Plate Boundaries

Divergent Plate Boundaries: Where Plates Pull Apart

Divergent plate boundaries are places where two tectonic plates are moving away from each other. They are often also called Constructive Margins or Constructive Boundaries because as the plates separate, magma from the mantle rises to fill the gap, creating new crust. This process is constantly renewing parts of the Earth's surface.

Most divergent boundaries are found on the ocean floor, forming mid-ocean ridges like the Mid-Atlantic Ridge. These ridges are underwater mountain chains created by the rising magma.

Sometimes, divergent boundaries can also occur on land, forming rift valleys, such as the East African Rift. A rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift.

Divergent boundaries are also associated with volcanic activity and shallow earthquakes, because the moving plates can crack and shift as magma pushes upward. Over millions of years, this slow process can reshape continents and oceans, showing just how dynamic our planet really is.

Features Assocaited with Divergent Boundaries:

Rifting (Rift Valleys)
Sea-floor spreading
Earthquakes
Volcanoes
Mountains

Convergent Boundaries: When Plates Collide

Convergent plate boundaries are places where two tectonic plates move toward each other. They are often also called Destructive Margins or Destructive Boundaries because as the plates collide, the crust is often destroyed or recycled. When they collide, the results depend on the type of crust involved:

Continental-Continental Convergent

When two continental plates collide, the crust crumples upward, forming huge mountain ranges like the Himalayas. The Himalayas are home to the tallest mountain in the world: Mount Everest.

Features Associated with C-C Convergent Boundaries:

Earthquakes
Mountains

2. Oceanic-Continental Convergent

An oceanic plate sinks beneath a continental plate in a process called subduction. Subduction is the process that happens at convergent plate boundaries when one tectonic plate sinks beneath another. This occurs when a denser plate collides with a less dense plate. In the case of oceanic-continental convergence, because the oceanic crust is more dense, it gets forced downward into the mantle, while the continental plate moves over the top.

Where the oceanic plate subducts beneath the continental plate, deep-ocean trenches form on the ocean floor just off the coast of continents. A deep-ocean trench is a long, narrow, depression that represents the deepest part of the ocean.

In addition, the subducting oceanic plate will melt as it is forced into the mantle, and the resulting magma bubbles back to the surface forming Continental Volcanic Arcs. The Andes Mountains in South America formed this way.

Features Associated with O-C Convergent Boundaries:

Earthquakes
Volcanoes
Mountains
Deep-ocean Trenches
Subduction
Continental Volcanic Arcs

3. Oceanic-Oceanic Convergent

Much like oceanic-continental convergence, when two ocean plates collide one subducts under the other. Because this time two ocean plates are involved, the older, denser ocean plate subducts beneath the younger, less dense ocean plate. Again, the plate subducting into the mantle melts and the magma bubbles to the surface. Because the subduction is taking place beneath the ocean, volcanos form on the ocean floor instead of a continent. This process leads to underwater volcanoes and in some cases creates islands. These islands are known as volcanic island arcs like The Caribbean Islands.

Features Associated with O-O Convergent Boundaries:

Earthquakes
Volcanoes
Mountains
Deep-ocean Trenches
Subduction
Volcanic Island Arcs
Island Formation

Transform Boundaries: When Plates Slide Side by Side

Transform plate boundaries are places where two tectonic plates move past each other horizontally. Instead of colliding (like at convergent boundaries) or pulling apart (like at divergent boundaries), the plates simply slide alongside one another. Because they do not create nor destroy crust, they are often also called Conservative Margins or Conservative Boundaries.

The sideways motion of transform boundaries creates fault lines, where the Earth’s crust cracks under stress. The most famous example is the San Andreas Fault in California. At transform boundaries, the constant grinding of the plates often causes frequent earthquakes. Unlike other plate boundaries, transform boundaries rarely create volcanoes because no crust is being created or destroyed.

Features Associated with Transform Boundaries:

Earthquakes

Hotspots (Intraplate Volcanism): Volcanoes Away from Plate Boundaries

Most volcanoes form along plate boundaries, but some are created in the middle of tectonic plates. These are called hotspot volcanoes. A hotspot forms when a plume of very hot mantle rock rises toward the surface. As the plume reaches the crust, it melts and creates magma that erupts as a volcano.

Unlike plate boundaries, hotspots stay in one place while the tectonic plate above them slowly moves. This movement creates a chain of volcanoes. The Hawaiian Islands are a perfect example: the Big Island of Hawaii is currently over the hotspot and has active volcanoes, while the older islands to the northwest formed earlier as the plate drifted.