Igneous Rocks & Plate Tectonics

The types and abundances of igneous rocks are closely related to tectonic setting. Basalt and gabbro are found most abundantly on the ocean floor; whereas, rhyolite and granite are most commonly located within continental interiors. The vast majority of all igneous rocks are produced along plate boundaries.

From Tarbuck and Lutgens

Magma Generation

• Most melting takes place between 10 and 200 km• Decrease in density allows liquid to rise• Believed to rise as tear-drop shaped bodies• Rises through fractures and rock assimilation• Solidifies as it moves into the cooler crust• Most movement ceases upon reaching rocks of similar density or when

overlying rocks form an impenetrable barrier• Some magma makes it to the surface where it erupts as lava

Magma Composition

• Magmas are a mixture of liquid, dissolved gasses, and crystalline solids (Gasses are lost upon eruption as lava)

• H2O and CO2 are the most abundant gasses• Consist mostly of 8 elements (0, Si, Al, Ca, Na, K, Fe, & Mg)• Vary from mafic (basaltic) through intermediate (andesitic) to

felsic (rhyolitic) depending upon source• Range in viscosity from highly fluid to a thick paste• Controlled by tectonic setting, composition of the original rock,

and time• Major processes include melting, mixing, assimilation, and

differentiation

Magmas are a mixture of liquid, dissolved gasses, and crystalline solids

Magma Composition (phases)

A large volume of gas (mostly H2O & CO2) is expelled during an eruption.

Magma Composition (gas)

Eight elements make up nearly 99% of a magma’s volume.

Magma Composition (liquid)

Magma Composition (solid)

Minerals that crystallize early can become separated from the rest of the melt if they have a significantly different density. Minerals with a higher density sink to the bottom of the chamber, while those with a lower density rise to the top.

Magmas vary in composition from mafic (basaltic) through intermediate (andesitic) to felsic (rhyolitic), which greatly affects the type of volcanic eruption due to differences in viscosity.

Basaltic Fissure Eruption

Andesitic(Composite)

ConeRhyolitic Dome

Magma Composition (mineralogy)

Highly fluid Very viscous (paste-like)

Controlled by tectonic setting, composition of the original rock, and time

From Tarbuck and Lutgens

Magma Composition(tectonic setting)

BasalticMagma

Andesitic &RhyoliticMagma

Magma Composition(source materials)

The final magma composition is influenced by:• Composition of the source rock• Composition of the country rock (assimilation)• Mixing with other magmas

Magma Composition (time)

The longer it takes for the magma to cool, the more differentiated it becomes.

Divergent Plate Boundaries

Divergent plate boundaries are characterized by basaltic volcanoes and gabbroic intrusions.

Divergent Boundary Volcanism

Volcanism along divergent plate boundaries is basaltic in composition and highly fluid in character.

Photo by W. W. Little

Shield Volcanoes

Because of the highly fluid nature of basaltic magma, it spreads laterally and flows for long distances upon eruption. The result is a broad, relatively flat structure made up of widespread, thin layers.

Photo by W. W. Little

Mauna Loa: World’s Largest Volcano

Mauna Loa extends from a depth of approximately 10,000 feet to an elevation of nearly 13,800 feet above sea-level.

Photo by W. W. Little

Columbia Plateau Flood Basalt

Types of Basaltic Lava Flows

Pahoehoe Flows

Pahoehoe flows are rich in volatiles (fluids) and deficient in silica and, therefore, are highly fluid in their flow characteristics.

Photo by W. W. Little

Columnar Joints

Columnar joints are shrinkage cracks that form as lava cools and contracts.

Photo by W. W. Little

Photo by W. W. Little

Aa Flows

Aa flows contain fewer volatiles (fluids) and, therefore, are stiffer and less fluid in their flow characteristics.

Photo by W. W. LittlePhoto by W. W. Little

Photo by W. W. Little

Fissure Eruptions

Fissures are similar to shield volcanoes, except that the vent is a long fracture rather than a crater.

Cinder Cones

Cinder cones are small, steep-sided (~330) volcanoes formed by the vertical accumulation of spatter that is shot out of the top of the volcano by escaping gasses.

Photo by W. W. Little

Spatter

Volcanic Bombs

As volcanic spatter moves through the air, it sometimes becomes streamlined, forming a feature called a volcanic bomb.

Photo by W. W. Little

Phreatic Cinder Cones

Intrusive Rock Bodies

• If it cuts across layers as a planar unit,

its called a dike. • If it is squeezed between layers, its

called a sill. • When the layers become arched over

the sill, its referred to as a laccolith. • Magma that cools within the volcano

forms a neck.• Magma that cools within the chamber

becomes a stock or a batholith,

depending upon size.

As magma makes its way toward the surface, it cuts through other rock bodies.

Volcanic Necks

A volcanic neck forms when magma cools within the throat of a volcano. It is more resistant to erosion than the outer part of the volcano.

Photo by W. W. Little

Photo by W. W. Little

Dikes

Dikes are tabular igneous rock bodies that cut across the layers of other rock units.

Sills

Sills are tabular igneous rock bodies that are squeezed between other rock layers.

Inverted Valleys

Basalt is fluid and flows down hill where it fills stream channels. Because basalt is more resistant to erosion than most rocks that form valley walls, it can be preferentially preserved and, eventually, cap the hill after the other rocks have been removed.

Underwater Eruptions

Pillow Structures

Underwater basalt flows cool almost immediately on the outer surface but remain fluid on the inside, forming a blob or pillow shape with a glassy outer shell and aphanitic interior. In some cases, the interior remains hollow.

Formation of Pillow Structures

As the fluid center continues to move, it breaks through the outer, brittle shell to form another pillow.

Structure of Oceanic CrustThe oceanic crust is relatively thin and simple in it’s structure and lithology. Ultramafic rocks of the upper mantle are overlain by gabrroic intrusions, which are overlain by sheeted dikes that filled volcanic fissure vents. The succession is capped by pillow basalt extruded onto the ocean floor and overlying pelagic sediment.

Oman Ophiolite

Oman Ophiolite

Moho

CRUSTMANTLE

Convergent Plate Boundaries

Convergent plate boundaries are characterized by andesitic and/or rhyolitic volcanoes and dioritic and/or granitic intrusions.

From Tarbuck and Lutgens

Ocean/Continent Boundaries

Ocean/continent convergent plate boundaries are characterized by both andesitic and rhyolitic volcanoes and accompanying intrusions. The type is determined by the degree of crustal assimilation and the cooling history.

Composite Volcano

Composite volcanoes form large, steep-sided mountains that are composed of a variety of materials, including rhyolite, andesite, tuff, breccia, and obsidian.

Photo by D. W. Little

Composite Volcanoes

Because of a high silica content, composite volcanoes are characterized by violent eruptions of ash and other pyroclastic material.

Photos by W. W. Little

Mount Shasta, CA

Pyroclastic Flows

Avalanches of ash and breccia are known as pyroclastic flows. These accumulate on the volcano slope as dipping layers.

Ash Falls

Ash is shot tens of thousands of feet into the air and settles to the earth to form tuff.

Lahars

A lahar is a flood/debris flow caused by a volcanic eruption.

Photo by W. W. Little

Photo by W. W. Little

Rhyolite Plugs

Following pyroclastic activity, composite volcanoes often begin to rebuild through rhyolite flows that plug the volcanic vent.

Rhyolite Flows

Rhyolite has a very high in silica content, making it very viscous. It flows more like a paste than a liquid and forms steep-sided domes.

Big Southern Butte, ID

The Big Southern Butte is a rhyolite dome that formed on the Snake River Plain about 300 ka.

Volcano Explosivity Index

Granitic Batholiths

Granitic batholiths are very large rock bodies that form through subsurface solidification of magma. As a result of faulting and isostatic adjustment, they can be brought to the surface for observation.

From Tarbuck and Lutgens

Ocean/Ocean Boundaries

Ocean/ocean convergent plate boundaries are characterized by andesitic volcanoes and dioritic intrusions.

Continent/Continent Boundaries

Continent/Continent convergent plate boundaries typically lack volcanoes and igneous intrusions.

Hot Spots and Mantle Plumes

Magma sources from the mantle can become stationary, forming hot spots over which the plate moves.

Caldera EruptionsCaldera eruptions form in association with composite volcanism and are the most violent of all volcanic eruptions. So much magma is removed from the chamber that it can no longer support the weight of overlying rock and collapses inward into the chamber.

http://www.youtube.com/watch?v=ZzFeZTVxDeY

Summary of Volcano Types

Summary of Magma Characteristics

Basaltic

• Very Hot (1000 °C) • Very Fluid (low viscosity) • High Density • Rich in Ca, Fe, & Mg

• Dark-colored

Andesitic Granitic

• Relatively Cool (700 °C) • Pasty (high viscosity) • Low Density • Rich in Si, Al, Na, & K

• Light-colored

Inte

rmed

iate

Pro

pert

ies

Deadliest Volcanic Eruptions Since 1500 A.D.Eruption Year Casualties Major Cause

Nevado del Ruiz, Colombia 1985 25,0001,3 Mudflows3

Mont Pelée, Martinique 1902 30,0001 (29,025)2 Pyroclastic flows2

Krakatau, Indonesia 1883 36,0001 (36,417)2 Tsunami2

Tambora, Indonesia 1815 92,0001,2 Starvation2

Unzen, Japan 1792 15,0001 (14,030)2Volcano collapse,

Tsunami2

Lakagigar (Laki), Iceland 1783 9,0001 (9,350)2 Starvation2

Kelut, Indonesia 1586 100,001  

Other Notable Eruptions Mount Pinatubo, Philippines 1991 3503 Roof Collapse3

Mount St. Helens, Washington 1980 573 Asphyxiation from ash

Kilauea, Hawaii 1924 11 Falling rock1

Lassen Peak, California 1915 4  Mount Vesuvius, Italy 79 A.D. 33,602 Pyroclastic Flow2

Sources 1 Tilling, Topinka, and Swanson, 1990, Eruptions of Mount St. Helens: Past, Present, and Future: U.S. Geological Survey General Interest Publication, 56p.

2 Blong, R.J., 1984, Volcanic Hazards: A Sourcebook on the Effects of Eruptions: Orlando, Florida, Academic Press, 424p.

3 Wright and Pierson, 1992, Living With Volcanoes: The U.S. Geological Survey's Volcano Hazards Program: U. S. Geological Survey Circular 1073, 57p.

4 Spall, H. (ed.), 1980, Earthquake Information Bulletin: July-August, 1980, v.12, no.4, 167p.

Volcanic Hazards

Infrastructure Burial

Lava flows

Ash Falls Scoria

Pyroclastic Flows

Flooding & Lahars

Benefits of Volcanism

Mitigation

Mammoth Mtn, eastern California

Trees killed by volcanic CO2 gas

CO2 from subsurface magma canaccumulate in depressions