Metamorphism

Question 1

What are the three main forms of metamorphism?

Answer 1

Contact/Thermal
Regional
Dynamic

Question 2

What does Metamorphism exclude?

Answer 2

Weathering, Diagenesis
and Lithification

Environments where
temperatures are below
200 – 250oC

Melting Of Rocks -
environments where
temperatures are above
650oC - 700oC

Environments less than
2km depth and at
pressures below 1000
bars (sedimentary zone)

Question 3

What’s the difference between stable and metastable minerals?

Answer 3

Minerals which are chemically unreactive at the earth’s surface are said to be stable. Most minerals are not chemically stable at the
earth’s surface and are thus referred to as metastable.

Rocks and minerals which are metastable will react chemically until they eventually become stable under their new conditions. This is
fundamental to all metamorphic processes.

Question 4

What are iso-chemical changes?

Answer 4

The bulk chemical composition of the parent rock and the metamorphic product are identical.
Both contain the same % Si, Al, O, Na etc.
The only loss from the system can be water as clay minerals are dehydrated by a rise in temperature

Question 5

What are metamorphical facies?

Answer 5

These are the specific minerals and rocks that form at certain temperature and pressure conditions.

Question 6

What are index minerals?

Answer 6

These are the minerals that give you clues as to the conditions that the rock formed under.

Question 7

How do Mudstone and Shale change due to metamorphism?

Answer 7

Both mudstone and shale are formed from a huge number of different clay minerals (Illite, Smectite and Montmorillonite). Therefore, both can have very complicated chemistry (e.g. Illite (K,H3O)(Al,Mg,Fe)2(Si,Al)4O10[(OH)2,(H2O)]). This means that the resultant mineralogical changes seen can be extremely varied depending on the P and T conditions that the rock is exposed to.

Question 8

Describe Contact/Thermal Metamorphism

Answer 8

Changes due to the action of heat alone
Associated with large scale igneous bodies
Batholiths and plutons of granite/gabbro
Example around the edges of the granites in S.W.
England (St. Austell, Bodmin etc.)
This involves the heating of the country rock
surrounding an igneous intrusion. The area affected by heat from a large intrusion such as a pluton is known as the metamorphic aureole.

Question 9

What is a metamorphic aureole/

Answer 9

The total volume of pre-existing ‘country rocks’ affected by heat from the intrusion. Grade of metamorphism decreases from the intrusion towards the edge of the aureole.

Question 10

Describe Thermal/Contact metamorphic textures.

Answer 10

If the country (parent) rock is different, then the resulting metamorphic rock will also be
different. All of these resultant metamorphic rocks
will be non-foliated. These rocks with tend to have Granoblastic texture. This is where grains
mutually adjust their boundaries in the solid state in an attempt to achieve textural equilibrium. They
will generally form triple junctions of 120o in order to achieve this equilibrium.

Question 11

What type of rock undergoes most change whilst undergoing Metamorphism?

Answer 11

Fine grained rocks undergo most change as they are composed of chemically complex clay minerals
such as kaolinite, illite, smectite, bentonite and
montmorillianite.

Question 12

Describe low grade thermal metamorphism with examples.

Answer 12

Low Grade – Spotted Rock

Increased temperature to 300 – 400oC

Partial recrystallisation occurs

New minerals occur as oval spots 2 – 5mm in
diameter

Spots have overgrown grains of the original rock

Relict structures such as bedding/lamination and
fossils may be evident

Question 13

Describe medium grade thermal metamorphism with examples.

Answer 13

Medium Grade – Chiastolite Hornfels

Increase in temperature to 400 – 500oC, results in
coarser grained rock

Extensive recrystallisation occurs

Needles of Chiastolite develop and show
Porphyroblastic texture. Up to 2cm long, 3mm in
diameter with square cross sections. Groundmass is mainly mica

Needles show random orientation, having
crystallised in the absence of pressure

No relict structures are evident

Question 14

Describe high grade thermal metamorphism with examples.

Answer 14

High Grade - Hornfels

Increase in temperature 500–600oC, results in grain
size >2mm

Hornfels shows hornfelsic texture-a tough, fibrous
and splintery-looking rock with a crystalline texture
(complete recrystallisation)

Andalusite often occurs as porphyroblasts

No evidence of any relict structures

Question 15

Describe contact metamorphism of limestone.

Answer 15

All types of limestone, including chalk are chemically simple rocks, comprising just calcium carbonate (CaCO3) in the form of the mineral calcite. No new can minerals form as there are only atoms of Ca, C and O present, instead calcium carbonate recrystallises into a coarser form. Limestone recrystallises to form marble
Grain size increases with grade. Low grade <1mm, Medium 1-2mm, High >2mm. All fossil detail and older structures are lost during
recrystallisation. Marbles will show Granoblastic texture, where all the crystals are roughly the same size. Crystals show triple point junctions with 120o angles between adjacent crystals. Pure limestone produce white marble with a sugary
texture.

Question 16

Describe contact metamorphism of sandstone.

Answer 16

Sandstones are chemically simple rocks comprising mainly quartz (silicon dioxide –SiO2). No new minerals form from pure sandstones as there are only atoms of Si and O present. Instead, quartz recrystallises into a coarser, crystalline form. Sandstone recrystallises to form metaquartzite, grain size increases with grade. Low grade <1mm, Medium 1-2mm, High >2mm.
Metaquartzite shows Granoblastic texture, where all the crystals are roughly the same size. Crystals show triple point junctions with 120o angles
between adjacent crystals. Metaquartzite can be distinguished from marble by testing with dilute hydrochloric acid or scratching with a steel nail. Metaquartzite does not react with acid and is not scratched by the steel nail (quartz vs calcite)

Question 17

Define Regional/Burial metamorphism.

Answer 17

This is where rocks are buried deeply under other sediments, or by earth movements. This most commonly occurs deep inside young mountain belts formed by continental collisions. As the rocks are deeply buried increasing pressures and temperatures are exerted on them. This can cause dramatic changes to the rocks leaving them almost unrecognisable. There are various grades of metamorphic rock depending on the amount of heat and pressure that they are put under.

Question 18

Describe low grade regional metamorphism with examples.

Answer 18

Low Grade – Slate

Occurs at 5 – 15 km depth, relatively high pressures but low temperatures < 300oC. Upper part of the subduction zone

New minerals mainly chlorite and biotite. These platy minerals are aligned at right angles to the principal stresses forming slaty cleavage

Grain size increased but crystals too small to see

At low grade, some relic sedimentary structures may be preserved such as bedding or lamination

Fossils may be present but will be deformed i.e.
stretched, elongated or compressed

Question 19

Describe medium grade regional metamorphism with examples.

Answer 19

Medium Grade – Schist

Formed under higher temperatures 400 to 500oC and at depths of 15 to 25 km

Higher temperature results in a much coarser crystal size 1–2mm and the growth of new minerals such as staurolite and garnet

Garnet crystals occur as porphyroblasts up to 5mm in diameter and often distort the foliation (Schistosity)

Overall texture is schistose, produced by long axes of micas aligned parallel and at right angles to the direction
of principal stress

Older sedimentary structures such as bedding,
laminations and fossils are completely destroyed

Question 20

Describe high grade regional metamorphism with examples.

Answer 20

High Grade – Gneiss

Formed under still higher temperatures and pressures, typically 450 to 650oC and at depths of 25 to 40 km

Higher temperatures result in a coarser crystal size, typically >2 mm

New minerals include kyanite and sillimanite along with quartz, feldspar and micas

Minerals have segregated into mineral-rich layers or bands and the texture is referred to as gneissose banding

Mineral rich layers are parallel and aligned at right angles to the principal stress direction Overall mineral composition is now very similar to
granite

Question 21

Describe very high grade regional metamorphism with examples.

Answer 21

Very High Grade – Migmatite

Migmatite means literally ‘mixed rock’ and comprises two distinct components. The rock is half metamorphic and half igneous

A foliated gneissose or schistose component and a
non-foliated crystalline granitic component

Further melting would yield a granitic or silicic magma and would then constitute the igneous phase of the rock cycle

Question 22

Describe Dynamic metamorphism.

Answer 22

The third type of metamorphism occurs is quite small areas where extremely high pressure conditions tend to occur for short periods of
time. It tends to involve only a rearrangement of the components already present in the rock, and is largely a matter of breaking up the rock into smaller pieces. This tends to occur in areas of shear deformation along fault planes. The product of this metamorphism depends on the depth at which faulting occurs as higher pressure deep in the crust increase the chances of ductile deformation. At shallow levels, brittle deformation
dominates and is characterised by rocks which are shattered to form fault breccia.

Question 23

Describe low grade dynamic metamorphism with examples.

Answer 23

Low Grade – Fault Breccia

Low to moderate pressures at shallow depths <5km below the surface. This occurs along fault planes where shearing of the rocks has occurred.
This often creates an easy path for groundwater to flow, causing mineralisation (calcite and quartz are
common) along the fault plane cementing the fault breccia together. Large clasts generally only produced by competent rocks such as sandstone and limestone. Fine grained (incompetent) rocks produce fault-gouge, a fine clayish material devoid of larger angular clasts. There is some disagreement amongst geologists as to whether fault breccia and fault gouge represent
true metamorphic rocks

Question 24

Describe medium grade dynamic metamorphism with examples.

Answer 24

Moderate to high pressure 5 – 10km depth. At deeper levels, where ductile deformation is more
important, the shearing action of fault movement
produces a fine grained metamorphic rock called
mylonite. It will often have lens shaped fragments in a streaky, fine grained matrix. This mylonitic texture is formed when the higher temperatures cause bonds between particles to breakdown and flowing to occur.

Question 25

Describe high grade dynamic metamorphism with examples.

Answer 25

Very high pressures, over 10km depth. Intense crushing/grinding generates frictional heat
to weld the microscopic angular particles together. In extreme cases frictional heating can initiate
localised melting and the formation of a glassy
texture.