Meta Rx Lab

Question 1

What are the controls over the metamorphic process?

Answer 1

The MINERAL COMPOSITION of the PARENT ROCK

The TEMPERATURE at which metamorphosis takes place

The AMOUNT AND TYPE OF PRESSURE during metamorphism

The types of fluids (MOSTLY WATER) that are present during metamorphism

The AMOUNT OF TIME AVAILABLE for metamorphism

Question 2

What are the two types of pressure relevant to metamorphism?

Answer 2

• Confining/lithostatic/hydrostatic
• Differential Stress

Question 3

What type of metamorphism occurs over a large geographic area?

Answer 3

Regional Metamorphism

Question 4

What type of metamorphism occurs due to contact with magma?

Answer 4

Contact Metamorphism

Question 5

What is the result of metamorphism regarding mineral composition?

Answer 5

The overall chemical composition generally remains constant.

Question 6

What is foliation in metamorphic rocks?

Answer 6

Reorientation/alignment of platy, bladed, or minerals perpendicular to the applied direction of pressure.

Question 7

True or False: Metamorphic rocks can begin to melt at temperatures greater than 700 °C.

Answer 7

True

Question 8

What is a migmatite?

Answer 8

A mixed rock formed from small amounts of granite magma mixed with metamorphic rock.

Question 9

What happens to grain size with increased temperature during metamorphism?

Answer 9

Grain Size Increases

Question 10

What is the sequence of minerals that are stable in metamorphic rocks as temperature increases?

Answer 10

• Clays
• Mica
• Hornblende
• Pyroxene

Question 11

What is a protolith?

Answer 11

The original, unmetamorphosed rock from which a given metamorphic rock is formed.

Question 12

What are the primary features that are usually destroyed during metamorphism?

Answer 12

Bedding and fossils

Question 13

What is the main difference between foliated and non-foliated metamorphic rocks?

Answer 13

Foliated rocks exhibit mineral alignment, while non-foliated rocks do not.

Question 14

What mineral assemblage is commonly found in Gneiss?

Answer 14

• Biotite
• Hornblende
• Pyroxene
• Quartz
• Feldspars

Question 15

Fill in the blank: The effects of increased pressure during metamorphism include an increase in ______.

Answer 15

Density

Question 16

What is the resulting texture of metamorphic rocks typically described as?

Answer 16

Holocrystalline

Question 17

Metamorphism of pre-existing rocks generally results in:

Answer 17

Loss of water (dehydration)

Loss of water and carbon dioxide (devolatization)

Increased density

Metamorphic reactions can cause the unstable, original minerals to be consumed, the chemical constituents of the rock rearranged, and new mineral assemblages to form.

The overall chemical composition of the resulting metamorphic rock will generally remain constant (for ex., a silica-rich protolith (parent-rock) will produce a silica-rich metamorphic rock).

Resulting texture is holocrystalline (made up entirely of crystals or crystalline particles)

Question 18

How can you distinguish marble from limestone?

Answer 18

Marble
* Can be fine to coarse-grained (=microscopic to
visible mineral grains), it depends on the grain size
of the original parent rock!
* Crystalline texture (meaning rock is
holocrystalline)
* No consistent foliation
* More dense and low porosity

Limestone
* Size of grain can vary from grained to course
grained—and depending on its origin, texture can
vary from clastic/grainy, bioclastic, and even
crystalline if formed by chemical precipitation or
other post depositional processes)
* Fossils/trace-fossils are commonly present
* Can exhibit bedding but usually massive
* More porous than marble and less dense

Question 19

How can you distinguish quartzite to sandstone?

Answer 19

Quartzite
* Medium to Coarse-grained
* Complete Crystalline texture (meaning rock is
holocrystalline)
* Denser and low porosity
* Usually devoid of sedimentary features (but
some may prevail)
* Very hard to scratch—quartz grains are
recrystallized and are no longer bounded or
held together by cement
* Breakage occurs across quartz grains (not
between or around them!)
* No consistent foliation present – although relic
bedding is common

Sandstone
* Visible grains
* Clastic/Grainy texture (grains of sand are cemented
together and individual quartz grains can be seen
with the naked-eye)
* Less dense and high porosity
* Bedding is common
* May contain fossils
* Easier to scratch—resulting in sand grains being
removed/flicked-off from the cement when scratched
* Breakage occurs between or around quartz grains
(not across them!)

Question 20

How can you tell the difference between slate and shale?

Answer 20

Slate
* Excellent rock cleavage due to mineral realignment (foliation) and recrystallization, and so
splitting occurs along foliation planes — for this
rock it is described as “slaty cleavage”
* Cleavage surface commonly display a noticeable
to moderate shine
* Holocrystalline
* A little harder than shale

Shale
* Rock breakage in shale is not as clean as slate
because splitting occurs along bedding planes, so it
breaks in a bulky manner—this breakage for SHALE
is described as fissile (which means to split easily)
* split surfaces appear dull
* clastic texture
* bedding and fossils/trace-fossils are commonly
present
* A little softer than slate

Question 21

How can you tell the difference between amphibolite and basalt?

Answer 21

Amphibolite
* coarse-grained (phaneritic, visible crystals)
* Prismatic/needle-like crystals of
amphibole/hornblende that show alignment

Basalt
* Fine-grained (aphanitic, no visible crystals)
* No layering alignment of minerals (minerals are
randomly arranged)

Question 22

Describe the changes that occur in metamorphic rocks as they increase in grade:
Slate to Schist:

Answer 22

Clays and submicroscopic micas of slate recrystallize into coarse-grained
micas and hornblende, resulting in the reduction of water content as hydrous minerals
lose water during recrystallization. Crystal size increases dramatically. Foliations become
more pronounced in schists and are easier to see with larger mica minerals. The slaty
cleavage in slate changes to schistosity in schist.

Question 23

Describe the changes that occur in metamorphic rocks as they increase in grade:
Schist to Gneiss:

Answer 23

In schist, micas and hornblendes change to pyroxene (although biotite
is also common in gneiss). Crystal size can increase. As metamorphism
proceeds/intensifies, foliation becomes more pronounced, and schistose texture (of
schist) is re-organized into banding (where there is clear segregation of light and dark
mineral bands) that occurs in gneiss

Question 24

How do you distinguish a marble from a quartzite?

Answer 24

Marble reacts to HCl, quartzite does NOT. Marble can
be scratched with a steel nail (sometimes a penny depending on the main carbonate mineral present,
i.e., calcite VS dolomite), quartzite is harder than a steal nail (and harder than glass or a streak plate as it
will scratch these tools too).

Question 25

How do you distinguish a schist from a gneiss?

Answer 25

Schist has abundant mica present (biotite and muscovite). Gneiss has no muscovite (although biotite is common). Although present, quartz and feldspar are not easy to see in schist but are easily visible in gneiss. Although both rocks are foliated, foliation is much more distinct/pronounced in gneiss because it occurs as banding where there is a segregation of light and dark minerals into layers (these bands can be thick or thin, and variable in continuity). Gneiss is harder than schist (gneiss has a hardness of 6-7 on Mohs hardness scale, and schist a 4-5)

Question 26

How do you distinguish a schist from a slate?

Answer 26

Slate has crystals invisible (microscopic) to the naked eye, whereas schist is composed of abundant, medium to coarse-grained micas that are easily visible and identifiable, and make the rock appear shinier and more lustrous. Foliation in schist may be wavy or refolded and is described as schistosity. Foliation in slate is very uniform (this is responsible for its thinsplitting behavior known as slaty cleavage---and this rock is commonly used as roof shingles and chalk boards). Schist is harder than slate (schist has a hardness of 4-5 on Mohs hardness scale, and slate 2.5- 4).

Question 27

How do you distinguish a schist from an amphibolite?

Answer 27

Schists are composed of abundant, visible, platy mica minerals often biotite, whereas amphibolite has abundant, needle-like, prismatic crystals of hornblende (amphibole). Both rocks show alignment of the minerals where these elongated, platy, prismatic or needle-like crystals (due to subjected pressures, shearing, stretching, etc.). This alignment can be different for both, schist exhibit clear foliation (planar layers), and amphibolite may or may not have foliation, but our samples show that the amphiboles are organized linearly.

Question 28

What is the difference between diagenesis and metamorphism?

Answer 28

Diagenesis involves changes in rocks at temperatures below 200°C and pressures below 300 MPa, whereas metamorphism occurs at higher temperatures and pressures, typically greater than 200°C and 300 MPa (Mega Pascals).

Question 29

What are the main factors controlling metamorphism?

Answer 29

The main factors are temperature, pressure, differential stress, and fluid presence. Changes in these factors cause mineralogical and textural changes in the protolith

Question 30

What is differential stress?

Answer 30

Differential stress occurs when pressure is not applied equally in all directions. It can cause deformation in rocks, affecting their texture and mineral orientation.

Question 31

What is metasomatism?

Answer 31

Metasomatism is the chemical alteration of a rock due to the presence of fluids, which speed up chemical reactions and lead to the formation of new minerals.

Question 32

How does time affect metamorphism?

Answer 32

How does time affect metamorphism?

Question 33

What is metamorphic grade?

Answer 33

Metamorphic grade refers to the relative temperature and pressure conditions under which a rock forms. Higher grade metamorphism occurs at higher temperatures and pressures.

Question 34

What minerals are common in low-grade metamorphic rocks?

Answer 34

Common minerals in low-grade metamorphic rocks include clay minerals, serpentine, and chlorite.

Question 35

What minerals are common in high-grade metamorphic rocks?

Answer 35

Examples of less hydrous minerals and non-hydrous minerals that characterize high grade metamorphic rocks: Muscovite - hydrous mineral that eventually disappears at the highest grade of metamorphism Biotite - a hydrous mineral that is stable to very high grades of metamorphism. Pyroxene - a non hydrous mineral. Garnet - a non hydrous mineral.

Question 36

What is retrograde metamorphism?

Answer 36

Retrograde metamorphism is the process where temperature and pressure decrease, causing the rock to revert to a less metamorphosed state, but it is less common due to slower chemical reactions at lower temperatures

Question 37

How does slate form?

Answer 37

Slate forms from low-grade metamorphism of shale, with minerals like chlorite and clay recrystallizing. It has a fine-grained texture and a characteristic cleavage.

Question 38

What is schistosity?

Answer 38

Schistosity refers to the texture of schist, where larger mineral grains, such as biotite and muscovite, form a foliated structure.

Question 39

What is the protolith of marble?

Answer 39

The protolith of marble is limestone or dolostone, which undergoes metamorphism to recrystallize into calcite or dolomite.

Question 40

What is blueschist?

Answer 40

Blueschist forms in subduction zones where high pressure and low temperature create unique minerals like glaucophane. It is characterized by a blue color and schistosity.

Question 41

Most regionally metamorphosed areas can be divided into zones where a particular mineral, called an index mineral, is characteristic of the zone. The zones are separated by lines (surfaces in three dimensions) that mark the first appearance of the index mineral. These lines are called _________ (meaning equal grade) and represent lines (really surfaces) where the grade of metamorphism is equal. A map of a regionally metamorphosed areas are can be seen in figure 8.16 of your text.

Answer 41

isograds

Question 42

What are metamorphic facies?

Answer 42

Metamorphic facies are groups of minerals that form under specific temperature and pressure conditions. They help determine the environment in which the metamorphism occurred. In general, metamorphic rocks do not undergo significant changes in chemical composition during metamorphism. The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism. Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to. This pressure and temperature environment is referred to as Metamorphic Facies.