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Popular classification of volcanoes
A popular way of classifying magmatic volcanoes is by their frequency of eruption, with those that erupt regularly called active, those that have erupted in historical times but are now quiet called dormant or inactive, and those that have not erupted in historical times called extinct. However, these popular classifications—extinct in particular—are practically meaningless to scientists. They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes.
Active
There is no consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of eruption. Given the long lifespan of such volcanoes, they are very active. By human lifespans, however, they are not.
Scientists usually consider a volcano to be erupting or likely to erupt if it is currently erupting, or showing signs of unrest such as unusual earthquake activity or significant new gas emissions. Most scientists consider a volcano active if it has erupted in the last 10,000 years (Holocene times)—the Smithsonian Global Volcanism Program uses this definition of active. Most volcanoes are situated on the Pacific Ring of Fire. An estimated 500 million people live near active volcanoes.
Historical time (or recorded history) is another timeframe for active. The Catalogue of the Active Volcanoes of the World, published by the International Association of Volcanology, uses this definition, by which there are more than 500 active volcanoes. However, the span of recorded history differs from region to region. In China and the Mediterranean, it reaches back nearly 3,000 years, but in the Pacific Northwest of the United States and Canada, it reaches back less than 300 years, and in Hawaii and New Zealand, only around 200 years.
As of 2013, the following are considered Earth's most active volcanoes
- KÄ«lauea, the famous Hawaiian volcano, has been in continuous, effusive eruption (in which lava steadily flows onto the ground) since 1983 and has the longest-observed lava lake.
- Mount Etna and nearby Stromboli, two Mediterranean volcanoes in "almost continuous eruption
- Mount Yasur, in Vanuatu, has been erupting "nearly continuously" for over 800 years
As of August 2013, the longest ongoing (but not necessarily continuous) volcanic eruptive phases are
- Mount Yasur, 111 years
- Mount Etna, 109 years
- Stromboli, 108 years
- Santa MarÃa, 101 years
- Sangay, 94 years
Other very active volcanoes include:
- Mount Nyiragongo and its neighbor, Nyamuragira, are Africa's most active volcanoes
- Piton de la Fournaise, in Réunion, erupts frequently enough to be a tourist attraction.
- Erta Ale, in the Afar Triangle, has maintained a lava lake since at least 1906.
- Mount Erebus, in Antarctica, has maintained a lava lake since at least 1972.
- Mount Merapi
- Whakaari / White Island, has been in a continuous state of releasing volcanic gas since before European observation in 1769.
- Ol Doinyo Lengai
- Ambrym
- Arenal Volcano
- Pacaya
- Klyuchevskaya Sopka
- Sheveluch
Extinct
Extinct volcanoes are those that scientists consider unlikely to erupt again because the volcano no longer has a magma supply. Examples of extinct volcanoes are many volcanoes on the Hawaiian – Emperor seamount chain in the Pacific Ocean (although some volcanoes at the eastern end of the chain are active), Hohentwiel in Germany, Shiprock in New Mexico, Zuidwal volcano in the Netherlands and many volcanoes in Italy like Monte Vulture. Edinburgh Castle in Scotland is famously located atop an extinct volcano. Otherwise, whether a volcano is truly extinct is often difficult to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct. Some volcanologists refer to extinct volcanoes as inactive, though the term is now more commonly used for dormant volcanoes once thought to be extinct.
Dormant and reactivated
It is difficult to distinguish an extinct volcano from a dormant (inactive) one. Dormant volcanoes are those that have not erupted for thousands of years, but are likely to erupt again in the future. Volcanoes are often considered to be extinct if there are no written records of its activity. Nevertheless, volcanoes may remain dormant for a long period of time. For example, Yellowstone has a repose/recharge period of around 700,000 years, and Toba of around 380,000 years.[19] Vesuvius was described by Roman writers as having been covered with gardens and vineyards before its eruption of 79 CE, which destroyed the towns of Herculaneum and Pompeii. Before its catastrophic eruption of 1991, Pinatubo was an inconspicuous volcano, unknown to most people in the surrounding areas. Two other examples are the long-dormant Soufrière Hills volcano on the island of Montserrat, thought to be extinct before activity resumed in 1995, and Fourpeaked Mountain in Alaska, which, before its September 2006 eruption, had not erupted since before 8000 BCE and had long been thought to be extinct.
Volcanic activity
Volcanic activity
http://volcano.oregonstate.edu/book/export/html/194
http://volcano.oregonstate.edu/book/export/html/194
Volcanic activity is the most powerful force in nature. Some volcanic eruptions are much more powerful than the largest nuclear explosion. Volcanoes have killed thousands of people and have created some of the most frightening events in human history.
Volcanic activity is the most powerful force in nature. Some volcanic eruptions are much more powerful than the largest nuclear explosion. Volcanoes have killed thousands of people and have created some of the most frightening events in human history.
Volcanoes have been the basis for myths and legends the world over.
Volcanoes have been the basis for myths and legends the world over.
Volcanoes are also responsible for much of the land we live on, 90% of all the continents and ocean basins are the product of volcanism. The air we breathe, and the water we drink have been produced by millions of years of eruptions of steam and other gases.
Volcanoes are also responsible for much of the land we live on, 90% of all the continents and ocean basins are the product of volcanism. The air we breathe, and the water we drink have been produced by millions of years of eruptions of steam and other gases.
The word volcano is derived from the name of the ancient Roman island of Vulcano which lies off the southwest coast of Italy. The Romans believed that Vulcan, the god of fire and the maker of weapons, used the volcano on that island to forge his weapons.
The word volcano is derived from the name of the ancient Roman island of Vulcano which lies off the southwest coast of Italy. The Romans believed that Vulcan, the god of fire and the maker of weapons, used the volcano on that island to forge his weapons.
Volcanoes are not alive but scientists use human terms to talk about volcanoes, such as active, alive, dormant, resting, sleeping, extinct, dead, lifetime, and restless.
Volcanoes are not alive but scientists use human terms to talk about volcanoes, such as active, alive, dormant, resting, sleeping, extinct, dead, lifetime, and restless.
The term Volcano has two definitions;
The term Volcano has two definitions;
1. An opening in the crust of the Earth in which molten rock called magma and gases can escape to the surface.
1. An opening in the crust of the Earth in which molten rock called magma and gases can escape to the surface.
2. The mountain that is formed from volcanic eruptions.
2. The mountain that is formed from volcanic eruptions.
This is a photo of the volcano Paricutin (Pear-A-Koo-Teen). Paricutin's cone formed from nine years of almost constant eruptions. Red hot cinders exploded from the main vent and landed near it building the cone higher and higher. This type of cone is called a cinder cone. You will learn more about the types of volcanic cones in the eighth lesson, "Volcanic Cones and Eruptions".
Volcanoes actually build themselves into a mountain with repeated eruptions. In 1943 a farmer in Mexico noticed that some cracks (fissures) in his corn field were growing wider and wider. The next day his field was engulfed by a growing volcanic cone (Light Green). During the week the cone grew 500 feet taller (Dark Green). Within a year (Dark Gray) Paricutin was over 1200 feet higher than the surrounding landscape. During the next eight years the volcano did not grow much taller but the cone's base grew wider and wider (Light gray). Paricutin stopped erupting in 1952 almost as fast as it started. The mountain has been silent since.
Volcanoes can build themselves into high mountains one day and in the case of Mt. St. Helens erupt violently blowing their top off the next day. Mt. St. Helens lost over 1300 feet of its summit during the eruption and simultaneous landslide of 1980.
Volcanoes are classified as active, dormant, and extinct. Active volcanoes are either currently erupting or have erupted in recorded history. There are over 500 volcanoes on Earth that fit this category today. Dormant or resting volcanoes are not currently erupting but are considered likely to do so. Mt. St. Helens had been dormant for one hundred twenty-three years before it erupted in 1980. Extinct or dead volcanoes have not erupted in recorded history and are not expected to erupt again.
The photo above is of beautiful Mt. St. Helens before it erupted on May 18, 1980. Mt. St. Helens was one of the most beautifully symetrical stratovolcanoes in the world. It was called "the Fuji of the west". Mount Fuji, in Japan, is the most photographed mountain in the world. The next card will show you what this mountain looked like shortly after the eruption. The lake in the foreground changed. The lake's level is now 150 feet higher because the landslide and eruption filled the bottom of the lake with rock, soil, and pyroclasts.
This is Mt. St. Helens four months after the eruption. Notice the loss of over 1300 feet of the summit. Also notice the total devastation of the beautiful forests and how Spirit Lake rose. Spirit Lake's surface was completely filled with trees that were blasted into the lake by the force of the eruption. The lake is now much more shallow, wider, and longer than before the eruption. Huge trees still float across the lake today.
The eruption left a crater over a mile wide and over 2000 feet deep. The mountain is still active today spewing small whisps of steam. A lava dome is growing in the bottom of the huge crater.
A lava dome is a steep mass of very thick and pasty lava that is pushed up from the main vent. The lava is so viscous (thick and pasty) that it does not flow but slowly rises higher with each movement of magma in the conduit. Think of toothpaste that is slowly squeezed and then stopped and then squeezed again from the tube. This is how the lava dome in Mt. St. Helen's was formed.
The dome's exterior surface is very rough with chunks of lava that were formed from small eruptions that broke the cooled and hardened surface into blocks.
The dome slowly "grew" larger and larger over a seven year period. An earlier dome started to form one month after the famous eruption when very thick lava (dacitic lava) rose into the crater from the magma chamber below. This dome was destroyed by an explosive eruption just a month later.
The large dome that is very visible today is over 900 feet tall (taller than an 80 story building) and over 3000 feet wide (10 football fields). As large as the lava dome is, it is still dwarfed by the huge crater that was the result of the 1980 eruption. Steamy whisps of steam are still visible from the dome telling us that the volcano's magma is filling the conduit, making the volcano still active today.
There are three ways that volcanoes form. Subduction Zone volcanoes form at the boundaries of two plates, one overriding the other. Subduction zone volcanoes are the most violent and destructive of the volcanic types. Mt. St. Helens, Mt. Pinatubo, Krakatoa, and Mt. Vesuvius are all famous explosive subduction zone volcanoes. Mid-ocean rift volcanoes form where two oceanic plates are spreading apart. There are more rift zone volcanoes than any other type.
These mid-ocean or rift zone volcanoes are the world's longest continuous mountain chain. This mountain chain encircles the entire Earth. It is more than 40,000 miles long.
The third way that volcanoes form occurs at a Hot Spot. Hot spots are usually found under oceanic crust, but can be located under continental crust. You will learn more about Hot Spot volcanoes in the lesson "Hot Spots-Yellowstone and Hawaii".
The diagram above shows the three ways that volcanoes form.
Predicting exactly when a volcano will erupt is next to impossible. Today geologists are becoming much more accurate in making the public aware that a volcano is showing signs that it may erupt in the near future.
In the months before Mt. St. Helens erupted geologists knew the mountain was getting restless. A magnitude 4.1 earthquake was recorded on March 20 (about 2 months before the large eruption). Many shallow earthquakes were recorded over the next seven weeks. Magma moving higher and higher inside the mountain was causing these earthquakes. As the magma rose it formed a large bulge on the north flank. This bulge was growing daily and the geologists knew that an eruption was soon to be.
What the authorities did was evacutate most of the people in and near the mountain. Some decided to stay. Almost everyone that was near the eruption was instantly killed. In all, 57 people died. Without the evacuation perhaps as many as 30,000 deaths would have been attributed to Mt. St. Helens fury.
The geologists in the photo are measuring a growing fissure near the lava dome in Mt. St. Helens crater. As magma rises the fissure will grow wider telling the geologists that the magma is rising again.
Scientists can not stop a volcano from erupting but with constant monitoring they can warn and evacuate people and save lives.
Many volcanoes erupt in very consistant patterns, while other volcanoes have no eruption pattern at all. This makes forecasting eruptions difficult.
What makes predicting eruptions even more difficult is the fact that many volcanoes start with one type of eruption pattern and then change eruption patterns as they grow older.
Some of the most powerful eruptions in recorded time have come from volcanoes that have been dormant for hundreds and even thousands of years.
Here we have geologists studying a tilt meter. A tilt meter is used to measure the growth of the lavadome in the foreground. The tiltmeter will show a different angle as the dome grows. With careful study the geologists can tell if magma is on the rise and that an eruption may occur in the near future
Volcanic Terms
The volcanic mountain in this picture is Mayo Volcano on the island of Luzon in the Philippines. Mayon is a beautiful example of a stratovolcano.
This is a model of the interior and exterior of a stratovolcano. The letters represent important terms that you need to know to understand how volcanoes are formed and how they work.
The letter A represents a magma chamber. Magma is molten rock that is located under the surface of the Earth. A magma chamber is usually located far beneath the surface of the Earth where an oceanic plate is driven down into the mantle by a continental plate. The oceanic plate melts as it desends into the upper layer of the mantle. Some ocean water gets trapped with the oceanic plate and is turned into steam by the intense heat.
The magma is less dense and under extreme pressures that force it up toward the surface. This molten rock and gas collects in a magma chamber until it can escape to the surface.
The letter B represents a Dike. Stratovolcanoes are built by many alternating eruptions of lava and ash. The magma below and inside the mountain exerts a lot of pressure on the crust and on the volcano itself. The magma pushes its way through small cracks in the crust and finally reaches the surface. This causes a dike to be produced.
A dike is an intrusion of magma that cuts through layers of already existing rock.
The letter C represents a Side vent. When the magma reaches the surface of the Earth it is then called lava. The lava leaving the side vent causes the volcano to add a layer of lava and usually a layer of ash with each eruption. These eruptions build the volcano higher and wider. Hawaii has volcanoes with many side vents that have built the islands with very wide bases. Some volcanoes on the other hand have few or no side vents. The materials that makes up the magma (gases, minerals, steam) determines how the magma will arrive at the surface. You will learn more about magma and lava in the next lesson "Lava Flows and Pyroclasts".
The letter D represents a conduit. A conduit is the main tube or pathway for the magma to reach the surface.
Devils Tower in Wyoming is an example of a cooled and hardened conduit.
This is a photo of Devils Tower National Monument. Devils Tower in Wyoming is an ancient conduit. The source for the magma moved and the magma in the conduit cooled and hardened into a very hard lava rock called basalt. The volcanic cone was made of softer volcanic materials probably ash and pumice that slowly eroded away leaving only the conduit standing. Today we know this ancient conduit as Devils Tower National Monument
The letter E represents the crater and main vent of a volcano. The crater is the bowl shaped opening located at the top of the volcano. The crater is also the steep sided walls made of hardened lava that surround the main vent. Lava can flow from the main vent, but not all volcanoes eject large amounts of lava. Some volcanoes explode molten rock and huge amounts of gas from the main vent.
Volcanoes are not always erupting and the crater may be a bubbling caldron of lava without enough pressure to erupt.
This photograph is of a volcanic cone. The crater is located at the top. The side vent is active and a lava flow is running down the side of the cone. A fissure is bringing the magma to the side vent. This photo is courtesy of Dr. Scott Rowland of the University of Hawaii.
You are looking at the inside of a volcanic crater. The steep walls were produced be many eruptions ejecting very liquid lava. This lava then lands on the crater walls building them higher and higher. The lava in the main vent is extremely hot (probably about 1800 degrees F.) The lava on top cools and hardens because the air that it is in contact with is so much cooler than the lava. This hardened lava will then be dragged back down under the surface and remelted. You probably noticed the same process if you have ever heated soup on the stove. If you did not keep stirring the soup it formed a "scum" on top.
The letter F represents layers of tuff and lava. When a volcano erupts it may eject lava, lava rock and ash. When stratovolcanoes are built some of the lava and ash lands and stays on the volcano building it higher and higher with each eruption. The ash hardens into a rock that is called tuff.
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