1. Volcanoes
Chapter 15 Section 2

2. What is a volcano
A vent in Earth’s crust through which melted or molten rock flows.
Below ground- magma
Above ground- lava

3. Volcanoes can form at plate boundaries
Convergent boundaries- denser plate sinks below the less dense plate.
Divergent boundaries- two plates pull apart especially at the mid-ocean ridge. ( 60 % of volcanoes occur here).

4. Volcano formation cont.
Can also occur at hotspots.
Hotspots are located away from plate boundaries.
Hawaiian Islands
Geologists speculate hot spots occur over rising convection currents.
Oldest volcano far from hot spot

5. Volcanoes found
The plate boundaries in the Ring of Fire ( this basically surrounds the Pacific Plate)
Hotspots
The Hawaiian Islands
Yellowstone National Park
Galapagos Islands

6. Types of volcanoes
Shield
Composite
Cinder Cone

7. Shield volcanoes Common along divergent boundaries
Common near oceanic hotspots
Large with gentle slopes

8. Composite Volcanoes Steep sided volcanoes Large
Result from explosive eruptions of andesitic lava rhyolitic lava
Convergent boundaries
Mount Ranier

9. Cinder Cone volcano
Small
Steep-sided
Erupt gas rich basaltic lava

10. Volcanic Eruption Magma chemistry determines how a volcano will erupt
Depends on the amount of water vapor in the magma
And- the amount of silica content.
Difference in silica affect the magma thickness or viscosity
Viscosity- the liquids resistance to flow.

11. Magma chemistry cont. Low silica content- low viscosity
Flows like warm maple syrup
Forms basalt
High Silica content- high viscosity
Flows like sticky toothpaste
Forms andesite and rhyolite

12. Dissolved gases All magmas contain dissolved gases
As the magma reaches the surface the dissolved gases can’t stay dissolved so they bubble or explode like a shaken soda bottle
In high viscosity lavas- the gases can escape so they tend to make explosive eruptions

13. Effects of volcanic eruptions
60 different volcanoes erupt each year
Lava flow
Ash fall
Pyroclastic flow
Mud flows

14. Lava flows Move slowly Rarely deadly Can be damaging
Often have to evacuate

15. Ash fall Ash is pulverized rock and glass Can reach heights of 40 km
Disrupt air travel
Can affect air quality and cause breathing problems
Can affect the climate by blocking out sunlight causing cooling

16. Mudflows Thermal energy can melt the snow and ice at the summit
This water can mix with mud and ash on the mountain
This forms mudflows or lahars

17. Pyroclastic flow Explosive volcanoes
Produce avalanches of hot moving gas, ash, and rock
Travel at speeds more than 100 km/hour

18. Predicting Eruptions Can be predicted Magma causes ground deformation
Series of earthquakes usually precedes eruption
Volcanic gas emission also increases
Ground water near volcano becomes acidic

19. Climate change
Ash can block sunlight making temperatures colder and altering weather
Sulfur dioxide emissions form sulfuric acid droplets which reflect sunlight into space lowering temperatures.