Igneous Rocks Tour

Introduction to Igneous Rocks    Intrusive Igneous Rocks    Igneous Intrusions/Plutons

Volcanic Igneous Rocks    Igneous Rocks Quiz    Credits





Volcanic or extrusive igneous rocks form when molten elements, called lava, erupt from Earth's interior through a volcanic vent or fissure and cool rapidly at the surface.    Eruptions tend to be fluid or explosive, depending on the:

1) amount of gases trapped within magma underground.   The more gas contained within magma, the more explosive it will tend to be as it erupts as lava at the low-pressure condition at Earth's surface.

2) amount of silicon dioxide present within the magma/lava.   The more silicon dioxide present, the thicker will be the body of molten elements, increasing the difficulty of trapped gases to escape.   So, when such magma is exposed to low-pressure conditions at the surface, the gases will suddenly and violently escape. producing an explosive volcanic eruption.

3) temperature of the magma/lava.   Higher-temperature molten elements tend to flow more readily than lower-temperature molten elements.   So, the lower the temperature of magma, the higher will be its viscosity, and the more likely it will erupt violently once it emerges at the surface as lava.  

In general, mafic magma/lava tends to be hotter and contain less silicon dioxide than felsic and intermediate magma/lava, so mafic eruptions tend to result in lava flows, but felsic and intermediate eruptions tend to be explosive and quite dangerous.


flowing/fluid eruptions - lava flows and features

Lava may flow from a volcanic vent, much like water flowing downhill.   But, lava tends to have more internal resistance to flowing (higher viscosity) than water, so it doesn't travel as far or as rapidly as water, but it can still cover surprisingly great distances (up to 30 miles) before cooling and solidifying.  

Mafic lava is much hotter than intermediate or especially felsic lava, so it is lower in viscosity than other lava flows.   Most lava-flow deposits on Earth are mafic/basaltic.

hot, fluid mafic lava erupts from small vent on Big Island, Hawaii; photograph by C. Heliker, USGS

cooling of flowing lava causes surface to darken and solidify; by J.D. Griggs, USGS

close view of cooling mafic lava flow, and formation of basalt; by T.N. Mattox, USGS

as lava cools at surface, it continues flowing underneath, forming ropy or pahoehoe texture; by T.N. Mattox, USGS

ropy nature of surface of basaltic lava flow; by Jack Green, CSULB Geological Sciences

a-a surface of lava flow formed as flow cools and loses trapped gases to the atmosphere; end of flow breaks up and solidifies into challenging surface for hiking; Jeff Brenner for scale

Big Obsidian Flow, Oregon; a rare example of fluid behavior by felsic lava; photograph credit to W. Scott, USGS

felsic/rhyolitic flows are very strange, with layers of pumice and obsidian contorted into bizarre shapes as the flows cooled rapidly while moving downslope; Jeff Brenner for scale

a lava tube can form when a thick lava flow begins to cool from the top down, and from the bottom up; hottest lava in center of flow eventually flows out, leaving a long, underground tube behind; from Hawaiian Volcanoes National Park archive

a small lava tube, partially collapsed, with thin roof; Pisgah Volcano, Mojave Desert

CSU Long Beach students who escaped from lava tube, and one still in the process; Pisgah Volcano, Mojave Desert

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explosive volcanic eruptions - eruptive columns, pyroclastic flows and features

eruptive columns

Explosive volcanic eruptions can send a vast column of gases, lava and rock fragments high into the atmosphere.   The gases are typically water vapor, carbon dioxide and sulfur dioxide.   The lava is mainly in the form of tiny ash particles, but can also include pea-sized particles call lapilli, as well as much larger blocks and bombs ejected from the volcano much like a cannon fires a projectile.   During the eruption, fragments of the volcano itself may be caught up in the eruption and hurled forcefully from the volcano.   All of these particles ejected from a volcano are referred to as pyroclasts (pyro = fiery origin, and clasts = fragments).

small volcanic vent releasing only gas, called a fumarole; yellow is sulfur condensing onto fumarole surface; by R.L. Chritiansen, USGS

minor gas release from lava dome within Mt. St. Helens in 1982; by L. Topinka, USGS

major eruptive column of Mt. St. Helens during 1980 eruption; by Post, USGS

steam and ash eruption from Soufriere Hills, a small but dangerous stratovolcano in the Carribean region; by M. Mangum, USGS, 1997


pyroclastic flows

Pyroclastic flows, referred to by French-speaking geologists as nuee ardentes, are exceedingly dangerous phenomena that can occur during an explosive volcanic eruption.   Pyroclastic flows can develop as gravity begins pulling pyroclasts back down onto a volcano.   Hot toxic gases are caught up in the descent, and the entire mass begins moving down the flank of a volcano at speeds of 50 to 80 miles per hour.   Pyroclastic flows can also form directly from a volcanic vent as frothy gas-charged lava overflows from the vent.   Autopsies of unfortunate humans caught up in pyroclastic flows indicate that the force of the flow impact is enough to kill a person.   In addition, the high-temperature gases (up to 1000۫ F) cause massive skin burns as well as searing of lungs.   Some corpses examined after a pyroclastic flow were filleted to the bone by the sharp-edged pyroclasts within the flow.   So, you may be able to outrun a lava flow, but it is unlikely that you could escape from a fast-moving and deadly pyroclastic flow.

beginning of pyroclastic flow due to gravity-collapse of eruptive column on Mt. St. Helens; by P.W. Lipman, USGS, 1980

pyroclastic flow racing down flank of Mt. St. Helens; by P.W. Lipman, USGS, 1980

steel reinforcement bars bent by force of a pyroclastic flow in Mexico; by R.I. Tilling, USGS, 1982

pyroclastic flow deposit; by D.A. Swanson, USGS, 1980


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Explosive volcanic eruptions sends lava and fragments of the volcano itself flying through the air.   All airborne volcanic fragments, referred to as pyroclasts, eventually fall to the surface where they weld together due to their high temperature, or simply compact together to form rock. Pyroclasts come in many sizes: the smallest are called ash, slightly larger are lapilli, and the biggest are called blocks or bombs.


The most abundant and smallest pyroclasts erupted from a volcano are referred to as ash.   Near the volcano, ash deposits can form layers many feet thick, but hundreds or thousands of miles away, the ash may form only  a dusting on cars, streets and plants.   Ash particles can become imbedded in a persons lungs and cause respiratory distress.   Ash can also clog air filters of cars and trucks, reducing their efficiency or permanently damaging the motors.   If enough ash is thrown high into the atmosphere, it can block incoming solar energy, reducing the amount of sunlight reaching Earth's surface, causing rapid global cooling and severe stress on entire ecosystems.

magnified ash particle - note jagged edges and porous surface which help ash to stay airborne for days after being erupted; by A.M. Sarna-Wojcicki, USGS

hand sample of ash (thousands of individual particles); by D.E. Wieprecht, USGS

ash erupting from Soufriere Hills Volcano; by M. Mangan, USGS, 1997

ash covering much of Clark Air Force Base during eruption of Mt. Pinatubo, Philippines; by W.E. Scott, 1991

  buildings collapsed under weight of ash, Clark Air Force Base, by W.E. Scott, 1991



Lapilli are pea to pebble-sized pyroclasts.   Since they are larger than ash particles, lapilli usually fall fairly close to their volcanic source, generally within a few miles.

lapilli and ash from eruption of Mt. St. Helen; by D.E. Wieprecht, USGS

larger lapilli ejected from PisgahVolcano, Mojave Desert

large lapilli samples, bordering on being classified as blocks (see below)


blocks and bombs

The largest pyroclasts are referred to as blocks (angular) or bombs (smooth and streamlined).   These pyroclasts generally range from the size of a golf ball up to the size of a beach ball, but very powerful eruptions can hurl blocks/bombs as large as a house up to a mile from the volcanic vent!

The shapes of volcanic bombs are developed as still-molten lava flies through the air after being ejected from a volcano.   Bombs can inflict severe damage if they strike your person.

Jonathan holds a bomb found at Pisgah Volcano, Mojave Desert.   Note the stream-lined shape of this pyroclast, indicating that it was a blob of molten lave that was shaped as it flew through the air.   At the left end of the bomb is where the tail broke off, probably as the bomb crashed to Earth's surface.

Terry poses next to a very large volcanic bomb.   Imagine the force required to eject this huge pyroclast from the volcano!

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There are many types of volcanoes.   Described below are the three most common volcanoes on Earth.

shield volcano - forms as mafic lava flows from a volcanic vent.   Initially this process will form only widespread lava flows and fields, with the thickest parts of lava flows forming closet to the vent, and the thinner parts of lava flows formed farthest from the vent.   With continued eruptions for thousands of years, a gently sloping, rounded volcano will form.   Its shape is somewhat reminiscent of a Roman shield laid on its side - hence the term "shield" volcano.   Though not the tallest volcanoes on Earth, shield volcanoes are immense in their bulk, with diameters of 40 miles or more.    The Big Island, Hawaii is largely composed of two large shield volcanoes, Mauna Loa and Mauna Kea, that have grown together over a period of hundreds of thousands of years.

Mauna Loa shield volcano; by J.D. Griggs, USGS

Mauna Loa in distance, shot from Mauna Kea; by T. Casadevall, USGS, 1979


stratovolcano - also referred to as composite volcano, is a tall, steep-sided volcano that forms due to the eruption of intermediate lava.   Alternating explosive and fluid eruptions pile up pyroclasts and then cover them with lava to preserve the steep flanks of these impressive volcanoes.   Mt. St. Helens and and Mt. Pinatubo are examples of stratovolcanoes that have erupted in the recent past.

Volcan (Mt.) Arenal, Costa Rica; view from the Volcano Observatory near the base of the volcano.   Arenal erupts daily, releasing pent-up pressure in small, explosive eruptions.   Photograph by Patrick Standingbear, with Leigh Standingbear as model.

typical eruption of Volcan Arenal; begins with a loud "boom", followed by eruptive cloud, and then a small lava flow down the flank of the volcano

Tabacon Hot Springs Resort, with Volcan Arenal looming in hte background.   The hot waters of this famous resort are naturally heated by magma underground.   Notice the person standing under the hot waterfall - you can almost hear the parrots and toucans flying overhead, and the monkeys screeching in the rain forest.

Mt. Rainier, with Tacoma Washington in the foreground; L. Tapinka, USGS

Mt. Shasta, northern California; this impressive stratovolcano is over 14,000 feet tall!

Wizard Island, Crater Lake National Park, Oregon; a small stratovolcano growing within a much larger collapsed volcano in the Cascade Range of volcanoes


cinder-cone volcano - forms as pyroclasts are forcefully ejected from a volcanic vent.   The conditions required to form these small, cone-shaped volcanoes seem to be that the magma below the volcano is relatively cool an gas-charged.   Cinder-cone volcanoes have brief eruptive histories that last a few days to a few months before becoming extinct.

Hawaii cinder-cone; J.P. Lockwood, USGS, 1975

Pisgah Volcano, Mojave Desert; mining operations have removed about 1/3rd of Pisgah

Geology students arrive by bus to explore Pisgah Volcano, with its mafic pyroclasts, lava flows and tubes.


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