When a rock is acted upon by pressure that is not the same in all directions, or by shear stress (forces acting to “smear” the rock), minerals can become elongated in the direction perpendicular to the main stress. The pattern of aligned crystals that results is called foliation.
Foliation can develop in a number of ways. Minerals can deform when they are squeezed, becoming narrower in one direction and longer in another.
Regional metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly those associated with convergent tectonic plates or the roots of previously eroded mountains.
Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism.
|The Formation of Foliated Metamorphic Rock: Slate --- Phyllite ---------- Schist ----- Gneiss --- Migmatite|
These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects, characterised by paired metamorphic belts.
The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane.
|Foliation that develops when minerals are squeezed and deform by lengthening in the direction perpendicular to the greatest stress (indicated by black arrows). Left- before squeezing. Right- after squeezing. Photo: Steven Earle|
If the parent rock (see Figure , above) was composed of several mineral types and the agents of metamorphism included directed and/or shear stress, a planar fabric called foliation will usually be present in the resulting metamorphic rocks.
Foliation may be expressed as (1) alternating layers of differing mineral composition or (2) parallel alignments of platy minerals. Foliation is usually developed during metamorphism by directed stresses; either in the compressional mode (perpendicular) or as shear (parallel).
In order to understand the development of metamorphic foliation, let's consider what would happen to a shale undergoing increasing temperature and pressure in a convergent plate boundary, for example. Recall that shale is a sedimentary rock , which consists mainly of clays. These clays have formed from the chemical weathering of many of our rock-forming minerals that we have examined in the igneous rocks. The clays are deposited as flat layers of microscopic clay flakes.
As metamorphism begins, the clay particles or flakes are slowly heated and "squished" by the increasing pressure. Generally the pressure imposed on the clay minerals is stronger in two directions than in others, and a compressional force begins to affect the minerals. Lower pressures cause only a slight change in the clay flakes in that they are compressed very close together and become much more densely packed.
Some of the clay flakes may begin to recrystallize into very small crystals of mica minerals. Any water remaining in the clay is driven off. During this process, a more dense platy metamorphic rock called slate will form. This rock very closely resembles shale; the difference can be determined by the fact that slate will "clink" like fine china when it is dropped, and only the shale will smell "muddy" when it is wet. Because the clays have begun to recrystallize into micas in slate, this rock has a microcrystalline texture.
As the process of metamorphism continues, additional pressure and temperature will cause the minerals to continue to recrystallize and become larger. The next stage in this transition will be the formation of a phyllite, which has slightly larger (but still microscopic) mica crystals than the slate. Because of the slightly larger crystals, the microcrystalline phyllite will display a characteristic "sheen" that resembles the shine of satin.
Continuing to increase the temperatures and pressures will allow the minerals to recrystallize into still-larger crystals. At this stage, the mica crystals will become visible on the surface of the rock (as may other minerals), and the rock will be called a schist and have what we describe as a crystalline texture.
Finally, temperatures and pressures may increase to the point that the mica crystals can recrystallize into higher temperature and pressure minerals such as feldspars and amphiboles. These minerals will occur as alternating layers of black and white visible crystals. This type of rock may also form from the metamorphic alteration of a felsic igneous rock. In either case, the rock is called a gneiss (pronounced "nice") and it displays a crystalline texture.
Classification of Metamorphic Rocks