Volcanic Volcanic EruptionsEruptions

Mt. St. HelenMt. St. Helen

What is a volcano?What is a volcano? A volcano is a vent A volcano is a vent or 'chimney' that or 'chimney' that connects molten rock connects molten rock (magma) from within (magma) from within the Earth’s crust to the Earth’s crust to the Earth's surface. the Earth's surface.

The volcano includes The volcano includes the surrounding cone the surrounding cone of erupted material.of erupted material.

vent

cone

magma chamber

conduit

What causes the magma What causes the magma to escape the mantle and to escape the mantle and

come up through the come up through the crust of Earth?crust of Earth?

Subduction Zone VolcanoesSubduction Zone Volcanoes Remember that subduction happens Remember that subduction happens

at convergent platesat convergent plates Divergent Zone VolcanoesDivergent Zone Volcanoes

This results in ridgesThis results in ridges Hot SpotsHot Spots

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

A A hotspothotspot is a location on the Earth's is a location on the Earth's surface that has experienced active surface that has experienced active volcanism for a long period of timevolcanism for a long period of time

What are Hotspot Volcanoes?

Photo: Tom Pfeiffer / www.volcanodiscovery.com

The Hawaiian island chain are examples of hotspot volcanoes.

What is the difference in What is the difference in magma and lava?magma and lava?

MagmaMagma Molten (melted) mantle beneath the Molten (melted) mantle beneath the

surface of the Earthsurface of the Earth LavaLava

Magma that reaches the surfaceMagma that reaches the surface

How and why do volcanoes How and why do volcanoes erupt?erupt?

Hot, molten rock (magma) is Hot, molten rock (magma) is buoyant (has a lower density than buoyant (has a lower density than the surrounding rocks) and will rise the surrounding rocks) and will rise up through the crust to erupt on the up through the crust to erupt on the surface.surface. Same principle as hot air rising, e.g. Same principle as hot air rising, e.g.

how a hot air balloon workshow a hot air balloon works When magma reaches the surface When magma reaches the surface

it depends on how easily it flows it depends on how easily it flows (viscosity) and the amount of gas (viscosity) and the amount of gas (H(H22O, COO, CO22, S) it has in it as to how it , S) it has in it as to how it erupts.erupts.

How and why do volcanoes How and why do volcanoes erupt?erupt?

Large amounts of gas and a high Large amounts of gas and a high viscosity (sticky) magma will form viscosity (sticky) magma will form an explosive eruption!an explosive eruption! Think about shaking a carbonated Think about shaking a carbonated

drink and then releasing the cap.drink and then releasing the cap. Small amounts of gas and (or) low Small amounts of gas and (or) low

viscosity (runny) magma will form viscosity (runny) magma will form an effusive eruptionan effusive eruption Where the magma just trickles out of Where the magma just trickles out of

the volcano (lava flow).the volcano (lava flow).

Ring of FireRing of Fire

The Ring of Fire has 452 The Ring of Fire has 452 volcanoes and is home to over volcanoes and is home to over 75% of the world's active and 75% of the world's active and dormant volcanoes dormant volcanoes

The Ring of Fire is a direct The Ring of Fire is a direct result of plate tectonics and the result of plate tectonics and the movement and collisions of movement and collisions of crustal platescrustal plates

EarthquakesEarthquakes

As with volcanoes, earthquakes are As with volcanoes, earthquakes are not not randomly distributed over the randomly distributed over the globeglobe

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

Figure showing the distribution of earthquakes around the globe

Where do they happen?Where do they happen?

Most often they happen where the Most often they happen where the plates meet (fault lines)plates meet (fault lines)

Sometimes in the middle of the plate Sometimes in the middle of the plate where the crust becomes very heavy where the crust becomes very heavy and drops like a sink hole. EX and drops like a sink hole. EX Mississippi River deltaMississippi River delta

The point The point where the where the earthquake earthquake starts is starts is called the called the focus. focus.

Where do earthquakes Where do earthquakes form? form?

Figure showing the tectonic setting of earthquakes

Earthquake key terms:Earthquake key terms:

Foot wallFoot wall:: The plate that doesn’t The plate that doesn’t move during an Earthquake.move during an Earthquake.

Hanging wallHanging wall: The plate that moves : The plate that moves during an Earthquake. during an Earthquake.

Fault planeFault plane: The plane along which : The plane along which the break between two plates the break between two plates occurs.occurs.

Fault line:Fault line: The line in the surface of The line in the surface of the Earth caused by the fault plane.the Earth caused by the fault plane.

How do they move?How do they move?

5 ways5 ways Strike-Slip QuakeStrike-Slip Quake

(happen at transform (happen at transform boundaries)boundaries)

NormalNormal- Footwall and the - Footwall and the hanging one is the one that hanging one is the one that moves.moves.

Reverse/Thrust Quakes-Reverse/Thrust Quakes- hanging wall moves uphanging wall moves up

How do they move?How do they move? Horst QuakesHorst Quakes

The The grabengraben are are the the downdropped downdropped blocks and the blocks and the horsthorst are the are the upthrown blocks upthrown blocks that lie next to that lie next to the graben the graben

Graben Graben QuakesQuakes

Strike-slip EarthquakesStrike-slip Earthquakes

Normal Earthquake:Normal Earthquake:

Reverse (Thrust) Reverse (Thrust) Earthquake:Earthquake:

Graben:Graben:

Horst:Horst:

How do you tell how severe How do you tell how severe an earthquake is?an earthquake is?

Earthquakes generate Earthquakes generate seismic waves which can be seismic waves which can be detected with a sensitive detected with a sensitive instrument called a instrument called a seismographseismograph . .

The The Richter ScaleRichter Scale is based is based in energy released as in energy released as measured by maximum measured by maximum wave amplitude on a wave amplitude on a seismographseismograph

Richter ScaleRichter ScaleRichter scale no. No. of earthquakes per

yearTypical effects of this magnitude

< 3.4 800 000 Detected only by seismometers

3.5 - 4.2 30 000 Just about noticeable indoors

4.3 - 4.8 4 800 Most people notice them, windows rattle.

4.9 - 5.4 1400Everyone notices them, dishes may break, open doors swing.

5.5 - 6.1 500 Slight damage to buildings, plaster cracks, bricks fall.

6.2  6.9 100Much damage to buildings: chimneys fall, houses move on foundations.

7.0 - 7.3 15Serious damage: bridges twist, walls fracture, buildings may collapse.

7.4 - 7.9 4 Great damage, most buildings collapse.

> 8.0 One every 5 to 10 yearsTotal damage, surface waves seen, objects thrown in the air.