Across our Earth, different plates collide with one another, move past one another, and pull away from one another. These movements between plates result in a wide variety of different plate boundaries, amazing landscapes, and dangerous hazards. Very broadly, plate interaction can be placed in one of three categories. Below, we will explore the first of these: Convergent Boundaries.

Convergent boundaries are places where two or more tectonic plates move toward each other. The process of convergence relies on compressional stress, which squeezes or pushes the two tectonic plates together. Convergent boundaries, more than any other, are known for building mountain chains. It is because tectonic plates collide with one another that Earth has seen periods of supercontinent formation, in which the continents as we know them today were once merged together into one landmass. The ancient continent of Pangaea is a well-known example of a supercontinent that once existed over 250 million years ago!

The key to predicting what will happen at a convergent boundary is understanding the density of each plate involved in the movement. You may tend to think of density, mass, and weight as all being similar. Whereas weight is the heaviness of an object relative to the planet’s gravitational field, mass is the amount of matter that object contains. Density is different; it is a measure of how tightly packed matter is within a given volume of a material, and it is usually measured in units of “mass per volume”.

Digging Deeper: Weight, Mass, and Density

We just read through a verbal description of weight, mass, and density, but what do they really mean? Notice that the description of weight is “relative to a planet’s gravitational field”. Earth is the only home we know, so we are a bit biased in assuming weight and mass tend to be the same thing in our daily lives, but that wouldn’t be helpful for an astronaut walking on the Moon!

Example: Astronaut Buzz Aldrin takes his first step on the Moon! A spacesuit tends to weigh a whopping 280 lbs, plus let’s assume Buzz was about 150 lbs at the time for a total weight of 430 lbs on Earth. The Moon’s gravity is only one-sixth of the Earth’s! So how much does Buzz weigh on the Moon?

430 lbs / 6 = 71.7 lbs          No wonder Buzz is feeling light!

However, Buzz’s mass on Earth with the spacesuit is about 195 kg. This remains the same on the Moon, in space, and wherever he may go – he just may experience “heaviness” differently in those places!

Let’s assume the volume of Buzz with the spacesuit on is roughly 90 L, or 90,000 cm³. The density of our astronaut (in grams/cm³) is:

Density = 195,000 g / 90,000 cm3 = 2.2 g/cm³

That is pretty dense. Humans without spacesuits are mostly water, and that’s a density of around 1 g/cm³ for comparison.

How does density affect plate motions at convergent boundaries? View the different types of plate collisions below to find out!

Oceanic-Continental Convergent Boundary

Oceanic-continental convergent boundaries occur when a tectonic plate primarily composed of oceanic lithosphere collides with a plate with continental lithosphere. As mentioned in a previous chapter, oceanic lithosphere, or crust, contains more iron and magnesium rich minerals, and it is therefore denser than continental lithosphere. When these very different plates converge, the plate with oceanic lithosphere will buckle beneath the continental plate, and sink into the hot asthenosphere. This process is called subduction [14].

When the subducting plate, which is also known as a slab, is subjected to the high temperatures of the asthenosphere, surface materials that were trapped in its rocks such as hydrated minerals undergo alteration and water vapor escapes. That water vapor lowers the melting temperature of the surrounding rock, and it produces magma. The magma rises and forms volcanic arcs in the mountains along the overlying continental crust.

What Happens to the Subducting Slab?

As the dense, oceanic lithosphere begins to subduct down into the mantle, it continues this journey at an average rate of 25 miles per million years or a half inch per year. This slab also pulls the neighboring ocean floor down into a trench. On average, the ocean floor is around 3-4 km deep. In trenches, the ocean can be more than twice as deep, with the Mariana Trench approaching a staggering 11 km!

Over tens of millions of years, the descending slab will eventually be destroyed in the Earth’s internal heat, although in some cases it will journey far into the mesophere in the lower mantle before this occurs. Today geophysicists image our mantle to find the remains of ancient slabs that were once part of Earth’s surface. All of this is proof that Plate Tectonics has been operating for billions of years, and it has enabled the Earth to become an efficient recycler!

Oceanic-Oceanic Convergent Boundary

An Oceanic-Oceanic convergent boundary describes a collision between two plates composed of oceanic lithosphere. Even though this boundary involves the same type of lithosphere, one of the plates will still subduct beneath the other. Oceanic lithosphere is notoriously dense; in some cases it is even denser than the asthenosphere, which means it never has trouble sinking into the mantle once the subduction process begins!

When two plates made of oceanic lithosphere meet, how can we predict which of the two will subduct? It almost always depends on age. With increasing age, oceanic lithosphere becomes colder and denser. Furthermore, the oceanic lithosphere becomes systematically older as it moves away from the spreading centers at Mid-Ocean Ridges, as you can see in the heat map below:

All of this means that the older, denser, and colder plate at an Oceanic-Oceanic convergent boundary will always subduct into the mantle!

Continental-Continental Convergent Boundary

A Continental-Continental convergent boundary describes the collision of two tectonic plates composed of continental lithosphere. Just like an Oceanic-Oceanic convergent boundary, this type of boundary involves the compression of the same type of lithosphere. However, there is one key difference; subduction never takes place at these boundaries.

The continental lithosphere is not very dense. In fact, compared to the underlying asthenosphere is buoyant! This means that if a force were to push continental lithosphere down into the mantle, the low-density lithosphere would push itself back up like a cork in a glass of water.

What happens at these convergent boundaries? Any underlying, dense oceanic lithosphere attached to the plate might sink, but the two plates of continental lithosphere will collide with one another. Within these collision zones, the lithosphere will deform, thicken, and events of extreme mountain-building will occur. Some of our most famous mountain ranges have been formed as a result of Continental-Continental Convergence! For example, watch the Himalayan mountain range (which contains Mt Everest) form in the video below (0:39 to end)!

Because continental lithosphere never subducts, very little of it has been destroyed in the mantle since it was created. Although continental crust can slowly become weathered down and eroded away by Earth’s atmosphere and hydrosphere, some of it has persisted for billions of years! As a result, geoscientists can piece together the early history of our planet using clues from rocks on our continents.

Something important to keep in mind is that Continental-Continental convergent boundaries do not “just” form! When the plates first collide, at least one of them typically contains oceanic crust. This is due to the fact that over 70% of our planet’s surface is covered by oceanic lithosphere. As a result, the convergent boundary will first form a subduction zone, but after all the oceanic lithosphere has finished subducting, the boundary becomes Continental-Continental. This is how supercontinents form!

***See 2.10  for Text and Media Attributions