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Oceanic and Continental Plates

Tiffany Leonard, Amanda Robb
  • Author
    Tiffany Leonard

    Tiffany has worked on science curriculum and lesson writing since 2015. She has her Master's in Geology from the University of Illinois and a Bachelor's in Geology and Physics from Carleton College. She taught geology courses while she was getting her MS and was a TA while at Carleton.

  • Instructor
    Amanda Robb

    Amanda has taught high school science for over 10 years. She has a Master's Degree in Cellular and Molecular Physiology from Tufts Medical School and a Master's of Teaching from Simmons College. She is also certified in secondary special education, biology, and physics in Massachusetts.

Learn about continental crust density and if ocean crust is denser than continental crust. Explore oceanic plate boundaries and which type of crust is more dense. Updated: 12/27/2021

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Oceanic and Continental Plate Tectonics

Plate tectonics is the theory that the surface of the Earth is covered by rigid plates. These plates consist of the lithosphere, which contains the crust and the uppermost rigid layer of the mantle. The plates move relative to each other on the surface, driven by convection, or cycling in the mantle.

The plates interact with each other in three ways:

  1. Divergent
  2. Convergent
  3. Transform

In addition, there are two types of plates: oceanic and continental. Oceanic plates are plates that lie primarily under the ocean. Continental plates are those containing continents.

As the plates interact with each other like when two continents collide together, they shape the topography, or relief, of the Earth's surface. They also can cause geologic events to take place, such as earthquakes and tsunamis. Earthquakes are defined as both slip along a fault or crack in the Earth, and the shaking motion felt by the release of energy caused by such a slip. Tsunamis are large sea waves that result from large earthquakes occurring near the top of the plate under the ocean.

Continental or Oceanic Plate Boundaries.

There are three different types of plate boundaries: divergent, convergent, and transform. These boundaries can occur beneath oceans or continents, though their behaviors can differ.

Plates interact at boundaries in one of three main ways, they spread apart, collide together, or slide past each other.

Diagram of different tectonic boundaries

Divergent Tectonic Boundaries

At divergent boundaries, the plates move away from each other. Most active divergent boundaries are located underneath seas or oceans. This is because, as the plates move apart, new material is added. This material comes from the mantle and creates oceanic crust rather than continental crust.

Spreading centers under oceans are called ridges. Examples include the Mid-Atlantic Ridge and the East Pacific Rise. They are topographic highs because the heat from the mantle causes the new crust to be more buoyant. As the crust ages and is pushed away from the ridge, it becomes colder and denser, forming the ocean basin.

Spreading centers under the ocean are called ridges, and new crustal material is produced at them.

Diagram of a ridge

Spreading can occur within continents as well. The current divergent boundary between the African and Arabian plates began with the two continents connected. When heat accumulates beneath a continent and causes spreading, the process is called rifting. The warm material will push into the continent and create a series of faults. The continent thins and a depression is formed. Eventually, the continent splits into two, and a new ocean forms.

Rifting occurs when hot material from the mantle rises underneath the continent and causes it to move apart.

Diagram of a Rift Zone

Convergent Plate Boundaries

At these boundaries, the plates collide with each other. For collisions that involve oceanic plates, subduction will occur. Subduction is the process of one plate sliding underneath another. The cold, dense, oceanic crust will sink under the more buoyant plate and be recycled into the mantle. Subduction zones create the largest earthquakes and are some of the most volcanically active areas in the world. They are also the regions where tsunamis are typically generated. The Aleutian Arc in Alaska was created by the subduction of one oceanic plate under another oceanic plate.

Oceanic crust will subduct beneath the less dense continental crust.

Diagram of a subduction zone

Continental crust is not dense enough to subduct. When two continents collide together, both plates have the same density and will create a large mountain range instead of a subduction zone. The Himalayas are an example of this type of mountain range, formed by the collision of the Indian and Eurasian plates.

Transform Plate Boundaries

At these boundaries, the plates slide alongside each other. There is no significant difference in behavior based on the type of plates involved in these boundaries. A well-known example of this boundary type is that between the Pacific and North American plates in California, where they create the San Andreas Fault.

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  • 0:04 What Are Plate Tectonics?
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What Is Oceanic Plate Density?

Oceanic plates are plates that are located beneath the ocean. As stated, oceanic plates are denser than continental ones. This is because they are made of denser rocks. Oceanic plates are typically composed of basalt. Basalts are mafic igneous rocks, which means they have a silica (quartz) content of less than 20% and are created by the cooling of lava. Basalts have a density of 3.0 g/cm3, or 200 lbs/ft3.

New oceanic plates are formed at spreading centers such as mid-ocean ridges. As the plates are pushed apart, new material is added from the hot mantle below. A typical sequence of the new oceanic crust has the following layers:

Composition Thickness
Layer 1 Sediments 0.4 km
Layer 2 Basalt 1.4 km
Layer 3 Gabbro and other intrusive igneous rocks 5.0 km

As oceanic crust is the newest crust on the surface of the Earth, it has an age range of 0 to 200 million years, quite young for an Earth that is 4.6 billion years old.

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Frequently Asked Questions

What is the oceanic plate made of?

Oceanic plates are made primarily of mafic igneous rocks, such as basalt. They have a silica content of less than 20%. The material that creates oceanic plates comes directly from the underlying mantle at ocean ridges.

What are the densities of continental and oceanic crust?

Oceanic crust is denser than continental crust. Oceanic crust has an average density of 3 g/cm3 of 200 lbs/ft3 while continental crust has an average density of 2.7 gm/cm3 or 162-172 lbs/ft3.

Is continental crust denser?

Continental crust is not denser than oceanic crust. It is more felsic, with a silica content of greater than 50%, and has an average density of 2.7 gm/cm^3, which is less than 3.0 gm/cm^3, the average density of oceanic crust.

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