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Volcanoes & Other Igneous Activity
Chapter 10
200
How & Where Volcanoes Form• What is a volcano?
– 1. opening in Earth’s crust through which molten rock (magma), gases, & ash erupt
– 2. land around opening in Earth’s crust
ES0901 Visualization
Strato-volcanoes
• What is the asthenosphere?– partially melted (“plastic”)
layer of the mantle that underlies the lithosphere
• Why is most of the asthenosphere solid?– because of the pressure
exerted on it by the lithosphere above
• some materials do melt– causes convection currents
» due to density differences
Origin of Magma sec. 10.2
• What 3 conditions allow for magma formation in the asthenosphere?– decrease
pressure• rifts
– increase temperature
• hot spot
– adding water• subduction
boundaries
Origin of Magma
• Where are volcanoes found?– along mid-ocean ridges/rifts– at subduction boundaries– over hot spots
Volcanism at a Subduction Zone Animation
Volcanism Along a Rift Zone Animation
Plate Tectonics & Igneous Activity sec. 10.3
• What is a hot spot?– areas of volcanic activity that
result from plumes of hot solid material that have risen from deep in the mantle
• as material rises, it melts due to decreasing pressure
• Do hot spots move or stay in the same place?– stay in the same place
• plate moves over it– how Hawaii formed
Volcanic Islands Forming Over a Hot Spot Animation
Plate Tectonics & Igneous Activity
• How do differences in magma affect volcanic activity?
Kilauea Mount St. Helens Yellowstone caldera
The Nature of Volcanic Eruptions sec. 10.1
• How does magma type/silica content affect a magma’s viscosity & how a volcano erupts?– felsic (rhyolitic) magmas
• thick & gooey• difficult for gases to escape• explosive
– mafic (basaltic) magmas• thin & runny• easy for gases
to escape• gentle, flowing
More on ViscosityThe Nature of Volcanic Eruptions
• What is lava?– magma that has reached Earth’s surface
• What factors affect lava flows:– on land?
• temperature & speed– hotter, basaltic lava flows quickly
» forms pahoehoe (smooth, ropelike surfaces)
– cooler, basaltic lava flows slowly & cools quickly» forms aa (rough, jagged surfaces)
– underwater?• contact w/ water cools outside faster than inside
– forms rounded, pillow-like crust pressure builds crust cracks lava pours out… cycle repeats
» forms pillow lava
The Nature of Volcanic Eruptions
• What are pyroclastic materials?– solid fragments ejected when trapped gases are
released from lava
• What is a pyroclastic flow?– when pyroclastic materials combine w/ hot gases
• dense, superheated cloud travels rapidly downhill
Volcanic Material
• What determines the shape & structure of a volcano?– nature of its eruptions & materials it ejects
• Types of volcanic landforms:– shield volcanoes
– cinder cones
– composite volcanoes (stratovolcanoes)
– calderas
– lava plateaus
Types of Volcanoes
Shield Volcanoes• runny, thin, basaltic lava
– builds up in layers• broad base, gently sloping sides
• form over (oceanic) hot spots
• molten lava thrown into air forming fragments– fragments accumulate
• small, cone-shaped mound, oval base
• often form in groups on side of larger volcanoes
Cinder Cones
Mount St. Helens
Mt. Shasta
Composite Volcanoes (Stratovolcanoes)
• gas-rich magma– explosive
• cause lahars (fast flowing mudflow)
• alternating layers of hardened lava flows & pyroclastic material
• large, steep, symmetrical cones
• form at subduction zones
• What is a caldera?– large crater-shaped depression
• How do calderas form?• formed when magma beneath a volcano
is released & top of volcano collapses– can fill w/ water lake
Other Volcanic Landforms
• What is a lava plateau?– flat, wide surface formed from basaltic lava
• How do lava plateaus form?• poured from a fissure in Earth’s surface & spread
across the land
Other Volcanic Landforms