What is Continental Crust?
The continental crust is the outermost layer of the earth’s lithosphere. It forms the landmasses, that is, the continental shelves and the continents on Earth. The continental crust is developed near the subduction zones at the boundaries between the oceanic and continental tectonic plates. The crust forms almost all the land surface of the Earth.
Oceanic and Continental Crust
The composition of the continental crust is mostly granitic in nature, and it is slightly lighter than the oceanic crust. The continental crust thickness comes to about 40 kilometers, that is, 25 miles roughly. However, the oceanic crust has a thickness of 6 kilometers approximately. There is a difference in the composition of continental crust and oceanic crust. Unlike the basaltic oceanic crust, it is rich in granite. The oceanic crust is rich in magnesium and iron. The density of Earth’s continental crust is 2.7 grams per cubic centimeter, whereas, the density of the oceanic crust is roughly 2.9 -3 grams per cubic centimeters.
The variations in the density of lithospheric rock have an evident impact on the elevations of oceanic and continental crust. Those parts of the continental crust that have a lower density floats quite high in the mantle due to their greater buoyancy. The average depth of the oceanic crust is 3790 meters and the average elevation of the continental crust above the sea level is 840 meters. The difference between the elevation and densities of oceanic crust vs continental crust has led to the formation of two main levels of Earth’s surface.
Continental Crust: Formation
After understanding the continental crust definition, let us now go through the formation of continental crust. The continental crust is mainly formed near the subduction zones. The oceanic lithosphere is converted into the mantle of the earth, at the sites of convergent boundaries between the oceanic and continental crust. The top layers of the oceanic plates are subducted under the margins of the continental crust. These scraped-off rocks from the oceanic crust add to the lateral growth of the continental crust. The continental margins are mostly outlined as volcanic arcs, and these volcanoes make an addition to the continental crust.
Island Arc: There are volcanoes aligned along the subduction zones located at the ocean basins. At these sites, the oceanic plates are piled up one above the other. These volcanic arcs along the oceanic subduction zones are known as island arcs. The composition of these island arcs comprises rocky materials ranging from the continental crust and oceanic crust. The density and thickness of the materials constituting the island arcs are likely to vary between that of the continental and oceanic crust. Apparently, the collection of the island arcs led to the formation of the first continent.
The age of continental crust, rather than the age of the oldest continental crust is about 260 million years. Also, the most ancient continental rock found on this planet happens to be 4 billion years old. The youngest of all ocean crusts is most likely to be found at the mid-ocean ridges, that is, at the sea floors at the sea centers. When the oceanic tectonic plates split apart, the molten magma from beneath oozes out to fill in the void created by the movement of the tectonic plates.
The continental crust is said to have been formed by re-elimination. It is a kind of accretionary process. Mostly, accretion is a process, in which small solid rock materials agglomerate to constitute large objects, such as the planets. Initially, the solid particles coming together are microscopic in nature and there is a disc of gas, as well. The total mass of the microscopic particles happens to be around 1 percent of the mass of the gas disc. This process of accretion is quite fast and efficacious.
When the oceanic tectonic plate begins to subduct under the continental tectonic plate, it starts to pull along magma, sediments from the ocean floor, and bigger rocky materials. There is a steady increase in the pressure and temperature with an increase in the oceanic depth. This subsequently leads to the melting of the rocky materials. As the rocks melt, the denser particles sink towards the center and move downward within the descending plates. The less-dense material, that is rich in silica, makes up the granulites and sticks to the continental plates’ bottom. These silica-rich granulites add to the mass of the continental crust.
Continental crust and oceanic crust are two of the most basic areas that are to be understood for the study of the movement of tectonic plates and the consequent phenomena. The continental crust supports the existence of life on the land. The continental crust lies just above the sea level with certain convergent sites with the oceanic crust. The definition, formation of continental crust, and comparison between oceanic vs continental crust are discussed above. Hence, a detailed study of the continental crust facilitates the research on the formation of various landmasses and phenomena like earthquakes and tsunamis.
FAQs on Continental Crust
1. What exactly is the Earth's continental crust?
The continental crust is the thick, outermost layer of our planet that forms the continents and the shallow seabeds near their shores (known as continental shelves). It is primarily made of lighter granitic rocks, which makes it less dense than the crust found under the oceans. This layer holds the oldest rocks on Earth, preserving billions of years of geological history.
2. What is the main difference between continental and oceanic crust?
The main differences are in their thickness, density, and what they are made of. Here’s a quick comparison:
- Thickness: Continental crust is much thicker, averaging about 35-40 km, while oceanic crust is thinner, at around 7-10 km.
- Density: Continental crust is less dense (about 2.7 g/cm³), which allows it to 'float' higher on the mantle. Oceanic crust is denser (about 3.0 g/cm³).
- Composition: Continental crust is mostly made of granite-like rocks (rich in silica and alumina), while oceanic crust is composed of denser basaltic rocks (rich in silica and magnesium).
3. What are the main types of rocks found in the continental crust?
The continental crust contains a wide variety of rocks. It is broadly divided into an upper layer, which is rich in granite and sedimentary rocks like sandstone and limestone, and a lower layer composed of metamorphic rocks like gneiss. Overall, its composition is often generalised as 'granitic' due to the prevalence of these less dense rock types.
4. How does the age of continental crust compare to oceanic crust?
Continental crust is significantly older than oceanic crust. The oldest parts of the continents are over 4 billion years old. In contrast, oceanic crust is constantly being recycled—created at mid-ocean ridges and destroyed in deep-sea trenches. Because of this, the oldest oceanic crust is only about 200 million years old.
5. If continental crust is thicker, why does it sit higher than the oceanic crust?
This is because of its lower density. Think of it like a large, thick log floating in water next to a small, thin rock. The log, despite being bigger, is less dense and floats high on the surface. Similarly, the less dense continental crust 'floats' higher on the semi-molten mantle beneath it compared to the much denser oceanic crust. This balancing act is known as isostasy.
6. How do collisions between continental crusts form mountains like the Himalayas?
When two plates carrying continental crust collide, neither one is dense enough to be easily pushed down into the mantle (a process called subduction). Instead, the immense pressure causes the rock layers to buckle, fold, and fault. This crumpling and upward thrusting of the crustal material is what creates massive, non-volcanic mountain ranges like the Himalayas.
7. Why is it important for us to study the continental crust?
Studying the continental crust is vital because it directly impacts our lives. It helps us:
- Understand Earth's long geological history and the movement of continents.
- Locate essential natural resources, including minerals, metals, fossil fuels, and fresh water.
- Predict and prepare for natural hazards like earthquakes and volcanic activity.
