In magma, something you’d only actually see

In this lesson, you will learn the meaning of the term ‘convergent boundary’ in the context of plate tectonics. A number of features and examples of convergent boundaries will be introduced. A short quiz will follow.

What is a Convergent Boundary?

Imagine the world is like an egg, with a shell that is cracked all over. Just like these cracked pieces of eggshell, the entire outer crust of the Earth is broken down into sections, called tectonic plates. But instead of resting on liquid-y egg yolk, these plates float on a sea of hot magma, something you’d only actually see in rare events like volcanoes. This magma is not a liquid – it’s actually solid, but under the incredible pressures beneath the Earth’s crust, even solids flow like honey.

The cracks in the egg are boundaries between plates. A convergent boundary, or destructive boundary, is where two plates are moving towards each other and colliding.

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The pressure and friction is great enough at these boundaries that the material in the Earth’s mantle can melt, and both earthquakes and volcanoes happen nearby.

Features of Convergent Boundaries

When two plates meet, one inevitably gives way and sinks below the other. This is called subduction.

The subduction zone is the area where two plates are sandwiched on top of each other, like a tectonic BLT. Exactly what happens during subduction depends on the type of tectonic plates involved. There are two kinds of plates: oceanic plates and continental plates.

Oceanic plates are exactly as they sound – the plates below the oceans. But they’re defined not so much by the oceans, but by the material they’re made of. Oceanic plates are made mostly of basalt and other mafic rocks. Continental plates are the part that lies above sea level, but again, the real definition is that the crust is made of granite and other felsic rocks.

When two continental plates collide, the impact is so significant that the material in the subduction zone is pushed upwards. This is how mountain ranges form, including even the biggest ones in the world, like the Himalayas.

When one oceanic and one continental plate meet, the denser oceanic plate subducts under the less dense continental plate. This forms a trench on the oceanic side, and volcanoes and mountains on the continental side.

When two oceanic plates meet, they form large trenches underwater. Material that is liquified under the pressure is pushed upwards, forming underwater volcanoes which can create islands.

Examples of Convergent Boundaries

The West Coast of South America is a convergent boundary between the Nazca Plate and the South American Plate. The collision of this oceanic and continental plate was how the Andes Mountains were formed.

Convergent boundaries can also form islands. The northern part of the Pacific Plate is subducting under the North American Plate, and the result is the formation of the Aleutian Islands.The Himalayas are the biggest and tallest mountains in the world, and they formed from the collision between two continental plates — the Eurasian Plate and the Indian Plate.

In fact, the collision continues to this day — the Himalayas are still growing!

Lesson Summary

The Earth’s crust is split, like an eggshell, into many sections called tectonic plates. Convergent boundaries are the borders between the plates that are moving towards each other and colliding. When this happens, one plate usually subducts beneath the other plate, and in the case of two continental plates, the collision can be so violent that the plates merge together.There are two types of plates: oceanic and continental. Oceanic plates are made mostly of basalt and other mafic rocks, whereas continental plates are made of granite and other felsic rocks.When two oceanic plates collide, ocean trenches and volcanoes can form. These volcanoes can lead to the formation of islands.

When two continental plates collide, large mountain ranges form. When one oceanic and one continental plate collide, the denser oceanic plate subducts beneath the continental plate. In this case, an underwater trench can form on the oceanic side, and volcanoes and mountains can form on the continental side.

Features of Convergent Boundaries

Convergent boundaries are where the tectonic plates meet.
Map of tectonic plates

Convergent Boundaries

Two Oceanic Two Continental One of Each
One plate subducts, forming trenches and volcanoes Material from subduction zone pushes up, forming mountains Oceanic plate subducts beneath the continental plate, forming mountains on the continental side and a trench on the oceanic side

Learning Outcomes

Draw upon what you’ve learned from this lesson on convergent boundaries when it’s time to:

  • Specify the cause of a convergent boundary
  • Point out two kinds of tectonic plates
  • Discuss the results of two oceanic plates colliding
  • Name some examples of convergent boundaries
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