The Earth's crust is fractured into giant pieces, called "tectonic plates." These plates move atop the Earth's mantle, a fluid layer of molten rock. When adjacent plates move horizontally across each other, a transform boundary is formed. Transform boundaries are responsible for forming distinct geological features, such as fault lines and oceanic fracture zones.
Types of Tectonic Boundaries
There are three primary types of tectonic boundaries. Divergent boundaries occur where two plates are spreading apart. These boundaries create new oceanic crust. Convergent boundaries occur where two plates are colliding together. Oceanic plates are forced under continental plates, forming subduction zones. Subduction zones destroy oceanic crust. Transform boundaries occur where two plates are sliding horizontally past each other. They are also referred to as conservative boundaries because crust is neither created nor destroyed.
One of the primary landforms that is produced by a transform boundary is a fault. Typically these faults produce what is known as "strike-slip faults." These faults build up pressure as friction prevents them from sliding. When the pressure exceeds the force of the friction, they violently slip. This slip produces powerful earthquakes. The San Andreas Fault in California is an example of a strike-slip fault. This fault line connects the East Pacific Rise, a divergent zone to the south, with the South Gorda, Juan de Fuca and Explorer Ridge, divergent zones to the north. Viewed from the air, the fault line is represented by a linear, shallow trough. From the ground, the fault line can be identified by several characteristic landforms, including long straight escarpments, narrow ridges and small ponds formed by settling.
Oceanic Fracture Zones
The majority of transform boundaries lie on the seafloor. These oceanic fracture zones form large valleys, or trenches, that connect spreading oceanic ridges. These features can extend anywhere from 100 miles to more than a 1,000 miles, reaching depths of up to five miles. The Clarion, Molokai and Pioneer fracture zones, located off the west coast of California and Mexico, are prime examples. While these zones are inactive, their scars provide a graphic reminder of the power transform boundaries pose to alter the Earth's landscape.
Complex Transform Boundary Features
The Dead Sea Rift represents the combination of a rift with a transform boundary. The rift itself, a continuation of the African Rift, forms the valley through which the Jordan River flows. However, this rift is also the location of a transform boundary, where the Arabian Plate is sliding past the Sinai/Israeli Plate. In this case, both plates are moving northward, but at different rates. This has created a strike-slip fault similar to the San Andreas Fault. This fault produced a major earthquake at its southern end in A.D. 363 that levelled the city of Petra. In 1202, an estimated 7.6 magnitude earthquake struck at the northern end, with an estimated 1 million fatalities. At the time of writing, the fault is missing an estimated 14 feet of slip, meaning another major earthquake is imminent.
- USGS: Understanding Plate Motions
- The Southern California Integrated GPS Network: Plate Boundaries
- USGS: The San Andreas Fault
- Trinity University; Plate Tectonics; Glenn C. Kroeger, Ph.D.
- Science Channel; Along the African Rift; February, 15, 2011
- Thinkstock/Comstock/Getty Images