1. Volcanoes Molten rock reaches Earth’s surface
Depending on viscosity and temperature, it either flows out or explodes

2. Why do volcanoes happen?
Subsurface materials heat up for various reasons
Liquid rock is less dense than solid, so it rises
Upward force from rising magma and melting from hot rock meeting cold produce gaps in overlying rock
When magma reaches surface, it is more dense than air, so it stays and cools

3. Lava types Basaltic lava -- very hot, not very viscous
Flood basalts - large areas covered by basaltic lava, e.g. Columbia River basalts, Deccan Traps, lunar maria
Granitic lava -- colder, more viscous
Tends to produce explosive eruptions

4. Columbia River Basalts, WA & OR

5. Devil’s Tower, WY

6. Basaltic flows
Pahoehoe (means "ropy") - highly fluid lava which has thin, glassy skin under which hot lava flows
Aa - forms after gases have departed and cooling has begun. Skin is big and chunky -- very sharp

7. Basaltic flows
Pillow basalt -- evidence of underwater eruptions -- surface chills quickly, but flow continues
Bubbles -- or vesicles -- gases exist in lava but stay in solution under pressure under earth

8. Lava Flow, Hawaii

9. Lava Toe, Hawaii

10. Pyroclastic Eruptions
Gas is trapped in magma, but magma is too viscous to flow through cracks
When pressure is released and gas comes out of magma, whole mountaintop can explode
Pyroclasts -- fire rocks

11. Pyroclastic Eruptions
Includes ash and fine material, but can be a lot bigger (one house sized piece traveled 10 km in one eruption)
Ash can stay aloft, entering upper atmosphere (e.g. Pinatubo)
If particles settle while still hot, they form tuffs -- welded together bits

12. Pyroclastic Eruptions
Pyroclastic flow -- big hazard near continental volcanoes - e.g. Japan, Mont Pelee on Martinique (1902)
Pyroclastic flow can be very hard to predict
Prof. Landes: "The Montagne Pelee presents no more danger to the inhabitants of Saint Pierre than does Vesuvius to those of Naples" -- died next day in eruption

13. Mont Pelee, West Indies 1902

14. Nuee Ardente
Pyroclastic Eruption

15. Eruption Styles
Lava Eruptions -- lava cone built by successive flows from central vent
Basalt -- creates shield volcanoes like Mauna Loa - big, broad gentle slopes
Rhyolite -- creates small dome in crater, plugs up areas below
Pyroclastic eruptions create concave cone with a summit vent
Stratovolcano -- alternating lava and pyroclasts (composite volcano) Fujiyama in Japan
Resurgent calderas
Phreatic explosions -- when magma meats lots of groundwater -- e.g. Krakatoa

16. Mount Saint Helens
Washington
Erupted
May 18, 1980

17. Mount St. Helens

19. Mount Saint Helens
Stratovolcano - mixture of lava eruptions and explosive ash eruptions
1980 eruption was very explosive
Mountain lost its top 400 meters of elevation within minutes

20. Before the eruption

21. After the Eruption

23. Mount Saint Helens Mud Flow

24. Krakatau August 26, 1883
Phreatic eruption of an entire island (English name is Krakatoa)
Loudest noise in recorded history (Heard in Australia 2000 km away)
Eruptive force of 100 million tons of TNT
5000 times the force as the first atomic bombs
36000 people drown in Tsunamis

26. Anak Krakatau