Metamorphic rocks are either igneous or sedimentary rocks that have been
altered by the action of pressure or heat. Marble is an example of a meta-
morphic rock, being limestone whose crystalline structure has been altered
by pressure or heat. Metamorphic rocks tend to be more resistant to weather-
ing agents than the original sedimentary rocks. The almost total loss of pore
space and structure reduces the ability of weathering agents to enter the
material.
Alteration of any rock, no matter what type, depends upon the ability of agents
of weathering to act upon the minerals of which the rock is composed. Alteration
tends to be concentrated on surfaces such as mineral grain boundaries. Certain
rock properties can either help or hinder this activity. Similarly, once altered,
the products of weathering need to be removed to enable further interaction
of rock minerals and weathering agents. For any stone the key properties for
its weathering behaviour are its mechanical strength, its solubility, its poros-
ity (a surrogate for the ability of agents of degradation to enter it) and the past
history of the rock (its memory). Stones which have a low mechanical strength
and which are prone to dissolve with high porosity are likely to adjust or
change rapidly upon exposure. These stones are likely to be degraded very rap-
idly as they adjust to their conditions of exposure. Stones that are less porous,
less soluble will respond more slowly to their conditions of exposure, thus
they will exhibit less degradation and not tend to reflect the prevailing expo-
sure conditions. The latter group may retain more memory, more former sur-
face forms and more weathered products, and so will always be unadjusted to
prevailing conditions. This means that a wide range of degradational forms
may be present upon a single building. Those present on each surface will
reflect the relative degree of adjustment to contemporary conditions and the
expression of the underlying characteristics of the stone.
2.2 Process
Processes of weathering have traditionally been divided into chemical, physical
and biological weathering. Usually, a range of terms such as haloclasty (salt
weathering), gelifaction (ice-induced weathering) and thermal stress are used
to distinguish different types of physical or chemical weathering. It may be more
appropriate to view these different forms of weathering as being the similar
effects of different weathering agents upon the stone. Ice and salt weathering,
for example, both operate to induce stresses within the stone and these stresses
produce the same effects whatever agent induces them. Variationsmay exist as
to where and how agents operate and so, in consequence, when and where
fractures occur. The mechanism by which the fracture occurs, however, is the
same – induced stress. It may be more appropriate to rethink chemical and
physical weathering as crystal lattice breakdown and stress/strain relationships.
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