Chapter 7: Metamorphism and Metamorphic Rocks

What is the fascinating characteristic of migmatite rock?

- A fascinating characteristic of migmatites is ptygmatic (pronounced "tigmatic") folding. These are folds look like they should be impossible because they are enveloped by rock which does not display the same complex deformation (Figure 10.23). - The answer to the ptygmatic fold mystery is that the folded layer is much stiffer than the surrounding layers. When squeezing forces act on the rock, the stiff layer buckles but the surrounding rock flows rather than buckling, because it isn't strong enough to buckle

What is Metamorphism?

- Metamorphism is the change that takes place within a body of rock as a result of it being subjected to conditions that are different from those in which it formed. - This involves the rock being deeply buried beneath other rocks, where it is subjected to higher temperatures and pressures than those under which it formed.

Why is the presence of water important in Metamorphism?

- The presence of water is important for two main reasons. First, water facilitates the transfer of ions between minerals and within minerals, and therefore increases the rates at which metamorphic reactions take place. aka the metamorphic process is sped up. - Secondly, water, especially hot water, can have elevated concentrations of dissolved substances, and therefore it is an important medium for moving certain elements around within the crust

What is migmatite?

A rock that has undergone partial melting. It is approximately half igneous and half metamorphic.

What is Seafloor (Hydrothermal) Metamorphism?

At an oceanic spreading ridge, recently formed oceanic crust of gabbro and basalt is slowly moving away from the plate boundary (Figure 10.26). Water within the crust is forced to rise in the area close to the source of volcanic heat, drawing in more water from further away. This eventually creates a convective system where cold seawater is drawn into the crust, heated to 200 °C to 300 °C as it passes through the crust, and then released again onto the seafloor near the ridge.

What are aureoles?

Aureole is a ring around an igneous intrusion. Deep magma bodies cool very slowly and turn into coarse-grained, plutonic rocks like granite and gabbro. During this time they bake a zone, or aureole, of contact metamorphism in the country rock.

What is hydrothermal alteration?

Chemical change in rocks due to interaction with hot water is called hydrothermal alteration

What is contact metamorphism and what does it produce?

Contact metamorphism happens when a body of magma intrudes into the upper part of the crust. Heat is important in contact metamorphism, but pressure is not a key factor, so contact metamorphism produces non-foliated metamorphic rocks such as hornfels, marble, and quartzite.

What is Dynamic Metamorphism?

Dynamic metamorphism is the result of very high shear stress, such as occurs along fault zones. Dynamic metamorphism occurs at relatively low temperatures compared to other types of metamorphism, and consists predominantly of the physical changes that happen to a rock experiencing shear stress. It affects a narrow region near the fault, and rocks nearby may appear unaffected.

What are the different types of Foliated Metamorphic rocks?

Four common types of foliated metamorphic rocks, listed in order of metamorphic grade or intensity of metamorphism are slate, phyllite, schist (pronounced "shist"), and gneiss (pronounced "nice").

Describe Gneiss

Gneiss (Figure 10.17) forms at the highest pressures and temperatures, and has crystals large enough to see with the unaided eye. Gneiss features minerals that have separated into bands of different colours. The bands of colours are what define foliation within gneiss. Sometimes the bands are very obvious and continuous (Figure 10.17, upper right), but sometimes they are more like lenses (upper left). Dark bands are largely amphibole while the light-coloured bands are feldspar and quartz. Most gneiss has little or no mica because it forms at temperatures higher than those under which micas are stable.

What are Hornfels?

Hornfels is another non-foliated metamorphic rock that normally forms during contact metamorphism of fine-grained rocks like mudstone or volcanic rocks. Hornfels have different elongated or platy minerals (e.g., micas, pyroxene, amphibole, and others) depending on the exact conditions and the parent rock, yet because the pressure wasn't substantially higher in any particular direction, these crystals remain randomly oriented.

What are metamorphic zones?

Index minerals have been used to map areas of higher or lower metamorphic intensity, called metamorphic zones. A metamorphic zone is a region bounded by the first appearance of an index mineral.

What does metamorphic zones tell us?

Knowledge of metamorphic zones makes it possible to draw conclusions about the geological conditions in which metamorphic rocks formed.

What is marble?

Marble (Figure 10.19) is metamorphosed limestone. When it forms, the calcite crystals recrystallize (re-form into larger blocky calcite crystals), and any sedimentary textures and fossils that might have been present are destroyed. If the original limestone is pure calcite, then the marble will be white. On the other hand, if it has impurities such as clay, silica, or magnesium, the marble could be "marbled" in appearance (Figure 10.19, bottom).

What are metamorphic rocks classified as?

Metamorphic rocks are broadly classified as foliated or non-foliated.

What are Metamorphic rocks?

Metamorphic rocks arise from the transformation of existing rock types, in a process called metamorphism, which means "change in form". The original rock is subjected to heat and pressure, causing profound physical or chemical change. The protolith may be a sedimentary, igneous, or existing metamorphic rock.

Do metamorphic rocks that form under low-pressure conditions or under the effects of confining pressure become foliated? Why or why not?

Metamorphic rocks that form under low-pressure conditions or under the effects confining pressure, which is equal in all directions, do not become foliated. In most cases, this is because they are not buried deeply enough, and the heat for the metamorphism comes from a body of magma that has moved into the upper part of the crust

What is metasomatism?

Metamorphism in which much of the change is derived from fluids passing through the rock is known as metasomatism.

What is contact metamorphism?

Metamorphism that happens because of proximity to magma is called contact metamorphism

retrograde metamorphism

Metamorphism that occurs as pressures and temperatures are decreasing; for retrograde metamorphism to occur, water must be added.

What are hydrated minerals ?

Minerals that contain water in the form of OH in their crystal structures

What are polymorphs?

Minerals with the same composition but different crystalline structures

Squeezing and heating alone (as shown in Figure 7.5) and squeezing, heating, and formation of new minerals (as shown in Figure 7.6) can contribute to foliation, but most foliation develops when what?

Most foliation develops when new minerals are forced to grow perpendicular to the direction of greatest stress (Figure 7.6).

When does non-foliated metamorphic rocks form?

Non-foliated metamorphic rocks do not have aligned mineral crystals. Non-foliated rocks form when pressure is uniform, or near the surface where pressure is very low. They can also form when the parent rock consists of blocky minerals such as quartz and calcite, in which individual crystals do not align because they aren't longer in any one dimension.

Describe Phyllite

Phyllite (Figure 10.15) is similar to slate, but has typically been heated to a higher temperature. As a result, the micas have grown larger. They still are not visible as individual crystals, but the larger size leads to a satiny sheen on the surface. The cleavage of phyllite is slightly wavy compared to that of slate

Why is pressure important in the metamorphic process?

Pressure is important in metamorphic processes for two main reasons. First, it has implications for mineral stability (Figure 7.3). Second, it has implications for the texture of metamorphic rocks.

What is Quartzite?

Quartzite (Figure 10.20) is metamorphosed sandstone. It is dominated by quartz, and in many cases, the original quartz grains of the sandstone are welded together with additional silica. Sandstone often contains some clay minerals, feldspar or lithic fragments, so quartzite can also contain impurities.

Describe Schist

Schist (Figure 10.16) forms at higher temperatures and pressures and exhibits mica crystals that are large enough to see without magnification. Individual crystal faces may flash when the sample is turned in the light, making the rock appear to sparkle. Other minerals such as garnet might also be visible, but it is not unusual to find that schist consists predominantly of a single mineral.

What is Shock Metamorphism?

Shock wave related deformation and heating during impact events Rocks act as a liquid

Describe Slate

Slate (Figure 10.14) forms from the low-grade metamorphism of shale. Slate has microscopic clay and mica crystals that have grown perpendicular to the maximum stress direction. Slate tends to break into flat sheets or plates, a property described as slaty cleavage

What are some example of non-foliated metamorphic rocks?

Some examples of non-foliated metamorphic rocks are marble, quartzite, and hornfels

What is parent rock and what is usually the parent rock of Metamorphic rocks?

The parent rock is the rock that exists before metamorphism starts. In most cases, this is sedimentary or igneous rock, but metamorphic rock that reaches the surface and is then reburied can also be considered a parent rock.

Why is temperature important?

The temperature that the rock is subjected to is a key variable in controlling the type of metamorphism that takes place.

There are 2 main types of metamorphic rocks, what are they?

There are two main types of metamorphic rocks: - those that are foliated because they have formed in an environment with either directed pressure or shear stress - and those that are not foliated because they have formed in an environment without directed pressure or relatively near the surface with very little pressure at all.

What are index minerals?

They are specific minerals within the metamorphic rocks (slate, schist, gneiss, etc) that are indicative of different grades of metamorphism. They are arranged in order of the temperature ranges within which they tend to stable.

Where is the potential for metamorphism the greatest?

While rocks can be metamorphosed at depth in most areas, the potential for metamorphism is greatest in the roots of mountain ranges where there is a strong likelihood for burial of relatively young sedimentary rock to great depths, as depicted in Figure 7.15.

What is the difference in the formation of slate and phyllite vs schist and gneiss?

While slate and phyllite typically form only from mudrock protoliths, schist and especially gneiss can form from a variety of parent rocks, including mudrock, sandstone, conglomerate, and a range of both volcanic and intrusive igneous rocks.

regional metamorphism

formation of metamorphic rock bodies that are hundreds of square kilometers in size

Subduction Zone Metamorphism

high pressure, low temperature metamorphism that occurs where sediments are carried to great depths by a subducting plate

Where is one place where magma is produced in abundance?

one place magma is produced in abundance, and where contact metamorphism can take place, is along convergent boundaries with subduction zones, where volcanic arcs form (Figure 10.31).