Volcanoes

1
Volcanoes
2
A Volcano is

• An opening in Earths crust through which molten
  rock, gases, and ash erupt.
• Also, the landform that develops around this
  opening.

3
Why Volcanoes form

• Volcanic Eruptions occur when magma rises to the
  surface. This will happen when the asthenosphere
  melts enough to flow. There are three things
  that can cause this
• 1- A decrease in pressure (like at a mid-ocean
  ridge or rift valley)
• 2- An increase in temperature (like at a hot
  spot)
• 3- An increase in the amount of water in the
  asthenosphere (like at subduction)

4
Where Volcanoes Form

• Most volcanoes occur at
• DIVERGENT boundaries, and at
• CONVERGENT boundaries that have SUBDUCTION.
• Hot Spots

5
Volcanoes at Divergent Boundaries

• Decrease in pressure as plates pull apart lets
  magma rise.
• These fissure volcanoes are located at the
  Mid-Atlantic Ridge and the Great African Rift
  Valley

6
Divergent- Great Rift Valley
7
Volcanoes at Convergent Boundaries (Subduction)

• Subduction increases the amount of water in the
  asthenosphere, which lowers the melting temp.
• As the denser oceanic crust is pushed lower, it
  melts into magma.
• When continental and oceanic plates converge, a
  volcano forms on land.
• When 2 oceanic plates converge together, a
  volcano forms an island.

Mount Hood/ Portland
8
Volcano- Convergent
Sibayak, Indonesia
Mt St. Helens
9
Hot Spots

• Sometimes, volcanoes occur at places that arent
  plate boundaries.
• We call these HOT SPOTS.
• Hot spots are areas where hot magma rises from
  deep in Earths mantle.
• Magma escapes where the crust is the thinnest or
  weakest.
• It starts out solid then it melts when it reaches
  areas of lower pressure.

10
Example of Hot Spot

• Volcanoes that make up Hawaii.

11
Types of Volcanoes

• There are 4 main types of Volcanoes
• Shield Volcanoes
• Cinder Cone Volcanoes
• Composite Volcanoes
• Fissure Volcanoes

12
Shield Volcanoes

• Shield Volcanoes form from runny lava (low
  viscosity) that tends to flow long distances
  before hardening. They generally have quiet
  eruptions.
• Volcano has a broad base and gently sloping
  sides.
• Volcano has a less explosive eruption than other
  types because the lava flows more easily
• Type of magma- Mafic Basalt black, runny lava
• Made of layers of lava
• Example Mauna Loa, Hawaii

13
(No Transcript)
14
Cinder Cones

• Form when molten lava is thrown into the air from
  a vent. As it falls, it breaks into fragments
  called ash or tephra that harden before hitting
  the ground.
• The ash and tephra make a cone-shaped mound.
• They are smaller than other types.
• Erupt explosively because magma is thick, which
  allows pressure to build up.
• Magma composition Felsic Rhyolite (light colored)

15
Example Krakatau, Indonesia
16
Cinder Cones
New Guinea
Iceland
17
Composite or Stratovolcano

• Form from alternating eruptions of quiet lava and
  explosive ash. The layers build up and make a
  moderate-sized volcano.
• Most common kind of volcano
• Made of layers of ash/tephra and lava.
• Magma composition- Andesite medium color

18
Example of Composite/Stratovolocano

• Mt. Pinatubo-Philippines

19
(No Transcript)
20
Fissure Volcanoes

• Form in long cracks where plates are pulled apart
  and near other volcanoes where the crust is
  weakened. Cinder Cone or Shield Volcanoes may
  also be nearby.

21
Yellowstone

• Check out how large some of the lava flows were
  from the Yellowstone Volcano!!!

22
How does a Caldera Form?

• A) A Volcanos pressure starts to build up
• B) The Volcano releases large amounts of Lava
• C) As the Lava is released, the pressure
  decreases and the volcanic mountain begins to
  collapse, forming a concave shape in the center
  of the volcano.
• D) The center of the volcanic mountain may begin
  to fill with water and form a lake.

23

• Crater Lake in Oregon State should actually be
  called Caldera Lake

24
Label the parts of a volcano

• Word Bank
• Main Vent/Crater
• Secondary Vent
• Magma Chamber
• Lava
• Ash Dust Cloud
• Tephra Bombs
• Cone

25
(No Transcript)
26
(No Transcript)
27
2 Types of eruptions

• Non explosive Build only enough pressure to
  allow lava to run down its sides.
• Explosive volcanoes that build enough pressure
  to blow its top, sending pyroclastic material
  into the air.

28
Non explosive eruption

• Mafic refers to rocks and magma rich in iron and
  magnesium.
• This type of lava that is very runny.
• As magma nears the surface there is little
  pressure, causing gasses escape easily.
• Magma low in Silica have quiet eruptions

29
Explosive eruptions

• Felsic means magma with high silica and feldspar
  content.
• Felsic magma traps water and gas bubbles, which
  leads to lots of pressure.
• Silica acts like a cork
• Explosive eruptions are caused by a build up of
  high pressure.
• Convergent zones contain lots of water, therefore
  have explosive eruptions.

30
Pyroclastic materials

• Material that is thrown into the air during an
  explosion.
• Volcanic bombs large blobs of magma that harden
  in the air.
• Lapilli pebble size rocks
• Volcanic ash tiny powder like material

31
(No Transcript)
32
Pyroclastic Flow

• A fast-moving current of hot gas and rock that
  reaches speeds moving away from a volcano of up
  to 700 km/h
• The gas can reach temperatures of about 1,000 C
• Pyroclastic flows normally hug the ground and
  travel downhill, or spread laterally under
  gravity.
• Their speed depends upon the density of the
  current, the volcanic output rate, and the
  gradient of the slope.
• They are a common and devastating result of
  certain explosive volcanic eruptions.

33
(No Transcript)
34
Mount Saint Helens
35
Four types of lava

• Aa lava that is thick and sharp
• Pahoehoe lava that forms thin crust and wrinkles
• Pillow lava lava that erupts under water, has a
  round shape
• Blocky lava cooler, lava that does not travel
  far from eruption, jagged when it dries.

36
Aa

• lava that is thick and sharp

37
Pahoehoe

• lava that forms thin crust and wrinkles

38
Pillow lava

• lava that erupts under water, has a round shape

39
Blocky lava

• cooler, lava that does not travel far from
  eruption, jagged when it dries.

40
(No Transcript)