Igneous

Question 1

What is an igneous rock?

Igneous 1

Answer 1

Rock: crystals of one or more mineral bound together in a mixture.

Igneous rocks comprise the ==majority of our accessible planet==.

Example:
- Granite: coarse grained/phaneritic
- Corse-grained/phaneritic: a rock whose grains (crystals or sediment particles) are roughly pea-sized or larger and are visible to the human eye

Minerals include:
- (Na) Plagioclase feldspar (white),
- K-feldspar (pink)
- and quartz
- +/- a black coloured (mafic) mineral (e.g., biotite)

Question 2

What are the differences between intrusive and extrusive rocks?

Igneous 1

Answer 2

Igneous rocks form when you crystallise a melt (associated with volcanoes).

1. Intrusive: where magma cools and solidifies within the Earth’s crust (plutonic)
2. Extrusive: where magma is expelled from the Earth’s interior and soldiifes quickly on the surface (volcanic)

Question 3

What are some types of igneous textures?

Igneous 1

Answer 3

1. APHANITIC:
   
   2. Basalt (structured lava of basalt composition):
      
      3. These rocks (i.e.,basalt) are typically extrusive rocks with an aphanitic texture due to very quick crystallisation and formation on the Earth’s surface which prevents the contained minerals from growing into large crystals.
   3. Andesite (lava containing lots of bubbles):
      1.These bubbles contain gas that didn’t escape from the liquid which were then quenched.
   4. Rhyolite(: lava that is nearly 100% glass (obsidian))
      
      1. Mafic rock (stands for magnesium and iron)
2. VESICULAR texture:
   
   1. Pumice: very frothy light-coloured cellular rock, full of interconnected gas bubbles (type of vesicular/pyroclastic rock)
      
      1. This texture is characterised by small cavities or holes called vesicles which are formed due to dissolved gases in the magma which escape as the pressure decreases (depressurisation) and rapid cooling occurs during extrusion.
      2. Due to high concentrations of vesicules, that means pumice is very bouyant and can float on water.
3. PYROCLASTIC texture:
   
   1. Pyroclastic rock: formed from fragments of chilled magma (pyroclastic = fiery fragments)
      
      1. These pyroclastic rocks form when explosive eruptions discharge lava into the air that wield together due to heat and pressure that results in fragmental, glassy material that can either fall in the form of volcanic ash, bombs or lapilli (rock fragments ejected from a volcano).

Question 4

Where can intrusive igneous rocks form?

Igneous 1

Answer 4

• Intrusive rock could form anywhere shown below, in the magma storage chamber or the overlying volcano feeder system:
  
  • where it can take thousands of years to crystallise due to its position underground surrounded by rocks which are a great thermal insulator.

Examples:
- Granite, diorite and gabbro are intrusive phaneritic-textured igneous rocks which are coarse-grained with their minerals clearly visible.

Question 5

Why do melts form?

Igneous 1

Answer 5

Temperatures of molten rock: 650 —1100C
- Where does Earth’s heat come from?
- Why isn’t the Earth all molten?

In regards to the Earth’s internal energy:
1. Kinetic energy of impacts:
1. the process of accretion where the impact of collision of smaller asteroids created a monumental amount of kinetic energy and heat that contributed towards its molten state. Additionally, the massive amounts of gravitational potential energy produced also steadily converted into heat while the planet cooled.
2. Latent heat of fusion:
1. the amount of energy in the form of heat that must be provided to a solid substance for it to convert its physical state into a liquid (melt) helped to preserve the Earth’s molten state
3. Radioactive elements:
1. the radioactive decay of unstable, short-lived radionuclides (i.e., uranium, thorium, etc)

• A geotherm charts how the temperature varies with depth (pressure) within the Earth:
  
  • the temperature required to melt given rock are about 650 to 1200°C at 10 to 50km below.
  • However, not all rocks melt at the same temperature. Every solid has its own energy threshold to which point the bonds move apart to form a .

The Earth’s geotherm is never above the solidus

Solidus: the temperature below which a magma or melt becomes completely solidified and crystallised into a solid rock.

To melt a mantle, you need to “disturb the geotherm”:
1. Add heat
1. If more energy was added from the Earth’s interior, it would exceed the solidus and result in partial melting.
2. Reduce pressure
1. If pressure was confined, it would require more energy to break (most important means of melting on Earth)
3. Change the (chemical) composition
1. Through volatile-assisted melting

Question 6

What is decompression (adiabatic) melting?

Igneous 1

Answer 6

• Decompression (adiabatic) melting: where the mantle rock undergoes partial melting due to a rapid decrease in pressure from upwelling of the mantle without a significant change in temperature.

1. Mid-ocean ridges (divergent plate boundaries):
   
   • Decompression melting forms mid-ocean ridge basalt (MORB)
     
     1. Upwelling of the mantle: As the plates diverge, the underlying mantle rises to plug the gap.
     2. Decrease in pressure: The pressure decreases as the mantle rises due to the reduction of the overburden.
     3. Decompression melting: The partial melting of the mantle occurs due to the rapid decrease in pressure which results in the magma ascending and generating magmatism that forms the ocean crust.
2. Continental rifts (e.g. East Africa):
   
   • Over time, it will become an ocean due to extension and creation of new oceanic crust depending on the degree of mantle upwelling that will eventually evolve into ‘oceanic spreading centres’ (i.e., the Red Sea)
     
     • Additionally, diverse composition is found through the volcanics due to their not being an existing single large heat source along the rift vally.
3. Mantle plumes (e.g., Hawaii):
   
   4. Mantle plumes: localised upwelling currents of solid rock within the mantle that are hotter and less dense than surrounding rock.

Heat-induced melting typically occurs at mantle plumes or hotspots where deep seated upwelling occurs,
- where the rock surrounding the plume is exposed to higher temperatures and the geothermal gradient crosses to the right past the solidus — with the rock beginning to melt.
- Both increased heat flow and decompression.
- Forms ocean-island basalt (OIB).

4. Volatile-assisted (flux) melting

Question 7

What is volatile-assisted melting?

Igneous 1

Answer 7

The process where the presence of volatile compounds (i.e., water, carbon dioxide or sulfur) lowers the melting temperature of solid rocks or minerals

Example:
- adding salt to water to lower the melting point of water (to stop roads from icing)

New ions increase the complexity of the chemical system reducing the energy required to break the bonds
- If you add volatiles to Earth’s mantle (H2O, CO2), you lower its melting temperature.

Question 8

What is volatile-assisted (flux) melting?

Igneous 1

Answer 8

• Volatile-assisted (flux) melting: process that occurs in subduction zones and island arcs, where the addition of water and other volatile components (i.e., carbon dioxide, sulfur dioxide and chlorine) to hot solid rock depresses its melting point to cause partial melting and the formation of magma.

Flux-melted magma produced through this melting produces many of the volcanoes in the subductions zones around the Pacific (i.e., Ring of Fire).
1. Oceanic lithosphere and hydrated minerals (when water ions bond with the crystal structure of silicate minerals) are contained within this subducting slab.
2. As the slab descends into the hot mantle, these hydrated minerals emit water vapour/volatile gases due to the increase in temperature.
3. These volatile gases are dissolved into the overlying asthenospheric mantle, resulting in a decreased melting point (temperature and pressure are unchanged)

Question 9

What is sub-solidus (volatile-induced) melting?

Igneous 1

Answer 9

Colligative properties of solutions:
1. Colligative properties that depend upon the concentration of solute molecules or ions, but not upon the identity of the solute.
1. They include:
2. freezing point depression,
3. boiling point elevation,
4. vapour pressure lowering,
5. and osmotic pressure.
2. The freezing point of pure water is 0°C, but that melting point can be depressed by the adding of a solvent (i.e., salt) — melting point decreases with decreasing purity

Question 10

How does the solidus and geotherm change for different tectonic environments?

Igneous 1

Answer 10

• Igneous rocks are intrinsically linked to plate tectonics.
• Plate tectonics are disturbing the geotherm.
  
  1. ~81% of magma annually erupted at divergent margins,
  2. 12% at convergent,
  3. 7% at hotspots.

Question 11

What are the tectonic relations to igneous activity (and features)?

Igneous 1

Answer 11

DIVERGENT:
1. Ocean-ocean: mid-ocean ridges
2. Ocean-continent: n/a
3. Continent-continent: East African rift

CONVERGENT:
1. Ocean-ocean: **ocean trench and island arc ** (e.g., Aleutians)
2. Ocean-continent: ocean trench and mountain range (e.g., Andes)
3. Continent-continent: mountain range with deformed crust (e..g, Himalayas)

TRANSFORM:
1. Ocean-ocean: part of a fracture crossing a mid-ocean ridge
2. Ocean-continent:
3. Continent-continent: (e.g., San Andreas fault)

Question 12

How is melting not restricted to the mantle?

Igneous 1

Answer 12

• Increased heat flow causes melting of country rock (sediments, metamorphic rock, igneous):
  
  • Mantle forming basaltic magma.

Continent-continent collision: mountain building events (“orogenesis”) — e.g., Himalayas, Alps, Caledonides (causes intense crustal thickening)
- Increase in heat and pressure in crust cause partial melting

Rule of thumb:
1. melt the mantle = basalt/gabbro
2. melt the crust = rhyolite/granite

Question 13

How does total melting rarely occur with rocks?

Igneous 1

Answer 13

All mantle melting is ‘sub-liquidus’

1. Congruent melting: melting of a compound when the composition of the liquid formed during melting is the same as that of the solid.
2. Incongruent melting: partial melting of a solid substance which results in the decomposition of a solid and liquid with different compositions compared to its original solid
   
   3. Different phases have different melting points.
   4. Rocks are multi-phase systems — each mineral has a different melting point.

• Additionally, during multi-phase/incongruent melting, a laccolith forms where a significantly large volume of melt with great enough buoyancy create a volcanic eruption.

Question 14

What is a laccolith?

Igneous 1

Answer 14

Laccolith:
- a body of intrusive igneous rock with dome-shaped upper surface that forms when magma rising through the Earth’s crust spreads out horizontally, splitting the layers of strata.

Question 15

How do you classify an igneous rock?

Igneous 2

Answer 15

1. Texture
2. Mineral/chemical composition

The combination of the minerals present, plus the texture of those minerals, allow us to classify igneous rocks.

• Glassy ⟶
• FInely crystalline (aphanitic) ⟶
• coarsely crystalline (phaneritic).

(Increasing grain size and decreasing rate of cooling)

Question 16

What is the cooling rate?

Igneous 2

Answer 16

Cooling rate:
- the rate at which magma or lava cools and solidifies — which affect the size and distribution of mineral crystals within the resulting igneous rocks.

• The rate is controlled by:
  
  1. The size/shape of the magmatic body
  2. Where it is forming

Question 17

How does cooling rate affect crystal size?

Igneous 2

Answer 17

The faster the cooling rate, the more crystals form at the expense of larger crystals.
- You can use crystal size of an igneous rock to assess if it cooled slowly or rapidly (or in between)

Question 18

What are magamtic intrusions?

Igneous 2

Answer 18

Magmatic/igneous intrusions:
- process by which magma crystallises and solidifies within the Earth’s crust which will force its way into/through the crust.

The intrusions will do this through:
1. existing planes of weakness,
2. by ‘stoping’ of the country rock if/when the intrusion grows.

Question 19

What are examples of magmatic intrusions?

Igneous 2

Answer 19

1. Sill:
   
   1. Sheet intrusion which is parallel and laterally moving to the present structures within host rocks which is concordant and exploits the plane of weakness underground.
2. Dike:
   
   1. Sheet tabular intrusions consisting of igneous rocks that crosscuts preexisting country rocks, where sedimentary processes can also produce sediment-filled crack (i.e., clastic or sedimentary dikes)
   2. Dikes and sill are often interconnected, all from one magma source that takes the path(s) of least resistance.
3. Batholiths (plutons):
   
   1. Large, deep-seated intrusions that form from cooled magma deep in the Earth’s crust which are composed of multiple masses (or plutons) which are bodies of igneous rock of irregular dimensions

Question 20

What is stoping?

igneous 2

Answer 20

Stoping:
- method by which intrusive igneous masses are supposed to make a way for themselves by breaking off blocks of the overlying rock and passing them downwards and backward until wholly or partially absorbed.

Question 21

What are the term for ranges in crystal sizes?

Igneous 2

Answer 21

1. > 15mm: super-coarse (pegmatitic)
2. 3-15mm: coarse
3. 1-3mm: medium
4. <1mm: fine

Question 22

What is the role of composition in igenous rock classification?

Igneous 2

Answer 22

Igneous rocks are predominantly composed of 9 elements:
1. Si (silicon)
2. O (oxygen)
3. Mg (magnesium)
4. Al (aluminium)
5. Fe (iron)
6. Ca (calcium)
7. K (potassium)
8. Na (sodium)
9. H (hydrogen)

By convention, we give the composition as oxides of the elements:
- SiO2, MgO, FeO, CaO, Al2O3, K2O, Na2O, H2O
- Event though these oxides are mostly not present in the magma or rock as distinct mineral phases:
- SIO2 ranges the most in igneous rocks:
- From ~45-75g/100g total (e..g, ~45 - 75 wt

Use this parameter to define broad groups of igneous rocks

Question 23

What are the four categories of magma?

Igneous 2

Answer 23

1. Felsic (or silicic) magma: 66-76% silica
2. Intermediate magma: 52-66% silica
3. Mafic magma: 45-52% silica
4. Ultramafic magma: 38-45% silica

Question 24

What are felsic rocks?

Igneous 2

Answer 24

Felsic rocks:
- Rocks that are made up of mostly felsic minerals (quartz, feldspars and muscovite)

Question 25

What are mafic rocks?

Igneous 2

Answer 25

Mafic rocks:
- Rocks that are made up of mostly mafic minerals (olivine, pyroxenes and amphiboles)

Question 26

What are intermediate rocks?

Igneous 2

Answer 26

Intermediate rocks:
- Rocks that have a mixture of mafic and felsic minerals (diorite and andesite)

Question 27

What is fractional crystallisation?

Igneous 3

Answer 27

Fractional crystallisation:
- sequential geochemical process of magmatic differentiation when magma cools and solidifies which leads to the separation of crystals (particularly the earliest-formed crystals, i.e., olivine and pyroxene) and formation of igneous rocks.

PROCESS:
1. During the initial stages of cooling, the earliest-formed mafic minerals (i.e., olivine and pyroxene which occurs between 1200C and 1300C) crystallise first and since they are of a higher density sink to the bottom of the magma chamber.
1. These mafic minerals remove iron- and magnesium- rich components out of the magma which alters the composition of the remaining magma to become more SIO2 (felsic or silica rich).
2. As the mafic magma continues to cool, the subsequent minerals (plagioclase) crystallise, which remove even more iron-rich and magnesium-rich components.
3. This process leaves behind a liquid of slightly different composition.

Question 28

What is the composition of a melt/source rock?

Igneous 3

Answer 28

The composition of a melt reflects the composition of the solid from which it was derived.
* Not all melts form from the same source rock, so not all melts have the same composition.
- A partial melt will always be more SiO2-rich than its source.

Question 29

What are two types of volcanic eruptions?

Igneous 3

Answer 29

1. Sticky:
   
   2. Gas-particle dispersion flows out of the vents.
2. Runny:
   
   1. Lava flows

Question 30

How does volcanic gas work?

Igneous 3

Answer 30

Gases are expelled as magma rises (Pressure drops)

• Style of gas escape controls the violence of the eruption:
  
  • Low viscosity (basalt) — easy escape; effusive eruption
  • High viscosity (rhyolite) — difficult escape; explosive release
• Gas bubbles in rock are called vesicles.

Question 31

What types of volcanic gas is present?

Igneous 3

Answer 31

• 1% to 9% of magma may be gas:
  
  1. Water (H2O) — the most abundant gas
  2. Carbon dioxide (CO2) — second in abundance
  3. Sulfur dioxide (SO2) — rotten-egg smell.
• Magma composition often controls gas content:
  
  • Felsic magmas have more gas;
  • Mafic magmas less.

Question 32

What is viscosity?

Igneous 3

Answer 32

Viscosity:
- the state of being thick, sticky and semi-fluid in consistency and the resistance of a fluid (i.e., magma or lava) to flow and change shape when subjected to stress or shear.

• Viscosity is a measure of internal friction and is a highly important factor in volcanology, where the differences in viscosity can affect the style in which the magma is erupted.
  > Higher viscosity = more explosive

Factors affecting viscosity:
1. Temperature:
1. Higher temperatures result in lower viscosity.

Question 33

What are the factors invovled in lava flows?

Igneous 3

Answer 33

• Flow style depends on viscosity, which depends upon:
  
  • Composition (especially silica [SiO2] content)
  • Temperature
  • Gas content
  • Crystal content

Question 34

What are basaltic lava flows?

Igneous 3

Answer 34

Basaltic lava flows contain mafic lava:
- very hot,
- low silica,
- and low viscosity

Basalt flows are often thin and fluid:
- They can flow rapidly (up to 30km per hour)

Question 35

What are andesitic lava flows?

Igneous 3

Answer 35

Andesitic lava flows:
- type of lava flow characterised by their high viscosity and high silica content.

Andesitic lava flows contain a higher SiO2 content which makes the lava highly viscous:
- Unlike basaltic lava flows, they do not flow rapidly.
- Instead, they mound around the vent and flow slowly.

The crust fractures into rubble, called “blocky lava”:
- which consists of smooth-sided, angular fragments of solidified lava.

Andesitic lava flows remain close to the vent.

Question 36

What are effusive lava flows?

Igneous 3

Answer 36

Effusive lava flows:
- type of volcanic eruption characterised by the steady and continuous outpourings of lava onto the ground.

Effusive eruptions occur when hot, runny basalt magmas at temperatures of around 1200C erupt through the surface.
- The dissolved gases escape easily due to the magma’s low viscosity which develops runny and gentle lava flows.

Example:
- (Andesitic magma — medium viscosity)

Question 37

What are pyroclastic eruptions?

Igneous 3

Answer 37

Different eruptive styles:
1. Pyroclastic fall (ask or tephra deposit)
2. Pyroclastic flows
3. Pyroclastic surges.

These can all occur during the same eruption.

Question 38

What is Bowen’s Reaction Series?

Igneous 3

Answer 38

The Bowen’s reaction series is the observation of the crystallisation sequence of a common silicate minerals from a typical basaltic magma undergoing fractional crystallisation.

This series only holds when the crystals have time to react.
- Slow cooling is intrusive
- Decreasing temperature results in more felsic phases forming.
- Remaining melt becomes more felsic

Question 39

Mid-ocean ridges (constructive margin):

Igneous 4

Answer 39

1. Style of melting: Decompression
2. Source of rock: Mantle
3. Major igneous processes: Partial melting
4. Major rock types formed: Basalt and gabbro
5. Mid-ocean ridges (divergent plate boundaries):
   
   • Decompression melting forms mid-ocean ridge basalt (MORB)
     
     1. Upwelling of the mantle: As the plates diverge, the underlying mantle rises to plug the gap.
     2. Decrease in pressure: The pressure decreases as the mantle rises due to the reduction of the overburden.
     3. Decompression melting: The partial melting of the mantle occurs due to the rapid decrease in pressure which results in the magma ascending and generating magmatism that forms the ocean crust.

80% to 90% of magmatism occurs on Earth.
- Opening of the plate boundary above mantle creates a void — mantle moves up to fill the void.
- Continuous spreading, so continuous source of magma:
- Limited fractional crystallisation
- Majority of melts are mafic

Additionally, the layers are stratified with the:
* pillow lava — (basalt),
* sheeted dykes — “feeder systems” (basalt)
* and magma chamber — slow crystallisation (gabbro)

Question 40

Subduction zones (destructive margin):

Igneous 4

Answer 40

1. Style of melting: Flux melting
2. Source of rock: Mantle and crust
3. Major igneous processes: Partial melting and fractional crystallisation
4. Major rock types formed: Basalt, andesite, rhyolite (and intrusive var.)

Flux melting:
1. Oceanic lithosphere and hydrated minerals (when water ions bond with the crystal structure of silicate minerals) are contained within this subducting slab.
2. As the slab descends into the hot mantle, these hydrated minerals emit water vapour/volatile gases due to the increase in temperature.
3. These volatile gases are dissolved into the overlying asthenospheric mantle, resulting in a decreased melting point (temperature and pressure are unchanged)

Final melts are often felsic or intermediate
- Hydration of mantle above subducting plate
- Subduction zones are where new continental crust is formed.

1. Thicker crust on overriding plate allows for magma storage = fractional crystallisation.
2. Increased heat flow from basaltic magmatism = partial melting of crust.

Question 41

Continent-continent collision:

Igneous 4

Answer 41

1. Style of melting: Increased heat flow
2. Source of rock: Crust
3. Major igneous processes: Partial melting and fractional crystallisation
4. Major rock types formed: Rhyolite and granite

Continental collision:
- the tectonic process of two continental plates at convergent boundaries approaching and colliding into each other via the driving force of plate motion and varying mantle dynamic processes.

1. Initial collision: this process involves the beginning of continent-continent convergence where the two tectonic plates collide and undergo deformation through actions such as thrusting.
   
   1. This further collision causes both crusts to rise and thicken which leads to increases in temperature and pressure, resulting in the partial melting of the crust to form granitic rocks and mountain ranges (“orogenesis”).

Sediments, metamorphic rocks, igneous rocks:
- Often, the melt is too viscous and deep enough in the crust, so it does not escape.
- This forms granite plutons/batholiths.

Question 42

Continental rift:

Igneous 4

Answer 42

1. Style of melting: Decompression
2. Source of rock: Mantle and crust
3. Major igneous processes: Partial melting
4. Major rock types formed: Basalt and rhoylite (bi-modal)

Continental rift:
1. Over time, it will become an ocean due to extension and creation of new oceanic crust depending on the degree of mantle upwelling that will eventually evolve into ‘oceanic spreading centres’ (i.e., the Red Sea)
2. Additionally, diverse composition is found through the volcanics due to their not being an existing single large heat source along the rift vally.

Majority of the melt is mafic:
- Some remelting of crust produces felsic volcanoes:
- Bimodal volcanism.

Question 43

Hot spots (mantle plumes)

Igneous 4

Answer 43

1. Style of melting: Decompression and increased heat flow
2. Source of rock: Mantle
3. Major igneous processes: Partial melting
4. Major rock types formed: Hot spot/mantle plume

Hot spots (mantle plumes)
- localised upwelling currents of solid rock within the mantle that are hotter and less dense than surrounding rock.

1. Heat-induced melting typically occurs at mantle plumes or hotspots where deep seated upwelling occurs,
   
   2. where the rock surrounding the plume is exposed to higher temperatures and the geothermal gradient crosses to the right past the solidus — with the rock beginning to melt.
2. Both increased heat flow and decompression eventually forms ocean-island basalt (OIB).

Majority of melts are basaltic (unless there is thick enough crust to allow differentiation)

Question 44

What are volcaniclastic deposits?

Igneous 5

Answer 44

Volcanoes often erupt large quantities of fragments:
- Volcaniclastic deposits include:
1. Pyroclastic debris: lava fragments (of all sizes) that freeze in air.
2. Preexisting rock: blasted apart by eruption.
3. Landslide debris: blocks that have rolled downslope.
4. Lahars: transported as water-rich slurries.

Question 45

What are volcani-sedimentary deposits?

Igneous 5

Answer 45

1. Volcanic debris flow: a mixture of water, volcanic ash and ice that rapidly moves downhill on the slopes of volcanoes.

This occurs where:
- Volcanoes are covered with ice and snow.
- Volcanoes are drenched in abundant rain.
- Volcanic debris flows move like wet concrete.

2.) Lahar: large mudflow/debris flow consisting of rock, ash mud and water that flows downhill under the weight of gravity.

Lahars can:
- can move very fast (~50 km per hour) and very far (~tens of km per second),
- be extremely destructive,
- and pose a very real hazard for people living near active volcanoes.

Question 46

What is pyroclastic debris?

Igneous 5

Answer 46

Pyroclastic debris:
- erupted fragmented and molten rock fragments from an explosive volcanic eruption.

Melts, crystals and country rock (lithic) fragments are fragmented by the rapid expansion of the exsolving gas from the melt:
- fragmented = blasted apart and blown from the vent

Question 47

What is pyrolastic rock classification?

Igneous 5

Answer 47

Large size range of pyroclastic material:
1. Volcanic bomb: >64mm
2. Lapilli: 3 - 64mm
3. Ash: <2mm

Question 48

What are the physical properties of magma?

Answer 48

The way a magma or lava deforms and flows is controlled by a number of properties:
1. Composition of melt:
2. Crystal content: present number of crystals in a melt acts to increase the viscosity of a melt
3. Gas content: if the gases are dissolved, they will act as network modifiers, decreasing viscosity (distrupts the bonding of the silicate oxygens)
1. If the gases exsolve, they will form bubbles which act against the flow.
4. Temperature: increasing temperature decreases the viscosity

Measuring the gas flux can allow us to determine volcanic activity.

All of these can vary greatly and can affect the style of eruption.

Question 49

What are volcanic materials?

Igneous 5

Answer 49

Volcanic materials:
- products of volcanic eruptions.

The products of volcanic eruptions come in three forms:
1. Lava flows: molten rock that moves over ground.
2. Pyroclastic debris: fragments blown out of a volcano.
3. Volcanic gases: expelled vapour and aerosols.

Question 50

What is a volcano?

Igneous 5

Answer 50

Volcano:
- a mountain built from magmatic eruptions and an erupting vent through which molten rocks surfaces as a clear result of tectonic activity.

Question 51

What are explosive mafic eruptions?

Igneous 5

Answer 51

Mafic eruptions:
- explosive eruptions containing mafic magma with:
1. low viscosity,
2. efficient degassing,
3. high magma flow rate

(i.e., Strombolian eruptions)
- This occurs where gas bubbles merge to form one large ‘slug’ which periodically rises through a magma-filled volcanic conduit and erupts onto the surface with bits of lava thrown into the air.
- Lava bombs (larger than about 3 inches in size) and scoria (smaller fragments) fall onto the surface which subsequently builds into a steep-sided volcanic cone.

Question 52

What are explosive felsic eruptions?

Igneous 5

Answer 52

Felsic eruptions:
- explosive eruptions containing felsic magma with:
1. more viscous magmas — more volcanic gases
2. Less easy to de-gas — more prone to explode
3. Explosive eruptions — that generate huge volumes of debris.

Debris includes:
1. Pumice — frothy volcanic glass.
2. Ash — fragments less than 2mm in diameter
3. Pumice lapilli — angular pumice fragments
4. Accretionary lapilli — rounded clumps of ash forming in moist air.

Question 53

What are volcanic cones?

Igneous 5

Answer 53

Volcanic cones:
- steep-sided hills/mountains built of layers of erupted lava flows and fragments of volcanic rocks that have piled up around a central vent.

There are three types of volcanic cones:
1. cinder cones,
2. composite cones,
3. and shield volcanoes

Question 54

What are pyroclastic eruptions?

Answer 54

Pyroclastic eruptions:
- explosive volcanic event characterised by rapid movement of hot, fragmented rock and gas down the slopes of a volcano.

Pyroclastic eruptions include different eruptive styles:
1. Pyroclastic fall (ash or tephra deposit):
2. Pyroclastic flows
3. Pyroclastic surges

Question 55

What are pyroclastic surges?

Igneous 5

Answer 55

Pyroclastic surges:
- low-density flow of pyroclastic materials that creates very energetic eruptions and is generally colder due to containing lots of water.

The resulting deposition of material over the topography is often cross-bedded and has intermediate sorting.

Question 56

What are pyroclastic flows?

Igneous 5

Answer 56

Pyroclastic flows:
- fast moving, high-density mix of lava blocks, ash, pumice and gases which move at very high speeds down volcanic slopes.

There are multiple ways pyroclastic flows form:
1. Gravitational collapse of the eruption column,
2. Lateral eruptions due to a release of pressure from the collapse of the edifice
3. Inclined blasts where magma is discharged from the base of an rising dome over the sides and down the slope of a volcano.

The characteristics of pyroclastic flows include:
- Avalanches of hot ash (200° to 450°C),
- Speeds of up to 300km per hour (decimation of everything in their path)

The resulting cloud of ash and debris fills the topography with poorly sorted material with no bedding.

Question 57

What are pyroclastic falls?

Igneous 5

Answer 57

Pyroclastic falls:
- uniform deposits of ejected volcanic material from plumes or eruptions (i.e., ash falls or tephra deposits)

The characteristics of pyroclastic falls include:
- Uniform thickness and sorting which is sorted and dispersed through wind direction that then covers and matches the topography
- (decreasing in both size and thickness the farther away it is from the source).

Question 58

What is a pumice flow?

Igneous 5

Answer 58

Pumice flow:
- type of high-density pyroclastic flow which is fast moving and contains coarse fragments of pumice, volcanic ash and lava blocks that is:
1. ejected through the eruption of pressurised magma.
2. This results in dissolved gases to quickly exsolve and form pumice deposits.

Question 59

What is a block and ash flow?

Igneous 5

Answer 59

Block and ash flow:
- type of pyroclastic flow — due to the gravitational collapse of a volcanic dome — which deposits a mixture of large, angular volcanic blocks and ash in small volumes.

Question 60

What is an eruption column?

Igneous 5

Answer 60

Eruption column:
- cloud of ascending super-heated volcanic gases and debris erupted directly from the vent of a volcano, usually rising kilometres into the air and spreading laterally across the atmosphere.