Igneous Rocks and Volcanoes

Igneous Processes and Volcanoes Key Concepts

• Magma origin is related to plate tectonics
• Bowen’s Reaction Series relates mineral crystallization and melting temperatures
• Cooling of magma leads to rock compositions and textures, used to classify igneous rocks
• Igneous landforms relate to their origin
• Partial melting and fractionation change magma compositions
• Silica content affects magma viscosity and eruptive style of volcanoes
• Volcano types, eruptive styles and compositions depend on plate tectonic settings
• Volcanic hazards exist

Igneous Rock Formation

• Liquid rock freezing into solid rock forms igneous rock
• Magma is molten rock in the ground
• Lava is molten rock on the surface
• Only Earth’s outer core is liquid
• Mantle and crust are naturally solid
• Pockets of magma form near the surface where geologic processes cause melting, becoming the source for volcanoes and igneous rocks

Extrusive vs. Intrusive Rocks

• Lava cools quickly on the surface and forms fine-grained extrusive (volcanic) rocks with microscopic crystals
• Extrusive rocks are often vesicular, filled with holes from escaping gas bubbles
• Volcanism is the process in which lava is erupted, ranging from smooth to explosive
• Magma cools slowly below the earth’s surface, forming coarse-grained intrusive (plutonic) rocks with larger crystals visible to the naked eye
• Cooling rates and grain sizes in igneous rocks help interpret the rock's geologic history

4.1 Classification of Igneous Rocks

• Igneous rocks are classified based on texture (grain size) and composition (mineralogy and chemistry)
• Texture relates to the cooling history of the magma
• Cooling history can also affect the composition of igneous rocks

4.1.1 Texture

• Texture describes the physical characteristics of the minerals, such as grain size

Intrusive Rocks

• Intrusive (plutonic) rocks cool slowly deep within the crust, resulting in rocks with a coarse-grained or phaneritic texture
• Individual crystals in phaneritic texture are readily visible to the unaided eye

Extrusive Rocks

• Extrusive (volcanic) rocks result when lava is extruded onto the surface or intruded into shallow fissures, cooling quickly
• Extrusive rocks have a fine-grained or aphanitic texture, where grains are too small to see without a microscope
• Rapidly cooling lava may not develop crystals, forming volcanic glass, a non-crystalline material
• Volcanic glass is common in volcanic ash and rocks like obsidian

Porphyritic Texture

• Porphyritic texture contains coarse-grained minerals (phenocrysts) surrounded by a fine-grained matrix (groundmass)
• Porphyritic texture indicates a multi-stage cooling history: slow cooling deep underground followed by faster cooling near the surface

Pegmatitic Texture

• Pegmatitic texture has very large crystals of minerals like feldspar, quartz, beryl, tourmaline, and mica formed from residual molten material expelled from igneous intrusions
• Pegmatitic texture indicates very slow crystallization
• Rocks with a pegmatitic texture are called pegmatites
• Cleavage sheets of pegmatitic muscovite mica were historically used as windows

Vesicular Texture

• Volatiles (dissolved gases) in magma bubble out of solution as the magma rises to the surface due to decreasing pressure
• Gas bubbles trapped in solidifying lava create a vesicular texture, with the holes specifically called vesicles
• Scoria is a type of volcanic rock with common vesicles

Pumice

• Pumice is a meringue-like froth of glass that occurs when volatile-rich lava is very quickly quenched
• Pumice is full of vesicles, making its density low enough to float

Volcanic Glass

• Volcanic glass forms when lava cools extremely quickly without forming crystals
• Obsidian is a rock consisting of volcanic glass
• Obsidian shows conchoidal fracture similar to quartz

Pyroclastic Texture

• Pyroclastic textures are formed when volcanoes erupt explosively, throwing lava, rock, ash, and gases into the atmosphere as tephra
• Pyro refers to the igneous source, and clastic refers to the rock fragments
• Pyroclastic texture has a chaotic mix of crystals, angular glass shards, and rock fragments
• Rock formed from tephra deposits is called tuff
• If fragments accumulate while still hot, they weld together, forming welded tuff

4.1.2 Composition

• Composition refers to a rock’s chemical and mineral make-up
• Igneous rock composition is divided into felsic, intermediate, mafic, and ultramafic groups
• These groups lie on a continuous spectrum with transitional compositions

Felsic Composition

• Felsic refers to light-colored silicate minerals, rocks, and magmas enriched in silicon, oxygen, sodium, and potassium
• Felsic rocks always contain a large amount of silica (SiO2)

Mafic Composition

• Mafic refers to minerals, rocks, or magmas rich in iron and magnesium
• Mafic minerals and rocks are dark-colored and contain less silica than felsic ones

Intermediate Composition

• Intermediate compositions fall between felsic and mafic

Ultramafic

• Ultramafic compositions are poorer in silica and richer in iron and magnesium than mafic compositions

Simplified Igneous Rock Nomenclature

• Rhyolite is felsic volcanic
• Dacite and andesite are intermediate volcanic
• Basalt is mafic volcanic
• Komatiite is ultramafic volcanic
• Granite is felsic plutonic
• Granodiorite and diorite are intermediate plutonic
• Gabbro is mafic plutonic
• Peridotite is ultramafic plutonic

Felsic Rocks

• Felsic rocks contain mostly light-colored minerals, mainly feldspar and silica (quartz)
• Felsic minerals are rich in silica
• Felsic rocks may contain minor amounts of dark-colored (mafic) minerals like amphibole and biotite mica
• Felsic igneous rocks contain 65-75% silica (SiO2) and are relatively poor in iron and magnesium

Intermediate Rocks

• Intermediate rocks have roughly equal amounts of light and dark minerals
• Intermediate rocks include plagioclase feldspar and amphibole
• Silica content of intermediate rocks is in the 55-60% range

Mafic rocks

• Mafic rocks contain abundant ferromagnesian minerals (magnesium and iron) plus plagioclase feldspar
• Mafic rocks contain dark minerals like pyroxene and olivine, rich in iron and magnesium and poor in silica
• Mafic rocks are low in silica, in the 45-50% range

Ultramafic Rocks

• Ultramafic rocks are composed mostly of olivine and some pyroxene
• Ultramafic rocks contain more magnesium and iron and less silica than mafic rocks
• Peridotite, the rock of the upper mantle, is an ultramafic rock
• Ultramafic rocks contain 40% or less silica

Continuous Spectrum

• Rhyolite refers to volcanic and felsic rocks
• Granite refers to intrusive and felsic rocks
• Andesite and diorite refer to extrusive and intrusive intermediate rocks
• Dacite and granodiorite have compositions between felsic and intermediate
• Basalt and gabbro are the extrusive and intrusive names for mafic igneous rocks
• Peridotite is ultramafic
• Komatiite is the fine-grained extrusive equivalent of peridotite
• Komatiite is rare because volcanic material direct from the mantle is uncommon
• Nature rarely has sharp boundaries, and rock classification imposes sharp names on a continuous spectrum

Granite

• Granite is a coarse-crystalline felsic intrusive rock identifiable by the presence of quartz
• Granite commonly has salmon pink potassium feldspar and white plagioclase crystals with visible cleavage planes
• Granite approximates the continental crust in density and composition

Rhyolite

• Rhyolite is a fine-crystalline felsic extrusive rock
• Rhyolite is commonly pink and often has glassy quartz phenocrysts
• Rhyolite is less common than granite because felsic lavas are less mobile
• Examples of rhyolite include lava flows in Yellowstone and altered rhyolite in the Grand Canyon of the Yellowstone

Diorite

• Diorite is a coarse-crystalline intermediate intrusive igneous rock
• Diorite has a Dalmatian-like appearance of black hornblende and biotite and white plagioclase feldspar
• Diorite is found in the Andes Mountains and the Henry and Abajo mountains of Utah

Andesite

• Andesite is a fine crystalline intermediate extrusive rock that is commonly gray and porphyritic
• Andesite is the fine-grained compositional equivalent of diorite
• Andesite can be found in the Andes Mountains and some island arcs

Gabbro

• Gabbro is a coarse-grained mafic igneous rock made mainly of mafic minerals like pyroxene and minor plagioclase
• Gabbro is less common than basalt because mafic lava is more mobile
• Gabbro is a major component of the lower oceanic crust

Basalt

• Basalt is a fine-grained mafic igneous rock that is commonly vesicular and aphanitic
• Basalt often has olivine or plagioclase phenocrysts when porphyritic
• Basalt is the main rock formed at mid-ocean ridges and is the most common rock on Earth's surface, making up the ocean floor

4.1.3 Igneous Rock Bodies

• Intrusive rocks are more common than extrusive rocks in the geologic record due to their durability
• Extrusive rocks are less durable due to small crystals, glass, and immediate exposure to erosion
• Landforms and rock groups originating from igneous rocks are usually intrusive bodies

Dikes

• Dikes (or dykes) are cross-cutting features formed when magma intrudes into a weakness like a crack or fissure and solidifies
• Dikes are discordant intrusions, vertical or at an angle relative to existing rock layers
• Dikes are important for dating rock sequences and interpreting geologic history
• Dikes are younger than the rocks they cut across and can be used to assign numeric ages to sedimentary sequences

Sills

• Sills are concordant intrusions that run parallel to sedimentary layers in the country rock
• Sills form when magma exploits a weakness between layers
• Sills are younger than surrounding layers and can be radioactively dated to study the age of sedimentary strata

Plutons

• A magma chamber is a large underground reservoir of molten rock
• The path of rising magma is called a diapir
• The processes by which a diapir intrudes into the country rock are not well understood
• Theories for diapir intrusion: overriding rock gets displaced, native rock is melted or broken off (stoping), or diapirs are a series of dikes
• Cooling diapirs form plutons, masses of intrusive rock that are often somewhat round

Batholiths

• Batholiths are extensive features formed when many plutons merge together
• Batholiths are found in the cores of many mountain ranges (e.g., Yosemite National Park in the Sierra Nevada)
• Batholiths are typically more than 100 km2 in area, associated with subduction zones, and mostly felsic in composition

Stocks

• A stock is a type of pluton with less surface exposure than a batholith, and may represent a narrower neck of material emerging from the top of a batholith
• Batholiths and stocks are discordant intrusions that cut through surrounding country rock

Laccoliths

• Laccoliths are blister-like, concordant intrusions of magma that form between sedimentary layers
• Laccoliths bulge upwards
• Lopoliths bulge downwards
• The Henry Mountains of Utah are a landform formed by laccoliths

4.2 Bowen’s Reaction Series

• Bowen’s Reaction Series describes the temperature at which minerals crystallize (when cooled) or melt (when heated)
• The low end of the temperature scale is 700°C, where all minerals crystallize into solid rock
• The upper end of the range is 1,250°C, where all minerals exist in a molten state
• These temperatures are for minerals crystallizing at standard sea-level pressure (1 bar)

Mineral Composition and Bowen's Series

• Ultramafic, mafic, intermediate, and felsic igneous rocks are listed from top to bottom
• Silica, sodium, aluminum, and potassium increase from ultramafic to felsic compositions
• Ferromagnesian components (iron, magnesium, and calcium) increase from felsic to ultramafic compositions
• Minerals near the top of the series (e.g., olivine and anorthite) crystallize at higher temperatures
• Minerals near the bottom of the series (e.g., quartz and muscovite) crystallize at lower temperatures

Norman L. Bowen

• Norman L. Bowen studied igneous rocks in the early 20th century and noticed that certain minerals always occur together, excluding others
• Bowen conducted experiments to study the temperature at which rocks cooled involving grinding rock combinations into powder, sealing them in metal capsules, heating, and cooling
• Experiments showed that common igneous minerals crystallize from magma at different temperatures
• Minerals occur together in rocks with others that crystallize within similar temperature ranges
• Mafic igneous rocks contain more mafic minerals and crystallize at higher temperatures than felsic rocks
• Felsic lavas erupt hundreds of degrees cooler than mafic lavas
• Bowen's work laid the foundation for understanding igneous petrology, described in his book, The Evolution of the Igneous Rocks (1928)

4.3 Magma Generation

• Magma and lava contain melt, solids, and volatiles
• The melt is made of liquefied ions from minerals
• The solids are crystallized minerals floating in the liquid melt
• Volatiles are gaseous components (water vapor, carbon dioxide, sulfur, and chlorine) dissolved in the magma

4.3.1 Geothermal Gradient

• The temperature of the Earth rises below the surface
• Earth's heat is caused by residual heat from its formation and ongoing radioactive decay
• The geothermal gradient is the rate at which temperature increases with depth
• The average geothermal gradient in the upper 100 km of the crust is about 25°C per kilometer of depth

Pressure and Temperature

• At 100 km deep, the temperature is about 1,200°C
• At the bottom of the crust (35 km deep), the pressure is about 10,000 bars
• At these pressures and temperatures, the crust and mantle are solid
• The solidus line slopes to the right because pressure increases the temperature needed to melt rock

Rock Melting

• Rock behavior crosses the solidus line to create magma in three ways: decompression melting, flux melting, and heat-induced melting
• Bowen’s Reaction Series shows that minerals melt at different temperatures
• Partial melting is when some minerals melt and some remain solid, which is typical for real-world magmas

Plate Tectonic Settings

• Partial melting occurs at different plate tectonic settings
• In a stable plate: there is no magma generation
• At a mid-ocean ridge: decompression melting occurs
• At a hotspot: decompression melting plus heat addition occurs
• At a subduction zone: flux melting takes place where water shifts the melting point

4.3.2 Decompression Melting

• Magma is created at mid-ocean ridges via decompression melting
• Convection currents cause the solid asthenosphere to flow beneath the lithosphere
• The upper lithosphere (crust) is a poor heat conductor, so the temperature remains about the same throughout the underlying mantle material
• Rising mantle material experiences decreasing pressure, which causes the melting point to drop
• Rock at the temperature of the geothermal gradient rises toward the surface, shifting past its melting point, and partial melting starts
• Rising magma cools and crystallizes to form new lithospheric crust

4.3.3 Flux Melting

• Flux melting (fluid-induced melting) occurs in island arcs and subduction zones when volatile gases are added to mantle material
• Flux-melted magma produces many of the volcanoes in the circum-Pacific subduction zones (Ring of Fire)
• The subducting slab contains oceanic lithosphere and hydrated minerals
• Increased temperature causes the hydrated minerals to emit water vapor and other volatile gases, expelled into the overlying asthenospheric mantle
• Volatiles dissolve into the asthenospheric mantle and decrease its melting point
• The mantle's melting point has been lowered by volatiles
• The solidus line shifts to the left of the geothermal gradient line, and melting begins

4.3.4 Heat-Induced Melting

• Heat-induced melting transforms solid mantle into liquid magma by applying heat
• Heat-induced melting occurs at mantle plumes or hotspots
• Rock surrounding the plume is exposed to higher temperatures, the geothermal gradient crosses the solidus line, and the rock begins to melt
• A small amount of magma is also generated by intense regional metamorphism, forming migmatite

4.4 Partial Melting and Crystallization

• Partial melting and crystallization processes can change the chemistry of magma
• Partial melting and crystallization explain the variety of igneous rocks on Earth

4.4.1 Partial Melting

• The mantle is composed of many different minerals, so it does not melt uniformly
• Minerals with lower melting points turn into liquid magma, while those with higher melting points remain solid crystals
• As magma rises and cools, it undergoes physical and chemical changes in a process called magmatic differentiation

Magma Chemistry

• Each mineral has a unique melting and crystallization temperature (Bowen’s Reaction Series)
• Partial melting creates magma with a different composition than the original mantle material

Mantle Rocks

• The chemistry of mantle rock (peridotite) is ultramafic, low in silicates and high in iron and magnesium
• When peridotite begins to melt, the silica-rich portions melt first because of their lower melting point
• This makes magma increasingly silica-rich: ultramafic mantle becomes mafic magma, and mafic mantle becomes intermediate magma
• Magma rises to the surface because it is more buoyant than the mantle

Crustal Rocks

• Partial melting occurs as existing crustal rocks melt in the presence of heat from magmas
• Melting existing rocks allows magma formed to be more felsic and less mafic than the pre-existing rock
• Early in Earth's history, silica-rich magmas formed, rose to the surface, and solidified into granitic continents
• The old granitic cores of the continents are called shields

4.4.2 Crystallization and Magmatic Differentiation

• Liquid magma is less dense than surrounding solid rock, so it rises through the mantle and crust
• As magma cools and crystallizes, magmatic differentiation changes the chemistry of the resultant rock towards a more felsic composition
• Magmatic differentiation happens via assimilation and fractionation

Assimilation

• During assimilation, pieces of country rock with a different, often more felsic, composition are added to the magma
• Solid pieces may melt, changing the composition of the original magma
• Unmelted country rocks within an igneous rock mass are called xenoliths

Magma Mixing and Rejuvenation

• Xenoliths are also common in the processes of magma mixing and rejuvenation
• Magma mixing occurs when two different magmas come into contact and mix
• Magmatic rejuvenation happens when a cooled and crystallized body of rock is remelted, leaving pieces of the original rock as xenoliths

Continental Lithosphere

• The continental lithosphere is felsic (granitic) and buoyant
• Mafic magma rises slowly through thick continental crust compared to oceanic plates
• Mafic magma tends to assimilate felsic rock, becoming more silica-rich as it migrates through the lithosphere, changing into intermediate or felsic magma by the time it reaches the surface
• Felsic magmas are much more common within continents

Fractional Crystallization

• Fractionation (fractional crystallization) increases magma silica content, making it more felsic
• As temperature drops within a magma diapir rising through the crust, some minerals crystallize and settle to the bottom of the magma chamber
• Remaining melt is depleted of those ions
• Olivine is a mafic mineral with a high melting point and a smaller percentage of silica
• When ultramafic magma cools, olivine crystallizes first and settles to the bottom of the magma chamber, making the remaining melt more silica-rich and felsic
• As mafic magma further cools, plagioclase and pyroxene crystallize next, removing more low-silica components, making the magma more felsic
• Crystal fractionation can occur in oceanic lithosphere
• Formation of more differentiated, highly evolved felsic magmas is largely confined to continental regions where the longer time to the surface allows more fractionation to occur

4.5 Volcanism

• When magma emerges onto the Earth’s surface, it is called lava
• A volcano is a type of land formation created when lava solidifies into rock

4.5.1. Distribution and Tectonics

• Most volcanoes are interplate volcanoes, located at active plate boundaries: mid-ocean ridges, subduction zones, & continental rifts
• Intraplate volcanoes are located within tectonic plates, far removed from plate boundaries
• Many intraplate volcanoes are formed by hotspots

Volcanoes at Mid-Ocean Ridges

• Most volcanism on Earth occurs on the ocean floor along mid-ocean ridges, a type of divergent plate boundary
• Most ocean floor volcanism is slow, gentle, and oozing
• One exception is the volcanoes of Iceland
• Diverging oceanic plates allow hot mantle rock to rise, releasing pressure and causing decompression melting
• Ultramafic mantle rock partially melts and generates basaltic magma
• Almost all volcanoes on the ocean floor are basaltic
• Most oceanic lithosphere is basaltic near the surface, with phaneritic gabbro and ultramafic peridotite underneath

Pillow Basalts

• When basaltic lava erupts underwater it emerges in small explosions and/or forms pillow-shaped structures called pillow basalts
• Seafloor eruptions enable underwater ecosystems to thrive in the deep ocean around mid-ocean ridges
• Tall vents emit black, hot mineral-rich water called deep-sea hydrothermal vents, also known as black smokers

Chemosynthesis

• Organisms utilize chemosynthesis instead of photosynthesis
• Certain bacteria turn hydrogen sulfide (H2S) into life-supporting nutrients and water
• Larger organisms may eat bacteria or absorb nutrients and water produced by bacteria living symbiotically inside their bodies

Volcanoes at Subduction Zones

• The second most common location for volcanism is adjacent to subduction zones, a type of convergent plate boundary
• Subduction expels water from hydrated minerals in the descending slab, which causes flux melting in the overlying mantle rock
• Subduction volcanism occurs in a volcanic arc
• The thickened crust promotes partial melting and magma differentiation, evolving mafic magma to more silica-rich magma
• The Ring of Fire is dominated by subduction-generated eruptions of silica-rich lava: andesite, rhyolite, pumice, and tuff

Volcanoes at Continental Rifts

• Some volcanoes are created at continental rifts, where crustal thinning is caused by diverging lithospheric plates
• Volcanism caused by crustal thinning without continental rifting is found in the Basin and Range Province
• Volcanic activity is produced by rising magma that stretches the overlying crust
• Lower crust or upper mantle material rises through the thinned crust, releases pressure, and undergoes decompression-induced partial melting
• Magma rises through the crust to the surface, erupting as basaltic lava
• Eruptions usually result in flood basalts, cinder cones, and basaltic lava flows
• Young cinder cones of basaltic lava are found in south-central Utah in the Black Rock Desert Volcanic Field

Hotspots

• Hotspots are the main source of intraplate volcanism
• Hotspots occur when lithospheric plates glide over a hot mantle plume, an ascending column of solid heated rock originating from deep within the mantle
• Mantle plume generates melts as material rises
• Ascending magma reaches the lithospheric crust and spreads out into a mushroom-shaped head
• Most mantle plumes are located beneath the oceanic lithosphere
• Early stages of intraplate volcanism take place underwater
• Basaltic volcanoes may build up from the sea floor into islands, such as the Hawaiian Islands
• Hotspots under a continental plate contacting hot mafic magma may cause the overlying felsic rock to melt and mix with the mafic material below, forming intermediate magma
• The Yellowstone caldera is an example of hotspot volcanism that resulted in an explosive eruption

Volcanic Chains

• A zone of actively erupting volcanism connected to a chain of extinct volcanoes indicates intraplate volcanism located over a hotspot
• Volcanic chains are created by the overriding oceanic plate slowly moving over a hotspot mantle plume
• The Hawaiian Islands on the Pacific Oceanic plate are the active end of a long volcanic chain that extends to the Emperor Seamounts
• The overriding North American continental plate created a chain of volcanic calderas that extends from Southwestern Idaho to the Yellowstone calderas

4.5.2 Volcano Features and Types

• Volcanoes are classified based on their shape, eruption style, magmatic composition, and other aspects

Volcano Features

• Magma chamber
• Lithosphere
• Conduit (pipe)
• Base of volcano
• Sill
• Diapir
• Layers of tephra
• Layers of lava
• Crater
• Parasitic cone
• Vents
• Crater rim

Viscosity

• Viscosity is the resistance to flowing by a fluid
• Low viscosity magma flows easily (basaltic volcanism in Hawaii)
• High viscosity magma flows slowly (felsic or intermediate)

Shield Volcanoes

• Shield volcanoes are the largest volcanoes
• Shield volcanoes have broad low-angle flanks, small vents at the top, and mafic magma chambers
• Side view resembles a medieval warrior’s shield
• Shield volcanoes are associated with hotspots, mid-ocean ridges, or continental rifts with rising upper mantle material
• Low-angle flanks are built up slowly from numerous low-viscosity basaltic lava flows
• Basaltic lava erupts effusively (small, localized, and predictable eruptions)
• Mauna Loa and Kilauea in Hawaii are examples of shield volcanoes
• Shield volcanoes are also found in Iceland, the Galapagos Islands, Northern California, Oregon, and the East African Rift

Martian Volcanoes

• Olympus Mons on Mars is the largest volcanic edifice in the Solar System
• Possibly extinct shield volcano covers an area the size of Arizona
• This may indicate the volcano erupted over a hotspot for millions of years, implying Mars had little, if any, plate tectonic activity

Basaltic Lava

• Basaltic lava forms special landforms based on magma temperature, composition, and content of dissolved gases and water vapor
• Two main types of basaltic volcanic rock have Hawaiian names: pahoehoe and aa

Pahoehoe Lava

• Pahoehoe is low-viscosity lava that flows easily into ropey strands

Aa Lava

• Aa is more viscous and has a crumbly blocky appearance

Lava Tubes

• Low-viscosity, fast-flowing basaltic lava tends to harden on the outside into a tube and continue to flow internally
• Once lava flow subsides, the empty outer shell may be left as a lava tube
• Lava tubes make famous caves in various locations

Fissures

• Fissures are cracks that commonly originate from shield-style eruptions
• Lava emerging from fissures is typically mafic and very fluid
• Some fissures are caused by volcanic seismic activity
• Some fissures are influenced by plate tectonics

Columnar Jointing

• Cooling lava can contract into columns with semi-hexagonal cross sections called columnar jointing
• Examples of columnar jointing include Devils Tower in Wyoming and the Giant’s Causeway in Ireland

Stratovolcanoes

• Stratovolcanoes (composite cone volcanoes) have steep flanks, a symmetrical cone shape, a distinct crater, and rise prominently above the surrounding landscape
• Composite refers to the alternating layers of pyroclastic fragments and solidified lava flows of varying composition
• Examples include Mount Rainier in Washington state and Mount Fuji in Japan
• Stratovolcanoes usually have felsic to intermediate magma chambers, but can even produce mafic lavas
• Stratovolcanoes have viscous lava flows and domes with explosive eruptions, which produce steep flanks

Lava Domes

• Lava domes are accumulations of silica-rich volcanic rock, such as rhyolite and obsidian
• Felsic lava is viscous and tends to pile up near the vent in blocky masses
• Lava domes often form in a vent within the collapsed crater of a stratovolcano and grow by internal expansion
• Mount Saint Helens has a lava dome inside of a collapsed stratovolcano crater
• Examples of stand-alone lava domes are Chaiten in Chile and Mammoth Mountain in California

Calderas

• Calderas are steep-walled, basin-shaped depressions formed by the collapse of a volcanic edifice into an empty magma chamber
• Calderas are large, with diameters of up to 25 km
• Caldera volcanoes are typically formed by eruptions of high-viscosity felsic lava with high volatiles content
• Crater Lake, Yellowstone, and the Long Valley Caldera are examples of volcanism
• Mount Mazama erupted in a huge explosive blast, draining the magma chamber and causing the top to collapse into a large depression that later filled with water to create the caldera at Crater Lake National Park in Oregon
• Wizard Island in the middle of Crater Lake is a later resurgent lava dome that formed within the caldera basin

Yellowstone

• The Yellowstone volcanic system erupted three times in the recent geologic past: 2.1, 1.3, and 0.64 million years ago
• Three eruptions created large rhyolite lava flows and pyroclastic flows that solidified into tuff formations
• Each eruption rapidly emptied the magma chamber, causing the roof to collapse and form a caldera
• The youngest of the three calderas contains most of Yellowstone National Park, as well as two resurgent lava domes
• Calderas are difficult to see today due to the amount of time since their eruptions, subsequent erosion, and glaciation
• Yellowstone volcanism started about 17-million years ago as a hotspot under the North American lithospheric plate near the Oregon/Nevada border
• The plate moved to the southwest over the stationary hotspot, leaving behind a track of past volcanic activities; Idaho’s Snake River Plain was created from this volcanism

Long Valley Caldera

• The Long Valley Caldera near Mammoth, California, is the result of a large volcanic eruption that occurred 760,000 years ago
• The explosive eruption dumped enormous amounts of ash across the United States
• The Bishop Tuff deposit is made of ash from this eruption
• The current caldera basin is 17 km by 32 km, large enough to contain the town of Mammoth Lakes, among many other features

Cinder Cones

• Cinder cones are small volcanoes with steep sides made of pyroclastic fragments ejected from a central vent
• Fragments are called cinders, and the largest are volcanic bombs
• Eruptions are usually short-lived, consisting of mafic lavas with a high content of volatiles
• Hot lava is ejected into the air, cooling and solidifying into fragments that accumulate on the flank of the volcano
• Cinder cones are found throughout western North America

Paricutin

• The eruption near the village of Parícutin, Mexico, started in 1943
• The cinder cone started explosively shooting cinders out of the vent in the middle of a farmer’s field
• The cone quickly built up to a height of over 90 m within a week, and 365 m within the first 8 months
• Basaltic lava poured out from the base of the cone after the initial explosive eruption of gases and cinders
• The cinder cone is not strong enough to support a column of lava rising to the top of the crater, so the lava breaks through and emerges near the bottom of the volcano
• Ashfall covered about 260 km2 and destroyed the nearby town of San Juan

Flood Basalts

• Flood basalts are a rare volcanic eruption type, unobserved in modern times, and are some of the largest and lowest viscosity types of eruptions known
• Famous examples include the Columbia River Flood Basalts, the Deccan Traps, and the Siberian Traps

Carbonatites

• Carbonatite eruptions are a product of carbonate-based magma and produce volcanic rocks containing greater than 50% carbonate minerals
• Carbonatite lavas are very low viscosity and relatively cold for lava
• Erupting lava is black, and solidifies to brown/grey rock that eventually turns white
• Only one actively erupting carbonatite volcano exists on Earth today: Ol Doinyo Lengai in the East African Rift Zone of Tanzania
• Carbonatites are mostly associated with continental rifting

4.5.3 Volcanic Hazards and Monitoring

• Common volcanic hazards include lava, explosions, landslides, pyroclastic flow, ash and volcanic gases
• On May 18, 1980, Mount Saint Helens erupted with an explosion and landslide that removed the upper 400 m of the mountain
• An eruption creates both lateral blasts and pyroclastic flows

Pyroclastic Flows

• Pyroclastic flows are a mix of lava blocks, pumice, ash, and hot gases between 200°C-700°C
• The turbulent cloud of ash and gas races down steep flanks at high speeds, up to 193 kph, into valleys
• Most explosive, silica-rich, high viscosity magma volcanoes (composite cones) usually have pyroclastic flows
• Tuff and welded tuff is often formed from pyroclastic flows

Examples of Pyroclastic Flows

• The Mount Ontake pyroclastic flow in Japan killed 47 people in 2014
• The flow was caused by magma heating groundwater into steam, which then rapidly ejected with ash and volcanic bombs
• The Mount Unzen eruptions in the early 1990s killed 41 people
• Mount Pelee erupted on the Caribbean Island Martinique with a violent pyroclastic flow that destroyed the entire town of St. Pierre and killing 28,000 people in 1902

Landslides

• Steep and unstable flanks of a volcano can lead to slope failure and dangerous landslides
• Landslides can be triggered by magma movement, explosive eruptions, large earthquakes, and/or heavy rainfall
• The 1980 Mount St. Helens eruption released a huge landslide that moved at speeds of 160-290 kph

Tsunamis

• If enough landslide material reaches the ocean, it may cause a tsunami
• A landslide caused by the Mount Unzen eruption reached the Ariaka Sea in 1792, generating a tsunami that killed 15,000 people
• The 1883 Mount Krakatau eruption generated ocean waves that towered 40 m above sea level, killing 36,000 people and destroying 165 villages

Tephra

• Volcanoes eject large amounts of tephra (ejected rock materials), most notably ash (tephra fragments less than 2 mm)
• Larger tephra is heavier and falls closer to the vent
• Blocks and bombs pose hazards to those close to the eruption
• Hot ash poses an immediate danger to people, animals, plants, machines, roads, and buildings close to the eruption
• Ash can travel airborne long distances away from the eruption site
• Heavy accumulations of ash can cause buildings to collapse
• Ash may cause respiratory issues like silicosis
• Ash is destructive to aircraft and automobile engines, which can disrupt transportation and shipping services
• In 2010, the Eyjafjallajökull volcano in Iceland emitted a large ash cloud, causing the largest air-travel disruption in northern Europe since World War II

Volcanic Gases

• As magma rises to the surface, confining pressure decreases, allowing dissolved gases to escape into the atmosphere
• Even volcanoes that are not actively erupting may emit hazardous gases, such as carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), and hydrogen halides (HF, HCl, or HBr)

Carbon Dioxide

• Carbon dioxide tends to sink and accumulate in depressions and basins
• Low-lying areas may trap hazardous concentrations of carbon dioxide
• The Mammoth Mountain Ski Resort in California, located within the Long Valley Caldera, is one such area of carbon dioxide-producing volcanism

Limnic Eruptions

• Limnic eruptions occur in crater lakes associated with active volcanism
• The water in these lakes is supercharged with high concentrations of dissolved gases
• If the water is physically jolted by a landslide or earthquake, it may trigger an immediate and massive release of gases
• An infamous limnic eruption occurred in 1986 at Lake Nyos, Cameroon, killing almost 2,000 people

Lahars

• Lahar is an Indonesian word to describe a volcanic mudflow formed from rapidly melting snow or glaciers
• Lahars resemble wet concrete and consist of water, ash, rock fragments, and other debris
• These mudflows flow down the flanks of volcanoes or mountains covered with freshly erupted ash, reaching speeds of up to 80 kph on steep slopes

Lahar Flows

• Several major cities, including Tacoma, are located on prehistoric lahar flows surrounding Mount Rainier in Washington
• Mount Baker in Washington has a similar hazard for lahar flows
• In 1985, a lahar from the Nevado del Ruiz volcano in Colombia buried the town of Armero and killed an estimated 23,000 people

Monitoring

• Geologists use various instruments to detect changes or indications that an eruption is imminent
• Different types of volcanic monitoring used to predict eruptions: earthquake activity, increases in gas emission, and changes in land surface orientation and elevation

Earthquake Activity

• Monitoring earthquake frequency, especially harmonic tremors, can detect magma movement and possible eruption

Gas Emission

• A rapid increase of gas