Tectonic Plates and Faults - Earthquake Engineering for Structural Design - page 145

Figure 5.1 The main tectonic plates (USGS, nd, Wikipedia, nd, Nelson, 2003)
Fault networks . The relative movements are produced along the boundaries with
velocity varying across the Earth, but with an average of the order of tens of
centimeters per year. In the region where the hot molten lava comes out, the
movement of the two boundaries is divergent and so some fault in the form of a rift
is created. Contrary, in the region where the cold rock mass goes into the Earth, the
movement of the two boundaries is convergent and, due to the collision along the
fault, some volcanoes and mountain chains are formed. Considering these relative
movements, different boundary types exist (Fig. 5.2):
- Divergent boundaries , when two plates move away from each other, due to
the ascension of molten lava, which fills the opened fault between the two
plates.
- Convergent boundaries, when two plates move slower in the front or in
contrary directions.
- Transform boundaries, when two plates move side-by-side along the same
fault with different velocities or in opposite directions.
Figure 5.3 shows the plate boundaries which are active at the present time. In
the following, each boundary type will be detailed, with the most important
examples.