Understanding Divergent, Convergent, and Transform Edges
Divergent Edges: Most of the diverging edges that produce the expansion of the plates are located along the crests of oceanic ridges. The plates move away from the ridge axis, and fractures once created are filled with molten rock rising from the hot asthenosphere beneath. The expansion of the plates and the ascent of magma add a new oceanic crust (lithosphere) between diverging plates. The extension of the crust is accompanied by alternating episodes of failure and volcanic formation. Adjacent to the axis of expansion, the crustal blocks are bounded by faults, and valleys are elongated, known as rifts. Continuous expansion of the rift valley has increased its depth, widening it into an ocean. At this point, the valley will become a linear, narrow sea, with a mouth similar to the current Red Sea.
Convergent Edges: Since the area of the Earth’s surface remains constant, the lithosphere must also be consumed. Areas of convergence between plates are where the lithosphere is subducted and absorbed into the mantle. When two plates converge, the front edge bends downward, allowing it to fall. The region that produces the decrease of an oceanic plate into the asthenosphere is called a subduction zone. As average oceanic plates slide past each other below, the plate bends, thus producing a trench that can be thousands of kilometers in length.
- Continental-Oceanic Convergence: The continental lithosphere is less dense than the oceanic lithosphere. For this reason, if a continental plate converges with an oceanic plate, the latter is introduced under the continental one. Sediments from the oceanic lithosphere are partially subducted, and most do not subduct. These sediments deform and accumulate, causing a prism known as an accretionary wedge. The prism and the accretionary wedge are formed from sediments that have not yet been accumulated, creating an elongated groove in the oceanic pit. Occasionally, fragments of the oceanic lithosphere that do not subduct ride over the continental lithosphere, leading to volcanic island arcs. The displacement of one plate with respect to the other is not continuous but occurs in jumps, which results in significant earthquakes. Subduction zones are the most seismically active regions. Earthquakes, depending on the depth of the seismic focus, are classified into the following types: shallow, intermediate, and deep. The subducting lithosphere of the oceanic plate and continental plate is cold. The friction with the continental lithosphere increases the temperature, lowering the melting point of the minerals. This allows for partial melting to occur, primarily of minerals rich in silica, which melt at lower temperatures. This magma can lead to volcanic eruptions.
- Oceanic-Oceanic Convergence: The characteristics include the subducting lithosphere with an inclination angle, where the two plates couple in a way that favors the subduction of oceanic sediments. As a consequence, there is no prior accretionary prism, and deep trenches (like the Mariana Trench) and associated magmatic arcs originate from islands.
- Continental-Continental Convergence: If a subducting plate has a section of oceanic lithosphere, the continental lithosphere will not subduct due to its lower density. After the collision of continental plates, rubbing occurs as one continental plate moves over another. This type of convergence has led to the formation of mountain ranges such as the Himalayas and the Alps.
Transform Fault Edges: These are characterized by horizontal sliding failures, where the plates move next to one another without producing or destroying lithosphere. The majority of transform faults connect two segments of a mid-ocean ridge. They are part of prominent lines in the oceanic crust known as fracture zones, with transform faults covered by extensions and inactive sections within the plates. These faults can extend over 100 km and are aligned with the ridge axis.