1. Chapter 13 Volcanoes 101 Videoclip

2. Section 1
Many volcanic eruptions are caused by moving plates- often on or around plate boundaries
Most volcanoes are found near the Ring of Fire along the Pacific Plate (see map)
Volcanoes are found:
Subduction zones
Mid Ocean Ridges
Hot spots (visual concepts-online book)

3. Subduction Zones Site of many volcanoes and trench formation
Plates slide underneath another plate

4. Reminds me of a song……..
Ring of Fire

5. Mid Ocean Ridges Divergent boundaries form new crust as lava cools
Most eruptions are not noticed because they are under water

6. Hot Spots Not on plate boundaries
Columns of hot material from the mantle-plumes
When the lithosphere drifts, hot spot moves to a different location, creating a new volcano and new continental crust ex) Hawaiian Islands

7. Hot Spots

8. Formation of Magma Liquid rock
Rock has a melting point at which it becomes liquid
Magma can form under 3 conditions:
Temperature exceeds melting point of minerals in rock
Pressure from surrounding rock is removed
Fluids such as water are added and lower the melting point

9. Magma Can be described as
Mafic-high in iron and magnesium –dark in color-forms oceanic crust
Felsic –high in silica-light colored-forms continental crust
See the visual concept on magma formation
Q: Why does magma rise to the surface?
Magma is less dense than surrounding rock

10. Volcanism Any activity in which magma rises to the surface
Magma is less dense than rock
Volume of magma increases-magma melts rock as it rises to the surface
Volcano-vent through which magma and gases travel to the surface
Lava-magma on the surface
Brain POP: Volcanoes

11. Intrusive Activity
When magma does not reach the Earth’s surface, the magma may cool and solidify inside the Earth
This formation is called a pluton
Devil’s Tower, Wyoming

12. Viscosity: resistance to flow
Eruption Types
Quiet
Explosive
Viscosity: resistance to flow

13. Lava has low viscosity-gases can escape easily-usually mafic lava
Quiet Eruptions
Lava has low viscosity-gases can escape easily-usually mafic lava

14. Lava Flows aa
Pahoehoe
Blocky
Pillow aa
Pahoehoe

15. Explosive Eruptions
Lava has high viscosity-high amount of trapped gases-usually felsic lava

16. Pyroclastic Materials
Rocks that form from lava during an eruption
Classified according to size
Volcanic dust-less than .25 mm
Volcanic ash- .26 mm to 2 mm
Lapilli- 2 mm to 64 mm
Volcanic Bombs-large clots of lava-rounded shape
Volcanic Blocks-the largest rock particles-solid rock blasted from the vent

18. Composite or Stratovolcano
Types of Volcanoes
Shield
Cinder Cones
Composite or Stratovolcano

19. Shield Volcanoes Broad at the base with gently sloping sides
Quiet eruptions, mafic lava, found at hot spots and diverging boundaries
Ex) Hawaiian
volcanoes

20. Cinder Cones
Usually the smallest in size, steep slopes, small explosive eruptions, felsic lava, converging boundaries

21. Composite or Stratovolcanoes
Large in size, quiet and explosive eruptions, steep slopes, both mafic and felsic lava, convergent boundaries

22. Composite Volcano in Washington
Mount St. Helens
Composite Volcano in Washington

23. Eruption May 18, 1980

24. Eruption May 18, 1980
USGS Video What information do scientists use to predict volcanic eruptions? Are eruptions a single event or a group of multiple eruptions? Before and After Photos

25. Calderas-How do they form?
Caldera Formation Demo-USGS
Crater Lake
Oregon