Southern England. Peter Friend

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      FIG 32. Diagram illustrating the movement processes of plates (not to scale).

      UNDERSTANDING SURFACE MOVEMENTS

      We have been considering the large movement systems that originate within the Earth. There are also more local movement systems operating on the Earth’s surface, which are linked to a very variable degree to the large-scale movements of plate tectonics. To explore this complex linkage further, it will be helpful to look now at different processes that may combine to cause particular local movements.

       Horizontal movements as part of convergence, divergence or lateral transfer

      Tectonic plates are recognised by their rigidity, so there is relatively little horizontal movement between points within the same plate compared to the deformation seen in plate boundary zones. This extreme deformation may involve folding and fracturing of the rock materials, addition of new material from below, or absorption of material into the interior during subduction.

      Nonetheless, deformation is not restricted solely to plate boundaries, and does occur to a lesser extent within the plates. In some cases, major structures that originally formed along a plate boundary can become incorporated into the interior of a plate when prolonged collision causes two plates to join. Southern England includes the remains of a former convergent plate boundary and contains many examples of structures of this sort (particularly around Dorset and the Isle of Wight). These structures have often been reactivated long after they first formed in order to accommodate forces along the new plate boundary via deformation within the plate. Conversely, changes of internal stress patterns can sometimes lead to the splitting of a plate into two, forming a new, initially divergent plate boundary. Many of the oil- and gas-containing features of the North Sea floor originated when a belt of divergent rift faults formed across a previously intact plate.

      It needs to be stressed that the patterns of deformation (fracturing and folding) due to these plate motions occur at a wide range of different scales, from centimetres to thousands of kilometres. Sometimes they are visible at the scale of an entire plate boundary, such as the enormous Himalayan mountain chain that marks the collision of India with Asia.

      The effects of features as large as plate boundaries on landscapes persist over hundreds of millions of years, long after the most active movement has ceased. For example, parts of southwestern England, Wales and the Scottish Highlands are underlain by bedrocks that were formed in convergent boundary zones of the past. The tin and lead mines of Cornwall owe their existence to a 300-million-year-old convergent plate boundary, where an ocean was destroyed as two plates converged and continents collided. The convergence released molten rock that rose in the crust and gradually cooled to form granite, while metals were precipitated in the surrounding crust as ‘lodes’ containing tin and lead (see Chapter 4).

      Mapping the patterns of bedrock exposed at the surface often reveals folds and faults that provide key information about the movements that have taken place during the past. Figure 33 provides a key to some of the terms commonly used to classify these structures as a step towards understanding the sorts of movement patterns that they represent. In broad terms, folds tend to indicate some form of local convergent movement, though they may be the result of larger movement patterns of a different kind. Normal faults tend to indicate divergent movements, at least locally, whereas reverse and strike-slip faults tend to indicate convergence. Two broad types of fold are distinguished: synclines are u-shaped downfolds, while anticlines are the opposite – n-shaped upfolds.

      Further mapping of folds and faults often reveals complex patterns of changing movements. In the example shown in Figure 34, divergent movements in an area of crust produce plastic deformation in the warmer lower crust, and faulting into a number of discrete blocks in the colder, more brittle, upper crust. This is then followed by an episode of convergent movement that results in closing up the upper crustal blocks and further flow in the plastic lower crust, causing crustal thickening and mountain building at the surface.

       Vertical crustal movements as part of other crustal movements

      The movement of lithospheric plates is the main cause of convergent and divergent movements affecting thousands of kilometres of the Earth’s surface. As shown in Figures 33 and 34, these horizontal movements are generally accompanied by vertical movements that can produce very large scenic features, such as a mountain belt or a rift valley. In this book we are primarily concerned with scenic features at a more local scale, so we now consider various other processes that may be important in creating vertical crustal movements without contributions from large-scale plate interactions.

      FIG 33. The most important types of folds and faults, and the local patterns of forces responsible.

       Vertical changes by erosion or deposition

      Addition or subtraction of material to the surface of the Earth is happening all the time as sediment is deposited or solid material is eroded. The field of sedimentology is concerned with the wide range of different processes that are involved in the erosion, transport and deposition of material, whether the primary agent of movement is water, ice, mud or wind. An important point is that few of these sedimentary processes relate directly to the large tectonic movements of the Earth’s crust that we have discussed above. Scenery is often produced by erosion of thick deposits that formed in sedimentary basins where material eroded from the surrounding uplands accumulated. One of the characteristic features of these thick deposits is their layered appearance, which is often visible in the scenery. Layering varies from millimetre-scale laminations produced by very small fluctuations in depositional processes, to sheets hundreds of metres thick that extend across an entire sedimentary basin. These thicker sheets are often so distinctive that they are named and mapped as separate geological units representing significant changes in the local environment at the time they were deposited.

      FIG 34. Example of a cross-section through the crust, showing how a divergent movement pattern (A) may be modified by later convergent movements (B and C).

       Vertical crustal movements resulting from loading or unloading СКАЧАТЬ