Our continents are part of the lithosphere, which is the uppermost layer of the earth (containing the crust). By the 1960s, the scientific consensus was that the lithosphere is divided into 12 large, tightly fitting plates and several small ones (see Fig. 2.2).
Since these so-called tectonic or crustal plates are riding on a semi-molten underlying material (asthenosphere) the plates are in permanent motion, moving 1 cm to 10 cm per year depending on the location. Six of the large plates bear the continents; the other six are oceanic. Note that plate boundaries and continents do not exactly coincide.
Where tectonic plates diverge (mainly mid-ocean) the semi-molten asthenosphere material can be driven to the earth surface. As a result new (oceanic) earth crust is formed resulting in the so-called oceanic ridges (Fig. 2.3). At other places, instead of divergence there is convergence. If an oceanic and continental plate meet, the denser oceanic plate dives under the continental plate (Fig. 2.3). This process of convergence creates mountains and oceanic trenches and is often accompanied by seismic and vol-canic activity (see Fig. 2.4).
The oceanic ridge system consists of the Mid-Indian Ridge, Mid-Atlantic Ridge and East Pacific Rise (see Fig. 2.4). The age of the crust on both sides of the mid-ocean ridges increases with distance from the ridge. The rates of plate divergence range from 1 to 3.5 \(cm/yr\) at the Mid-Atlantic ridge to 9 to 15 \(cm/yr\) at the East Pacific Rise in the southeastern Pacific. Nowadays, the movements are measured with the aid of satellites using very accurate geodetic positioning systems (such as Differential Global Positioning System (DGPS)).
Figure 2.5 shows the deep rift valley that runs along the axis of the Mid-Atlantic Ridge and becomes visible at the surface on Iceland in a very spectacular way.