Volcanoes and Other Igneous Activity

Origin of magma Magma originates when essentially solid rock, located in the crust and upper mantle, melts

Origin of magma Factors: –Role of heat Earth’s natural temperature increases with depth (geothermal gradient) is not sufficient to melt rock at the lower crust and upper mantle Additional heat is generated by –Friction in subduction zones –Crustal rocks heated during subduction –Rising, hot mantle rocks

Origin of magma Factors: –Role of heat –Role of 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: –Role of heat –Role of pressure –Role of volatiles Primarily water Cause rock to melt at a lower temperature Play an important role in subducting ocean plates

Origin of magma Factors: –Role of heat –Role of pressure –Role of volatiles –Partial melting Igneous rocks are mixtures of minerals Melting occurs over a range of temperatures Produces a magma with a higher silica content than the original rock

Where do volcanoes form? Volcanoes form at: –Hot Spots (10% of all volcanic activity)

Where do volcanoes form? Volcanoes form at: –Hot Spots (10%) –Spreading Centers (80% of all volcanic activity)

Where do volcanoes form? Volcanoes form at: –Hot Spots (10%) –Spreading Centers (80%) –Convergent Plate Boundaries (10% of all volcanic activity) Ocean–Continental Ocean – Ocean

Plate tectonics and igneous activity Global distribution of igneous activity is not random –Most volcanoes are located on the margins of the ocean basins (intermediate, andesitic composition) –Second group is confined to the deep ocean basins (basaltic lavas) –Third group includes those found in the interiors of continents

Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma –Convergent plate boundaries 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)

Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma –Divergent plate boundaries Produces the greatest volume of volcanic rock –Lithosphere pulls apart –Less pressure on underlying rocks –Partial melting occurs –Large quantities of fluid basaltic magma are produced

Plate tectonics and igneous activity Plate motions provide the mechanism by which mantle rocks melt to form magma –Intraplate igneous activity Activity within a rigid plate Plumes of hot mantle material rise Form localized volcanic regions called hot spots –the Hawaiian Islands and the Columbia River Plateau

Lithospheric Plates

Volcanoes of the World

Volcanic Eruptions Factors that determine the violence of an eruption –Composition of the magma –Temperature of the magma –Dissolved gases in the magma The above three factors actually control the viscosity of a given magma which in turn controls the nature of an eruption

Volcanic Eruptions Viscosity is a measure of a material's resistance to flow

Volcanic Eruptions Factors affecting viscosity –Temperature hotter magmas are less viscous

Volcanic Eruptions Factors affecting viscosity –Temperature –Composition (silica content) High silica – high viscosity (e.g., rhyolitic lava) Low silica – more fluid (e.g., basaltic lava

Volcanic Eruptions Factors affecting viscosity –Temperature –Composition –Dissolved gases (volatiles) Mainly water vapor and carbon dioxide Gases expand near the surface Provide the force to extrude lava Violence of an eruption = how easily gases escape

Viscosity, Temperature and Water Content of Magmas Rock TypeBasaltAndesiteRhyolite SiO 2 content45-55%55-65%65-75% Magma temperature 1,000 – 1,250ºC800 – 1,000 ºC ºC ViscosityLowHigh Gas escape from magma EasyDifficult Eruptive style PeacefulExplosive increasing

Plate-Tectonic Setting of Volcanoes Revisited Why more volcanic activity at spreading centers? –Low SiO 2 content –High temperature –Low pressure as plates pull apart Fluid basaltic lavas generally produce quiet eruptions Why less volcanic activity at subduction zones? –High SiO 2 content –Lower temperatures –Higher pressures Highly viscous lavas produce more explosive eruptions

Materials associated with volcanic eruptions Lava flows –Molten rock that has flowed out onto the Earth’s surface –Types of basaltic lava Pahoehoe lava (resembles braids in ropes) Aa lava (rough, jagged blocks)

Pillow basalts

Materials associated with volcanic eruptions Gases –One to 5 percent of magma by weight –Mainly water vapor and carbon dioxide

Materials associated with volcanic eruptions Pyroclastic materials –Ash and dust – fine, glassy fragments –Pumice – from "frothy" lava –Cinders – "pea-sized" –Lapilli – "walnut" size –Particles larger than lapilli Blocks Bombs

Bomb is approximately 10 cm long

Materials associated with volcanic eruptions Nuée Ardente (or pyroclastic flow) –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 Lahar –volcanic mudflow

A nueé ardente on Mt. Saint Helens

A lahar along the Toutle River near Mt. St. Helens

Volcanoes General features –Conduit, caries gas-rich magma to the surface –Vent, the surface opening (connected to the magma chamber via a pipe) –Crater, steep-walled depression at the summit Caldera (a summit depression greater than 1 km diameter) –Parasitic cones –Fumaroles

Side vent Volcanic bombs Sills Lava flow Dikes Eroded cone Lava pavement (cracked/broken) Cinder cones Ash and tephra Explosive eruption Pyroclastic flow (nuée ardente) Old lava dome Lavas Sedimentary rocks Laccolith Metamorphic rocks Contact metamorphism Magma chamber Granite intrusion (older/cold) Mud flows (older) Lava cone Fracturing Sequential ash and lava layers Chimney

Types of volcanoes Shield volcano –Low Viscosity, Low Volatiles, Large Volume –Broad, slightly domed –Primarily made of basaltic (fluid) lava –Generally large size –Hawaiian Islands

Types of volcanoes Cinder cone –Low Viscosity, Medium Volatiles, Small Volume –Built from ejected lava fragments –Steep slope angle –Rather small size –Frequently occur in groups

Sunset Crater, Flagstaff, Arizona

Types of volcanoes Composite cone (or stratovolcano) –High Viscosity, High Volatiles, Large Volume –Most are adjacent to the Pacific Ocean –Large in size –Interbedded lavas and pyroclastics

Mt. St. Helens – a typical composite volcano

Mt. St. Helens following the 1980 eruption

Mt. St. Helens Rock Glacier Dome New Dome Vent

Mt. St. Helens, October 1,

Types of Volcanoes Composite cone (or stratovolcano) –Often produce nuée ardente –May produce a lahar - volcanic mudflow

Eruptions in the Cascades Ranges

A size comparison of the three types of volcanoes

Other volcanic landforms Calderas –High Viscosity, High Volatiles, Very Large Volume –Steep walled depression at the summit formed by collapse –Size exceeds one kilometer in diameter

Other volcanic landforms Calderas –Famous (or infamous) collapsed calderas: Long Valley, California (Mammoth) Crater Lake (Mount Mazama), Oregon Yellowstone, Wyoming Krakatau, Indonesia, 1883 Santorini and the Lost Continent of Atlantis

Formation of Crater Lake

Caldera Formation

Crater Lake, Oregon

Other volcanic landforms Fissure eruptions and lava plateaus –Low Viscosity, Low Volatiles, Very Large Volume –Basaltic lava extruded from crustal fractures –Incredibly large volumes of lava pour out of fissures over 2-3 million years –Can affect global climate

Formation of Flood Basalts

The Columbia River basalts

Other volcanic landforms Lava Domes –Bulbous mass of congealed lava –Most are associated with explosive eruptions of gas-rich magma –One is currently developing in Mt. St. Helens

Other volcanic landforms Volcanic pipes and necks –Pipes are short conduits that connect a magma chamber to the surface –Volcanic necks (e.g., Ship Rock, New Mexico) are resistant vents left standing after erosion has removed the volcanic cone

Formation of a volcanic neck

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