Volcanoes & Other Igneous Activity

Chapter 10

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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