Fig. 3.0.1: Types of plate boundaries (modified after Frisch & Meschede, 2021)

There are three different types of plate boundaries (Fig. 3.0.1):

• Constructive or divergent plate boundary
• Destructive or convergent plate boundary
• Conservative plate boundary, transform fault

When two plates move away from each other, it is called a diverging plate boundary or a constructive one. This is the case, for example, at a mid-oceanic ridge, where there is a spreading zone and where new oceanic crust is created, so to speak, constructed –
therefore constructive plate boundary.

Diverging plate boundaries initially form as a rift in a breaking apart continental plate (Fig. 3.0.2). Asthenospheric material rises and when the rupture is complete and the plates continue to move away from each other, new oceanic crust is formed at a newly initiated spreading center (Fig. 3.0.3). As a result, a new ocean can evolve there, as happened, for example, with the Atlantic at the beginning of the Jurassic period. The Atlantic Ocean, which is now several thousand kilometers wide, developed from a narrow rift between Europe and America.

Converging plates move towards each other, they collide with each other. If at least one of the two plates is an oceanic plate, a subduction zone develops (Fig. 3.0.4). The oceanic lithosphere descends into the Earth’s mantle and is pulled down into the deep Earth’s mantle.

If an ocean is completely subducted, a collision between two continental plates occurs, i.e. both plates that collide with each other consist of continental crust (Fig. 3.0.5). But: continental crust cannot be subducted because it is too light. That’s why in such cases a continent-continent collision occurs, which creates mountains.

At a conservative plate boundary, two lithospheric plates slide past each other, with neither new being constructed, nor being returned to the Earth’s mantle, or colliding with each other. A transform fault is created (Fig. 3.0.6). This type of plate boundary is a very special type because, once the principle of movement at transform faults was understood in the 1960s, it finally helped the plate tectonic theory to achieve its breakthrough.

The figure shows the global distribution of plate boundaries (Fig. 3.0.7). The long lines are dominated by diverging and converging plate boundaries, but the transform faults act as a geometric compensation in many places. Without the transform faults, such a plate pattern with bends and curves would not be possible.