Oceanic Crust vs Continental Crust: What’s the difference?
Our planet earth is estimated to be around 4.5 billion years old. During this time, the earth has been constantly evolving and changing.
The continents we see today have not always been in the same place, and the landmasses have constantly shifted over time.
Oceans and continents are what make up the earth’s surface. However, there is much more than just that.
What makes up the continents and oceans aren’t just rocks and water, but the types of crust that make up the earth’s surface.
Continental and oceanic crusts are what make up the earth’s lithosphere, which is the solid outermost layer that makes up the earth. The crust is a thin and brittle outer layer that sits on top of the earth’s mantle.
That said, oceanic crust and continental crust are significantly different from one another. Here, we will take a look at the differences between oceanic crust and continental crust.
What is Oceanic crust?
Oceanic crust is the topmost layer of the tectonic plates’ oceanic basins. It forms from the cooling and solidification of molten magma that erupts from submarine volcanoes.
It is made up of pillow lavas and a dike complex in the upper oceanic crust and troctolite, gabbro, and ultramafic cumulates in the lower oceanic crust.
The crust sits on top of the mantle’s hardened topmost layer. The crust and mantle together make up the oceanic lithosphere.
It is 6-7 km thick on average but can be up to 30 km thick in areas of active volcanism.
The thickness of the crust varies due to the different densities of the materials it is made up of. The lower density oceanic crust floats on the mantle’s denser mantle material.
The density of the mantle increases with depth. The crust is denser than the mantle material it sits on top of because it is made up of heavier materials, like rocks that contain iron and magnesium.
The average density of the oceanic crust is 3.0 g/cm3 with a range from 2.7 to 3.3 g/cm3.
Despite the fact that no whole section of the oceanic crust has been brought to the surface and examined, scientists have many ways of learning about its composition and structure.
They use seismic waves to map the crust’s thickness and density. Seismic waves are created when an earthquake occurs. The waves travel through the different layers of the Earth and are recorded by seismometers.
Scientists also study samples of oceanic crust that have been brought to the surface by volcanoes or other geological processes.
Fortunately, oceanic crust is much simpler in structure than continental crust, so scientists have an easier time understanding it.
Oceanic crust is substantially simpler than continental crust and can be separated into three levels. According to mineral physics investigations, at lower mantle pressures, the oceanic crust grows denser than the surrounding mantle.
Layer 1
Layer 1 is typically 0.4 km thick. It is composed of unconsolidated or semi-consolidated sediments that are typically thin or absent near mid-ocean ridges but thicken further away from the ridge.
Unlike deep-sea sediments, which are formed of microscopic shells of marine creatures, mainly calcareous and siliceous, near the continental edges are made of volcanic ash and terrigenous sediments brought by turbidity currents.
Layer 2
Layer 2 is broken into two sections: layer 2A, a 0.5 km thick highest volcanic layer of glassy to finely crystalline basalt, typically in the form of pillow basalt, and layer 2B, a 1.5 km thick layer formed of diabase dikes.
Layer 3
Layer 3 is composed of coarse-grained gabbro and cumulate ultramafic rocks generated by delayed cooling of lava beneath the surface. It accounts for over two-thirds of the volume of the oceanic crust and has a thickness of nearly 5 km.
Oceanic crust is continuously being created at mid-ocean ridges through the process of seafloor spreading.
As molten magma rises to the surface and cools, a new oceanic crust is formed. This oceanic crust is pushed away from the ridge by the formation of new magma.
As it moves away from the ridge, the oceanic crust becomes cooler and denser. This drives convection currents in the mantle, which helps to move the tectonic plates around on Earth’s surface.
The oceanic crust is constantly being recycled back into the mantle at subduction zones. Subduction zones are areas where one tectonic plate is being pushed underneath another.
As the oceanic crust sinks into the mantle, it is heated and melted. This molten material then rises back up to the surface and forms a new oceanic crust at mid-ocean ridges.
This cycle of formation and destruction of oceanic crust is known as the Wilson cycle.
Using seismic data, scientists have been able to map all three layers of oceanic crust. The age of the oceanic crust can also be determined using seismic data.
The oldest oceanic crust is found near mid-ocean ridges, while the youngest is found at subduction zones. The average age of oceanic crust is between 20 and 200 million years old.
What is Continental crust?
The Continental crust is the topmost layer of the Earth’s continental landmasses. It consists of igneous, metamorphic, and sedimentary rocks that have undergone a long process of formation, modification, and erosion.
This layer of sedimentary and metamorphic rocks is what gives continentals their distinct, raised appearance. The average density of continental crust is 2.7 g/cm3.
Continental crust is mainly made up of Al-Si minerals, like feldspar, mica, and quartz. It also contains a higher percentage of iron and magnesium than oceanic crust.
Continental crust is 30-70 km thick but can be up to 200 km thick in areas of active mountain building. The thickness of the crust varies due to the different densities of the materials it is made up of.
The average density of the continental crust is 2.7 g/cm3 which is about the same density as granite. That said, the density of the continental crust can range from 2.0 to 3.0 g/cm3.
What are the differences between oceanic crust and continental crust?
There are several key differences between oceanic crust and continental crust:
Composition
The chemical compositions and lithologies of Earth’s oceanic and continental crusts are different.
Oceanic crust is made up of igneous rocks that have a lower percentage of iron and magnesium than continental crust. On the other hand, continental crust is made up of a higher percentage of iron and magnesium than oceanic crust.
The most abundant minerals in the oceanic crust are clinopyroxene, orthopyroxene, and olivine. The continental crust is composed of minerals such as quartz, feldspar, and mica.
Thickness
The oceanic crust is thinner and denser than the continental crust. The average thickness of the oceanic crust is 6-7km, while the average thickness of the continental crust is 30-70km.
Density
Oceanic crust is thinner, denser, and made up of different materials than continental crust. The average density of oceanic crust is 3.0g/cm3, while the average density of continental crust is 2.7g/cm3.
As a result, oceanic crust is more buoyant than continental crust and will float on top of the mantle if it is subducted.
Origin
Oceanic crust forms at mid-ocean ridges and is continuously recycled back into the mantle at subduction zones. Continental crust is made up of recycled oceanic crust and sediments that have been deposited on the continents over time.
Covered Area
The Continental crust accounts for approximately 40% of the Earth’s surface area and 70% of the volume of the Earth’s crust. This means that there is more continental crust than oceanic crust.
Age
Unlike oceanic crust most of which is less than 200 million years old, the age of continental crust can range from 4.4 billion to over 4.5 billion years old. This tells us that continental crust is recycled much less often than oceanic crust.
While most continental crusts has been existing for a long time, a new continental crust occurs occasionally.
For example, the westward movement of the North American plate has scraped up a band of ancient islands, coral reefs, portions of the ocean floor, and other debris along the plate’s western edge.
Terranes that have been accreted in this way are new expansions to the original continent. The movement created various colors and textures in the rocks that make up the Great Basin of North America.
