Divergent spreading boundaries

A plate can move relative to another in three different ways (Fig. 23.2). Two plates may separate across a divergent or spreading boundary, where the gap is

Fig. 23.2 The three principal plate interactions and the moving forces. The single arrows show the direction of plate movement, and the double arrows indicate the plate movements and forces related to the sliding of the plate away from the ridge and its sinking into the mantle. At a spreading boundary the plate movement is in opposite directions; at a convergent boundary or subduction zone, one plate moves below another; at a transform fault, slipping (thick line segments) occurs as one plate moves parallel to another. As a plate ages and cools, the thickness of the cold lithosphere increases, thus increasing the gravitational force on the plate segment as it sinks through the mantle. After van Andel (1992).

Fig. 23.2 The three principal plate interactions and the moving forces. The single arrows show the direction of plate movement, and the double arrows indicate the plate movements and forces related to the sliding of the plate away from the ridge and its sinking into the mantle. At a spreading boundary the plate movement is in opposite directions; at a convergent boundary or subduction zone, one plate moves below another; at a transform fault, slipping (thick line segments) occurs as one plate moves parallel to another. As a plate ages and cools, the thickness of the cold lithosphere increases, thus increasing the gravitational force on the plate segment as it sinks through the mantle. After van Andel (1992).

continuously being filled by upwelling material. The present-day spreading boundaries are generally those between oceanic segments of the plates, and they appear as long, uniformly high, ridges (Fig. 23.1). A closer look (through our hypothetical "transparent" ocean) reveals the "filling" process, which takes place in a narrow valley (termed a rift valley) along the uppermost ridge. The depth of the valley varies inversely to the spreading rate, from 0.1 km at a spreading rate ~ 10 cm yr-1 to ~ 1 km at ~ 1 cm yr-1. One or several fissures occur in the floor of the valley and erupt basaltic lava at a rate that is much higher in fast-spreading rifts than in slow ones. The thickness of the layer newly formed from solidified magmas, the basaltic oceanic crust, varies from 1 to ~ 10 km, with a mean value close to 7 km (Humler etal, 1999).

The basaltic magmas are generated by the partial melting of "fertile" peridotite, the dominant rock type of the mantle discussed in Chapter 17. The melting occurs, without heating, through decompression as hot mantle rock rises below the ridge and crosses its solidus. Harzburgitic (or "depleted" peridotitic) rocks, which are residues of this partial melting, underlie the basaltic crust and constitute a subo-ceanic lithosphere up to ~ 100 km thick.

The depth of the ocean floor increases, along with the age of the crust, away from the ridge axis. This is due to cooling both of the oceanic crust as it ages and of the suboceanic lithosphere underlying it, so that they become denser. Fine silt is sedimented onto the basaltic crust. Naturally, the thickness of this sediment cover increases away from the ridge.

Although we see from our virtual satellite that at present the divergent boundaries are generally between oceanic segments of the plates (Fig. 23.1), rifting could start not only via the spreading of an oceanic floor (e.g. the west boundary of Nazca) but also as continental break-up. Thus, the fit of the shapes of some opposing continental coastlines and their geological structure and other features, e.g. S. America and Africa, led Wegener (1915) to propose the drift of these continents away from each other.

The mean speed of a plate's motion (e.g. ~ 1 cm yr-1) and its size (~ 108 cm, both in principle measurable from our satellite) allow the age of the oceanic crust to be estimated at an average ~ 100 Myr. This age, the thickness of the oceanic basaltic crust (~ 7 km), its density (~ 3 g cm-3) and its total surface area, ~ 2.9 x 1018 cm2 (about two-thirds of the total surface of the oceans) together give the present-day rate of ridge magmatism, 6 x 1016 g yr-1. This is by far the most productive magmatic process on Earth (Chapter 24).

Convergent boundaries

The spreading of the plates on the Earth's spherical surface inevitably causes collisions, and the plate boundaries where this occurs are termed colliding or convergent (Fig. 23.2). The colliding plates may interact in two different ways.

In the course of collision one plate can turn downwards and sink back (be subducted) into the mantle; this generally takes place if another colliding plate is thicker and stronger, for example in the collision of oceanic and continental plates (e.g. the oceanic Pacific and continental Eurasian plates). Subduction, however, also takes place when two oceanic plates collide (e.g. in the south-western Pacific Ocean). Subduction consumes most of the oceanic crust and lithosphere that originated at the spreading boundary, along with its sediments. Some of this highly diverse material is released from the subducting slab as fluids and/or melts, which enter the overlying mantle and stimulate its partial melting. The resulting magmas mainly erupt in volcanic arcs, chains of volcanoes parallel to the subduction zone, but can also evolve to silica-rich compositions and ultimately form continental crust. The arc volcanic chains are clearly visible from our satellite; the circum-Pacific volcanic belt presents an example. Finally the remaining slab sinks deep into the mantle, introducing chemical and isotopic heterogeneities in this otherwise relatively homogeneous reservoir; a "mantle wedge" develops between the two plates (Fig. 23.2).

Continued convergence of the oceanic lithosphere can culminate in the collision of two continental masses. As this material has lower density than the oceanic crust and mantle, such a collision cannot be accommodated by subduction. Compression results in local thickening of the crust via intense rock deformation and faulting. This in turn leads to isostatic uplift: orogeny, or the building of mountain chains. An example, the Himalayan chain, also visible from our satellite, is a direct consequence of the collision of the Indian and Eurasian plates.

Continental crustal matter is further evolved by three major processes that operate in and below these mountain chains. First, the high heat flow and deep fluid circulation in this thickened crust can lead to melting, producing granitic magmas that intrude into higher crustal levels, thus augmenting the upper crust. Second, the destruction of the mountains by weathering and erosion is another important process leading to the formation of sedimentary rocks. Third, crustal rocks, whether originally magmatic or sedimentary, that are subjected to heating and high pressures within mountain belts undergo changes in their mineral composition known as meta-morphism. Apart from normally involving the loss of a hydrous component (which escapes as fluid), this process is largely isochemical, but it can drastically change the mechanical properties of rocks and their density, both of which affect continental tectonic processes. All these features are clearly seen in the Himalayan chain.

Transform boundaries

A transform boundary occurs when two plates move approximately parallel to each other but in different directions, or in the same direction but with different velocities. In either case the plates slip past each other with relatively weak interaction (Fig. 23.2). Also, different segments of a given plate may move relative to each other, for example owing to a uniform rate of spreading on the spherical Earth, and the separation between such segments is termed a transform fault. They are perpendicular to oceanic ridges, often offsetting the ridge itself. Transform boundaries and faults process matter in a very minor way compared with spreading ridges and subduction zones and are therefore not discussed further here.

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