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