Geology Reference
In-Depth Information
Plate 6.4
Weathered balustrade on the Ashmolean Museum, Oxford, England. The balustrade has now been cleaned.
(
Photograph by Heather A. Viles
)
10 microns per year in an urban industrial site (Attewell
and Taylor 1988).
In the last few decades, concern has been voiced over
the economic and cultural costs of
historic buildings
being attacked by pollutants in cities (Plate 6.4). Geo-
morphologists can advise such bodies as the Cathedrals
Fabric Commission in an informed way by study-
ing urban weathering forms, measuring weathering
rates, and establishing the connections between the two
(e.g. Inkpen
et al
. 1994). The case of the Parthenon,
Athens, was mentioned at the start of the chapter.
St Paul's Cathedral in London, England, which is built
of Portland limestone, is also being damaged by weath-
ering (Plate 6.5). It has suffered considerable attack by
weathering over the past few hundred years. Portland
limestone is a bright white colour. Before recent cleaning,
St Paul's was a sooty black. Acid rainwaters have etched
out hollows where it runs across the building's surface.
Along these channels, bulbous gypsum precipitates have
formed beneath anvils and gargoyles, and acids, partic-
ularly sulphuric acid, in rainwater have reacted with the
limestone. About 0.62 microns of the limestone surface
is lost each year, which represents a cumulative loss of
1.5 cm since St Paul's was built (Sharp
et al
. 1982).
Salt weathering
is playing havoc with buildings of
ethnic, religious, and cultural value in some parts of the
world. In the towns of Khiva, Bukhara, and Samarkand,
which lie in the centre of Uzbekistan's irrigated cotton
belt, prime examples of Islamic architecture - including
mausolea, minarets, mosques, and madrasses - are being
ruined by capillary rise, a rising water table resulting
from over-irrigation, and an increase in the salinity of the
groundwater (Cooke 1994). The solution to these prob-
lems is that the capillary fringe and the salts connected
with it must be removed from the buildings, which might
be achieved by more effective water management (e.g.
the installation of effective pumping wells) and the con-
struction of damp-proof courses in selected buildings to
prevent capillary rise.
Weathering plays an important role in releasing trace
elements from rocks and soil, some of which are beneficial
to humans and some injurious, usually depending on the
concentrations involved in both cases. It is therefore rele-
vant to
geomedicine
, a subject that considers the effects
of trace elements or compounds in very small amounts -
usually in the range of 10 to 100 parts per million
(ppm) or less - on human health. For example, iodine is
essential to the proper functioning of the thyroid gland.
