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Convergent plate boundaries

Convergent plate boundaries come in several flavors, but they share one thing in common - plate collisions! Take a look at the differences between the three examples on this page.

Collision between oceanic and continental lithosphere

Continental vs. Oceanic Plate Convergence

In a contest between a dense oceanic plate and a less dense, buoyant continental plate, guess which one will sink? The dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere and a subduction zone is born! Where the two plates intersect, a deep trench forms.

Geologists aren't sure how deep the oceanic plate sinks before it completely melts, but we do know that it remains solid far beyond depths of 100 km beneath the Earth's surface.

When the subducting oceanic plate sinks deeper than 100 kilometers, huge temperature and pressure increases make the plate ‘sweat'. Well, not exactly, but the uncomfortable conditions force minerals in the subducting plate to release trapped water and other gasses. The gaseous sweat works its way upward, causing a chain of chemical reactions that melt the mantle above the subducting plate.

This hot, freshly melted liquid rock (magma) makes its way toward the surface. Most of the molten rock cools and solidifies in huge sponge-like magma chambers far below the Earth's surface. Large intrusive rock bodies that form the backbones of great mountain ranges such as the Sierra Nevada form by this process.

Some molten rock may break through the Earth's surface, instantly releasing the huge pressure built up in the gas-rich magma chambers below. Gasses, lava and ash explode out from the breached surface. Over time, layer upon layer of erupting lava and ash build volcanic mountain ranges above the simmering cauldrons below.

The Juan de Fuca plate dives beneath North America

An example of this kind of convergence produces the spectacular volcanic landscape of the Northwest. Off the coast of Oregon, Washington, Alaska and Canada a small plate, the Juan de Fuca, dives beneath North America. This type of convergent plate boundary, called a subduction zone, is known for producing historic earthquakes of great magnitudes.

Look for curved volcanic mountain ranges with deep trenches alongside and it's a safe bet that you're looking at a subduction zone.

Collision between oceanic and oceanic lithosphere

Oceanic vs. oceanic plate convergence

In a contest between a dense oceanic plate and a less dense, buoyant continental plate, you know that it’s the dense oceanic plate that sinks.

What happens when two dense oceanic plates collide? Once again, density is the key!

Remember that oceanic plates are born at midocean ridges where molten rock rises from the mantle, cools and solidifies. Little by little, as new molten rock erupts at the mid-ocean ridge, the newly created oceanic plate moves away from the ridge where it was created. The farther the plate gets from the ridge that created it, the colder and denser ('heavier') it gets.

When two oceanic plates collide, the plate that is older, therefore colder and denser, is the one that will sink.

The rest of the story is a lot like the continental vs. oceanic plate collision we described above. Once again, a subduction zone forms and a curved volcanic mountain chain forms above the subducting plate. Of course, this time the volcanoes rise out of the ocean, so we call these volcanic mountain chains island arcs. The Aleutian Peninsula of Alaska is an excellent example of a very volcanically-active island arc.

Collision between continental and continental lithosphere

Continental vs. continental plate convergence

By this time, you understand enough about plates to guess that when the massive bulk of two buoyant continental plates collide there is bound to be trouble!

The Himalayan mountain range provides a spectacular example of continent vs. continent collision. When two huge masses of continental lithosphere meet head-on, neither one can sink because both plates are too buoyant.

It is here that the highest mountains in the world grow. At these boundaries solid rock is crumpled and faulted. Huge slivers of rock, many kilometers wide are thrust on top of one another, forming a towering mountain range. The pressure here is so great that an enormous piece of Asia is being wedged sideways, slipping out of the way like a watermelon seed squeezed between your fingers.

Click here to learn more about the collision that is creating the Himalayas

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