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Environmental Science > 4.1 - Volcanology > Flashcards

4.1 - Volcanology Flashcards

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1
Q

Cumbre vieja eruption in 2021

A

La cumbre vieja is a volcanic ridge in La Palma in the Canary Islands.

  • The volcano began to erupt on 19th September 2021
  • It produced Lava fountains, lava flows and volcanic gas
  • There was an evacuation of around 7000 residents (there were no injuries but it did $800 million in damage)
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2
Q

2022 Hunga Tonga-Hunga Ha’apai eruption

A
  • The submarine volcano located in the South Pacific, 65km north of Tongas main island (Tongatapu)
  • In December 2021 to early January, several violent volcanic eruptions took place and there was a major eruption on January 15th 2022
  • The major explosive eruption created a plume that reached heights of 50km (half way to space)
  • The blast was equivalent to 10 million tons of TNT
  • There was a pressure wave picked up around the world
  • The ferocity of the eruption was most likely linked to the magma-water interaction
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3
Q

Why do people live near volcanoes?

A
  • It offer extremely fertile soils
  • It provides geothermal energy resources = free green energy
  • It provides mineral resources (Cu, Zn, Fe, Au, Ag ores)
  • Volcanoes can attract tourists which helps support the local economy
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4
Q

What is a volcano?

A
  • It is an erupting vent through molten rock surfaces
  • They are the surface expression of melting processes (that take place in planetary interiors)
  • On earth these melts are silicate magmas
  • Volcanism is mostly (but not always) related to plate boundaries
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5
Q

What are the 3 forms that volcanic eruptions come in?

A
  1. Lava flows = molten rock that moves over the ground
  2. Pyroclastic debris = fragments blown out of a volcano
  3. Volcanic gases = expelled Vapor and aerosols
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6
Q

How is intrusive rock formed?

A

It forms when magma intrudes into the lithosphere or rock, the magma cools and solidifies underground to form igneous intrusions such as dikes, sills and batholiths

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7
Q

How is extrusive rock formed?

A

Extrusive igneous rock is formed when magma exits and cools above the earths surface. These form at erupting volcanoes and oozing fissures

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8
Q

What are the temps required to melt a rock?

A

650-1100 degrees Celsius

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9
Q

How do you melt the mantle?

A

You have to disturb the geothermal to melt the mantle:

  • Add heat
  • Reduce pressure!!!!!! (Most important)
  • Change the composition
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10
Q

What is the geotherm gradient?

A

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the earths interior.

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11
Q

What is decompression (adiabatic) melting?

A
  • It is a Geologically sudden decrease in Pressure
  • Suddenly, molecules in the rock have a lot more room to move about
  • They have a relatively high T for that amount of room
  • Melting occurs
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12
Q

What is the geotherm?

A

It is the temperature as a function of depth

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13
Q

What is the liquidus?

A

The conditions at which a rock completely melts

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14
Q

What is the solidus?

A

The T and P conditions at which a rock starts to melt

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15
Q

Where does decompression (adiabatic) melting take place?

A
  • Mid ocean ridges = Decompression melting forms mid ocean ridge basalt (MORB)
  • Continental rifts = eg. East Africa over time will become an ocean
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16
Q

What is volatile assisted melting?

A

It is when there is a change to the composition of the system eg. By adding salt (NaCl) to water to lower the melting point

  • 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, C02) you lower its melting temperature
17
Q

How does melt genesis take place?

A

It requires an increase in temperature to melt a solid into a liquid.

This tales place at mantle plumes where there is a hot spot in the mantle which causes the crust to melt.

18
Q

What type of plate margin does the majority of magma erupt from?

A
  • 81% of magma erupts at divergent margins
  • 12% of magma erupts at convergent plate margins
  • 7% of magma erupts at hotspots
19
Q

How may the driver of melting change over time?

A

Eg. In east Africa the magma melts due to a plume (high T) but this melt will change to decompression melt as the plate thins.

(Tectonics disturb the geotherm)

20
Q

What is the cause of surface volcanism?

A

It is the result of the mantle melting.

  • there is also submarine volcanism, as well as volcanism on other planets eg. Io (moon of Jupiter)
21
Q

What are the 2 types of volcanic eruptions?

A
  1. Explosive
  2. Effusive
22
Q

What are the 2 magma properties that control an extrusive eruption style?

A
  1. Rheology (Viscosity)
  2. Dissolved volatile content
23
Q

What is viscosity?

A

It is a measure of a fluids resistance to flow (units= Pa.s)

24
Q

Rheology in silica polymerisation with high conc. of network formers

A

The high concentration of network formers (Si02, Al203, and Fe203 molecules) means there are more bonds between oxygens.

This means there needs to be big energy to deform the network.

This creates a high viscosity of magma (rhyolite)

25
Q

Rheology in magma might a high concentration of network modifiers

A

When there is a high concentration of network modifiers (Mg0, Ca0, Na20 and K20 molecules) it means that there is a small amount of bonds between oxygen.

This means that a smaller amount of energy is required to break the bonds/ deform the network

This means the magma is less viscous (basalt)

26
Q

What is viscosity controlled by?

A
  • Chemical composition = more Si02 means the magma is more viscous
  • Temperature = higher temperature means the magma is less viscous
  • Dissolved gas content (volatiles, eg. Water) = higher dissolved gas content means the magma is less viscous
  • Crystal content = the higher the crystal content, the more viscous the magma will be
27
Q

What is the most viscous magma on earth?

A

Rhyolites which have >70% Si02 are the most viscous magmas on earth (thickest)

28
Q

What does magma/lava flows viscosity depend on?

A
  1. Composition - especially silica (Si02)
  2. Temperature
  3. Gas content (water)
  4. Crystal content
29
Q

What are the 4 categories of magma?

A
  1. Felsic (or silica) magma = 66-76% silica (more viscous)
  2. Intermediate magma = 52=66% silica
  3. Magic magma = 45-52% silica
  4. Ultramafic magma = 38-45% silica (less viscous)
30
Q

What are magfic (basaltic) lava flows?

A

They are very hot, with low silica and have a low viscosity.
(They are thin and fluid)

  • They can flow rapidly (up to 30km an hour)
  • They can flow for long distances (up to several hundred km)
    (Most flows measure less than 10km)
  • The long distance flows are facilitated by lava tubes
31
Q

What is a ‘ropy’ or ‘pahoehoe’ lava?

A
  • It is a basalt with glassy, ropy texture
  • It forms when extremely hot basalt forms a skin
  • With flow, the skin is rolled into ropy ridges and furrows
  • It forms when flow is hotter and closer to a vent
32
Q

What is ‘Blocky’ or ‘aa’ lava?

A
  • It is a basalt that solidifies with a jagged, sharp, angular texture
  • With flow, the lava crumbles into shards and fragments
  • forms further away from the vent

(aa is what u say when you walk across this material barefoot)

33
Q

What is submarine magic (basaltic) lava like?

A

A pillow basalt forms which are round blobs of basalt cooled in water.

  • Underwater, the basalt skin cools instantly forming a pillow
  • The pillow surface is cracked, quenched glass
  • Lava pressure ruptures a pillow to form the next blob
  • The process repeats to form a mound of pillow basalts
  • They are common at the mid ocean ridge
34
Q

What is silicic (rhyolite) lava like?

A
  • High viscosity magmas erupted onto surface at relatively slow ascent rates
  • Rhyolite lava tends to form short domes and spines
  • The lava rarely flows, piles around the vent and there is intermittent explosive activity
35
Q

What are effusions of silicic magma?

A

A variety of types of lava dome, related to yield strength eg. Mount St. Helens lava dome

  • Blake (1999) classification of lava domes is based primarily on morphology
36
Q

What are intermediate (andesitic) lava flows like?

A

They have a higher si02 content which makes andesitic lavas viscous

  • Unlike basalt, they dont flow rapidly and instead form a mound around the vent and flow slowly
  • The crust fractures into rubble, called blocky lava
  • Andesitic lava flows remain close to the vent
    Eg. Kanaga volcano

Environmental Science (15 decks)

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