Introduction to Volcanology (1) Flashcards
Where does most volcanism arise from?
Plate tectonics
What is the outer layer of the Earth called?
the Lithosphere
What is the lithosphere split up into?
Plates
The movement of what causes volcanism?
Tectonic plates
Explain the four stages of Subduction zone Volcanism
- More dense oceanic crust subjects beneath less dense continental crust
- At a certain depth the subjecting plate begins to melt.
- Batches of magma rise through the continental crust
- Magma may collect in storage zones before rising through a conduit and resulting in an eruption.
Explain the two stages of Intra Plate (“Hot spot”) Volcanism
- A mantle plume rises from the core-mantle boundary
2. Over time it will penetrate the oceanic crust and erupt to form a shield volcano
What are the two main types of volcano?
Stratovolcanoes and Shield Volcanoes
What is a stratovolcano?
These are built from layers of viscous lava flows and pyroclastic rocks from explosions. The concave profile can produce a “postcard volcano”.
What are shield volcanoes?
Layers of fluid lava flows, convex profile and can be enormous.
Give an example of a Shield Volcano and an interesting fact
Mauna Kea has a volume 100 times greater than a typical stratovolcano
Give an example of a stratovolcano
Mayon Volcano in the Philippines
What are the two types of Magmas?
Low viscosity and high visocity
Give an example of low viscosity magma
Basalt
What type of volcano does Basalt give rise to?
Basaltic
What type of volcano does low viscosity magma create?
Shield Volcanoes
Give two examples of high viscosity magma
Andesite and Rhyolite
What type of volcanism do high viscosity magmas give rise to?
Silicic Volcanism
Where is high visocity magma most commonly produced?
Subduction zone settings
What type of volcanoes does high visocity magma create?
Stratovolcanoes
Which compound defines the viscosity of magmas?
Silicate
What is the chemical formula for silicate? What does it mean?
SiO4. One silicon atom with four oxygen atoms surrounding it.
What is formed when silicate combines?
A lattice
How does the lattice affect the magma?
It makes it more viscous.
Which type of magma has more SiO4? High or low visocity?
Silicic. High Viscosity
Which type of magma has less SiO4? High or low visocity?
Basaltic. Low viscosity
What happens to gas flow in silicic magmas?
It is impeded
What happens to gas flow in basaltic magmas?
It can move more freely
What causes explosively in volcanism?
Gases within the magma
What three elements make up magma?
Melt (molten rock), Crystals (solid rock) and Gas
What temperature can the gas within the magma reach?
1000*C
Where does the gas come from?
A solution (it exsolves) from the magma as pressure decreases during magma ascent
What is the term used for when gas is released at the surface?
Degassing
How can gas within magma be compared to a champagne bottle?
Both have explosive potential
What can changes in degassing cause?
Abrupt transitions in activity
Why is it important to understand which gas is being emitted?
In order to determine how we measure and detect gas
What are the three major constituents to Volcanic gas?
- Water Vapour (H2O)
- Carbon Dioxide (CO2)
- Sulphur Dioxide (SO2)
Where are gases usually emitted from?
Summit craters or calderas from a volcanic conduit
Which two ways can gases be emitted from a volcano?
- Quiescently during passive degassing
2. More violently during explosive degassing
Is it easier or more difficult for gases to move independently within high viscosity magma?
More difficult
Where can gas sometimes escape from?
Through fractures in a volcano or through networks of other bubbles
What can degassing cause changes to?
The magma chemistry
Does degassing increase or decrease viscosity?
Increase
What happens if gas can’t escape from a conduit?
It will pressurise and cause explosions
What happens when gas rises faster than magma?
Bubbles can form and coalesce
What happens when bubbles coalesce within the magma?
They grow in size and can lead to passive degassing, lava flows or Strombolian volcanism
What happens when gas rises at the same rate as the magma?
The mixture is ejected into the atmosphere to form a low explosivity “lava fountain”.
What happens when gas emits without lava?
The magma and gas rises, the gas is passively emitted, the magma then sinks back down the conduit during convection
What are the three distinct behaviours of degassing in basaltic volcanism?
- Gas rises faster than magma
- Gas and magma rise at the same rate
- Gas emission without lava
