2. VOLCANOES AND VOLCANISM SUBAQUEOUS ERUPTIONS Submarine eruptions Quiet and effusive. May produce submarine shield volcanoes. If deep enough, water pressure prevents gas escape. Produces pillow structures. Shallow eruptions can be explosive. Contact of water with hot magma produces lots of steam and consequently tephra.

3. VOLCANOES AND VOLCANISM SUBGLACIAL ERUPTIONS Mafic lava erupts under glaciers. Causes rapid melting of snow and ice. Large amounts of meltwater is freed. Bardabunga and Grimvötn volcanoes erupted under Vatnajökull on Iceland in 1996. Outburst flood occurred. Termed JOKULHLAUP.

4. VOLCANOES AND VOLCANISM JOKULHLAUP

5. VOLCANOES AND VOLCANISM PYROCLASTIC ERUPTIONS Involve viscous, gas-rich magmas. Produce large amounts of tephra. Area receives large amounts of hot ash, nuée ardente, and hot lahars.

6. VOLCANOES AND VOLCANISM Mt. Vesuvius & Herculaneum, Italy, 79 A.D.

7. VOLCANOES AND VOLCANISM With little gas or water felsic magmas will produce a VOLCANIC DOME in the crater. If pressure is not released, massive eruptions occur. Produces CALDERA. Mt. Mazama, OR Krakatau, Indonesia Mt. Desert Island, ME Mt. St. Helens, WA

8. VOLCANOES AND VOLCANISM Mt. St. Helens, WA

9. Crater Lake and Wizard Island (Mt. Mazama), OR

11. VOLCANOES AND VOLCANISM PYROCLASTIC VOLCANIC CONES Structure and shape depends on magma type. Intermediate and felsic volcanoes produces large amounts of tephra. Tephra generally accumulates close to vent. Forms steep-sided cinder piles. The cinders are covered with the next lava flow event. Process repeats. Produces COMPOSITE or STRATOVOLCANO. Possesses alternating layers of lava and pyroclastics. Mt. Hood, OR, Mt. Ranier, WA, Mt. Shasta, CA

12. VOLCANOES AND VOLCANISM PYROCLASTIC VOLCANIC CONES COMPOSITE or STRATOVOLCANO Mt. Hood, OR

13. VOLCANOES AND VOLCANISM PYROCLASTIC CONES or CINDER CONES Composed entirely of volcanic cinders. Generally small volcanoes <1000 m (~3000’) in height. Have steep sides. No intervening lava between cinders. Easily eroded. Can form from any type of magma as long as enough gas exists to keep blowing particles up into the atmosphere. Sunset Crater, AZ and Paricutin, Mexico

14. VOLCANOES AND VOLCANISM PYROCLASTIC CONES or CINDER CONES Sunset Crater, AZ

15. VOLCANOES AND VOLCANISM PYROCLASTIC CONES or CINDER CONES Paricutin, Mexico

16. Volcano Explosivity Index (VEI)