1. CO: VOLCANOES LO: Describe the types of volcanoes, which type of plate boundaries create volcanoes and why volcanoes occur there.

2. DEFINITIONS
VOLCANO - IS A MOUNTIAN FORMED WHEN MAGMA FROM THE MANTLE RISES TO THE EARTH’S SURFACE.
MAGMA-IS MOLTEN ROCK THAT IS BENEATH THE EARTH’S SURFACE.
LAVA- IS MOLTEN ROCK THAT IS ON OR ABOVE THE EARTH’S SURFACE.

3. HOW A VOLCANO FORMS 1. Magma starts to rise to the Earth’s surface
because it is less dense than the surrounding rock.
2. The magma works its way to the surface through
cracks (known as faults) in the Earth’s crust.
3. Once the magma reaches the surface a
volcano is formed.

4. Where Do Volcanoes Form?
1. DIVERGENT PLATE BOUNDRIES
2. CONVERGENT PLATE BOUNDRIES
3. HOT SPOTS

5. DIVERGENT PLATE BOUNDARY VOLCANO
At divergent boundaries crustal plates form deep cracks called rifts. Magma rises to the surface through these rifts. This occurs along mid-ocean ridges and within continents
EXAMPLE:
The Edfel Volcano on
the island of Iceland .

7. Edfel Volcanic eruption in 1973

8. CONVERGENT PLATE BOUNDARY VOLCANOES
At convergent boundaries magma that forms in the subduction zone rise to the surface through weaknesses or cracks the earth’s crust, forming a volcano.
Mt. Shasta in California
is a convergent boundary
volcano. So is Mt. Saint
Helens.

10. The Cascade Mountain Range in the Pacific Northwest features several active and dormant volcanoes.
These volcanoes formed over the subduction zone where the Juan de Fuca plate meets the North American plate

13. HOT SPOT VOLCANOES
Hot spots are place where the mantle is hotter than the rest of the mantle. Volcanoes form when magma from these spots rises to surface. Hot spots can occur in the middle of tectonic plates.
Hawaiian islands formed over a hot spot.
Yellowstone also is on top of
a hot spot.

14. PACIFIC RING OF FIRE
Most of the worlds active volcanoes occur at the plate
boundaries that surround the pacific ocean. This forms
a circle known as the pacific ring of fire.

15. VOLCANOES (PART 2)

16. TYPES OF ERUPTIONS
Volcanic eruptions range from violent and explosive to relatively quiet. Two factors effect this:
1) Composition of the magma.
( how much silica is in the magma)
2) How much gas is trapped in the volcano

17. Composition of the Magma
BASALTIC MAGMA (MAFIC)- the more basaltic the magma is the smoother it flows and the more quiet the eruption.
GRANITIC MAGMA (FELSIC) - is thick and does not flow easily. Granitic magma plugs up the vent of the volcano allowing pressure to build. When the pressure becomes to great an explosive eruption results

18. TYPES OF VOLCANOES SHEILD VOLCANOES CINDER CONE VOLCANO
COMPOSITE (Strato) VOLCANO

19. SHEILD VOLCANOES Broad base and gentle slopes
Form as a result of quiet eruptions of basaltic (runny) magma.
Hawaiian volcanoes are examples

22. CINDER CONE VOLCANOES Form from explosive eruptions.
Steep slopes of tephra and ash.

27. COMPOSITE CONE VOLCANOES
Form from combinations of explosive and quiet eruptions
Steep slopes made of alternating layers of tephra and lava.
Highest of all volcanoes

30. Colima in Mexico

31. Mount Hood in Oregon

32. Mount Rainier in Washington State

33. Mount Shasta in Northern California

34. Mount St. Helens and Spirit Lake before the 1980 Eruption
Mount St Helens and Spirit Lake in 1982

35. Mt. St. Helens

36. Mt. Saint Helens

37. Mt. Saint Helens

38. Mt. Saint Helens

39. Volcanic Hazards
More than 50 volcanoes in the United States have erupted one or more times in the past 200 years. The most volcanically active regions of the Nation are in Alaska, Hawaii, California, Oregon, and Washington. Volcanoes produce a wide variety of hazards that can kill people and destroy property. Large explosive eruptions can endanger people and property hundreds of miles away and even affect global climate. Some of the volcano hazards, such as landslides, can occur even when a volcano is not erupting. -- Myers, et.al., 1997