4.1 - Volcanology Flashcards
Cumbre vieja eruption in 2021
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)
2022 Hunga Tonga-Hunga Ha’apai eruption
- 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
Why do people live near volcanoes?
- 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
What is a volcano?
- 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
What are the 3 forms that volcanic eruptions come in?
- Lava flows = molten rock that moves over the ground
- Pyroclastic debris = fragments blown out of a volcano
- Volcanic gases = expelled Vapor and aerosols
How is intrusive rock formed?
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
How is extrusive rock formed?
Extrusive igneous rock is formed when magma exits and cools above the earths surface. These form at erupting volcanoes and oozing fissures
What are the temps required to melt a rock?
650-1100 degrees Celsius
How do you melt the mantle?
You have to disturb the geothermal to melt the mantle:
- Add heat
- Reduce pressure!!!!!! (Most important)
- Change the composition
What is the geotherm gradient?
Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the earths interior.
What is decompression (adiabatic) melting?
- 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
What is the geotherm?
It is the temperature as a function of depth
What is the liquidus?
The conditions at which a rock completely melts
What is the solidus?
The T and P conditions at which a rock starts to melt
Where does decompression (adiabatic) melting take place?
- Mid ocean ridges = Decompression melting forms mid ocean ridge basalt (MORB)
- Continental rifts = eg. East Africa over time will become an ocean
What is volatile assisted melting?
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
How does melt genesis take place?
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.
What type of plate margin does the majority of magma erupt from?
- 81% of magma erupts at divergent margins
- 12% of magma erupts at convergent plate margins
- 7% of magma erupts at hotspots
How may the driver of melting change over time?
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)
What is the cause of surface volcanism?
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)
What are the 2 types of volcanic eruptions?
- Explosive
- Effusive
What are the 2 magma properties that control an extrusive eruption style?
- Rheology (Viscosity)
- Dissolved volatile content
What is viscosity?
It is a measure of a fluids resistance to flow (units= Pa.s)
Rheology in silica polymerisation with high conc. of network formers
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)
