READ: Types of Plate Boundaries

The red line is the San Andreas Fault. On the left is the Pacific Plate, which is moving northeast. On the right is the North American Plate, which is moving southwest. The movement of the plates is relative to each other.

Ocean-Continent Convergence

Oceanic crust may collide with a continent. The oceanic plate is denser, so it undergoes subduction. This means that the oceanic plate sinks beneath the continent. This occurs at an ocean trench (Figure below). Subduction zones are where subduction takes place.

As you would expect, where plates collide there are lots of intense earthquakes and volcanic eruptions. The subducting oceanic plate melts as it reenters the mantle. The magma rises and erupts. This creates a volcanic mountain range near the coast of the continent. This range is called a continental arc. The Andes Mountains, along the western edge of South America, are a volcanic arc (Figure below).

Ocean-Ocean Convergence

Two oceanic plates may collide. In this case, the older plate is denser. This plate subducts beneath the younger plate. As the subducting plate is pushed deeper into the mantle, it melts. The magma this creates rises and erupts. This forms a line of volcanoes, known as an island arc (figure below). Japan, Indonesia, the Philippine Islands, and the Aleutian Islands of Alaska are examples of island arcs (figure below).

What do you see at a continent-continent convergent plate boundary? Big mountains! The best place to see two continental plates converging is in the Himalaya Mountains. These mountains are the highest above sea level on Earth. They are very popular with mountain climbers.

Another type of convergent plate boundary is when two continental plates collide. Continental lithosphere is low in density and very thick. Continental lithosphere cannot subduct. So when two continental plates collide, they just smash together. This is just like what happens if you put your hands on two sides of a sheet of paper and bring your hands together. The material has nowhere to go but up (figure below)! Earthquakes and metamorphic rocks result from the tremendous forces of the collision. But the crust is too thick for magma to get through. As a result, there are no volcanoes at continent-continent collision zones.

Mountain Building

Continent-continent convergence creates some of the world’s largest mountains ranges. The Himalayas (figure below) are the world's tallest mountains. They are forming as two continents collide. The Appalachian Mountains are the remnants of a larger mountain range. This range formed from continent-continent collisions in the time of Pangaea.

Summary

• Divergent plate boundaries in the ocean create mid-ocean ridges. This is where new seafloor is created by upwelling magma.
• Divergent plate boundaries rift a continent apart. Eventually a new ocean will form between the two continents.
• Iceland is a location where we can see a mid-ocean ridge above sea level.
• At transform plate boundaries, two plates move in opposite direction.
• Transform faults are the site of massive earthquakes.
• The San Andreas Fault is the boundary between the Pacific and North American plates. It is the site of massive earthquakes.
• When two plates come toward each other, they create a convergent plate boundary.
• If at least one plate is oceanic, there will be subduction.
• Subduction of a plate leads to melting and volcanism.
• An island arc is a line of volcanoes on an oceanic plate. A continental arc is a line of volcanoes on a continental plate.
• Continental crust is too buoyant to subduct.
• When two continental plates converge, they smash together and create mountains.
• The amazing Himalaya Mountains are the result of this type of convergent plate boundary.
• The Appalachian Mountains resulted from ancient convergence when Pangaea came together.