1. Volcanoes

2. The cause of it all… What causes volcanoes to erupt???
The shift in the Earth’s plates are what causes volcanoes to form.
As the plates join or separate some of the molten rock is exposed.
viscosity, resistance of a fluid to flow.

3. 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.
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).

4. Explosive Rapidly escaping gas bubbles rip apart magma, fragmenting it.
Explosive volcanic eruptions can be catastrophic
Erupt 10’s-1000’s km3 of magma
Send ash clouds >25 km into the stratosphere
Have severe environmental and climatic effects
Hazardous!!!
Above: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska

5. Explosive Eruptions Three products from an explosive eruption Ash fall
Pyroclastic flow
Pyroclastic surge
Pyroclastic flows on Montserrat, buried the capital city.

6. Effusive magma leaks out onto the surface passively as lava flows.
Effusive eruptions are characterized by outpourings of lava on to the ground.
Hawaii
Courtesy of

7. Where do volcanoes erupt???
Convergent plate boundaries – where two of Earth’s plates join together RING OF FIRE
Divergent plate boundaries – where two of Earth’s plates move apart MID OCEAN RIDGES
Hot spots – active areas below the earths crust

8. Structure of a Volcano magma central vent side vent lava old layers
of lava
ash cloud
Earth’s crust

9. Structure of a Volcano
Magma – the molten, or liquid-like, rock within the Earth
Lava – magma that reaches Earth’s surface
Vent – an opening through which molten (liquid-like) rock flows onto Earth’s surface
Fissures: side vents, usually accompanying the main vent.
Crater/caldera: large, depression created by the emptying of the magma at great speed. The volcano collapses creating a large crater.
Ash cloud: smoke and debris usually consisting dust, hot rock fragments (tephra), gases, water vapor, and other particles.

10. Types of Volcanoes Shield volcanoes Composite volcanoes
There are three major types of volcanoes:
based on composition of
magma
Shield volcanoes
Composite volcanoes
Cinder cone volcanoes

11. Shield Volcanoes
The magma inside a shield volcano is rich in iron and magnesium and is very fluid.
Since the magma is very fluid, the lava coming out of the volcano tends to flow great distances.
When shield volcanoes erupt, the flowing lava gives the volcano the shape of a gently sloping mountain.

12. Shield Volcanoes
Eruptions of shield volcanoes are mild and can occur several times.
Happens in HOT SPOTS (place where earth’s crust is weak and lava may flow continuously)
Mauna Loa in Hawaii is an example of a shield volcano.

13. Shield Volcanoes

14. Composite Volcanoes
The magma inside a composite volcano is rich in silica and much thicker than magma from a shield volcano.
Gases get trapped inside this thicker magma.
Eruptions from composite volcanoes can be flowing lava or explosions. The explosive eruptions come from the trapped gases and produce cinders and ash.

15. Composite Volcanoes
These different types of eruptions are what give composite volcanoes their alternating layers of lava and cinders.
Composite volcanoes have much steeper slopes than shield volcanoes.
Happens at subduction zones
Mount Fuji in Japan and Mount St. Helens in the USA are examples of composite volcanoes.

16. Composite Volcanoes

17. Cinder Cone Volcanoes
The magma inside a cinder cone volcano has large amounts of gas trapped in it.
Eruptions from cinder cone volcanoes are violent and explosive because of all the gas trapped in the magma.
The large amounts of hot ash and lava thrown out of the vent fall to the ground forming the cone shape that these volcanoes have.

18. Cinder Cone Volcanoes
Cinder cone volcanoes are usually only active for a short time and then become dormant (inactive).
Paricutin in Mexico is an example of a cinder cone volcano.

19. Cinder Cone Volcanoes

20. Volcanic Products
Pyroclastic flows / falls
Lahars
Lava
Volcanic ash

21. Pyroclastic flows
Pyroclastic flows are mixtures of hot gas, ash and other volcanic rocks travelling very quickly down the slopes of volcanoes.  They are one of the most dangerous hazards posed by volcanoes.
     Pyroclastic flows are so hot and choking that if one is caught in one the person will certainly be killed.  Because these flows are very fast they cannot be out-runned!

22. Pyroclastic Fall Ash load Collapses roofs Brings down power lines
Kills plants
Contaminates water supplies
Respiratory hazard for humans and animals

23. Lahars Hot volcanic activity can melt snow and ice
term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys.
Hot volcanic activity can melt snow and ice
Melt water picks up rock and debris
Forms fast flowing, high energy torrents
Destroys all in its path

24. Volcanic ash
Volcanic ash is a volcanic rock which is exploded from a vent in fragments less than 2mm in size. 
Volcanic ash-particles are like small sharp glass-particles that damage anything they come across. 
During heavy ash-rains houses and buildings may collapse, people and animals may die by lack of oxygen.

25. Pahoehoe Lava
Pahoehoe lavas flow smoothly and are often formed by small volumes of hot, fluid basalt.  The higher the volume of lava emitted the faster the current. 
.  When the pahoehoe lava flow cools, it often solidifies to a smooth surface.

26. Aa Lava
Aa flows are emitted from the vent at high rates ranging to 50km an hour, often with much lava fountaining. 
Aa flows are animated with sporadic bursts of energy.  They may push down houses, walls and forests. 
However, the hallmark of aa lava flows is the very rough surface it produces when it cools and solidifies.

27. Lava flow differences

28. Seismicity Deformation Gas Output Volcano Monitoring
Volcano Observatories are set up on all active volcanoes that threaten the human population. These are designed to monitor and potentially to predict the eruptive behavior of the volcano in question.
Seismicity
Deformation
Gas Output
These three are the most important in monitoring active volcanoes.

29. Volcanic Gas
Volcanic gas is contained within magma.  As the magma rises to the Earth's surface the gases are released.  Becau se some gases are toxic they can suffocate people. 

30. Seismometers
Seismometers are instruments that measure and record motions of the ground, including those of seismic waves generated by earthquakes.
When a volcano is ready to erupt, the magma chamber starts filling up, creating cracks in the surrounding rocks, i.e. earthquakes

31. Global Impact
When a volcano erupts it throws out a lot of ash.  At short notice this ash can be very harmful to the environment, but on the long term the ash layer, which contains many useful minerals, will be converted to a very fertile soil.
Volcanoes provide resources for energy extraction, also called geothermal resources.  Heat from the earth's crust is being converted to energy.  The big advantages to this type of energy are that it is very clean and the resources are nearly inexhaustible.

32. Global Impact
Sea Floor Spreading- new crust is formed at the bottom of the ocean floor.

33. Final Exam Focus Question
Most of the gas erupted from a volcano is steam, the remainder is…
A) hydrogen sulfide B) Carbon Dioxide
Although volcanic ash is talked about a lot, the most significant global effects are produced by:
A) heat from lava flows
B) melting of glaciers during eruptions
C) Destruction of vegetation

34. Final Exam Focus
Why do earthquakes cause such large death tolls in 3rd world countries?
A) more tsunamis B) poor hospitals
C) stronger earthquakes
D) weak but heavy construction materials
What is a tsunami?
A) measure of energy released by an earthquake
B) A seismic sea wave

35. Final Exam Focus Why do volcanoes occur at subduction zones?
A) compression heats the rocks
B) The descending slab begins to melt
C) tension opens cracks so material from the core can rise.