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`LESSON 3 METARMOPHIC ROCKS

Factors Controlling Metamorphism

In most cases, the overall chemistry of the metamorphic rock is very similar to that of the parent
rock. A quartz sandstone, for example, will metamorphose into a rock that contains a high
percentage of silica. A calcite‐rich rock such as limestone can metamorphose only into a
calcium‐rich metamorphic rock. A quartz sandstone cannotmetamorphose into a calcium‐rich
rock.

Temperature and pressure.

Temperature and pressure are important factors in determining the new minerals thatform in a
metamorphic rock. Different minerals form under different pressure and temperature conditions.
As pressures and temperatures change, a mineral reaches theedge of its stability field and breaks
down to form new minerals that are stable in the new pressure‐temperature field. Higher‐
temperature minerals tend to be less dense than lower‐temperature minerals. The higher
temperatures also speed up the chemicalreactions that take place during metamorphism.

Water.

The amount of water available for metamorphic reactions and the length of time involved are
important factors in how quickly and intensely metamorphism proceeds. Metamorphic textures
and minerals are most likely formed over 10 to 20 million years orlonger.

Geostatic pressure.

The geostatic pressure, or confining pressure, is the pressure that is equally appliedto all sides
of a deeply buried mass of rock. Geostatic pressure increases with depth.

Differential stress.

Differential stress is usually the result of tectonic forces applied to a body of rock from
different directions. This stress “stretches out” the rock mass into an elongate shape (Figure).
Generally, the greater the differential stress, the greater the degree of stretching. Components of
the rock, such as crystals, fragments, or pillow structures, will also be stretched out, often to the
point where they are difficult to recognize.
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Figure 1: Differential Stress

Compressive stress.

In contrast, a compressive stress is applied from directly opposite directions and

compresses and flattens the rock mass (Figure 2).

Figure 2 :Compressive Stress

Shearing.

Shearing is related to differential stress and forces parts of the rock mass (usually minerals) to
align or grow along a shear plane. Shear planes become zones of weakness along which
mineral grains are subjected to crushing or recrystallization. Water can enter rocks along shear
planes, which speeds up the metamorphic chemicalreactions.

Foliation.

Prolonged compressive stress and differential stress and/or shearing forces the mineral grains in a
metamorphic rock to form parallel layers or bands. This resulting alignment iscalled foliation.
New metamorphic minerals crystallize along this foliation. The angle of the foliation is related to
the direction of the stress and may cross‐cut the original bedding in the rock. A foliation can be
so prominent that the original bedding is impossible to identify.

A rock has a slaty cleavage if it splits easily along abundant, parallel foliation planes.
A schistose foliation is more massive and is identified by coarser‐grained minerals that have
grown along the foliation planes. A schist can also be broken along foliation planes, but they
are more widely spaced than those in a slate. A gneissic texture is common in intensely
metamorphosed rocks where pressures and temperatures were so high that the rock became
plastic, or soft, allowing new minerals to form distinctive light and dark bands.
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Types of Metamorphism

There are two major kinds of metamorphism: regional and contact.

Regional metamorphism.

Most metamorphic rocks are the result of regional metamorphism (also

called dynamothermal metamorphism). These rocks were typically exposed to tectonic forces
and associated high pressures and temperatures. They are usuallyfoliated and deformed and
thought to be remnants of ancient mountain ranges.

Metamorphic grades.

The different groups of minerals, or assemblages, that crystallize and are stable at thedifferent
pressure and temperature ranges during regional metamorphism distinguish distinct
metamorphic grades, or faces. The grades are usually named for the dominant minerals or
colors that identify them (Figure 1).

Figure 1: Regional Metamorphic Rock Facies

In general, proceeding from low grade (lower pressure and temperature) to highgrade
(higher pressure and temperature), the following facies are recognized:

 Zeolite: low temperature, low pressure

 Prehnite‐pumpellyite: low temperature, low‐medium pressure
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 Greenschist: low‐medium temperature, low‐medium pressure

 Blueschist: low‐medium temperature, high pressure
 Amphibolite: medium‐high temperature, medium‐high pressure
 Granulite: high temperature, high pressure

Contact metamorphism.

Contact metamorphism (also called thermal metamorphism) is the process by whichthe

country rock that surrounds a hot magma intrusion is metamorphosed by the high heat flow
coming from the intrusion. The zone of metamorphism that surrounds the intrusion is called the
halo (or aureole) and rarely extends more than 100 meters into the country rock. Geostatic
pressure is usually a minor factor, since contact metamorphism generally takes place less than
10 kilometers from the surface.

Metamorphic Rock Types

Metamorphic rocks are classified by texture and by mineral composition.

Foliated metamorphic rocks.

If a rock is foliated, its name is determined by the type of foliation present and the dominant
minerals—for example, a kyanite schist. If the minerals are segregated into alternating light‐
colored and dark‐colored layers, the rock is called a gneiss. Slates aregenerally fine‐grained,
dark‐colored, metamorphosed sedimentary rocks that split easilyalong slaty foliations and were
formed under low‐grade temperature and pressure conditions. Phyllites are slightly more
metamorphosed than slates and contain mica crystals that impart a glossy sheen. A schist is
coarser grained than phyllite or slate and has aligned minerals that can be identified with the
naked eye. Some varieties of schist are mica, garnet‐mica, biotite, kyanite, and talc schist. A
schistose rock composed of the mineral serpentine is called a serpentinite.

Migmatites form when temperatures are hot enough to partially melt the rock. Themagma is
sweated out, or injected, as layers between foliation planes in the rock.

An example of the categories a shale would pass through as temperatures and pressures increase
(from low grade to high grade) is as follows: shale/slate/phyllite/micaschist/gneiss/migmatite.

Nonfoliated metamorphic rocks.

If a rock is not foliated, its name is derived from its chemical composition. A quartz‐rich rock
such a sandstone, for example, is called a quartzite when it has been metamorphosed. A
metamorphosed limestone is called a marble. When rocks (especially shales and basalts) are
affected by contact metamorphism, they often develop a texture called hornfels. A hornfels rock
is characterized by evenly distributed,very fine‐grained mica crystals that give it a more massive,
equigranular appearance.
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Hydrothermal Rocks

Hydrothermal essentially means “hot water.” Hydrothermal rocks are those rocks whose
minerals crystallized from hot water or whose minerals have been altered by hotwater passing
through them. Thus, these rocks are distinct from metamorphic rocks, which are created by solid‐
state mineral transformations. In fact, many hydrothermal rocks (such as those that form from hot
springs and geysers or crystallize as veins in cracks in other rocks) actually build up in layers,
much as sedimentary rocks do.

Veins result when hot water moves through cracks in the bedrock of the crust. The water
leaches elements from the rocks it passes through. Various minerals are precipitated on the sides
of the crack as the temperatures decrease. The shape andorientation of the minerals depends on
the temperature, pressure, and rate of flow. When all the available space in the crack has been
filled with mineral deposits, the crack is sealed and the vein is complete.

The water involved in hydrothermal processes is usually either seawater that is moving
downward through oceanic crust near midoceanic ridges or meteoric water. Meteoric water is
water that is derived from the atmosphere as rain or snow and that moves down into the bedrock
from the earth's surface. Water trapped in the original sedimentsduring deposition and
lithification (connate water) can also be included in hydrothermalreactions but is not a major
source of hydrothermal fluid. Magmatic water derived frommagmas is also a minor component.

The water is heated to very high temperatures as it moves deeper into the crust. It eventually
rises again, often removing elements from the rocks it passes through andcarrying them in
solution. As the hot water rises toward the surface, it begins to cool.This temperature drop
induces a number of chemical reactions, and hydrothermal minerals are precipitated.

Metasomatism is the process by which hot‐water solutions carrying ions from an outside source
move through a rock mass via fractures or pore space. Some of the rockmass is usually dissolved
away, and the ions introduced by the water are incorporated into the new minerals that
precipitate. Unlike metamorphism, metasomatism can significantly change the overall chemistry
of the parent rock. Elements commonly addedduring metasomatism are iron, sodium, potassium,
oxygen, and silica. Easily soluble elements, such as calcium and magnesium from limestones,
are often dissolved and carried away, creating more room for new chemical reactions.

Metamorphism and Plate Tectonics

Metamorphic rocks result from the forces active during plate tectonic processes. The collision of
plates, subduction, and the sliding of plates along transform faults create differential stress,
friction, shearing, compressive stress, folding, faulting, and increasedheat flow. The tectonic
forces deform and break the rock, creating openings, cracks,
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faults, breccias, and zones of weakness along which magmas can rise. Generally speaking, the
greater the tectonic forces, the higher the pressures and temperatures affecting a rock mass and
the greater the amount of resulting structural deformation andmetamorphism.

Metamorphism Defined

When rocks are subjected to deep burial, tectonic forces such as folding, and high pressures and
temperatures, the textures and mineral compositions begin to change. This process, called
metamorphism, is the solid‐state transformation (no melting) of arock mass into a rock of
generally the same chemistry but with different textures and minerals.

Usually the metamorphic rock looks quite different from the original rock, called the
parent rock or protolith. Metamorphic rocks often show contorted patterns of
folding that indicate they were soft enough to bend (plastic deformation). Folding is achieved
by the application of great pressure over long periods. The intensity of the metamorphism
increases with increasing temperature and/or pressure, and the highest“grade” of metamorphism
approaches partial melting of the rock, almost completing therock cycle.