Extras for quiz 5

Seite 1

Extras for quiz 5:

Metamorphic rocks (meta = change; morph = form) are those in which the constituent minerals have changed from a previous state while they are still in a coherent solid state.

Igneous rocks which solidify from a melt

Sedimentary rocks, which forms from lithified sediment.

Metamorphism is the process by which the mineralogy and arrangement of rocks (textures) are modified at depth in the Earth in response mainly to changes in temperature and pressure. The key idea in metamorphism is change in the solid state. Occurs well below the Earth's surface Metamorphism produces new minerals that are stable under the conditions operating within the earth's crust.

A protolith is the original, unmetamorphosed rock from which a given metamorphic rock is formed (proto-: first; lithos: rock; both Greek). For example, the protolith of a slate is a shale or mudstone.

Metamorphism occurs at temperatures and pressures between those at which sedimentary rocks form and those at which rocks begin to melt.

The lower limit of metamorphism is at temperatures of around 200°C and pressures of 2 to 3 Kb.

The upper limit of metamorphism is at temperatures of around 700 to 900°C and at pressures around 12 Kb.

Migmatites - Rocks that show the onset of partial melting, that is, they have a mixture of igneous and metamorphic textures

Low-grade metamorphism refers to processes that occur at temperatures below 200 - 320°C

High-grade metamorphism refers to metamorphic processes at high temperature greater than 550°C and high pressure.

The nature of a metamorphic rock will depend on

the original composition of the rock,

the P-T conditions that the rock has been subjected to,

the amount and composition of fluids in pore spaces, and

the amount of time required for reactions to take place

Causes of metamorphism are heat, pressure and fluids

Generally an increase in pressure leads to a reduction in volume, and a consequent increase in density.

The growth of new minerals may also form that have a denser structure.

Pressure vs. Stress and the creation of Foliation

Stress is defined as a force per unit area. Load pressure (or uniform stress or confining pressure or lithostatic pressure) is uniform, i.e. acting equally in all directions. In the Earth it is caused by the weight of the overlying rocks and therefore increases with depth.

Directed pressure (or differential stress) is non-uniform, i.e. it is not equal in all directions. It is caused by tectonic forces. The amount of directed pressure is unrelated to depth of burial. Directed pressure generally causes flattening of mineral grains and the development of platy minerals (especially micas). The alignment of numerous micas will produce a foliation.

A bar is a measure of the pressure of the atmosphere on the earth's surface.

Metamorphism requires pressures greater than 1 Kb, but as the temperatures required for metamorphism do not normally occur until above 10 km.

Seite 2

The spaces between minerals are known as the pore spaces. This pore space is filled with watery fluids, known as intergranular fluid. The fluid may be a liquid, or, at high temperatures, a vapour. The fluid is largely water, but also contains salts and volatiles and other elements.

Intergranular fluids play an important role during metamorphism.

1. Fluids act as catalysts

2. Fluids transfer heat

3. Fluids transport dissolved solids

The released water combines with the intergranular fluid. As a result rocks subjected to high temperature metamorphism tend to have fewer hydrous minerals (e.g. micas, amphiboles) than those subjected to low temperature metamorphism. As pressure increases, the rock density increases and the amount of pore space decreases, driving the intergranular fluid out of the rock. Quartz veins are most common, but precipitation of economic deposits (such as gold) may also occur.

Any chemical reaction needs time. Mineral reactions are exceptionally slow. Generally, metamorphic rocks that have undergone higher pressures and temperatures for longer periods of time tend to be coarser grained than their lower grade counterparts.

Metamorphic changes that take place while pressure-temperature conditions are increasing occur when fluid is abundant and are relatively rapid. This is termed prograde metamorphism.

Any changes that occur when pressure-temperature conditions are declining are much slower, as a large percentage of the intergranular fluid has been lost. Such change is known as retrograde metamorphism.

As metamorphism involves the recrystallisation of minerals into more stable phases, the overall chemical composition of the metamorphic rock must be similar to that of the original parent rock.

Seite 3

Metamorphic changes

Metamorphism involves three types of related changes: Textural, Minerological, and Chemical.

Textural changes

Heating and increasing pressures will change the texture of a rock (its grain size and shape).

Generally, increases in temperatures result in the formation of larger mineral grains.

Schistosity is a type of foliation found in medium to coarse grained rocks. The platy minerals are large enough to be distinguished with a hand lens.

Remembering that most of the materials of the earth are made up of only a few elements and that silicon (Si) and oxygen (O) are the most abundant elements found in crustal rocks

Intergranular fluids are able to transport material from and to the rock.

If there is no chemical change at all during metamorphism, the process is isochemical metamorphism.

If there has been some addition or leaching of chemical components, the process is termed metasomatism.

Seite 4

The main types of metamorphism are contact, regional, burial, dynamic and metasomatic.

Contact Metamorphism is characterised by high temperature, low pressure conditions, and so is also known as thermal metamorphism.

As the metamorphism envelopes an igneous intrusion, the zone of metamorphism is called a metamorphic aureole . This type of metamorphism produces a recrystallisation with no preferred mineral elongation direction, i.e. foliation, resulting in a massive, hard rock of fused, interlocking grains. The rock is a called a hornfels.

A quartz sandstone will recrystallise to a quartzite. A pure limestone will recrystallise to a marble.

Burial metamorphism extends over wide areas and is produced by the pressures and temperatures involved with deep burial of rocks or sediment. There is no directed pressure, so there is no preferred mineral elongation.

Regional metamorphism is characterised by a range of temperature and pressure conditions. This type of metamorphism is induced by an increasing geothermal gradient as rocks are buried and differential stress is created due to the collision of tectonic plates. Thus regional metamorphism is widespread in continental collision zones. The key diagnostic feature of regional metamorphic rocks is the development of a foliation due to the differential stresses.

Dynamic metamorphism Is characterised low temperatures and high pressures. This type of metamorphism is produced by a mechanical breakage, rather than recrystallisation. It is the crushing and grinding that occurs in fault zones.

Metasomatism (also called hydrothermal metamorphism)is commonly associated with contact metamorphism, as the fluids from the cooling pluton combine with convection currents of groundwater and pass through the adjacent rocks.

Seite 5

Similarly, large-scale regional metamorphism may result in the production of large volumes of metamorphic fluids.

Large-scale metasomatism is also operating at the mid-ocean ridges. Seawater penetrates into the diverging plates and is heated. A convection current is formed, and as the heated seawater rises (as a vapour) it alters the ultramafic mantle rocks. Olivine and pyroxene are altered to serpentine, chlorite, epidote and talc. This is known as serpentinisation.

metal-rich plumes, known as black smokers