Volcanoes Review

Is a volcano always a mountain? -- fissure eruption

What are volcanoes?

What are volcanoes?

• Volcano- a vent or fissure on the Earth’s surface where lava and gases flow

• Volcanic eruption- movement of ash, lava, or gases out of a volcano

Where do volcanoes form?

Where do volcanoes form?• Most volcanoes erupt on Earth’s surface,

near plate boundaries

– When an oceanic plate subducts, water & gases combine with the crust to decrease rock’s melting point

– Melted rock rises & forms a line of volcanic mountains along the edge of a plate

• Some volcanoes form in the middle of a tectonic plate

How does magma form?

How does magma form? • Subduction causes heat and pressure

above the mantle, and that causes rock to melt into magma

• Magma forms due to– rising temperature– reduced pressure

– added gases (H2O)

These things decrease rock’s melting point

How does lava form?

• Since magma is less dense, it rises up through weak spots in the crust

• Near the surface, less pressure causes magma to expand

• Magma escapes from vents as lava- magma that flows onto Earth’s surface

What are the 2 types of magma?

1. Non-Explosive (mafic)- low silica (Si and O), higher magnesium (Mg) and iron (Fe), it is runny and flows easily

{ think of oceanic crust }

2. Explosive (felsic)- lots of silicates (Si and O), it’s relatively cooler, more sticky and full of gases

{ Continental crust has both mafic & felsic magma }

What are tephra? Tephra- refers to ash, lapilli, and rocks

• Pyroclastic flows- hot gases and rock that rush downhill (1000oC and 700kph)

• Pyroclastic materials- magma broken into pieces due to rapidly expanding gases

They can form when erupting lava cools & solidifies mid-airEXAMPLE

Pyroclastic material are: - volcanic dust (1/4mm) - ash (less than 2mm) - lapilli “little stones” - volcanic bombs - volcanic blocks (house-sized)

What are tephra? Tephra- refers to ash, lapilli, and rocks

• Pyroclastic flows- hot gases and rock that rush downhill (1000oC and 700kph)

• Pyroclastic materials- magma broken into pieces due to rapidly expanding gases

They can form when erupting lava cools & solidifies mid-airEXAMPLE

Pyroclastic material are: - volcanic dust (1/4mm) - ash (less than 2mm) - lapilli “little stones” - volcanic bombs - volcanic blocks (house-sized)

What is left when the magma is gone?

Caldera- crater hole formed when a

magma chamber empties and collapses

Crater Lake, in Oregon, formed about 7,690 BP

Let’s review.

1.What is a volcanic eruption?

2.What happens when an oceanic plate subducts?

3. What are the

2 types of magma?

What do volcanoes form as?There are 3 types of volcanoes that you must know

1. shield 2. strato-composite 3. cinder cone

How are eruptions classified?Volcanic eruptions are classified as quiet or explosive

Each of the 3 types of volcanoes erupt this way

1. shield 2. strato-composite 3. cinder cone

Why is this one called a shield volcano?

Why is this one called a shield volcano?

Shield volcano:

• Formed by low-silica lava that flows great distances

• They have a flat, broad profile and quiet eruptions

The volcano with many names.Stratovolcano,

Composite cone or Stratocone:

Built from both, lava flows (quiet) and layers of ash and cinder (explosive)

They are steep, but not large like a

shield volcano

Why is this one called a cinder cone?

Why is this one called a cinder cone?

Cinder cone volcano:• Has explosive eruptions, such that lava is thrown

into the air and rains down as ash or cinder

What is a volcanic hot spot?

What is a volcanic hot spot?

The hot spot is a fixed mantle plume, and the plate moves slowly over it

• Kauai 5 mya

• Oahu 3 mya

• Maui 1 mya

Example- Hawaiian Islands, a line of islands, shield volcanoes made by a hotspot in the middle of the Pacific Plate

Active, Dormant, or Extinct?

How do you know if your friendly neighborhood volcano is working,

resting, or retired?

What are the stages of activity?When volcanoes erupt... expanding gases pushes

magma from the magma chamber…. solids, liquids, & gases can erupt

• active volcano = at least one eruption in the past 10,000 years

An active volcano might be erupting or dormant

• dormant volcano = active volcano that is not erupting, but will likely erupt again

extinct volcano = no eruptions in at least 10,000 years & not expected to erupt in the future

NOTE some extinct volcanoes have become active

What are the stages of activity?When volcanoes erupt... expanding gases pushes

magma from the magma chamber…. solids, liquids, & gases can erupt

• active volcano = at least one eruption in the past 10,000 years

An active volcano might be erupting or dormant

• dormant volcano = active volcano that is not erupting, but will likely erupt again

• extinct volcano = no eruptions in at least 10,000 years & unlikely to erupt again in the future

NOTE some extinct volcanoes have become active

What do volcanoes create?• A chance to study,

a window inside Earth

• Landforms such as, mountains, plateaus, and calderas

• Mineral-rich soils that help plants to grow

• Damage, to ecosystems… an opportunity for new plants & animals to start up

Of the 3 types of volcanoes that you must know, which one does not belong, here?

Caldera Cinder cone

Shield Composite

Of the 3 types of volcanoes that you must know, which one does not belong, here?

The Cascade Mountain Range is actually a series of volcanoes

Mt. St. Helens, Mt. Rainer,Mt. Adams, & Mt. Hood

What is this picture saying? Outgassing of volcanoes also form climate change

What is this picture saying? Outgassing of volcanoes can make climate change

Was Mt. Pinatubo the largest eruption?

Resources

http://pubs.usgs.gov

http://commons.wikimedia.org

http://gregvaughn.photoshelter.com/image/I0000O6J1HZKTbAY

--- fissure eruption near Pu'u O'o in the east rift zone of Kilauea

http://geologycafe.com/class/week4.html Gavilan College -- Geology

Volcanoes !

Shield volcano:

• Formed by the eruption of lava that can flow great distances

• They have a flat, broad profile and quiet eruptions

Mauna Loa

Stratovolcano, composite cone, stratocone

Example- Mount St. Helens

Mount St. Helens erupted violently in 1980

• The elevation changed from 9,677 feet to 8,363 feet

• There were flows of rock and ash at over 175 mph

before and after

Is there a 4th type of volcano?Supervolcano:• A large caldera that builds over thousands of

years, and then explodes with devastating force

• hotspot (over mya)

Yellowstone