Plate Tectonics: Boundary Types and Associated Geological Phenomena

Seismic Discontinuities and Plate Tectonics

Seismic Discontinuity

A Seismic Discontinuity is a boundary between Earth's interior layers, identified by the different behavior of seismic waves passing through the material (changes in velocity and trajectory direction of the wave).

Geological Processes at Plate Boundaries

1. Constructive Boundaries (Divergent)

Oceanic Ridges

These boundaries create new oceanic lithosphere. The following characteristics are present:

• They are fracture zones where mantle material rises to the surface, causing intense fissural volcanism.
• This volcanism produces large volumes of material that create new basaltic oceanic crust.
• Convection currents tend to separate the two sides of the fracture (Seafloor Spreading).
• The depression occupied by the fracture is called a rift.
• Ocean water infiltrates, contacts incandescent rocks, and is ejected at very high temperatures, forming hydrothermal vents.

2. Passive Boundaries (Shear)

Transform Faults

Oceanic ridges are discontinuous and zigzagging fractures. In this case, a shear zone called a transform fault appears. The main feature is that it produces strong seismicity.

3. Destructive Boundaries (Convergent)

Subduction Zones

As oceanic lithosphere moves away from the ridge, it cools and becomes denser. Eventually, it has enough density to sink into the mantle.

• Subducting Plate: This is the oceanic plate that sinks.
• Overriding Plate: The plate that remains on the surface (can be oceanic or continental).

In both cases, the following occurs:

1. Oceanic lithosphere is destroyed.
2. Intense seismicity is produced.
3. Magmatism is produced by the fusion of basalt.
4. Thickening of the overriding plate produces a mountain range (thermal orogeny) or an alignment of volcanic islands (island arc).
5. Metamorphism occurs.

Collision Boundaries (Orogenic Collision)

When collision occurs between two continental plates, one rides over the other, and the convergent movement stops. Several geological processes occur:

1. Sediments accumulated between the two plates are stacked and deformed, forming an orogenic collision suture zone.
2. Metamorphism and magmatism are produced.
3. Causes strong seismicity.

Summary of Plate Boundary Processes