Chapter 6: section 2 Notes
Geologists classify eruptions as quiet or explosive.
 Whether an eruption is quiet or explosive depends on the magma’s silica content
 Whether the magma is thin and runny or thick and sticky.
 Silica- material found in magma that is formed from the elements oxygen and silicon.
Quiet Eruptions Low in silica
 Thin, runny lava
 Gas bubbles out gently
 Lava oozes quietly from the vent
 May flow for many kms
Pahoehoe-fast-moving, hot lava that is thin and runny, looks like wrinkles or coils of rope.
Aa- cooler, slow-moving lava, thinker, hardens as a rough surface with jagged chunks.
Fissures- when lava flows out of long cracks on the volcano’s side.
Explosive Eruptions Magma is high in silica content
 Magma is thick and sticky
 Builds up in the pipe, plugging it like a cork
 Dissolved gases build up creating pressure until the volcano explodes
Pyroclastic flow- ash, size of dust, cinders, pebble size, bombs, size of cars, is a type of explosive
eruption that hurls these things out.
Geologist use terms like active, dormant, extinct to describe a volcano’s stage of activity.
 Active or live, volcano that is erupting or showing signs of erupting.
 Dormant- volcano that will awaken and become active in the future.
 Extinct- volcano that is unlikely to erupt again.
Hot spring- forms when water deep underground is heated by a nearby body of magma or by hot rock.
Geyser- a fountain of water and stream that erupts from the ground.
Monitoring Volcanoes1. Tiltmeters- measure the change in elevation caused by magma raising underground.
2. Gas levels- measuring gases escaping from the volcano
3. Temperature- increase in temperature in underground water
4. Many small earthquakes in the area