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Warm Up 1. What is an earthquake?  2. What are the different types of earthquakes?  3. What are the different types of boundaries?  4. What are the layers of the atmosphere?  5. What are the layers of the Earth? 

Volcanic Eruptions and Hazards

Mt. St. Helen

What is a volcano? ď Ź




ď Ź

magma chamber

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.

What causes the magma to escape the mantle and come up through the crust of Earth? 

Subduction Zone Volcanoes –

Divergent Zone Volcanoes –

Remember that subduction happens at convergent plates This results in ridges

Hot Spots –

These can pop up anywhere the crust is weak and thin, even in the middle of plates

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

The Hawaiian island chain are examples of hotspot volcanoes.

Photo: Tom Pfeiffer /

The tectonic plate moves over a fixed hotspot forming a chain of volcanoes.

The volcanoes get younger from one end to the other.

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.

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).

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.

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 ď Ź enough heat is lost so that the magma cools and is no longer buoyant ď Ź

Pacific Ring of Fire

Volcanism is mostly focused at plate margins

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

Explosive Eruptions 

   

Explosive volcanic eruptions can be catastrophic Erupt 10’s-1000’s km3 of magma Send ash clouds > up to 25 km into the stratosphere Have severe environmental and climatic effects Hazardous!!!

Mt. Redoubt

Above: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska

Explosive Eruptions 

Three products from an explosive eruption –

Ash fall  The

Pyroclastic flow 

fallout of rock, debris and ash

Pyroclastic flows are hot, turbulent, fast-moving, high particle concentration clouds of rock, ash and gas.

Pyroclastic surge  they

don’t have a high concentration of particles and contain a lot of gas

Pyroclastic flows on Montserrat, buried the capital city.

Direct measurements of pyroclastic flows are extremely dangerous!!!

Pyroclastic Flow - direct impact

Courtesy of

Pyroclastic Flow - burial

Pyroclastic Flow - burns

Pyroclastic Flow - lahars    

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

ď Ź

From above you can see their fine grainsize and the distinctive slight ridges and grooves that show which way the mass of hot dusty air was moving.

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

Effusive Eruptions ď Ź

Effusive eruptions are characterized by outpourings of lava on to the ground.

Hawaii Courtesy of

Lava Flow It is not just explosive volcanic activity that can be hazardous. Effusive (lava) activity is also dangerous. ď Ź Lava flows have temperatures in excess of 200 degrees Celsius ď Ź

How can scientists tell if there is going to be an eruption?

Volcanoes Give Signs ď Ź

"These signs may include very small earthquakes beneath the volcano, slight inflation, or swelling, of the volcano and increased emission of heat and gas from vents on the volcano," ď Ź said

U.S. Geological Survey (USGS) Volcano Hazards Program coordinator John Eichelberger.

Deformation Monitoring 

“Tiltmeters” are used to measure the deformation of the volcano –

The tiltmeters measure changes in slope as small as one part per million. A slope change of one part per million is equivalent to raising the end of a board one kilometer long only one millimeter!

Deformation Monitoring ď Ź

Tiltmeters can tell you when new material enters the magma chamber.



Note the presence of earthquakes in relation to the deformation. Often it is a combination of events that fore-warns of an eruption.

Gas Monitoring 

Commonly gas output from a volcano increases or changes composition before an eruption. – As magma rises to the surface it releases (exsolves) much of its gas content. – This can be measured

Types of volcanoes: ď Ź

Composite Volcanoes-are a mixture between shield volcanoes and cone volcanoes, their eruptions are explosive.

Types of volcanoes: Composite Volcanoes  Shield Volcanoes-are low and flat and have small, flowing eruptions 

Types of volcanoes: Composite (Strato) Volcanoes  Shield Volcanoes  Cinder Cones - Cones are the tallest and largest volcanoes, and they have VERY explosive eruptions. 

Types of volcanoes: Composite Volcanoes  Shield Volcanoes  Cinder Cones  Spatter Cones 

Types of volcanoes: Composite Volcanoes  Shield Volcanoes  Cinder Cones  Spatter Cones  Super (Complex) Volcanoes 

In Summary.. Volcanoes are extremely hazardous.  However, the volcano can be studied, monitored and understood.  Each volcano is different, and offers a unique set of dangers  Plans may be emplaced to help control potential damage. 

Post-lecture Question: ď Ź

What should geologists do about volcanic eruptions in the future? 1. Study volcanoes to find out more about how and why they erupt 2. Monitor the volcanoes 3. Develop hazard mitigation plans 4. Understand the population around volcanoes, i.e. why do people choose to live near volcanoes? 5. Education

Warm Up 1. What is a divergent boundary? Draw one.  2. What is a convergent boundary? Draw.  3. What is subduction? Draw.  4. What is a transform boundary? Draw.  5. What is the lithosphere?  6. What are the layers of the atmosphere? 


ď Ź

As with volcanoes, earthquakes are not randomly distributed over the globe Figure showing the distribution of earthquakes around the globe

ď Ź

At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur.

Where do they happen? Most often they happen where the plates meet (fault lines) ď Ź Sometimes in the middle of the plate where the crust becomes very heavy and drops like a sink hole. Ex: Mississippi River delta ď Ź

ď Ź

ď Ź

Friction causes the plates to get stuck, building pressure and when the pressure if released, an earthquake occurs. The point where the earthquake starts is called the focus.

Where do earthquakes form?

Figure showing the tectonic setting of earthquakes

Earthquake key terms:  Foot

wall: The plate that doesn’t move during an Earthquake.  Hanging wall: The plate that moves during an Earthquake.  Fault plane: The plane along which the break between two plates occurs.  Fault line: The line in the surface of the Earth caused by the fault plane.

How do they move? 5 ways  Strike-Slip Quake (happen at transform boundaries)  Normal- Hanging wall moves down. (divergent boundaries)  Reverse/Thrust Quakeshanging wall moves up (convergent boundaries.) 

How do they move?

Horst Quakes  A horst represents a block pushed upward by the faulting, and a graben is a block that has dropped due to the faulting.  Graben Quakes 

Strike-slip Earthquakes

Normal Earthquake:

Normal Earthquake:

Reverse (Thrust) Earthquake:



Warm Up 1. What is an earthquake?  2. Where do earthquakes happen?  3. What is a strike slip earthquake?  4. What is a normal earthquake?  5. What is a reverse earthquake?  6. What is a horst earthquake?  7. What is a graben earthquake? 

How do you tell how severe an earthquake is? Earthquakes generate seismic waves which can be detected with a sensitive instrument called a seismograph . ď Ź The Richter Scale is based in energy released as measured by maximum wave amplitude on a seismograph. ď Ź

Richter Scale Richter scale no.

No. of earthquakes per year

Typical effects of this magnitude

< 3.4

800 000

3.5 - 4.2

30 000

4.3 - 4.8

4 800

Most people notice them, windows rattle.

4.9 - 5.4


Everyone notices them, dishes may break, open doors swing.

5.5 - 6.1


Slight damage to buildings, plaster cracks, bricks fall.

6.2 6.9


Much damage to buildings: chimneys fall, houses move on foundations.

7.0 - 7.3


Serious damage: bridges twist, walls fracture, buildings may collapse.

7.4 - 7.9


Great damage, most buildings collapse.

> 8.0

One every 5 to 10 years

Detected only by seismometers Just about noticeable indoors

Total damage, surface waves seen, objects thrown in the air.

Seismic Activity 

Earthquake activity commonly precedes an eruption – Result of magma pushing up towards the surface – Increase volume of material in the volcano shatters the rock – This causes earthquakes

Seismic Activity Earthquake activity is measured by Seismographs Seismographs are stationed on the flanks of the volcano These record the frequency, duration and intensity of the earthquakes and report it back to the volcano observatory.

Earthquakes Large volumes of magma moving through the shallow crust can cause large earthquakes. ď Ź This can lead to building collapse, slope failure and avalanches ď Ź


Destruction after a volcanic induced earthquake in Japan

Pyroclastic flow ď Ź

A pyroclastic flow is a fluidized mixture of solid to semi-solid fragments and hot, expanding gases that flows down the flank of a volcanic edifice. The features are heavierthan-air emulsions that move much like a snow avalanche, except that they are fiercely hot, contain toxic gases, and move at phenomenal, hurricane-force speeds, often over 100 km/hour. They are the most deadly of all volcanic phenomena.

Pyroclastic Surge ď Ź

Pyroclastic Surge: A more energetic and dilute mixture of searing gas and rock fragments is called a pyroclastic surge. Surges move easily up and over ridges; flows tend to follow valleys. They are relatively thin; usually no more than about 10 cm thick, except in local "drifts" (like snow).

Deeper Learnings About Volcanoes and Earthquakes  

Grow your knowledge about volcanoes and earthquakes.

Deeper Learnings About Volcanoes and Earthquakes  

Grow your knowledge about volcanoes and earthquakes.