Introduction to Igneous Rocks Intrusive Igneous Rocks Igneous Intrusions/Plutons
Volcanic Igneous Rocks Igneous Rocks Quiz Credits
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
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
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
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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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!
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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