Magmatic And Metamorphic Processes

Question 1

What are the main three components of the QAPF diagram and when is it typically used?

Answer 1

Q - Quartz, A - alkali feldspar, P - plagioclase feldspar

It is used when the mafic mineral content is less than 90%. There are separate diagrams for plutonic and volcanic rocks.

Question 2

What is the structure of the diagram that is used to classify ultramafic rocks, where the mafic mineral content is greater than 90%?

Answer 2

Olivine [top], orthopyroxene [left], clinopyroxene [right]

The top half of the diagram classifies rocks as peridotites and the bottom half classifies them as pyroxenites.

Question 3

What elemental concentrations does the alumina saturation ratio take into account?

Answer 3

The ratio of Al2O3 compared to the total ratio of Na2O + K2O + CaO
If the ratio is more than 1, the rock is oversaturated
If the ratio is less than 1, the rock is undersaturated

Question 4

What are the three groups we can classify rocks on based on their alumina saturation?

Answer 4

Peraluminous, Metaluminous, Peralkaline

Question 5

What does the TAS plot show?

Answer 5

This is a classification diagram for rock types, where the two axes are the total alkalis [Na2O + K2O] and the SiO2 weight %.

Question 6

What are the key textures, mineralogy and composition for a basalt?

Answer 6

Texture - fine grained, can be porphyritic
Mineralogy - plagioclase, clinopyroxene and opaques, quartz [rare]
Composition - 45-52% SiO2, 4-12% MgO

Question 7

What are the rock types for silica saturated basalts?

Answer 7

Silica undersaturated - alkali basalts
Silica saturated - tholeiites
Silica oversaturated - Quartz tholeiites

Question 8

Where is basalt erupted?

Answer 8

Mainly at MORs - average production rate is 20km3 per year
They can be erupted on both continents and in the oceans. The moon also has basalt; the dark areas are basalt filled maria [basins].
Venus and Mars [olympus mons] also show large scale basalt volcanism.

Question 9

What are the physical properties of basalt magma - density and viscosity?

Answer 9

Density - 2.6-2.8 g/cm3, greater than andesite or rhyolite

Viscosity - Hawaiian tholeiite magma: 600-700 Pa.s at 1130 degrees, 100 Pa.s at 1200 degrees

Question 10

What is the difference in rock textures for intrusive and extrusive rocks?

Answer 10

Intrusive - coarse grained or phaneritic with a grain size range of 5-50mm
Extrusive - fine grained or aphanitic with a grain size of less than 1mm

Question 11

What is the main method of melting for basalt magma?

Answer 11

Decompressional melting

Question 12

What is fractional crystallisation?

Answer 12

When a magma cools, separation of the liquid and crystals occurs which causes the magma to evolve.

Question 13

Where does basalt come from?

Answer 13

It must come from an magnesium rich source because basalt is a magnesium rich rock. Experimental melting of mantle type rocks produces basalt.

Question 14

What affect does the lithosphere have on melting?

Answer 14

If the lithosphere is sufficiently thick, it can stop melting altogether and it can block its path of ascent.

Question 15

What happens with melting at ternary phases?

Answer 15

Melting begins at the triple point, otherwise known as the ternary eutectic, and initially produces melt with the eutectic composition. Melting may continue until one phase is exhausted, which is usually clinopyroxene.
The eutectic melt from the melting of peridotite is an MgO rich basalt.

Question 16

What is differentiation? And what is an ideal location for this process to occur?

Answer 16

Differentiation is the process where a magma evolves during crystallisation. An ideal location for this to occur is within magma chambers.

Question 17

What is a parental lava/magma?

Answer 17

The most primitive member of a suite of related igneous rocks, that can be considered as the parent of the suite.
It will have the highest MgO.

Question 18

What is a primary magma?

Answer 18

A magma that is in equilibrium with its source. Primary magmas are rarely erupted at the surface without undergoing differentiation.

Question 19

What is a magma chamber?

Answer 19

This is simply a place where magma is stored. This may be a melt inclusion, a sill of dyke-like body or a major intrusion in the crust or upper mantle.
A simple magma chamber has a hot, primitive magma at the base and a cooler, evolved magma at the top.

Question 20

What is Skaergaard?

Answer 20

It is a gabbro intrusion that is Tertiary in age [55Ma] and is located in East Greenland. It is now thought to be a lopsided lopolith.
It has been intruded into both Archaean gneiss [top] and tertiary basalts [bottom]. It exhibits a layered series with four zones.

Question 21

What are the four types of crystal layering found in Skaergaard?

Answer 21

Modal - layers with different mineralogies
Phase - appearance or disappearance of minerals
Cryptic - mineral changes throughout the whole intrusion
Rhythmic – patterns are seen within the layers

Question 22

What are cumulus and intercumulus minerals?

Answer 22

Cumulus [primocrysts] - these grew first, I.e plagioclase, olivine, augite
Intercumulus - grow between the cumulus minerals, I.e Quartz, orthopyroxene

Question 23

What is the classic cumulus theory?

Answer 23

This theory states that crystals are grown in one region of the magma body and accumulate everywhere else. Crystals migrate through density currents and gravity.

Question 24

What is the relationship between potassium content and distance from the subduction zone?

Answer 24

The K content increases with increasing distance from the subduction zone.

Question 25

What are the properties of andesite? [texture, SiO2 content, colour]

Answer 25

They are usually porphyritic and purplish in colour. Their SiO2 content ranges between 57 and 63%.

Question 26

What are the typical properties of andesitic magma? [density, viscosity, eruption temperatures, mineralogy]

Answer 26

Density - 2.4-2.5 g/cm3, which is less than basalt
Eruption temperature - 930-1200 degrees Celsius
Viscosity - 900 degrees=107 Pa.s, 1200 degrees=1000 Pa.s, thick flow
Mineralogy - plag,cpx,amphibole,biotite & FeTis. Ol & Quartz - rare

Question 27

What is the main method of formation of andesite magmas?

Answer 27

Fractional crystallisation of basalt magmas

Along with partial melting of wet basalts [amphibolites]

Question 28

What is the evidence that supports this method for andesite formation? [fractional crystallisation of basalt]

Answer 28

Complex phenocryst assemblages [zoned plag]
Resorbed amphiboles with reaction coronas - medium pressure
Ascending magma bodies - evidence of polybaric fractionation [occurs over pressure intervals]
Low viscosity allows crystal melt segregation

Question 29

What is the evidence against this process? [fractional crystallisation of basalt]

Answer 29

Where are all the cumulates?
Andesite has a high liquidus temperature so the parental basalt would need to be even hotter
The proportion of basalt to andesite would have to be at least 2-1

Question 30

What are the differences between low and high pressure melting of basalt?

Answer 30

Low - initial melt is rhyolitic, as melting increases the melt becomes more basic, until the basalt is totally molten
High - at 40kb, the wet andesite is in equilibrium with garnet only. Therefore during melting, the melt needs to be dry, which requires a lot of energy, like in young oceanic crust that is still hot.

Question 31

What is the main mineralogy for granites and rhyolites?

Answer 31

The two major phases are Quartz and alkali [potassic] feldspar. They most likely will contain pyroxene and biotite with small proportions of amphibole and muscovite.

Question 32

What is the main mineralogy for syenites and trachytes?

Answer 32

Less Quartz than granites and rhyolites and may even be Quartz free. They sometimes contain pyroxene with small amounts of amphibole.

Question 33

What is the difference in composition between granites&rhyolites and syenites&trachytes?

Answer 33

Granites and rhyolites have abundant SiO2 whereas syenites and trachytes have higher alkali/silica ratios so they are less silica saturated.

Question 34

What are the differences in localities of granites, rhyolites, syenites and trachytes?

Answer 34

Granites and rhyolites - continental, collision zones, subduction zones, hotspots and rift zones.
Syenites and trachytes - continental or oceanic, intraplate settings such as ocean islands.

Question 35

What are the physical properties of granite, rhyolites, syenites and trachyte magmas?

Answer 35

Viscosity - 800 degrees dry =1010-1111 Pa.s, 1000 degrees dry - 108 Pa.s
Density - 2.2g/cm3 at 800-1200 degrees, less than basalt
Temperature - dry: 1100 degrees on eruption, wet: 600-700 degrees

Question 36

What are the two ideas for granite formation?

Answer 36

Magmatists- granitic magmas, evidence: large volumes of rhyolitic magmas, experimental work shows that melting in the KAS system produces granitic melts
Granitisers- formed by fluids that convert the country rock.

Question 37

What are the four types of granites found in the granite alphabet?

Answer 37

I type - igneous rock protolith
S type - sedimentary rock protolith
M type - fractionation of mantle derived melts
A type - anorogenic, alkaline, anhydrous

Question 38

What are the different characteristics for the four types of granite?

Answer 38

I types - Metaluminous, contain amphibole
S types - peraluminous, may contain albuminous phases such as muscovite, andalusite and cordierite. May also contain very s,all amounts of mafic minerals
A type - often anhydrous, tend to be hypersolvus

Question 39

What is the evidence we have for hydrous conditions?

Answer 39

Hypersolvus and subsolvus textures indicate hydrous conditions. Feldspar textures can be used to indicate whether a granite of syenite crystallised under wet or dry conditions.
Hypersolvus textures are small, white, crack lines seen in thin section that appear like roots.

Question 40

What is the primary idea for granite genesis?

Answer 40

Most granites probably result from melting of the crust during the intrusion of hot basaltic melts. Many granite bodies are probably thick sills or sill-like bodies, rather than spheres.

Question 41

How do I type granites and rhyolites form?

Answer 41

Heat incoming from a basalt intrusion creates an igneous [amphibolite] protolith. Once it is all melted, you get hydrous melts that often crystallise out amphibole. They are typically explosive if they reach the surface.

Question 42

How do s type granites and rhyolites form?

Answer 42

Heat incoming from a basalt intrusion and/or radioactive decay creates a sedimentary protolith. Once it is all melted, you get hydrous melts that often crystallise muscovite and other Al rich minerals. These are also explosive if they reach the surface.

Question 43

What are two reasons why intrusive rocks may not make it to the surface?

Answer 43

Overlying rocks may block their pathways

As they reach the surface, it may already be crystallised, in which case it will already be solid so will not move.

Question 44

How do pyroclastic deposits form?

Answer 44

They may form from fragmentation of magma by internal gas expansion, or by interaction with water

Question 45

What are the subdivisions for classification of pyroclastic deposits based on size?

Answer 45

> 256 - deposit: bombs and blocks, rock: agglomerate/breccia
64 - deposit: lapilli, rock: lapilli deposit/tuff
2 - deposit: ash, rock: ash deposit/tuff

(Sizes in mm)

Question 46

What is the collective term for pyroclastic deposits?

Answer 46

Tephra

Question 47

What are pyroclastic fall deposits?

Answer 47

Typically well sorted due to winnowing, and drape the topography like snow. They have variable grain sizes and show good bed forms.
They typically form when there is an explosive eruption with an eruption column to aid wide dispersal. Degassing of the magma is needed.

Question 48

What does the solubility of magmatic volatiles depend upon? And what other factor affects magmatic solubility?

Answer 48

The solubility of volatiles is dependent upon pressure, as higher pressures mean they are more soluble. The melt composition affects the solubility, as more silicon melts are more soluble.

Question 49

Which magmas are most viscous, and what affect do volatiles have on viscosity?

Answer 49

Silicic magmas are the most viscous, and adding volatiles can reduce the viscosity.

Note at high pressures, silicic magmas can actually contain the highest amounts of volatiles.

Question 50

What are the four main eruption types, and what sort of deposit do they produce?

Answer 50

Hawaiian [basaltic] - scoria fall
Strombolian [basaltic] - scoria fall
Plinian [intermediate/silicic] - pumice fall
Phreatomagmatic [any composition] - scoria or pumice fall

Question 51

What are pyroclastic flow deposits?

Answer 51

These are surface flows of high particle concentrations in a gas solid dispersion. They are gravity controlled and tend to follow the topography. They may be erosive, can occur at various temperatures and will always form poorly sorted deposits. They can often show a eutaxitic texture with flattened pumice fragments.

Question 52

What are ignimbrites?

Answer 52

A deposit or rock formed from a pumiceous pyroclastic flow, irrespective of the degree of welding or size. They can be welded or unweeded and can be ash flow tuffs or pumice flow deposits.

Question 53

What are lahars?

Answer 53

These are volcanic mud flows and remobilised, poorly consolidated volcanic deposits. Melting of ice caps, bursting of summit lakes and heavy rainfall can be strong contributors. These may occur during or long after an eruption and can be deadly.

Question 54

What is the role of the lithosphere?

Answer 54

It is the thermal boundary layer to our convecting mantle system. It acts to prevent melting by putting a lid or cap on the melt column