Volcanoes!

Chapter 10

Origin of Magma Magma originates when solid rock,

located in the crust and upper mantle, melts.

Factors that influence the formation of magma include…1. Heat2. Pressure3. Water Content

Origin of Magma Factors that influence the formation of

magma• Heat

• Additional heat is generated by • Friction in subduction zones• Crustal rocks heated during subduction • Rising, hot mantle rocks

Origin of Magma Factors that influence the formation of

magma• Pressure

• Increase in confining pressure causes an increase in melting temperature

• Drop in confining pressure can cause decompression melting

• Lowers the melting temperature • Occurs when rock ascends

Origin of Magma Factors that influence the formation of

magma• Water Content

• Cause rock to melt at a lower temperature • Plays an important role in subducting ocean

plates

Volcanoes and Plate BoundariesGlobal distribution of igneous activity is

not random!• Most volcanoes form along divergent and

convergent plate boundaries.• Some volcanoes form far from plate

boundaries above “hot spots”.

Locations of some of Earth’s major volcanoes

Volcanoes and Plate Boundaries Divergent Boundary Volcanism

• The greatest volume of volcanic rock is produced along the oceanic ridge system

• Lithosphere pulls apart • Mantle rock rises to fill the gap between

the plates. • Decompression melting occurs forming

magma.

Volcanoes and Plate Boundaries Convergent Boundary Volcanism

• Descending plate partially melts • Magma slowly rises upward • Rising magma can form…

• Volcanic island arcs in an ocean (Aleutian Islands)

• Continental volcanic arcs (Andes Mountains)

Volcanoes and Plate Boundaries Intraplate Volcanism

• Activity within a rigid plate NOT near a plate boundary

• Plumes of hot mantle material rise• Form localized volcanic regions called hot

spots • Examples include…

• Hawaiian Islands (middle of the Pacific Plate)• Yellowstone National Park (Middle of the N.

American Plate)

The Nature of Volcanic Eruptions

Factors Affecting Eruptions1. Viscosity of the magma

• Viscosity is a measure of a material’s resistance to flow

• Maple syrup vs. water

The Nature of Volcanic Eruptions Factors Affecting Eruptions

Viscosity of magma• Temperature (hotter magma is less viscous) • Composition (silica content)

• High silica content – high viscosity (e.g., rhyolitic lava) this creates an explosive eruption.

• Low silica content – more fluidlike (e.g., basaltic lava) quiet eruption.

The Nature of Volcanic Eruptions

Factors affecting Eruptions 2. Dissolved Gases

• Gases trapped in the magma • Violence of an eruption is related to how

easily gases escape from magma • Easy escape from fluid magma • Viscous magma produces an explosive

eruption

Volcanic MaterialLava flows

• Basaltic lavas are more fluid (low silica content)

• Types of lava • Pahoehoe lava (resembles braids in ropes) • Aa lava (rough, jagged blocks)

Gases • One to five percent of magma by weight• Mainly water vapor and carbon dioxide

A pahoehoe lava flow

A typical aa flow

Volcanic Material Pyroclastic materials “Fire fragments”

• Types of pyroclastic material• Ash and dust – fine, glassy fragments • Pumice – from “frothy” lava• Lapilli – “walnut” size • Cinders – “pea-sized” • Particles larger than lapilli

• Blocks – hardened lava • Bombs – ejected as hot lava

Volcanic bombs on Kilauea volcano in Hawaii

Volcano Formation 1. Begins when a fissure or crack

develops in the crust as magma is forced to the surface.

2. Repeated eruptions of lava or pyroclastic material eventually build a mountain called a volcano.

Three Main Types of Volcanoes

1. Shield Volcanoes2. Cinder Cones3. Composite Cones

Types of Volcanoes

Shield volcano• Broad, slightly domed (like a warriors shield)• Primarily made of basaltic (fluid) lava • Generally large in size • e.g., Mauna Loa and Mauna Kea in Hawaii

Shield volcano

Types of Volcanoes

Cinder cone • Built from ejected lava fragments that

harden in the air• Steep slope angle • Small in size

A cinder cone near Flagstaff, Arizona

Types of Volcanoes Composite cone (or stratovolcano)

• Most are adjacent to the Pacific Ocean along the ‘Ring of Fire’ (e.g., Mt. Rainier)

• Magma is viscous with a high silica content • Large size• Composed of layers of lavas and

pyroclastic material • Most dangerous and explosive volcano

Composite volcano

Mount St. Helens – a typical composite volcano

Mount St. Helens following the 1980 eruption

A size comparison of the three types of volcanoes

Other Volcanic Landforms Calderas

• Steep-walled depression at the summit • Formed by collapse • Nearly circular • Size exceeds 1 kilometer in diameter

Lava plateaus • Fluid basaltic lava erupts from crustal

fractures called fissures • e.g., Columbia Plateau

Crater Lake, Oregon, is a good example of a caldera

The Columbia Plateau

Other volcanic landforms

Volcanic pipes and necks • Volcanic necks (e.g., Ship Rock, New

Mexico) Landform made of magma that hardened in a volcano’s pipe and later exposed by erosion.

• Pipes are short conduits that connect a magma chamber to the surface

Formation of a volcanic neck

Volcanic HazardsTypes of volcanoes

• Composite cone (or stratovolcano)• Often produce

• Fiery pyroclastic flow made of hot gases infused with ash

• Flows down sides of a volcano at speeds up to 200 km (125 miles) per hour

• May produce a lahar – volcanic mudflow

Pyroclastic flow Mount St. Helens

A lahar along the Toutle River near Mount St. Helens

Intrusive igneous activity Most magma crystallizes within Earth’s crust An underground igneous body is called a

pluton *Pluto is Roman God of the underworld*

Types of Plutons• Sills - Magma flows between sedimentary layers• Laccoliths - ‘lens shaped’ Magma pushed the overlying

rock layers upward.• Dikes - Magma moves into fractures and cuts across

rock layers.

Intrusive igneous structures exposed by erosion

A sill in the Salt River Canyon, Arizona

Intrusive igneous activity • Batholith

• Largest intrusive body• Much larger than a pluton• Surface exposure 100+ square

kilometers• Frequently form the cores of mountain

ranges

A batholith exposed by erosion