust component is much shallower, generally measuring only 5 to 10 kilometers thick.
The Mohorovičić discontinuity, or Moho, marks the compositional boundary between the crust and the underlying mantle. The depth of the Moho varies significantly between the two lithospheres, reflecting their different crustal thicknesses. Beneath the continents, the Moho averages 32 kilometers deep, but beneath the ocean floor, it sits at a shallow average depth of about 8 kilometers.
These differences in thickness and density cause the two lithospheres to “float” differently on the viscous asthenosphere below, a concept known as isostasy. The thick continental lithosphere has greater buoyancy, causing it to ride higher and form elevated landmasses. The thinner, heavier oceanic lithosphere floats lower, forming the deep basins that hold the world’s oceans.
Contrasting Chemical Makeup and Density
The chemical makeup of the crustal component is the primary reason for the contrasting behavior of the two lithospheres. Continental crust is predominantly felsic, meaning it is rich in lighter elements like silicon and aluminum. Common rocks are granitic, which are generally light-colored and have a low density.
This felsic composition results in continental crust having a lower average density, typically around 2.7 grams per cubic centimeter. This lower density contributes to its greater buoyancy, preventing it from sinking easily back into the mantle.
The oceanic crust is mafic, containing higher concentrations of heavier elements such as magnesium and iron. This material is primarily basaltic rock, which is darker and significantly denser than continental rock.
The density of oceanic crust averages about 3.0 grams per cubic centimeter, a geologically meaningful difference compared to continental crust. This higher density is a primary driver in plate tectonics, causing oceanic lithosphere to subduct beneath the less-dense continental lithosphere when the two collide.
Age and Geological Longevity
The two lithospheres differ dramatically in their age and permanence. The continental lithosphere is incredibly stable and old, holding the oldest known rocks on Earth. Fragments of continental material have been dated to over four billion years old, providing a preserved record of the planet’s deep history.
Continental material is relatively permanent because its low density makes it too buoyant to be easily forced down into the mantle. When continental plates collide, the material is crumpled and pushed upward to form mountain ranges, continually adding to the continental mass. This formation involves complex, slow accretion and differentiation over vast stretches of time.
In contrast, the oceanic lithosphere is geologically young and constantly being recycled. It forms continuously at mid-ocean ridges through the upwelling and cooling of magma from the mantle. As it moves away from these ridges, the lithosphere cools, thickens, and becomes progressively denser.
Because of this continuous cycle of creation and destruction, the oldest oceanic lithosphere is rarely older than 200 million years. When it reaches a subduction zone, its high density causes it to plunge beneath the adjacent plate, where it is reabsorbed into the mantle. This rapid, continuous process ensures the ocean floor is constantly renewed, unlike the long-lived continents.