Geology Reference
In-Depth Information
field example is the Skiddaw Granite in the Lake
District, UK, where the crystalline hornfels zone extends more than 3 km
from the exposed granite outcrop (the granite shallowly underlies the
ground surface). The limit of metamorphism can be traced up to about
5 km away from the exposed granite; the metamorphic aureole, mea-
sured at right angles to the granite, is about 1 km thick (Institute of
Geological Sciences, 1971).
Marble forms from the metamorphism of limestone through heat
and pressure, often during mountain-building processes. Metamorph-
ism of sandstone can form quartzite, in which the sand grains are
welded together. This rock is often extremely strong and abrasive to
drills and tunnel boring machines.
Minor intrusive rocks such as dykes and sills, though very hot when
emplaced, often cause little metamorphism, because of their relatively
small volumes, as illustrated in
Figure 3.18
for a basalt dyke cutting
limestone.
Regional pressure during mountain building can impose a marked
cleavage or schistosity perpendicular to the maximum compre-
ssive stress. Rock types formed in this way range from relatively
country rock. A good
