Geoscience Reference
In-Depth Information
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 hun-
dreds of millions of years, long after the most active movement has ceased. For ex-
ample, 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 col-
lided. 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' con-
taining 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 re-
verse 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 build-
ing 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 Fig-
ures 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 topic 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.
