1. Volcanoes Review
Is a volcano always a mountain? -- fissure eruption

2. What are volcanoes? 2

3. 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 3

4. Where do volcanoes form?4

5. 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 5 5

6. How does magma form? 6 6

7. 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 7 7

8. 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 8

9. 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 }

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

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

12. 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

13. 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?

14. What do volcanoes form as?
There are 3 types of volcanoes that you must know
1. shield strato-composite cinder cone

15. How are eruptions classified?
Volcanic eruptions are classified as quiet or explosive Each of the 3 types of volcanoes erupt this way
1. shield strato-composite cinder cone

16. Why is this one called a shield volcano?16

17. Why is this one called a shield volcano?
Formed by low-silica lava that flows great distances
They have a flat, broad profile and quiet eruptions 17

18. 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

19. Why is this one called a cinder cone?

20. 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 20

21. What is a volcanic hot spot?

22. 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

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

24. Active, Dormant, or Extinct?
How do you know if your friendly neighborhood volcano is working, resting, or retired?

25. 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

26. 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

27. 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

28. Of the 3 types of volcanoes that you must know, which one does not belong, here?
Caldera Cinder cone
Shield Composite 28

29. Of the 3 types of volcanoes that you must know, which one does not belong, here?29

30. The Cascade Mountain Range is actually a series of volcanoes
Mt. St. Helens, Mt. Rainer,
Mt. Adams, & Mt. Hood

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

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

33. Was Mt. Pinatubo the largest eruption?

34. Resources
--- fissure eruption near Pu'u O'o in the east rift zone of Kilauea
Gavilan College -- Geology

35. Volcanoes ! 35

36. Formed by the eruption of lava that can flow great distances
Shield volcano:
Formed by the eruption of lava that can flow great distances
They have a flat, broad profile and quiet eruptions
Mauna Loa

37. 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

38. 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 38