Geology Reference
In-Depth Information
Between the mineral sheets: Foliation, or maybe
not
Pressure is one of the causes of metamorphism. The squeezing of rock minerals under
conditions of high pressure forces them to change. Two types of pressure are applied to
metamorphic rocks and illustrated in Figure 7-15. Indirect pressure pushes on the rocks
from all sides, compacting the materials and removing any spaces between crystals or
particles. Direct pressure comes from two opposite directions and elongates the minerals
into parallel layers.
Figure 7-14: Index
minerals in shale
metamorphism.
The elongation of minerals by direct pressure creates a texture specific to metamorphic
rocks called foliation. Foliation occurs when the minerals line up in thin layers under the
application of direct pressure. The minerals are compressed or reshaped into long, lin-
ear forms, illustrated in Figure 7-16.
Figure 7-15: Indir-
ect and direct
pressure.
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