G1 : Rock Forming Processes KI2 (metamorphic Rocks)

Question 1

What is metamorphic grade

Answer 1

Describes the temperature and pressure conditions under which metamorphic rocks form

Question 2

Metamorphic aureole

Answer 2

A region surrounding an igneous intrusion in which the country rocks have been recrystallised and changed by heat from the intrusion

Question 3

Spotted rock

Answer 3

Formed by low grade metamorphisms in the outer part of a metamorphic aureole

Question 4

Index minerals

Answer 4

Metamorphic minerals, which are so stable under specific temperature and pressure conditions. They indicate the metamorphic grade

Question 5

What is contact metamorphism

Answer 5

When country rock is affected by heat from a large igneous intrusion. Because temperature differences between the country rock and the intrusion are greater at shallow depths in the earths crust, where pressure is low, contact metamorphism is described as high temperature, low pressure.

Question 6

What does high temperature and low pressure lead to

Answer 6

The formation of altered, recrystallised, unfoliated ricks in a zone surrounding the intrusion.

Question 7

How does distance from a metamorphic aureole impact metamorphism

Answer 7

Temperature decreases with distance from the contact with the intrusion for this reason the effects of contact metamorphism are greatest near to the contacts and decrease with distance. The outer part of the metamorphic aureole is only weakly metamorphosed and forms spotted rock

Question 8

How does colour and hardness change in a metamorphic aureole

Answer 8

It gets darker and harder towards the centre

Question 9

What do index minerals do in a metamorphic aureole

Answer 9

Index minerals are metamorphic minerals, which are stable under specific temperature and pressure conditions, they indicate the metamorphic grade.
Because contact metamorphism is caused by temperature only, an increase in grade represents a thermal gradient

Question 10

What does the regional metamorphism of shale produce and with what minerals

Answer 10

Low grade : slate (chlorite)
Medium grade : schist (kyanite)
High grade : gneiss (sillimanite)

Question 11

Regional metamorphism in orthoquartzite and limestone

Answer 11

Produces the same products and contact metamorphism - meta quartzite and marble. Each of these rocks is monomineralic as they are composed of one mineral only quartz and calcite respectively. The minerals are equidimensional, so they cannot align under pressure

Question 12

Grade and metamorphic minerals

Answer 12

Low grade metamorphic rocks contain a high proportion of hydrous minerals (clay minerals and chlorite contain h2o in their crystal structure). High grade metamorphism takes place at higher temperatures. H2O is lost and anhydrous minerals (e.g. garnet) become more common. However, biotite, a hydrous mineral, is stable at a very high grade if metamorphism

Question 13

Why is metamorphic grade not the same in contact and regional metamorphism

Answer 13

In regional metamorphism the metamorphic grade is made up of both temperature and pressure but only temperature is in contact

Question 14

Why does the metamorphism of shake produce so much more information

Answer 14

Clay minerals can have many different chemical changes so can produce more rock types

Question 15

Index minerals and contact metamorphism

Answer 15

-Biotite is the low grade and is found in spotted rock
-Andalusite is the low to medium temperature, low pressure alumina silicate polymorph found in andalusite slate. It indicates the medium grade
-with increasing grade, it follows a path from andalusite to sillimanite on the alumina silicates polymorph phase diagram, sillimanite indicated high grade grade and is found in hornfels
-kyanite the high pressure, low temperature alumina silicates polymorph, is not found in contact metamorphic rocks due to the lack of pressure

Question 16

Where did George barrow map and when

Answer 16

In 1893 he mapped a sequence of highly deformed regionally metamorphosed rocks in the south eastern part of the Scottish highlands, this metamorphism and deformation occurred during the closure of the Iapetus ocean and during the Caledonian orogenic, about 400ma, these Precambrian rocks are known as the dalradian supergroup

Question 17

What pattern did George barrow notice

Answer 17

There was a pattern to the occurrence of metamorphic minerals. He used the first appearance of these minerals, which he termed index minerals, to plot isograds and map metamorphic zones. Some of the minerals that crystallise at low grades are stable at higher grades so more than one index minerals may be found in one rock

Question 18

What was George barrow able to map

Answer 18

Metamorphic zones using index minerals and isograds which define the boundaries of the zones. Although he did not do all the mapping personally, the system he devised was names after him and the zones are called barrovian zones

Question 19

What does the chlorite zone represent

Answer 19

Low grade regional metamorphism. The rock is skate where most of the rock has recrystallised but some clay minerals may still exist

Question 20

Where do schists develop

Answer 20

As a result of increasing temperatures and pressure and can be found in both bigots and garnet zones. The crystal sizes increase with metamorphic grade. Schists formed at lower temperatures and pressures are composed of quartz, Muscovite and biotite mica.

Question 21

Medium grade metamorphism

Answer 21

Results from higher temperatures and pressure and many schists formed at this grade contain garnet, and less commonly, kyanite porphyroblasts

Question 22

Where are the alumina silicates polymorphs kyanite and sillimanite found

Answer 22

In regional metamorphic rocks

Question 23

Kyanite zone

Answer 23

Kyanite is typically found in gneisses and the kyanite zone represents high grade regional metamorphism. With increasing metamorphic grade, regional metamorphism follows a oath from kyanite to sillimanite, shown in the alumina silicates polymorph phase diagram

Question 24

The silimanite zone

Answer 24

High grade regional metamorphism with very high temperatures and pressures. The rocks a gneisses. A maximum temperature if about 700c and pressure if 700mpa. This pressure exists at a depth of 25km below the surface of the continental crust. It gives us a geothermal gradient of 28Ckm-1

Question 25

Why are quartz and plagioclase feldspar not useful as index minerals

Answer 25

They are stable throughout the whole range of grades

Question 26

Why are there changes in mineral assemblages

Answer 26

Due to change in temperature and pressure conditions.

Question 27

What’s a metamorphic facies

Answer 27

Metamorphic minerals that were formed under similar pressures and temperatures

Question 28

What can rocks of a particular assemblage be linked to

Answer 28

Certain tectonic setting and times and can be used to reconstruct the conditions of metamorphism

Question 29

What does the sequence of metamorphic facies seen in an area depend on

Answer 29

The geothermal gradient during metamorphism

Question 30

Prograde metamorphism

Answer 30

The recrystallisation of a rock in response to an increase in the intensity of metamorphism

Question 31

Why is andalusite not formed by contact metamorphism of pure limestone

Answer 31

Pure limestone os monomineralic so does not contain any impurities such as clay minerals which contain alumina silicates

Question 32

Why are clay rich sediments the most useful in mapping metamorphic zones

Answer 32

Good indicator of environment of formation and facies and show a latter to the occurrence of these minerals

Question 33

Why are metamorphic rocks classified mainly based on their textures

Answer 33

Grain size and orientation tell us a lot about the conditions of metamorphism

Question 34

What happens if rocks are subjected to directed pressure

Answer 34

A preferred orientation for the minerals develops at 90 degrees to the pressure. if the minerals are Flat or Plath, foliation is produced. Foliation results from pressure and is a characteristic of rocks formed. H regional metamorphism

Question 35

Dalradian

Answer 35

Regionally metamorphosed group of rocks originally deposited in the late Precambrian, they are found in Scotland and Ireland

Question 36

Inclusion

Answer 36

An early formed mineral enclosed by one that grew later

Question 37

Unfoliated

Answer 37

The random orientation of minerals in a metamorphic rock

Question 38

Hornfels

Answer 38

Fine grained, hard, splintery, granoblastic metamorphic rock formed when shale is completely recrystallised close to the contact with an igneous intrusion

Question 39

What is slaty cleavage

Answer 39

Rocks with slaty cleavage will split into thin sheets along the cleavage okayed. It occurs in fine grained rocks formed by low grade regional metamorphism
-form in rocks consisting of platy minerals such and clay, chlorite and micas
-these minerals align at 90 to the direction of maximum pressure during metamorphism
-usually parallel to axial planes of folds
-cannot occur in rocks with rounded grains such as quartz in sandstone
-relic structures may be found

Question 40

What is schistosity

Answer 40

Texture found in phylites (fine grained) and schists (medium grained) and form by regional metamorphism. Results from the alignment of flat, platy minerals, commonly Muscovite mica and 90 to direction of maximum pressure during metamorphism. Light coloured Muscovite mica is concentrated into thin parallel bands giving the rock a characteristic shiny appearance where fk
At surfaces of mica are visible
Garnet porphyroblasts may be present
Both schists and phyllites are dalradian rocks

Question 41

What is gneissose banding

Answer 41

Found in gneisses (high grade regional - coarse grained) formed when light (usually quartz and feldspar) and dark coloured minerals (usually biotite and mafic minerals) are separated into bands. The mica rich layer is foliage and the pale layer is granoblastic. The bands may be contorted or folded but are roughly 90 to the maximum oressure

Question 42

What is porphyroblastic texture

Answer 42

Both regional and contact, porphyroblasts are large crystals that grow during metamorphism and are surrounded by finer grained ground mass. Metamorphic rocks that contain these are described as porphyroblastic.
Pyrite porphyroblasts in slate (often cubic crystals)
Garnet porphyroblasts in schist may contain inclusions (provides clues about the timing of events during formation)

Question 43

Granoblastic texture

Answer 43

Unfoliated fabric by thermal metamorphism. Randomly orientated, equidimensional crystals usually in rocks with few or one minerals.
E.g. hornfels, marble and quartzite.

Question 44

Crenulation cleavage

Answer 44

When several foliations are present but not all in the same direction. In rocks such as phyllite, schist and some gneisses where two or more stress directions cause the formation of one or more foliations superimposed in earlier ones, giving a wavy appearance to the texture. This allows us to determine the number if phases of metamorphism that have affected a rock

Question 45

Fabrics stress and time during mountain building

Answer 45

High temperatures increase the rate of ions diffusing between minerals, process is sped up by water (diffuse more rapidly), these reactions take place of ma so temperature and pressure conditions must remain for a long time.

Question 46

How do conditions change during mountain building

Answer 46

Several periods of deformation. Minerals that form early in metamorphism sometimes grow over existing sedimentary features and thus can help to identify the parent rock. This evidence can be defrocked by deformation and later metamorphic recrystallisation and pressure and temperature change and increase. Using a microscope allows more detailed information about the relationship between the growth of new minerals and the effects of deformation to be studied.

Question 47

What happens if stress is applied to rocks rapidly

Answer 47

E.g. fault zones
Mineral grains suffer brittle fracture and are crushed and ground mechanics,lay as the fault moves thus process is cataclasis. It produces a variety of textures ranging from coarse grained angular class in a fine grained matrix, to rock consisting almost entirely of finely crushed particles. These fabrics are characteristics of dynamic metamorphism, but will occur in fault zones as part of mountain building

Question 48

What happens if stress is applied to rocks slowly

Answer 48

Over a long period at low strain rates, the rock does not fracture but deforms gradually. Although temperature is important, foliation develops largely in response to stresses. Later phases of deformation may produce foliation with a different orientation and by recognising these differences in metamorphic fabric, a history of changing pressure and temperature during regional metamorphism can be determined

Question 49

Spotted rock

Answer 49

In the outer part of the metamorphic aureole, temperatures are lower. Some recrystallisation occurs causing clusters of dark minerals to grow on separate spots. Iron, carbon, or biotite mica will form the spots. The rock in this outer part of the aureole is called spotted rock and is formed by low grade metamorphism

Question 50

Andalusite rocks/slate

Answer 50

Further away from the contact, where heat is less intense, medium grade metamorphism occurs. Clusters of a new metamorphic mineral andalusite form porphyroblasts. Thus partly recrystallised rock is andalusite slate or rock

Question 51

Hornfels

Answer 51

Close to the contact with the intrusion, temperatures are high and so high grade metamorphism occurs, shake is completely recrystallised to form a fine grained, hard, splintery, granoblastic, metamorphic rock called hornfels.

Question 52

Contact metamorphism

Answer 52

-Associated with major igneous intrusions such as plutons and batholiths
-the volume of country rock altered by heat from the intrusion is know as the metamorphic aureole
-recrystallisation occurs with a random orientation of minerals (non foliated)
-volume of magma affects size of aureole (small have baked margins)
-not occur below 200c
-mafic 1200c and felsic 850c (contains more volatiles)
-aureoles narrower in sandstone that shake
-limestone contains carbonate which may react with hydrothermal fluids to create skarn

Question 53

Regional metamorphism

Answer 53

Slate, schist, phyllite, gneiss, migmatites. Occurs when temperature and heat become too great and a mineral e.g. clay has to adapt to the new environment producing chlorite. Occurs in a large scale over regions in orogenic events and volcanic arcs. The process can affect igneous rocks and sedimentary rocks, but they produce different results.

Question 54

Migmatite

Answer 54

A rock composed of metamorphic host material that is streaked or veined with granite rock; the name means ‘mixed rock’ such rocks are usually gneissic (banded) and felsic rather than mafic in composition; they may occur on a regional scale in areas of high grade metamorphism

Question 55

Dynamic metamorphism

Answer 55

High pressure (cataclasis) and low temperature (along major fault zones pieces of crust are forced to slip past each other)
Often involves a change in texture of the rock with little to no change in minerals content. The resulting texture is that of finely pulverised rock powered called mylonite. There may be a thin layering of the rock (parallel to the fault zone)
Produces fault breccias and fault gouge