Volcanic Eruptions

Mt. St. Helen

What is a volcano? A volcano is a vent or 'chimney' that connects molten rock (magma) from within the Earth’s crust to the Earth's surface. A volcano is a vent or 'chimney' that connects molten rock (magma) from within the Earth’s crust to the Earth's surface. The volcano includes the surrounding cone of erupted material. The volcano includes the surrounding cone of erupted material. vent cone magma chamber conduit

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

A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time A hotspot is a location on the Earth's surface that has experienced active volcanism for a long period of time What are Hotspot Volcanoes? Photo: Tom Pfeiffer / The Hawaiian island chain are examples of hotspot volcanoes.

What is the difference in magma and lava? Magma Magma Molten (melted) mantle beneath the surface of the Earth Molten (melted) mantle beneath the surface of the Earth Lava Lava Magma that reaches the surface Magma that reaches the surface

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. 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 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 (H 2 O, CO 2, S) it has in it as to how it erupts. When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H 2 O, CO 2, 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! 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. 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 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). Where the magma just trickles out of the volcano (lava flow).

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

Earthquakes

As with volcanoes, earthquakes are not randomly distributed over the globe As with volcanoes, earthquakes are not randomly distributed over the globe At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. At the boundaries between plates, friction causes them to stick together. When built up energy causes them to break, earthquakes occur. Figure showing the distribution of earthquakes around the globe

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

The point where the earthquake starts is called the focus. 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. Foot wall: The plate that doesn’t move during an Earthquake. Hanging wall: The plate that moves 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 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. Fault line: The line in the surface of the Earth caused by the fault plane.

How do they move? 5 ways 5 ways Strike-Slip Quake (happen at transform boundaries) Strike-Slip Quake (happen at transform boundaries) Normal- Footwall and the hanging one is the one that moves. Normal- Footwall and the hanging one is the one that moves. Reverse/Thrust Quakes- hanging wall moves up Reverse/Thrust Quakes- hanging wall moves up

How do they move? Horst Quakes Horst Quakes The graben are the downdropped blocks and the horst are the upthrown blocks that lie next to the graben The graben are the downdropped blocks and the horst are the upthrown blocks that lie next to the graben Graben Quakes Graben Quakes

Strike-slip Earthquakes

Normal Earthquake:

Reverse (Thrust) Earthquake:

Graben:

Horst:

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

Richter Scale Richter scale no.No. of earthquakes per year Typical effects of this magnitude < Detected only by seismometers Just about noticeable indoors Most people notice them, windows rattle Everyone notices them, dishes may break, open doors swing Slight damage to buildings, plaster cracks, bricks fall Much damage to buildings: chimneys fall, houses move on foundations Serious damage: bridges twist, walls fracture, buildings may collapse Great damage, most buildings collapse. > 8.0One every 5 to 10 years Total damage, surface waves seen, objects thrown in the air.