1. Igneous Activity

2. The Nature of Volcanic Eruptions Factors that determine whether a volcano erupts explosively or gently –Magma’s composition –Temperature –Dissolved gases These factors affect viscosity –Higher silica content—higher viscosity –Higher temperature—lower viscosity Gases provide force

3. The Nature of Volcanic Eruptions Three kinds of magma –Basaltic—50% silica, least viscous, tendency to form lavas, high melting temperature –Andesitic—60% silica, intermediate on all properties –Granite—70% silica, most viscous, tendency to form pyroclastics, low melting temperature

4. What is Extruded During Eruptions? Lava Flows –Pahoehoe—ropy basaltic lava flows –Aa—basaltic lava flow that leaves sharp edges

5. What is Extruded During Eruptions? Gases –1-5% of the total weight of magma –70% water vapor –15% carbon dioxide –5 % nitrogen –5% sulfur –Chlorine, hydrogen, and argon –Natural source of air pollution

6. What is Extruded During Eruptions? Pyroclastics (fire fragments) –Fine ash particles (contain lots of gas) –Welded tuff—ash shards fused together –Pumice—larger pieces of welded tuff –Cinders—pea sized –Lapilli (little stones)—walnut-sized –Blocks—particles larger than lapilli –Bombs—ejected as incandescent lava

7. Volcano Types Shield –Broad, slightly domed structure –Primarily basaltic lava flows, only a small percentage of pyroclastic material –Mauna Loa—largest –Kilauea—active today

8. Volcano Types Cinder Cones –Built from ejected lava fragments –Steep angle –Rather small –Paricutin, Mexico

9. Volcano Types Composite Cones (Stratovolcano) –Most in the Ring of Fire –Large, nearly symmetrical –Formed from relatively viscous andesitic lavas –Can suddenly change eruptive style to pyroclastic flows –Mt. Fuji –Mt. Vesuvius

10. Volcano Types Composite Cones (con’t.) –Nuee ardente Fiery cloud that can race up to 125 miles per hour –Lahars Mudflow—ash and debris combine with water

11. Other Volcanic Landforms Calderas –Crater that exceeds one kilometer in diameter –Most are formed following explosive eruptions –They form when the summit of a volcano collapses into the partially emptied magma chamber below –Crater Lake in Oregon Vents –Pipelike pathway from a magma chamber to a crater

12. Other Volcanic Landforms Volcanic neck—when most of a cinder cone erodes, leaving the rock that occupies the vent Fissure—fractures in the crust that pour forth lava Lava plateaus—flood basalts that come from fissures and cover extensive areas Pyroclastic flows—come from vents at high speeds and blanket extensive areas