1. Volcanic Eruptions and Hazards

2. What is a volcano?
A mountain or hill, typically conical, having a crater or vent through which molten rock (magma), rock fragments, hot vapor, and gas are or have been erupted from the earth's crust
The volcano includes the surrounding cone of erupted material.
vent
cone
A volcano is a place on the Earth’s surface where hot, molten rock (called magma) breaks through. As we will see there are many different types of volcanoes and material that is erupted.
However, in general a volcano is classed as “active” if it erupts lava, rock, gas or ash, or if it shows seismic (earthquake) activity.
A volcano is dormant if it hasn't erupted for a long time (less than 1 million years) but could again one day.
An extinct volcano will never erupt again.
Presenters; go through the different parts of a volcano:
Magma: Molten rock beneath the surface of the earth.
Magma chamber: The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.
Conduit: A passage followed by magma in a volcano.
Vent: The opening at the earth's surface through which volcanic materials issue forth.
Cone: A volcanic cone built entirely of loose fragmented material (pyroclastics) and (or) lava flows erupted from the vent. Erupted material builds up with each eruption forming the cone.
conduit
magma chamber

3. What causes the magma to escape the mantle and come up through the crust of Earth?
Subduction Zone Volcanoes
Remember that subduction happens at convergent plates
Divergent Zone Volcanoes
This results in ridges
Hot Spots
These can pop up anywhere the crust is weak and thin, even in the middle of plates

4. What are Hotspot Volcanoes?
A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time
Firstly, what are hotspot volcanoes and how do they form?
A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time.
The source of this volcanism is a mantle plume of hot mantle material rising up from near the core-mantle boundary through the crust to the surface (see left diagram).
A mantle plume may rise at any location in the mantle, and this is why hotspot volcanoes are independent from tectonic plate boundaries.
The Hawaiian island chain are an example of hotspot volcanoes (see right photograph).
The Hawaiian island chain are examples of hotspot volcanoes.
Photo: Tom Pfeiffer /

5. The volcanoes get younger from one end to the other.
The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.
Hotspot’s commonly form volcanic island chains (like the Hawaiian islands). These result from the slow movement of a tectonic plate over a FIXED hotspot.
Persistent volcanic activity at a hotspot will create new islands as the plate moves the position of the “old” volcanic island from over the hotspot.
Therefore at one end of the island chain you see the youngest, most active volcanic islands (directly over the hotspot) and along the island chain the extinct volcanoes become older and more eroded (see diagram).
This way geologists can use hotspot volcano chains to track the movement of the tectonic plate through time.
The volcanoes get younger from one end to the other.

6. How and why do volcanoes erupt?
Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface.
Same principle as hot air rising, e.g. how a hot air balloon works
When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts.
Why Do Volcanic Eruptions Occur?
High temperature of the Earth’s interior
Melting of lower crust and mantle = molten rock = magma
At depths > 20 km the temperature = 800-1,600 degrees Celsius
The density of the magma is less than the crustal rock, therefore it rises to the surface
Source of this heat?
Residual from the cooling of the Earth (& solar system)
Radioactive decay
Convection in the mantle
- Brings hot rock up from near the interior of the Earth and returns cooler material towards the centre of the Earth for reheating.
Shock/impact melting
E.g. meteorite impacts produce instantaneous heat and melting from high energy collisions
Two styles of volcanic eruption: Explosive and Effusive (see next slides for further descriptions of each)
Explosive: where rapidly escaping gas bubbles (= vesicles) rip apart the magma, fragmenting it.
Effusive: where the magma leaks out onto the surface passively as lava flows.
- NOTE: Some effusive eruptions involving highly viscous lava may turn into explosive eruptions. If the magma is too viscous (sticky) it can block up the volcanic vent, trapping gas inside the volcano. If this gas builds up enough to break through the blockage an extremely dangerous explosive eruption may form.
- Some of the most explosive eruptions have formed this way, e.g. Pinatubo 1991

7. How and why do volcanoes erupt?
Large amounts of gas and a high viscosity (sticky) magma will form an explosive eruption!
Think about shaking a carbonated drink and then releasing the cap.
Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruption
Where the magma just trickles out of the volcano (lava flow).
Why Do Volcanic Eruptions Occur?
High temperature of the Earth’s interior
Melting of lower crust and mantle = molten rock = magma
At depths > 20 km the temperature = 800-1,600 degrees Celsius
The density of the magma is less than the crustal rock, therefore it rises to the surface
Source of this heat?
Residual from the cooling of the Earth (& solar system)
Radioactive decay
Convection in the mantle
- Brings hot rock up from near the interior of the Earth and returns cooler material towards the centre of the Earth for reheating.
Shock/impact melting
E.g. meteorite impacts produce instantaneous heat and melting from high energy collisions
Two styles of volcanic eruption: Explosive and Effusive (see next slides for further descriptions of each)
Explosive: where rapidly escaping gas bubbles (= vesicles) rip apart the magma, fragmenting it.
Effusive: where the magma leaks out onto the surface passively as lava flows.
- NOTE: Some effusive eruptions involving highly viscous lava may turn into explosive eruptions. If the magma is too viscous (sticky) it can block up the volcanic vent, trapping gas inside the volcano. If this gas builds up enough to break through the blockage an extremely dangerous explosive eruption may form.
- Some of the most explosive eruptions have formed this way, e.g. Pinatubo 1991

8. Types of Volcanoes
An active volcano is a volcano that has had at least one eruption during the past 10,000 years. An active volcano might be erupting or dormant.
An erupting volcano is an active volcano that is having an eruption...
A dormant volcano is an active volcano that is not erupting, but supposed to erupt again.
An extinct volcano has not had an eruption for at least 10,000 years and is not expected to erupt again in a comparable time scale of the future.

9. Why do volcanoes stop erupting?
all the trapped volatile gasses have degassed and there is no longer sufficient pressure to drive the magma out of the Earth. OR
enough heat is lost so that the magma cools and is no longer buoyant

10. Volcanism is mostly focused at plate margins
Pacific Ring of Fire
This map shows the margins of the Pacific tectonic plate and surrounding region. The red dots show the location of active volcanism. Notice how the majority of the volcanism is focused in lines along the plate boundaries? For this region this area is known as the “Pacific Ring of Fire”.
But why are all of the volcanoes located at the plate margins?
Volcanism is mostly focused at plate margins

11. Ring of Fire
The Ring of Fire has 452 volcanoes and is home to over 75% of the world's active and dormant volcanoes
The Ring of Fire is a direct result of plate tectonics and the movement and collisions of crustal plates