Tectonic Plates and Faults - Earthquake Engineering for Structural Design - page 147

5.2.3 Divergent Boundaries

In plate tectonics, the Divergent boundaries (also known as constructive or
extensional boundaries) are a linear fracture between two tectonic plates which are
moving away from each other, rising convection currents (Nelson, 2003,
Wikipedia, nd). The rising current pushes up on the bottom of the lithosphere,
lifting it and flowing laterally beneath it. The magma (liquid rock) seeps upward to
fill the cracks. In this way, a new crust is formed along the boundary. There are
two divergent boundary types.
Oceanic divergent boundary (Fig. 5.4a) occurs beneath oceanic lithosphere, the
rising convection producing a mid-oceanic ridge. Extensional forces stretch the
lithosphere and produce a deep fissure. When the fissure opens, pressure is reduced
on the super-heated mantle material below. It responds by melting and the new
magma flows into the fissure. The magma then solidifies and the process repeats
itself. The Mid-Atlantic Ridge (Fig. 5.4b) is a classic example of this type of plate
boundary (Wikipedia, nd). It is the result of the separation of some tectonic plates:
the North American plate from the Eurasian plate in the North Atlantic and the
South American plate from the African plate in the South Atlantic. These plates are
still moving apart, so the Atlantic is growing at the ridge, at a rate of about 5-10 cm
per year in the East-West direction. The volcanic country of Iceland, which
straddles the Mid-Atlantic Ridge, offers scientists a natural laboratory about the
process of spreading between North American and Eurasian plates. There are also
other important oceanic divergent boundaries
Continental divergent boundary occurs beneath a thick continental plate (Fig.
5.5). Generally, the pull-apart is not vigorous enough to create a clean, single break
through the thick plate. As the two plates pull apart, normal faults develop on both
sides of the rift and the central blocks slide downwards. Early in the rift forming,
streams and rivers will flow into the sinking rift valley to form a long linear lake or
a shallow sea. The East Africa Rift Valley is a classic example of this plate
boundary. This rift is in a very early stage of development, but the Northern part of
the rift, the Red Sea, is in a more complete stage of evolution, showing the
separation of Saudi Arabia from the African continent. Geologists believe that, if
this spreading continues, the new Somalia plate will break away and Eastern Africa
will become an island, as happened for Madagascar a long time ago.

5.2.4 Convergent Boundaries

In plate tectonics, the Convergent boundaries (also known as destructive
boundaries) are the actively deforming region where two tectonic plates move
towards one other (Nelson, 2003, Wikipedia, nd). This motion of two plates
produces the subduction of one plate beneath the other, or the collision. In the first
case the less dense plate rides over the denser plate, which is subducted. In the
second case, if the densities are similar, the plate's collision occurs. Generally, the
oceanic plates are more dense that the crust plates. In function of these differences
in density, there are the following convergent boundaries.