Metamorphic Rocks

Describe the process of metamorphism Describe possible changes that result from metamorphism. Include: (i) texture (ii) volume change (iii) chemical change Describe the result of selected rocks being metamorphosed. Include: (i) limestone to marble (ii) sandstone to quartzite (iii) shale to slate (to phyllite to schist to gneiss) (iv) granite to gneiss Contrast the two types of metamorphism. Include: (i) contact (ii) regional Describe the locations where contact metamorphism occurs. Include: (i) beneath lava flows (ii) adjacent to magma intrusions (iii) dykes and sills Describe how contact metamorphism can be used to distinguish between a buried lava flow and an intrusion of magma. Describe the locations where regional metamorphism occurs. Include: (i) areas of mountain building (ii) subduction zones Identify metamorphic rocks

Metamorphism

Metamorphism literally means to “change form.”

Metamorphism refers to a change in mineral composition and texture of a rock which is subjected to

increased temperature and pressure within Earth.

These changes occur at various depths beneath Earth’s surface, from 2 - 3 km depth down deep to the

crust - mantle boundary. Metamorphism can act on any rock type including Igneous, Sedimentary, and

even other Metamorphic rocks.

Metamorphism most often occurs in one of two geologic settings;

1) During mountain building (regional metamorphism)

2) Surrounding masses of molten rock (contact metamorphism.)

Changes that result from metamorphism Include: (i) Texture (ii) Volume change (iii) Chemical change

Metamorphism of rocks can vary in degree from a rock that is slightly changed, low grade

metamorphism (Shale to Slate), to a rock that undergoes such change that the identity of the original

rock cannot be determined, high grade metamorphism (Granite to Gneiss).

During metamorphism the rock undergoing change must remain in a solid state as the changes take

place, for once it melts it is then considered igneous activity.

Metamorphism often changes texture by;

1) Re-crystallization of minerals, and 2) Reorganization of minerals

Metamorphism does not involve a melt and that there is no re-crystallization of minerals from such melt. It is usually the chemically active fluids that serve to dissolve minerals and serve as the medium for the crystallization of new minerals.

Recrystallization of minerals.

Pressure causes minerals to recrystallize which generally forms larger, more dense crystals.

The recrystallization process causes some metamorphic rocks to display visible crystals with no visible

evidence of the original mineral grains. For example, quartz sandstone changes to form quartzite.

This is common during low-grade metamorphism and often produces a non-foliated texture.

Agents of Metamorphism: Three Metamorphic agents cause rocks to change;

1) Increased heat

2) Increased pressure

3) Chemically active fluids

During metamorphism, rocks are often subjected to all three agents at the same time.

Heat:

Most important agent of metamorphism. It provides the energy to drive the chemical reactions that

re-crystallize minerals.

Rocks are heated by burial beneath the surface (30oC/km) and by intrusions of molten material rising

from below. Certain minerals, such as clay, when buried only a few kilometers re-crystallize to become

stable, whereas, other minerals, such as igneous minerals are stable at higher temperatures and

pressures and must be buried deeper (20 km or more) before metamorphism will occur.

Involved in both regional and contact metamorphism

Pressure:

Pressure on rocks is mainly caused by;

1) The weight of the material above which causes compression forces to act equally in all directions.

2) Stress caused by compressional forces as tectonic plates push against one another.

These forces are directional and cause the rock to be squeezed which may result in

folds and a foliated texture.

Pressure increase with depth beneath Earth’s surface.

Involved primarily with regional metamorphism.

Chemically Active Fluids:

Has a strong influence on the metamorphism of rocks.

Water located in pore spaces of rocks is perhaps the most common fluid involved in metamorphism.

Water helps move the ions through the solid rock allowing the rock to re-crystalize into a more stable

structure. It also helps in ion exchange between minerals which is responsible for the formation of

completely different minerals. For example Garnet.

The degree of metamorphism and the effect of each metamorphic agent vary from one environment to

another. The result can be;

1) Low-Grade Metamorphism

Rocks are subjected to small changes in temperatures and pressures which result in slight changes

to the rock. Example: Shale changes to form slate.

2) High-Grade Metamorphism

Rocks are subjected to extreme temperatures and pressures (close to that at which rocks melt) which

result in great changes to the rock.

Example: Granite changes to form Gneiss.

Sample Problem

List the agents which cause metamorphism to occur and explain how it can change quartz sandstone to

form quartzite.

Answer: Agents of metamorphism include, heat, pressure and chemically active fluids. Quartz

sandstone is composed of sand grains cemented together and may contain water in its pore spaces.

Heat and pressure causes the quartz grains to re-crystallize. The chemical re-crystallization of the

minerals is accelerated because of the activity of fluids (water) in the rock which moves the ions around.

This causes larger, denser interlocking crystals to form. This is an example of a non-foliated texture.

Metamorphic Textures

The degree of metamorphism is reflected in the rocks texture and mineralogy.

METAMORPHISM CHANGES TEXTURE

Two textures are formed as a result of metamorphism;

1) Foliated Texture ( Allignment of minerals)

Results when the minerals of a rock are brought into parallel alignment during recrystallization.

Produces a banded or layered appearance.

Directional stress causes minerals to align when re-crystallizing. The orientation of these minerals is

usually perpendicular to the compressional forces.

This gives the resulting metamorphic rock a layered appearance.

This is common during high-grade metamorphism and often produces a foliated texture. Example:

Gneiss.

2) Non-Foliated Texture

Results when the minerals of a rock are recrystallized creating uniform sized crystals with no visible

foliation. This occurs when limestone (consist of pure calcite) undergoes metamorphism and the

smaller calcite crystals combine to form larger interlocking crystals. This process of recrystallization of

calcite forms Marble.

Foliated: show layering or banding of different minerals created by the alignment of minerals during

directional stress and recrystallization (Ex: schist, gneiss)

Non foliated: a mass or network of interlocking crystals consisting of one mineral with no visible layering

or banding (Ex: marble, quartzite)

Note: Metamorphic rocks are generally classified as;

1) Foliated Rocks

2) Non-Foliated Rocks

Before

Metamorphism

After

Metamorphism

Foliated Rocks Include:

1) Slate

Forms from low-grade metamorphism of shale.

Fine-grained foliated rock. But foliation is not visible.

Has excellent rock cleavage and splits easily. This property makes slate useful for tiles and billiard tables

2) Phyllite

Forms from intermediate-grade metamorphism of slate.

Fine-grained foliated rock, with visible foliation.

New minerals are often formed. For example, Garnet.

3) Schist

Forms from high-grade metamorphism of phyllite.

Coarse-grained foliated rock, with distinct foliation.

These rocks are “platy” and can be split into flakes or slabs.

New minerals are often formed. For example, Garnet.

4) Gneiss

Forms from high-grade metamorphism of schist.

Coarse-grained foliated rock, with distinct foliation.

These rocks display elongated and granular minerals which give the rock a dark and light banded

appearance.

Most common minerals are quartz and feldspar.

Non - Foliated Rocks Include:

1.) Marble

Coarse crystalline network of calcite grains that form as a result of recrystallization. The parent rock

of marble is Limestone.

During recrystallization of limestone, bedding, fossils, and other sedimentary features are

destroyed.

Marble is used for statues and gravestones.

2. Quartzite

Forms when silica sand grains and silica cement recrystallize forming a coarse grained network of

silica. The parent rock of quartzite is quartz sandstone.

Moderate to high-grade metamorphism fuses the sand grains. Sometimes outlines of the original

grains may be seen, a feature called ghosting.

Quartzite is a very hard rock.

Metamorphism

These changes take thousands to millions of years to complete. During this period of change minerals

within the rocks flow in solid state and can change on the atomic scale.

Metamorphism most often occurs during one of two geologic events;

1) mountain building (regional metamorphism)

2) intrusion of molten rock (contact metamorphism)

Regional Metamorphism

Occurs where tectonic plates collide, mainly at ocean – continent collision boundaries.

Locations where regional metamorphism occurs Include:

(i) Areas of mountain building

(ii) Subduction zones

This type of environment produces extensive mountain building.

The greatest volumes of metamorphic rocks are formed in this way.

This type of metamorphism is caused by two main conditions;

1) extreme pressure, and 2) extreme heat.

Pressure on rocks within mountain environments is mainly caused by;

1) The weight of the material above, and 2) stress caused compressional forces as tectonic plates

push against one another.

This pressure causes rocks to deform and flow as if they were plastic. The minerals within the rock re-

crystallize and reorganize to form metamorphic textures. Folding is a common feature found in rocks

that have undergone regional metamorphism.

Heat responsible for regional metamorphism within mountain environments is produced as a result of;

1) Geothermal gradient which states that temperature increases 30 degrees Celsius for each

kilometer below the surface, and

2) Intrusion of hot molten rock called magma which causes the surrounding rock to heat up as

heat from magma radiates outward.

Contact Metamorphism

This type of metamorphism occurs when rock is in contact with, or near, a mass of magma (Heat).

Locations where contact metamorphism occurs: Include: (i) Beneath lava flows (ii) Adjacent to magma intrusions (iii) Dykes and sills

The changes are caused primarily by the high temperatures of the molten rock, which in effect “bake”

the surrounding rock.

Examples include Marble and Hornfels.

Contact Metamorphism is indicated by the “xxxx” in the

diagram.

Note: These x’s can allow you to determine the difference between a buried lava flow and an intrusion of magma. If there is no evidence of contact metamorphic at the top then the lava must have reached the surface and cooled there. If there is evidence that the rocks atop the magma have been changed via contact metamorphism then it must be a magmatic intrusion!

Formation of Sedimentary Rocks

Sedimentary rocks account for approximately 5% of the crust. The upper 16 km of Earth.

Regardless of this low percentage, approximately 75% of all rocks exposed at the surface is sedimentary

rock.

Factors leading to the formation of sedimentary rocks include;

1) Erosion and weathering

2) Transportation

3) Deposition

4) Lithification-compaction and cementation

Erosion and Weathering

In reference to the Rock Cycle, the origin of sedimentary rocks begins with the processes of weathering

and erosion.

Pre-existing rocks can be broken down both mechanically or chemically to produce sediment.

Mechanical Weathering happens when a material is broken into smaller pieces which still retain the

characteristics of the original material. Example: freeze – thaw action.

Water freezing cracks fragments of rock

Chemical Weathering happens when a material is altered by chemical action and a new material is

produced which is chemically different than the original material.

Examples: Oxidation reactions - Iron rusting Decomposition of limestone (statues and headstones)

Dissolution- Dissolving minerals by a liquid agent (such as water) example: Halite (salt)

Oxidation-Chemical reaction that causes loss of electrons

Hydrolysis- Water reacts with silicate and carbonate minerals

WATER: most important agent!

Alterationschemical weathering that causes physical changes to rocks

Rock corners are eroded more rapidly than sides (more surface area at corners) spheroidal weathering!

Transportation of Sediment

Erosional agents (such as, water, wind, and ice,) influenced by gravity, transport the sediment to new

locations where it is deposited.

Sediment is broken down during the transport phase. This process is called Abrasion.

Three main agents tend to transport the sediment, these being

1) Water (rivers and groundwater)

2) Ice (glaciers

3) Wind

Deposition of Sediment

Eventually the sediment is deposited in lakes, river valleys, seas, and more frequently the oceans.

In dry areas of the world, sediment (sand and silt) can accumulate in areas called deserts.

Lithification of Sediment

Following deposition, sediment may become lithified, which means turned to solid rock.

Sediment is commonly lithified by two processes;

Compaction: when the sediment size is small (silt and clay), the weight from the layers of sediment

above can compact the lower layers forming solid sedimentary rock.

Cementation: Over long period of time mineral matter dissolved in groundwater pass through pore spaces and cement (stick) sediment particles together to form solid sedimentary rock. Example of cements in sedimentary rocks include calcite, silica, and iron oxide.

Classification of Sedimentary Rocks

Sedimentary rocks usually originate in water environments, either oceans, lakes, or river beds.

Sedimentary rocks are grouped as;

1) Clastic 2) Chemical 3) Biochemical (Organic)

1. Clastic detrital sedimentary rocks are made of fragments of pre-existing rocks that were weathered, eroded, and deposited.

2. Chemical sedimentary rocks are due to the chemical processes of evaporation and precipitation.

3. Biochemical sedimentary rocks originate from organic remains.

Clastic and chemical are the most common and make up the majority of sedimentary rocks found on

Earth’s surface.

Clastic Sedimentary Rocks----- pieces (clasts) of pre-existing rock

The clastic sedimentary rocks that could form depend on the agent(s) of erosion as well as the depositional environments. They are as follows:

1. Fluvial (rivers, streams) - breccia, conglomerate, sandstone, siltstone, shale, and mudstone

2. Lagoonal - siltstone, shale, and mudstone 3. beaches - conglomerate, sandstone 4. deep ocean/marine - includes turbidites (conglomerate, sandstone, siltstone, shale), but

is dominated by chemical sedimentary rocks 5. shallow marine - various types of clastic sedimentary rocks

Consist of solid particles from weathered rocks. These rock fragments include pebbles, sand, silt and

clay. The rock fragments are a result of physical weathering.

Clay minerals and Quartz fragments are the most common type of sediment produced.

clay minerals (weathering of feldspar)

quartz (resistant to weathering)

Presence of minerals (like feldspars) indicates:

- little weathering

- short transport

Primary distinguishing factor in identifying clastic sedimentary rocks: Particle Size!

as seen in the table below….

Particle Name Sediment Name Rock Name

Boulder Pebble Gravel Conglomerate or Breccia

Sand Sand Sandstone

Silt Clay Mud Shale & Siltstone

1. Conglomerate

Consist of poorly sorted rounded gravel size particles.

Form in shoreline and river environments where mixtures of sediments

are deposited.

The rounded rock fragments suggest that the sediment was transported

great distances. This allowed angular edges to be eroded to produce

rounded fragments.

Deposited by strong, turbulent currents: big flooding rivers, steep

streams (near mountains), glaciers

2. Breccia- conglomerate with angular grains- didn’t travel far

Consist of poorly sorted, angular gravel size particles.

Form in shoreline and river environments where mixtures of sediments

are deposited.

The angular rock particles suggest that the sediment was not

transported far from the place where it originated. Thus angular

fragments.

3. Sandstone

Composed of sand grains and consist of well sorted sand size particles. Mostly quartz (strong &

chemically stable

2nd most abundant sedimentary rock

deposited by moderate currents:

- rivers & deltas

- beaches

- wind (sand dunes)

Form in shoreline environments (deltas) where sand is deposited.

Sand size sediment is a result of erosion due to wind, water, and ice acting on rock fragments over a

long period of time.

Shale & siltstone

silt & clay-sized particles (mud, clay, silt compacted to form thin layers.

Form in deeper water environments where clay is transported and

deposited.

Commonly contain organic material within the original clay or mud

which form fossils.

over 1/2 of all sedimentary rocks

particles too small to identify w/ eye

deposited in quiet (slow moving) water

- deep ocean & continental slope

- lakes

- floodplains (siltstone

raw material for brick, tile, pottery, china

Size of clasts (particles)

common name

size

Detrital rock

Gravel Bigger than 2mm Conglomerate or breccia

Sand Sand sized Sandstone

Silt, mud, clay Very fine grained Shale or siltstone

What determines the size of the particles?

The velocity of streams and rivers will also determine type of sediment that is transported, called the

stream load.

The ability of a stream to erode and transport material depends on its velocity.

Several factors determine the velocity of a stream

1) stream gradient

The slope of the stream channel expressed in terms of the vertical drop over a certain distance.

Example: Mountain streams have a high gradient.

Streams gradient changes over its length.The steeper the gradient, the greater the energy.

2) shape, size, and roughness of the channel

The channel controls the flow of a stream and river. Water experiences friction at the base and this will

effect the velocity of the stream.

Large channels have more efficient flow because less water is in contact with the channel.

Large boulders in a channel tend to slow water flow.

3) discharge

The volume of water flowing past a certain point at a given time.

Discharge from streams are not constant and are controlled by other factors such as rainfall & runoff.

If amount of water in the stream increases, the velocity, width, and depth all increase.

Stream Load: How much sediment is transported?

Rivers can carry the eroded sediment, called the stream load, in the following ways;

Stream transport mechanisms:

in solution (dissolved in water)

in suspension

bed load (along bottom)

1) Invisible Load

Material dissolved within the water.

Ground water carries dissolved minerals to the streams and streams may dissolve minerals when it

moves.

2) Suspended Load

Visible cloud of sediment including sand, silt, and clay which is suspended and carried within the water.

The largest part of a streams load is carried this way

3) Bed Load

Larger particles that are bounced rolled and slide along the stream bed.

These particles only move when water velocity is high.

Velocity also determines Size of the sediment that is transported (sorting)

If the velocity of the stream is slow, only the very small (slit and clay) particles are moved.

If the velocity of the stream is fast, larger particles can be transported by the moving water.

The sediment can be sorted according to size depending on the velocity of the moving water. Seen

where streams enter lakes.

Sorting: Degree of similarity in particle size (Sorting does not apply to chemical rocks.)

High Velocity → Low Velocity

Breccia, Conglomerate → Sandstone → Siltstone → Shale

Chemical Sedimentary Rocks

evaporation and precipitation from solution to form “chemical sediments”

precipitation: Process where chemicals dissolved on solution, fall out of solution and forms a solid

material. Most common in shallow water environments.

evaporation :Process where there is a change in state from a liquid to a gas. Chemicals dissolved in the

liquid (water) are left behind as a solid material.

These rocks form as a result of chemical weathering dissolving chemicals and transporting it in solution.

When conditions are right, these dissolved chemicals change back into a solid through the processes of

precipitation and evaporation.

Precipitation may occur as a result of physical processes, or indirectly through life processes of water-

dwelling organisms. Sedimentary rock formed in this way is referred to as Biochemical.

Many organisms extract dissolved minerals to form shells and when they die the shells accumulate on

the sea floor and form a rock called Coquina.

These rocks usually form in water environments such as lakes and shallow seas or oceans.

The chemical sedimentary rocks that are associated with the environments are as follows: Shallow marine - gypsum, halite, sylvite, limestone, dolomite Deep marine/ocean - limestone, dolomite, chert Cave – travertine (stalactites and stalagmites).

Some examples of chemical sedimentary rocks include evaporates such as:

(i)Halite (ii)Gypsum (iii)Sylvite And precipitates such as: (i)Limestone (ii)Dolomite (iii)Travertine

Evaporites

Water evaporates and dissolved particles are deposited. Mostly marine (ocean) rocks, but some lakes.

1) Rock salt (Halite) – (from by evaporation)

Consist of the mineral Halite.

Forms by evaporation of shallow seas and lagoons that have high concentrations of halite in

solution. The mineral precipitates out of solution as the water evaporates.

Common use is table salt and road salt.

2) Rock Gypsum - (form by precipitation and evaporation)

Consist of the mineral Gypsum.

Forms by evaporation of shallow seas and lagoons that have high concentrations of gypsum in

solution. The mineral precipitates out of solution as the water evaporates.

Common use is plaster and gyproc (drywall).

3) Sylvite- Potassium Chloride deposits.

Precipitates:

4) Limestone (Calcite) - (form by precipitation)

10% of all sedimentary rocks (by volume)

Most abundant chemical sedimentary rock

Composed primarily of calcite (calcium carbonate CaCO3

Formed by marine organisms (corals, clams, algae)

Some deposited directly out of ocean or other waters

Most abundant chemical precipitate rock which forms in shallow marine waters.

Often contain shell fossils.

5) Dolomite: is a carbonate mineral

Dolomite appears to form in many different types of environment and can have varying structural,

textural and chemical characteristics. It forms white, gray to pink, commonly curved (saddle shape)

crystals

6) Travertine:

Travertine is a form of limestone deposited by mineral springs, especially hot springs. Travertine often has a fibrous or concentric appearance and exists in white, tan, and cream-colored varieties. It is formed by a process of rapid precipitation of calcium carbonate, often at the mouth of a hot spring or in a limestone cave. In the latter, it can form stalactites and stalagmites, It is frequently used in Italy and elsewhere as a building material.

Stalactites are icicle-like pendants that hang from the ceilings of caverns and form where water seeps through cracks above. Stalagmites form on the floors of caverns and reach upwards towards the ceilings. Both are composed of precipitated travertine (i.e. a type of limestone).

Organic Sedimentary Rocks

These rocks form as a result of once living material accumulating to form solid rock.

(i) coquina (ii) chaulk (iii) chert (iv) limestone (coral) (v) coal

1. Coquina - It is formed when corals and seashells are consolidated into a rock. The component

shells and corals retain much of their shape, and are generally poorly cemented. Coquina was

once used as a building material, although the artificial material of the same name consists of

concrete stucco in which pieces of sea shells are pressed.

Coquina is mostly calcite, although other shell forming minerals (such as aragonite or apatite) are also present.

2. Chaulk: is a soft, white, porous sedimentary rock, a form of limestone composed of the mineral calcite. Calcite is calcium carbonate or CaCO3. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite plates (coccoliths) shed from micro-organisms called coccolithophores. It is common to find chert or flint nodules embedded in chalk. Chalk can also refer to other compounds including magnesium silicate and calcium sulfate.

3. Chert is a sedimentary rock composed mostly of the mineral chalcedony—cryptocrystalline

silica, or quartz in crystals of submicroscopic size. (more below) Chert can form in parts of the

deep sea where the tiny shells of siliceous organisms are concentrated, or elsewhere where

underground fluids replace sediments with silica. Chert nodules also occur in altered limestones.

4. Limestone is a sedimentary rock composed mainly of calcium carbonate (CaCO3), usually calcite,

sometimes aragonite. Additionally it may contain considerable amounts of magnesium

carbonate (dolomite). If limestone’s are built up from corals and coral fragments, they are called

coral lime stones.

5. Coal: The most common organic rock is coal, which forms when plant material in water

saturated environments (swamps) die and accumulate to form peat. As peat is buried it

compresses and eventually changes to form coal.

-There are different kinds of coal, depending on formation process

- major fuel for power plants even today.

The Formation of Coal Involves Several Steps:

Formation of Peat. Swamps are areas where organic matter from plants accumulate. As the plants die

and get buried they compact to become peat. With time and more compaction, almost all of the water

is lost and three different grades of coal result.

Formation of Lignite Coal.Compaction of the peat due to burial drives off volatile components like water

and methane, eventually producing a black-colored, organic-rich coal called lignite .Soft brown coal

which consist of about 40% carbon and do not burn efficiently

Formation of Bituminous Coal.Further compaction and heating results in a more carbon- rich coal called

bituminous coal. Soft coal which consist of about 85% carbon and burns readily but produces a lot of

smoke.

Formation of Anthracite Coal. If the rock becomes metamorphosed, a high grade coal called anthracite is

produced. Hard dark coal which consist of 90% to 95% carbon and burns very hot and clean. Forms as a

result of metamorphic conditions Anthracite coal produces the most energy when burned.

PEAT

LIGNITE

BITUMINOUS

ANTHRACITE