The Earth’s surface is covered by the crust, a relatively thin, rigid outer shell broken into large tectonic plates. This outermost layer exists in two primary forms: oceanic crust and continental crust. Continental crust is the foundation of all landmasses and continental shelves. Understanding its composition and physical properties explains why continents stand tall above sea level.
The Defining Characteristics of Continental Crust
The crust found under the continents is primarily defined by its composition, which is broadly described as felsic. This term refers to rocks rich in lighter elements like silicon and aluminum, with the most common rock type being granite or granodiorite. This chemical makeup contrasts sharply with the crust under the oceans, which is much richer in heavier elements like iron and magnesium.
A direct consequence of this mineral composition is the crust’s relatively low density, averaging about 2.7 grams per cubic centimeter. This lower density grants the continental crust significant buoyancy when resting on the denser mantle below. The principle of isostasy dictates that a thicker, less dense mass will float higher, similar to how an iceberg floats in water.
Continental crust is remarkably thick, with an average depth ranging between 30 and 40 kilometers. This thickness can increase dramatically in regions of mountain building, such as beneath the Himalayas, where it can reach up to 70 kilometers. The combination of lower density and greater thickness allows the continents to maintain their elevation above the underlying mantle and the oceanic crust.
Contrasting Continental and Oceanic Crust
The fundamental differences between continental and oceanic crust dictate the geology and topography of the planet. While continental crust is felsic and granitic, oceanic crust is mafic, meaning it is composed mainly of dark, dense basalt and gabbro. This difference in composition translates directly into a density contrast, with oceanic crust possessing a higher average density of about 2.9 to 3.0 grams per cubic centimeter.
This density differential means oceanic crust is easily consumed back into the Earth’s interior during subduction. When a denser oceanic plate collides with a less dense continental plate, the oceanic crust sinks beneath the continent into the mantle. The continental crust, being buoyant, resists subduction and remains at the surface, making it geologically permanent.
Continental crust contains some of the oldest rocks on Earth, with sections dating back over four billion years. This longevity results from its resistance to subduction and recycling. Conversely, oceanic crust is constantly being created at mid-ocean ridges and destroyed at subduction zones. Therefore, it is geologically young, rarely exceeding 200 million years old.
The Internal Architecture and Boundaries
The continental crust is not a uniform block but is divided into distinct structural layers. The upper crust is more brittle, composed of lower-density, silica-rich rocks like granite, along with sedimentary and metamorphic rocks. Below this lies the lower crust, which is denser, more ductile, and transitions toward a more mafic composition, deforming under intense heat and pressure.
The base of the continental crust is marked by the Mohorovičić discontinuity, commonly called the Moho. This boundary is identified by a sharp increase in the velocity of seismic waves. This change is caused by the transition from less dense crustal rocks to the much denser, peridotite-rich rocks of the underlying upper mantle. The depth of the Moho beneath the continents averages about 35 kilometers, plunging deeper beneath mountain ranges.
The crust and the rigid uppermost layer of the mantle together form the lithosphere, the thick, mechanically strong layer that makes up the Earth’s tectonic plates.