Biogeography in the Sub-Arctic. Группа авторов

Чтение книги онлайн.

Figure 1 The bathymetry of the North Atlantic, based on satellite sea‐surface altimetry, model DNSC08. The symmetrical disposition of the mid‐Atlantic ridge relative to the bounding continents is well exhibited.

      Source: Based on Anderson, O.B. & Knudsen, P. 2009.

      Because of this mechanism the spreading (and thus growth of the oceanic plate) does not take place continuously but in a jerky, spasmodic manner. For example, generation of a new fissure 1 m wide every 50 years would correspond to an averaged spreading rate of 20 mm/year. Hence, whilst the spreading ocean floors can be crudely considered as analogous to moving walkways, it is intermittent with intervals of tens to hundreds of years intervening between the intrusion of one dyke and the next. The relatively hot rocks composing the axial region have a lower density so that they rise to form the mid‐ocean ridges. With age and cooling they subside to the depths of the abyssal plains.

The mid‐ocean ridges are offset by faults of a type known as transform faults. The motions along these fractures (which can have offsets ranging up to hundreds of kilometres) are dominantly horizontal. In the North Atlantic there are several of these major fractures that subdivide the mid‐ocean ridge between Greenland and Norway. The most prominent are the Senja fracture zone to the south of Spitzbergen and the Jan Mayen fracture zone at ~52°N.

Magnetic Anomalies

As a result of processes in the Earth's core, the magnetic field spontaneously reverses at irregular intervals averaging at 500,000 years. The direction of magnetization is recorded within the oxide minerals that crystallize from the magmas, much as in a compass needle. As they cool, these minerals retain a memory of the field at the time of their formation. As sea‐floor spreading proceeds, alternating strips of ‘normally magnetized’ (i.e. with the magnetic field as it is currently) and those with ‘reversed magnetization’ are left on the ocean floor (Figure 2, see Plate section). It was the discovery of these ocean floor magnetic anomalies by Vine and Matthews (1963) that provided confirmation of Hess's (1962) ocean‐floor spreading hypothesis. Vine and Matthews demonstrated that continuous growth of the ocean floor, in conjunction with the periodic reversals, produces distinct magnetic stripes, developed as mirror‐images on either side of the spreading ridge.

      These magnetic signatures record the past positions of the ridge axis and, since specific ages can now be assigned to the reversals, the age of the ocean floor from new‐formed crust to oldest (first‐formed) crust can be determined.

Mantle Plumes

      Masses of mantle rock rise convectively when their composition and/or temperature confers buoyancy. Temperature, however, is regarded as the dominant factor and great volumes of abnormally hot rock are thought to detach periodically from deeper parts of the mantle to rise as so‐called mantle plumes. Although the concept of mantle plumes dates back to the early 1970s, it remains controversial with opponents of the idea referring simply to foci of high temperature phenomena as ‘hot‐spots’ and denying that such plumes arise from the deep mantle (e.g. Anderson 2005).

      The idea that mantle plumes have a mushroom‐like shape, with a massive plume head and an extremely hot tail, is based on fluid‐dynamical studies. The most voluminous magmatic events during the Earth's history have been related in space and time to an impact at the base of the lithosphere by such a plume head. Such a ‘hot‐spot’ model was outlined for the North Atlantic by White (1988), postulating a central plume causing raised asthenospheric temperatures across a wide region. A review of conflicting hypotheses with respect to the North Atlantic by Meyer et al. (2007) concluded that it would be difficult to present a model explaining both the records of igneous activity and associated uplift (discussed below) without appealing to an up‐rise of hot mantle beneath the lithosphere. Because the various arguments cannot be rehearsed in this chapter the author adopts a partisan attitude, regarding the plume model as the more robust and better supported by evidence.

The Iceland Plume

The sudden and massive onset of magmatic activity in what may be broadly termed the north Atlantic region during the early Cenozoic (i.e. in the Palaeocene and Eocene epochs, between 66 and 34 Ma) has been attributed to a dramatic increase in the temperature of the underlying mantle starting at around 62 Ma. The most plausible explanation for this is that it was due to the arrival at shallow levels of an abnormally hot mantle body, namely a mantle plume. However, some sea‐mounts in the Rockall Trough, dated at 70+/−1 Ma, may indicate some precursor plume activity commencing in the Cretaceous (O'Connor et al. 2000).