Metamorphic rocks

Question 1

What is metamorphism?

Answer 1

The isochemical process by which rocks are changed by either heat, pressure or both.

Question 2

Where does metamorphism fit into the rock cycle?

Answer 2

After compaction and before melting.

Question 3

What are the three types of metamorphism?

Answer 3

Regional= high pressure and low to high temperature.
Contact= low pressure and high temperature.
Dynamic= low temperature and low pressure.

Question 4

How does temperature affect metamorphism?

Answer 4

Higher temperatures occur near igneous intrusions and temperature increases with depth due to geothermal gradient, therefore there is more recrystallisation and new mineral growth (higher grade).

Question 5

How does pressure affect metamorphism?

Answer 5

At plate boundaries there are continental collisions and subduction zones, where there is more foliation (higher grade-only regional metamorphism) and pressure steadily increases with depth.

Question 6

How does time affect metamorphism?

Answer 6

The longer rock undergoes heat and/or pressure, there is more metamorphism.

Question 7

Define characteristics of slate?

Answer 7

-Low grade regional metamorphism
-Foliated (slaty cleavage)
-Partially recrystallised
-Mineralogy= muscovite mica and clay

Question 8

Define characteristics of schist?

Answer 8

-Foliated (schistosity)
-Medium-grained (0.05-2mm)
-Partially recrystallised
-Mineralogy= muscovite mica and garnet (porphyroblast)
-Medium grade regional metamorphism

Question 9

Define characteristics of gneiss?

Answer 9

-Coarse grained (>2mm)
-High grade regional metamorphism
-Foliated (gneissose banding)
-Totally recrystallised
-Mineralogy= hornblende, biotite mica, quartz and feldspar

Question 10

Define characteristics of spotted rock?

Answer 10

-Low grade contact metamorphism
-Temp= 200-350 degrees celcius
-Partial recrystallisation
-New minerals occur as oval spots
-Relic structures such as bedding, lamination and fossils may be evident

Question 11

Define characteristics of hornfels?

Answer 11

-High grade contact metamorphism
-Temp= 500-600 degrees celcius
-Coarser grained >2mm
-Tough, fibrous and splintery looking rock
-Crystalline texture
-Andalusite porphroblasts
-No evidence of any relic structures

Question 12

Define characteristics of limestone?

Answer 12

-Recrystallise to form marbles through contact metamorphism
Contact and regional metamorphism
-All fossils and older structures destroyed during recrystallisation
-Marbles show granoblastic texture= crystals relatively equal sizes
-Grain size increases with grade= low grade <1mm, medium 1-2mm, high >2mm
-Chemically simple rocks= just calcite

Question 13

Define characteristics of sandstone?

Answer 13

-Contact metamorphism to form metaquartzite
-Chemically simple rocks, mainly quartz and no new minerals form from pure sandstones
-Grain size increases with grade= low grade <1mm, medium 1-2mm, high >2mm
-All fossils and older structures destroyed during recrystallisation
-Marbles show granoblastic texture= crystals relatively equal sizes

Question 14

What is slaty cleavage texture?

Answer 14

Fine grained rocks formed by low grade regional metamorphism. Split into thin sheets along cleavage planes and only in rocks with platy minerals at any angle to bedding.

Question 15

What is porphyroblastic texture?

Answer 15

Occurs in both regional and contact metamorphic rocks and grow during metamorphism, so are essentially crystals surrounded by a finer grained groundmass.

Question 16

What is granoblastic texture?

Answer 16

Unfoliated texture that occurs due to contact metamorphism, here pressure is not a factor and the granoblasts are randomly orientated and equidimensional crystals, which are sometimes only one mineral.

Question 17

What is schistosity texture?

Answer 17

Found in schists and is 90 degrees to the direction of maximum pressure during metamorphism and results from alignment of flat, platy minerals.

Question 18

What is gneissose banding texture?

Answer 18

Formed in gneisses and bands may be contorted/folded at 90 degrees to the direction of maximum pressure during metamorphism, it is formed when light and dark coloured minerals are separated into bands.

Question 19

What happens with metamorphism and plate tectonics at convergent margins with subduction zones?

Answer 19

The plates move towards each other and collide. There is high pressure as the oceanic plate is subducted, which leads to magma generation and high contact metamorphism. The temperature and pressure is highest here at the edge of the magma generated and/or igneous intrusions.

Question 20

What happens with metamorphism and plate tectonics at convergent margins (continental-continental)?

Answer 20

The two continental plates move towards each other and collide. There are high temperatures and pressures acting over long periods of time due to the plates buckling and colliding. Temperature and pressure are highest in the middle of fold mountain ranges.

Question 21

What is contact metamorphism?

Answer 21

Changes due to temperature alone. Associated with large igneous intrusions such as plutons and batholiths. Example around the edges of the granites in S.W. England and the Lake District. The temperature of contact metamorphism ranges from 200-1000+ degrees celcius.

Question 22

What is a metamorphic aureole?

Answer 22

The volume of rock that has undergone recrystallisation due to heat from the intrusion. The zone of recrystallisation has to be 50m+ wide to be plotted onto a 1:50,000 BGS geology map.

Question 23

How does the size of an intrusion affect the size of an aureole?

Answer 23

-Larger volume intrusions take longer to cool and produce larger aureoles.
-Minor intrusions like dykes and sills produce baked margins (less than 50m wide) rather than true aureoles.

Question 24

What are paired metamorphic belts?

Answer 24

Sets of parallel linear rock units that display contrasting metamorphic mineral assemblages. These paired belts develop along convergent plate boundaries where subduction is occurring.

Question 25

When and where were the Dalradian Supergroup deposited, what type of plate boundary were they formed at and which orogeny metamorphism occurred?

Answer 25

-Deposited 400 Ma.
-Deposited in the Southern-Eastern Scottish Highlands and Caledonian Mountains in Norway.
-Deformed at a convergent plate boundary.
-Caledonian orogeny (Scottish Mountain) was the building period.

Question 26

What is an index mineral?

Answer 26

A metamorphic mineral that is stable over a particular pressure and temperature range. They can therefore indicate the metamorphic grade.

Question 27

What is an isograd?

Answer 27

A line on a map joining points of equal metamorphic grade.

Question 28

What is a metamorphic zone?

Answer 28

The area between two isograds. The zone is named after the lower grade isograd. All locations within a metamorphic zone experience the same metamorphic grade.