Plate Boundaries

Plate boundaries are where different plates come together on Earth’s surface. There are three main types: convergent, divergent, and transform.

The red arrows in the map show the directions the different plates are moving. Arrows moving towards each other are convergent boundaries; arrows moving away from each other are divergent boundaries; and arrows moving alongside each other are transform boundaries.

Convergent

Convergent boundaries happen where two plates collide. To envision this, take your imaginary Oreo, and break the top cookie in half. Move them towards each other over the cream filling. Congratulations, you have created a convergent plate boundary!

In a convergent boundary, one plate slides underneath the other, forming a subduction zone. Sometimes, the plates will get ‘caught’ against each other, building tension and pressure as the plates continue to shift. When the plates finally shift, this can cause a major earthquake called a megathrust earthquake. When these earthquakes happen under oceans, the sudden displacement of water will cause a tsunami.

The crumpling and folding of the upper plate against the bottom plate causes orogenesis, or mountain formation (e.g. Himalayas, Alps, Rocky Mountains). As a result, convergent boundaries where volcanic arcs are commonly found – for instance, the Pacific plate rim has volcanic arcs along the Andes Mountains, Japan, and Indonesia. Consider also the prevalence of active volcanoes in the Cascade Mountain range in the Pacific Northwest! Volcanoes tend to be more prevalent because molten rock from the lower mantle can make its way past the crust.

At a convergent plate boundary, one plate (the subducting plate) goes underneath the other plate (the overriding plate), forming a subduction zone. Numerous hazards are associated with convergent boundaries and subduction zones, including volcanic activity, megathrust earthquakes, and tsunamis. As a result, convergent boundaries are considered the most dangerous plate boundary.

Divergent

Divergent boundaries happen where two plates are separating or ‘rifting’. Take the two halves of your imaginary Oreo cookie. Move then over the cream filling so they move away from each other. Congratulations, you have created a divergent plate boundary!

Rifting happens under oceans where Earth’s crust is thinnest. As the two plates move away from each other, magma (molten rock) from the mantle comes up to fill the gap. As it hits the ocean water, it immediately cools to form rock. This process is known as sea floor spreading.

The red zones show divergent plate boundaries where sea floor spreading is occurring.

Divergent boundaries also form a landform known as a mid-ocean ridge - these are underwater mountain systems formed as a result of sea floor spreading. 

Because Earth's crust is thinnest underneath oceans (only 3 miles!), magma rises from the mantle at divergent plate boundaries to form new crust. Underground mountain systems called mid-ocean ridges and rift valleys are created as new crust forms.

Transform

Transform boundaries happen when two plates are sliding past each other. Take your imaginary Oreo halves, and move them so the sides rub against each other, each moving in a different direction. Congratulations – you have created a transform plate boundary!

Transform boundaries and faults move horizontally, with the plates sliding alongside each other.

As with megathrust earthquakes, plates at a transform boundary rub against each other, creating friction. Sometimes the plates can get stuck against one another. Pressure builds until the stress overcomes the friction and the plate boundaries (or fault lines) break loose). This sharp release of energy produces seismic waves, causing an earthquake. For this reason, earthquakes are fairly frequent along transform boundaries.

The San Andreas fault is one example of a transform plate boundary which runs through the state of California. As a result, parts of California experience frequent earthquake activity.