CHAPTER 3.3: METAMORPHIC ROCKS

Introduction

Metamorphism - the process of changes in texture and mineralogy of pre-existing rock due to changes in temperature and/or pressure.

Metamorphic means ‘change of form’.

The rocks formed due to the transformation of pre-existing igneous or sedimentary that has been buried deeply within the crust because of the movements of lithospheric plates.

These rocks are subjected to changes in the temperature, pressure and chemical environments inside the earth's crust and thus become unstable.

The minerals undergo recrystallization forming new minerals and new rocks either physically or chemically and the texture, color, structure and chemical composition are modified.

The processes that cause these changes are known as metamorphism (meta-change; morphe - form/shape).

Causes and Changes during Metamorphism

Metamorphism causes changes in texture (recrystallization, alignment of platy minerals) and mineralogy (growth of new minerals that are more stable).

The processes of compaction and recrystallization change the texture of rocks during metamorphism.

Compaction

The grains move closer together.

The rock becomes denser.

Porosity is reduced.

Example: clay to shale to slate

Recrystallization

Growth of new crystals.

No changes in overall chemistry.

New crystals grow from the minerals already present.

A preferred orientation of minerals commonly develops under applied pressure.

Platy or sheet-like minerals such as muscovite and biotite become oriented perpendicular to the direction of force.

This preferred orientation is called foliation.

Metamorphic Textures

Consist of three:

(1) Foliation(2) Lineation(3) Non - foliated or granular

Foliation

A broad term referring to the alignment of sheet - like minerals.

Schistosity – alignment of large mica flakes, as in a mica schist derived from the metamorphism of shale.

Slaty cleavage – alignment of very fine - grained micas, as in a slate derived from the metamorphism of shale.

Phyllitic structure – alignment of fine - grained micas, as in a phyllite.

Gneissic banding – segregation of light and dark minerals into distinct layers in the rock, as in a gneiss.

Lineation

Refers to the alignment of elongated, rod - like minerals.

Lineation is a texture commonly seen in the metamorphic rock amphibolites derived from the metamorphism of basalt.

E.g. amphibole, pyroxene, tourmaline, kyanite, etc.

Non - foliated or granular

Those which are composed of equidimensional grains.

There is no preferred orientation. The grains form a mosaic.

E.g. quartz or calcite.

Mineral changes in Metamorphic Rocks

The process consist of:

(1) Recrystallization(2) Formation of new minerals

Recrystallization

Rearrangement of crystal structure of existing minerals.

Commonly many small crystals merge to form larger crystals.

Clay in shale becoming micas in slate, phyllite, and schist.

Formation of new minerals

A number of metamorphic minerals which form during metamorphism and are found exclusively (or almost exclusively) in metamorphic rocks.

Garnet - dark red dodecahedrons (12 sides).

Staurolite - brown lozenge -shaped minerals, commonly twinned to form "fairy crosses". State mineral of Georgia.

Agents of Metamorphism

The principal agents of metamorphism are:

(1) Temperature(2) Pressure(3) Chemically Active Solution

Temperature

The major cause and important agent of metamorphism.

The temperature in the crust at a depth of 15 km is approximately 300°C.

It is noted also that the temperature increases as the depth increase.

This temperature is sufficient for recrystallization for some minerals to begin.

As the rock temperature rises, minerals begin to change from solid state to liquid state and amount of pore fluid in rocks increases.

Heat reduces the ability of rock to withstand deformation and increase the rate of chemical reactions which facilitate the production of new minerals, i.e. new atomic arrangement.

Heat is provided by the nearby intrusions of magma or associated with compression of the crust.

Pressure

The effect of pressure varies at different depths in the crust.

Rocks at shallow depths are relatively cold and brittle, so they can be altered, e.g. fracture or crack when subjected to high pressures.

At greater depths, rocks are much softer because of the high temperatures.

Under action of pressure, they tend to deform by plastic flow.

In the region of plastic deformation, pressure influences the types of new minerals formed and are more tightly packed atomic structure and thus has greater density.

Pressure is derived from deep below the earth surface and also associated with the collision of tectonic plates.

Chemically Active Solution

Rocks that crystallize during the metamorphism does not actually melt but occur in a solid phase state.

The minerals are greatly facilitated by movement of small amounts of liquid or gaseous solutions through the rock which acts as a medium of transport for ions.

These solutions which travel through the pores and cracks of the rock add and remove various ions and molecules as the reactions occur.

In this way new chemical constituents can be brought in contact with mineral grains so that they may diffuse through the mineral structures during recrystallization.

Water may also react as solvent to form another mineral and it can be derived from:

(1) Entrapped water in parent sedimentary rocks at time of deposition

(2) Large watery liquid and vapors from magma

(3) Small amount of water from hydrous mineral

Types of Metamorphism

Three types of metamorphism are:

(1) Contact metamorphism(2) Regional metamorphism(3) Dynamic metamorphism

Contact metamorphism

Contact metamorphism occurs when country rocks are surrounded by igneous intrusion and altered by intruding magma.

Physical changes such as recrystallization occur due to contact metamorphism when original minerals in country rock are permeated by magmatic fluid.

For example, limestone intruded by hot magma may be altered from distance of few inch to several miles from line of contact between the two rocks.

Heat is the most significant influence in contact metamorphism.

Metamorphic rock formed in a baked-zone of the altered country rock.

Contact metamorphism

Igneous Intrusion

Regional metamorphism

The zones of rock alteration are much more extensive (large scale) than rocks altered under contact metamorphism.

Regional metamorphism occurs due to the effects of both pressure and temperature.

The rock layers undergo structural deformation (folded, crushed or fractured) due to great pressures exerted on it and therefore results in the obliteration of any indication of fossils or stratification and realignment of mineral grains.

Regional metamorphism must occur deep within the crust, at least at depths of 10 km or more and is known to be responsible for the forming of mountain ranges.

Specific group of minerals present in rock can be used to infer a certain metamorphic grade. Rocks subjected to high temperatures and pressures are of high grade.

Under various metamorphic grades, different minerals can be produced from the same original rock. For example, kyanite, sillimanite and andalusite have the same chemical composition but different internal structures.

Dynamic metamorphism

Dynamic metamorphism is produced by variable strain, variable pressure, variable temperature and high fluid pressure and normally occurs in active fault zones.

Dynamic metamorphism is metamorphism of rock masses caused primarily by stresses that yield relatively high strain (deformation) rates. More simply, it is metamorphism resulting from deformation.

Temperatures during dynamic metamorphism are typically elevated and may be caused by the deformation process.

Fluids commonly contribute to the metamorphic process, both by altering chemistry and by aiding recrystallization.

Dynamic metamorphism occurs in fault zones when country rock is ground up and partially recrystallized.

Rocks formed by dynamic metamorphism display a sugary texture, moderate foliated texture, and small round rock fragments within the foliation.

Rocks types of Dynamic Metamorphism such as breccias and mylonite.

Types of Metamorphism

Characteristics/Grades of Metamorphic Rocks

Under metamorphism, rocks may undergo changes in their mineral composition leading to formation of new minerals and changes in textures.

Texture of metamorphic rocks is subdivided into:

(1) Foliated Metamorphic Rock(2) Non Foliated Metamorphic Rock

Foliated Metamorphic Rock

During mountain building, rocks are deformed and exposed to both increased pressures and increased temperatures.

At such high temperatures and pressures, rocks can actually flow plastically producing a parallel alignment of mineral grains.

Mineral alignment may also be formed under these extreme conditions, by the growth of crystals with an orientation perpendicular to the direction of greatest pressure.

The parallel alignment of minerals in a metamorphic rock is called foliation

Orientation of platy mineral grains with respect to direction to highest pressure during metamorphism

Cont’d

Metamorphic foliations may be: (1) Slaty(2) Phyllitic(3) Schistose(4) Gneissic

It is important to remember that foliation is completely gradational.

No sharp boundaries exist between the different types.

Slaty

Slaty foliation is a foliation in which microscopic platy minerals have a parallel alignment (Figure 3.57).

This parallel alignment causes the rock to tend to break along parallel planes forming sheet-like pieces and called rock cleavage.

Slaty metamorphic rocks are also finely crystalline (Fine texture, rocks characteristically split into thin slabs) and tend to appear dull.

Rocks with a salty foliation are formed at low pressures and temperatures and are considered low grade metamorphic rocks.

Slate can be converted to phyllite which is coarser grained than shale if the metamorphism grade is increased.

Slaty Outcrop

Phyllitic

Phyllitic foliation is similar to slaty foliation except that the crystals are slightly larger while still fine.

Rocks with a phyllitic foliation tend to be shiny, sometimes wrinkled or folded rock cleavage.

These rocks are formed under slightly higher pressures and temperatures than slaty metamorphic rocks and are of low to medium grade.

Schistose

Schistose foliation is a foliated texture in which the rock is dominated by visible platy minerals such as micas which are in a parallel to sub-parallel orientation (Figure 3.58).

Schistose metamorphic rocks often have a poor rock cleavage.

The mineral grains are large enough to be seen by the unaided eye (medium to coarse grained and coarsely banded metamorphic rock).

The foliation consists of alternate bands of light and dark colored minerals. Light colored minerals are mainly composed of quartz and feldspar whereas the dark layers contain biotite, hornblende, augite and other minerals.

Rocks having a schistose foliation form under conditions of high pressures and medium temperatures (from silicic igneous rocks as well as various types of sedimentary rocks) and are considered medium grade metamorphic rocks.

An Outcrop of Schist

Gneissic

Gneissic foliation is a coarsely foliated texture in which there are alternating layers which are dominated by different minerals.

Rocks with a gneissic foliation are produced by exposure to high pressures and temperatures and are considered high grade metamorphic rocks.

Non Foliated Metamorphic Rock

Non - foliated metamorphic rocks have no preferred orientation of mineral grains.

The grains are also usually of equal size within any particular rock sample.

This texture is common in contact metamorphic rocks but may also be seen in some regionally metamorphosed rocks (i.e. marble and quartzite).

These rocks are usually massive and granular in texture. Non-foliated metamorphic rocks may be of high, medium, or low grade. The most common non - foliated metamorphic rocks are quartzite and marble.

Some common igneous and sedimentary rocks and their metamorphic equivalent

Original rock Metamorphic rockSedimentary:

SandstoneShale

Limestone

QuartziteSlate, phyllite, schist

MarbleIgneous:

Granitic textured igneous rockCompact textured igneous rock

GneissSchist

A simplified flowchart showing the origin of some of the common metamorphic rock

Classification of Metamorphic Rocks

The classification of many metamorphic rocks is based on metamorphic texture which depends on crystal size and, if present, foliation.

Foliated metamorphic rocks are named according to their type of foliation and any visible minerals which may be present.

A rock with a schistose foliation and containing significant proportions of garnets and micas might be called garnet-mica schist.

A rock with a gneissic foliation and containing the same minerals as granite may be called a granitic gneiss.

Some low-grade metamorphic rocks are named by adding the prefix "meta" to the flame of their protolith. For example, a meta-conglomerate is the low grade metamorphic equivalent of a conglomerate.

Other metamorphic rocks are named on the basis of mineral composition. Marble is a metamorphic rock composed almost entirely of calcite or dolomite.

A quartzite is formed by the recrystallization of sandstone under metamorphic pressures and temperatures and consists mainly of quartz.

Quartzite can be distinguished from quartz sandstone by the fact that quartzite breaks through grains while quartz sandstone breaks between grains.

Engineering in Metamorphic Rock Terrains

The engineering characteristics of metamorphic rocks can be generalized into two basic types:

Unaltered and unfractured non foliated metamorphic rocks are considered strong materials and posses similar engineering properties to intrusive igneous rocks. Few limitations for foundations, tunnels and dams and remain stable for vertical excavation slopes.

Foliated metamorphic rocks are more similar to sedimentary rocks because of their tendency to fail along specific planes. Foliation planes in this instance are similar to bedding planes. The orientation of foliation planes with respect to a natural slope or excavation therefore becomes critical to the stability of the material.

In a way similar to the igneous and sedimentary rocks, the ultimate behavior of a metamorphic rock mass depends upon the degree and orientation of fractures and the weathering characteristics. These properties must be ascertained prior to construction of each individual engineering project.

ROCK CYCLE

Q & AQ & A

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