Volcanoes Chapter 9 300

How & Where Volcanoes Form Sec. 1 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

Magma Formation 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

What 3 conditions allow for magma formation in the asthenosphere? –decrease pressure rifts –increase temperature hot spot –adding water subduction boundaries Magma Formation

Volcano Formation 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

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 Volcano Formation Volcanic Islands Forming Over a Hot Spot Animation

How do differences in magma affect volcanic activity? Magma & Erupted Materials Sec. 2 Kilauea Mount St. Helens Yellowstone caldera

Types of Magma How does magma type/silica content affect a magma’s viscosity & how a volcano erupts?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 Viscosity

Lava Flows 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)forms pahoehoe –cooler, basaltic lava flows slowly & cools quickly »forms aa (rough, jagged surfaces)forms aa –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 lavapillow lava

Ash & Rock Fragments 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 gasses dense, superheated cloud  travels rapidly downhill

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 Volcanic Landforms Sec. 3

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 Calderas

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 Volcanic Plateaus