The San Andreas Fault, a transform boundary, extends for approximately 800 miles through California.
Convergent, divergent and transform boundaries represent areas where the Earth’s tectonic plates are interacting with each other. Convergent boundaries, of which there are three types, occur where plates are colliding. Divergent boundaries represent areas where plates are spreading apart. Transform boundaries occur where plates are sliding past each other.
Oceanic vs. Continental Convergent Boundaries
When oceanic plates collide with continental plates, the denser oceanic plate is forced under the lighter continental plate. This process has three geological results. The continental plate is lifted upwards, creating mountains. As the oceanic plate subducts, a trench is formed. Finally, as the descending plate melts, it leads to volcanic activity on the surface of the continental plate. This is occurring where the oceanic Nazca Plate is subducting under the South American Plate, creating the Andes Mountains and the Peru-Chile Trench.
Oceanic vs. Oceanic Convergent Boundaries
When two oceanic plates collide, the older denser plate subducts. The results of this tectonic collision are similar to those involving oceanic and continental plates. A deep trench is formed on the seafloor. For example, the formidable Marianas Trench has been formed by the subduction of the Philippine Plate under the Pacific Plate. There is also undersea volcanic activity, which over time can form island chains. The Aleutian Peninsula in Alaska is an example of this type of island arc.
Continental vs. Continental Convergent Boundaries
When continental plates collide into one another, neither plate can subduct under the other because they are equally light and buoyant. Instead, they are pressed together under intense pressure. This pressure creates buckling and slipping, both vertically and horizontally. This is the process by which the largest mountains on Earth have been formed. For example, when the Indian and Eurasian Plates collided around 50 million years ago, the result was the formation of the Himalayas and Tibetan Plateau.
Divergent boundaries occur where plates are spreading apart. This spreading is caused by convective forces in the molten magma below them. As they slowly spread apart, this fluid basalt lava fills the gap and quickly solidifies, forming new oceanic crust. When this occurs with continental plates, a rift valley is formed, such as the East African Rift. When this occurs with oceanic plates, a ridge is formed on the seafloor, such as the Mid-Atlantic Ridge. Iceland actually sits atop the Mid-Atlantic Ridge. Eventually, the island will be split into two separate land masses.
Transform boundaries occur where plates are sliding past one another. They are also called conservative boundaries because crust is neither destroyed nor created along them. Transform boundaries are most common on the seafloor, where they form oceanic fracture zones. When they occur on land, they produce faults. These fracture and fault lines typically connect offsetting divergent zones. For example, the San Andreas Fault connects the South Gorda divergent zone, north, to the East Pacific Rise, to the south. On the north end, this fault continues out into the Pacific Ocean as the Mendocino Fracture Zone. Along the San Andreas Fault, the Pacific Plate is moving to the northwest and the North American Plate is moving to the southeast.