How Density & Buoyancy Affect Plate Tectonics

Instructor: Amanda Robb
In this lesson we'll be exploring the forces behind plate tectonics. We'll look at different types of plates and their properties, including density and buoyancy, and how it affects the results of plate interactions.

What are Plate Tectonics?

Imagine hiking in the Himalayas. Snow covered mountains soar up into the sky all around you. The mountains are so high in fact that they extend above the cloud line. Very few living things are able to survive at this intense altitude. But, why are the Himalayas so high, while other areas of the Earth are so low and flat? Why are some areas prone to Earthquakes while others have remained stable for millions of years?

The answer is plate tectonics. Plate tectonics is the theory that explains the movement of the Earth's crust, or lithosphere. The lithosphere is made of large areas called plates. These plates are rocky and maintain a rigid shape. Underneath the lithosphere is the asthenosphere, which is viscous, melted rock. The plates in the lithosphere drift around on the currents of the asthenosphere, moving about one to two inches per year. Although this might not sound like much, this tiny amount of movement is responsible for all volcanos, earthquakes, and tsunamis we feel on Earth.

Types of Plates

Plates come in two forms, oceanic and continental. Oceanic plates lie under oceans. They are denser compared to continental crust, meaning they have more mass per unit volume. Continental crust is just the opposite. It is located under land masses and is less dense than oceanic plates.

The reason for the differences in density is the composition of rock in the plates. Oceanic plates are made of dark basalt rock, like the type that makes up the black rocks and sand in Hawaii. Continental plates, on the other hand, are made of rocks similar to granite, like the countertops in your home might be made of. These rocks are less dense, and thus the continental plates are also less dense.

But, why do we care about plate density? Things that are more dense tend to sink. Think of throwing a penny into a pond versus a plastic bottle cap. Although relatively the same size, the penny is much heavier because copper is more dense than plastic, thus the penny sinks and the plastic floats. The plastic is more buoyant, meaning it is more likely to float due to its low density.

When two plates come in contact with each other through plate tectonics, scientists can use the density of the plates to predict what will happen. Whichever plate is more dense will sink, and the less dense plate will float over it. The exact result depends on which types of plates are interacting. Let's look at some examples.

Oceanic and Continental Plates

Think back, which plates are more dense, oceanic or continental? Oceanic plates are made of basalt rock, so they are denser. So, what do you think will happen when an oceanic plate and a continental plate collide? The oceanic plate is denser and sinks due to its lower buoyancy. It is sucked into the asthenosphere and is melted deeper into the Earth, called a subduction zone. The continental plate is less dense and floats over the top of it since it is more buoyant.

The beautiful volcanos of the Pacific Northwest are an example of a convergent boundary between oceanic and continental plates. The Juan de Fuca plate under the Pacific Ocean is driving under the North American plate creating seismic activity under the Pacific northwest coast.

Mount Rainer is formed by the subduction zone between the Juan de Fuca plate and the North American plate
Mount Rainer

Oceanic and Oceanic Plates

So, it's clear who's going under in a battle between an oceanic and continental plate, but what happens if two oceanic plates collide? It turns out that all oceanic plates aren't created equally. New oceanic plate crust is created when two plates slide away from each other. Molten magma spews into the surface and cools into new rock. As the plates spread further apart, the rock cools further and becomes denser.

Whenever two oceanic plates collide, one will be older, thus colder, and more dense. This plate will slide under the newer, less dense plate and again a subduction zone will form creating volcanos arising from the ocean, called a volcanic arc.

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