Transcript Igneous Activity - Pleasant Grove Middle School

Igneous Activity
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
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
What is Extruded During
Eruptions?
• Lava Flows
– Pahoehoe—ropy basaltic lava flows
– Aa—basaltic lava flow that leaves sharp
edges
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
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
Volcano Types
• Shield
– Broad, slightly domed structure
– Primarily basaltic lava flows, only a small
percentage of pyroclastic material
– Mauna Loa—largest
– Kilauea—active today
Volcano Types
• Cinder Cones
– Built from ejected lava fragments
– Steep angle
– Rather small
– Paricutin, Mexico
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
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
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
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