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9.8: Volcanoes - Geosciences


WHERE VOLCANOES ARE LOCATED

Volcanoes are a vibrant manifestation of plate tectonics processes. Wherever mantle is able to melt, volcanoes may be the result.


Figure 1. World map of active volcanoes.

See if you can give a geological explanation for the locations of all the volcanoes in figure 1. What is the Pacific Ring of Fire? Why are the Hawaiian volcanoes located away from any plate boundaries? What is the cause of the volcanoes along the mid-Atlantic ridge?

Volcanoes erupt because mantle rock melts. This is the first stage in creating a volcano. Remember from the chapter “Rocks” that mantle may melt if temperature rises, pressure lowers, or water is added. Be sure to think about how melting occurs in each of the following volcanic settings.

Convergent Plate Boundaries

Why does melting occur at convergent plate boundaries? The subducting plate heats up as it sinks into the mantle. Also, water is mixed in with the sediments lying on top of the subducting plate. This water lowers the melting point of the mantle material, which increases melting. Volcanoes at convergent plate boundaries are found all along the Pacific Ocean basin, primarily at the edges of the Pacific, Cocos, and Nazca plates. Trenches mark subduction zones, although only the Aleutian Trench and the Java Trench appear on the map in figure 1.

Remember your plate tectonics knowledge. Large earthquakes are extremely common along convergent plate boundaries. Since the Pacific Ocean is rimmed by convergent and transform boundaries, about 80% of all earthquakes strike around the Pacific Ocean basin (the ring of fire). Why are 75% of the world’s volcanoes found around the Pacific basin? Of course, these volcanoes are caused by the abundance of convergent plate boundaries around the Pacific.

The Pacific Ring of Fire is where the majority of the volcanic activity on the Earth occurs. A description of the Pacific Ring of Fire along western North America is a description of the plate boundaries.

  • Subduction at the Middle American Trench creates volcanoes in Central America.
  • The San Andreas Fault is a transform boundary.
  • Subduction of the Juan de Fuca plate beneath the North American plate creates the Cascade volcanoes.
  • Subduction of the Pacific plate beneath the North American plate in the north creates the Aleutian Islands volcanoes.

The Cascades are shown on this interactive map with photos and descriptions of each of the volcanoes.

This incredible explosive eruption of Mount Vesuvius in Italy in A.D. 79 is an example of a composite volcano that forms as the result of a convergent plate boundary:

Divergent Plate Boundaries

Why does melting occur at divergent plate boundaries? Hot mantle rock rises where the plates are moving apart. This releases pressure on the mantle, which lowers its melting temperature. Lava erupts through long cracks in the ground, or fissures.


Figure 2. A volcanic eruption at Surtsey, a small island near Iceland.

Volcanoes erupt at mid-ocean ridges, such as the Mid-Atlantic ridge, where seafloor spreading creates new seafloor in the rift valleys. Where a hotspot is located along the ridge, such as at Iceland, volcanoes grow high enough to create islands (figure 2).

Eruptions are found at divergent plate boundaries as continents break apart. The volcanoes in figure 3 are in the East African Rift between the African and Arabian plates.


Figure 3. Mount Gahinga, a mountain in Uganda, located in the East African Rift valley.

Volcanic Hotspots

Although most volcanoes are found at convergent or divergent plate boundaries, intraplate volcanoes are found in the middle of a tectonic plate. Why is there melting at these locations? The Hawaiian Islands are the exposed peaks of a great chain of volcanoes that lie on the Pacific plate. These islands are in the middle of the Pacific plate. The youngest island sits directly above a column of hot rock called a mantle plume. As the plume rises through the mantle, pressure is released and mantle melts to create a hotspot (figure 4).


Figure 4. (a) The Society Islands formed above a hotspot that is now beneath Mehetia and two submarine volcanoes. (b) The satellite image shows how the islands become smaller and coral reefs became more developed as the volcanoes move off the hotspot and grow older.

Earth is home to about 50 known hot spots. Most of these are in the oceans because they are better able to penetrate oceanic lithosphere to create volcanoes. The hotspots that are known beneath continents are extremely large, such as Yellowstone (figure 5).


Figure 5. Prominent hotspots of the world.

A hot spot beneath Hawaii, the origin of the voluminous lava produced by the shield volcano Kilauea can be viewed here:

How would you be able to tell hotspot volcanoes from island arc volcanoes? At island arcs, the volcanoes are all about the same age. By contrast, at hotspots the volcanoes are youngest at one end of the chain and oldest at the other.

VOLCANIC LANDFORMS AND GEOTHERMAL ACTIVITY

Volcanoes are associated with many types of landforms. The landforms vary with the composition of the magma that created them. Hot springs and geysers are also examples of surface features related to volcanic activity.

Landforms From Lava

Volcanoes and Vents

The most obvious landforms created by lava are volcanoes, most commonly as cinder cones, composite volcanoes, and shield volcanoes. Eruptions also take place through fissures (Figure 6). The eruptions that created the entire ocean floor are essentially fissure eruptions.


Figure 6. A fissure eruption on Mauna Loa in Hawaii travels toward Mauna Kea on the Big Island.

Lava Domes

When lava is viscous, it is flows slowly. If there is not enough magma or enough pressure to create an explosive eruption, the magma may form a lava dome. Because it is so thick, the lava does not flow far from the vent (figure 7).


Figure 7. Lava domes are large, round landforms created by thick lava that does not travel far from the vent.

Lava flows often make mounds right in the middle of craters at the top of volcanoes, as seen in figure 8.


Figure 8. Lava domes may form in the crater of composite volcanoes as at Mount St. Helens

Lava Plateaus

A lava plateau forms when large amounts of fluid lava flows over an extensive area (figure 9). When the lava solidifies, it creates a large, flat surface of igneous rock.


Figure 9. Layer upon layer of basalt have created the Columbia Plateau, which covers more than 161,000 square kilometers (63,000 square miles) in Washington, Oregon, and Idaho.

Land

Lava creates new land as it solidifies on the coast or emerges from beneath the water (figure 10).


Figure 10. Lava hitting seawater creates new land.

Over time the eruptions can create whole islands. The Hawaiian Islands are formed from shield volcano eruptions that have grown over the last 5 million years (figure 11).


Figure 11. A compilation of satellite images of the Big Island of Hawaii with its five volcanoes.

Landforms From Magma

Magma intrusions can create landforms. Shiprock in New Mexico is the neck of an old volcano that has eroded away (figure 12).


Figure 12. The aptly named Shiprock in New Mexico.

Hot Springs and Geysers

Water sometimes comes into contact with hot rock. The water may emerge at the surface as either a hot spring or a geyser.

Hot Springs

Water heated below ground that rises through a crack to the surface creates a hot spring (figure 13). The water in hot springs may reach temperatures in the hundreds of degrees Celsius beneath the surface, although most hot springs are much cooler.


Figure 13. Even some animals enjoy relaxing in nature’s hot tubs.

Geysers

Geysers are also created by water that is heated beneath the Earth’s surface, but geysers do not bubble to the surface — they erupt. When water is both superheated by magma and flows through a narrow passageway underground, the environment is ideal for a geyser. The passageway traps the heated water underground, so that heat and pressure can build. Eventually, the pressure grows so great that the superheated water bursts out onto the surface to create a geyser (figure 14).


Figure 14. Castle Geyser is one of the many geysers at Yellowstone National Park. Castle erupts regularly, but not as frequently or predictably as Old Faithful.

Conditions are right for the formation of geysers in only a few places on Earth. Of the roughly 1,000 geysers worldwide and about half are found in the United States.

LESSON SUMMARY

  • Most volcanoes are found along convergent or divergent plate boundaries.
  • The Pacific Ring of Fire is the most geologically active region in the world.
  • Volcanoes such as those that form the islands of Hawaii form over hotspots, which are melting zones above mantle plumes.
  • Viscous lava can produce lava domes along a fissure or within a volcano.
  • Lava plateaus form from large lava flows that spread out over large areas.
  • Many islands are built by or are volcanoes.
  • Igneous intrusions associated with volcanoes may create volcanic landforms.
  • When magma heats groundwater, it can reach the surface as hot springs or geysers.

REFLECTION QUESTIONS

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Over 75% of the volcanic activity on Earth occurs underwater and recent heightened unrest from many submarine volcanoes has raised serious concerns regarding the level of risk posed to local communities. The overall goal of this Special Issue of Geosciences is to evaluate the potential of combining innovative and emerging technologies to enable breakthrough developments in understanding the impact of disastrous submarine volcanic hazards on society. Specifically, this Special Issue aims to provide an outlet for rapid, widely-accessible publication of peer-reviewed studies promoting an integrated approach to underpin new concepts (e.g., for risk monitoring protocols or civil hazard planning), next-generation commercial products (e.g., for in situ sensors or imaging instrumentation), and innovative services (e.g., for education/training or early-warning systems for society) for submarine volcanoes.

This Special Issue aims to cover, without being limited to, the following areas:

Identification of submarine volcanic hazards such as: Volcanic eruptions, volcanic earthquakes, submarine landslides, hydrothermal emissions and volcanogenic tsunamis.

Exploration of optimal monitoring technologies and state-of-the-art methods, providing new insights for further exploration and potential exploitation of submarine volcanoes, which are hosting significant hydrothermal deposits, minerals and fauna.

Volcanic crisis management, general public awareness and preparedness, for a better understanding of the hazards and impacts of submarine volcanoes.

Prof. Paraskevi V. Nomikou
Guest Editor

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Mount Vesuvius

Italy’s Vesuvius has been a menacing figure since an eruption in 79 CE buried the city of Pompeii. Over the last 17,000 years, the volcano has gone through eight major explosive eruptions that were followed by large pyroclastic flows, according to the Smithsonian Institute/USGS Global Volcanic Program database. Vesuvius’ last known eruption occurred in 1944. The Italian government has multiple plans prepared for a possible eruption in the future. At least six million people live in the vicinity of Vesuvius, according to the database.


Abstract

The isotope composition of He and its relationship to CO2 have been investigated in gases sampled on 12 active volcanoes of Japan. The highest 3 He 4 He ratios reach the lower limit of the mid-ocean ridge basalt (MORB) range and have been recorded at Kusatsu-Shirane Volcano, which presented a phreatic activity during the sampling period, and at Satsuma-Iojima Volcano, where the highest temperature (786°C) was recorded. 3 He 4 He variations (3.1·10 −6 –9.8·10 −6 ) suggest that, although 3 He comes from the upper mantle, part of 4 He is released from the crust in the volcano area. Time elapsed after volcanic events and distance from volcanics centers appear the major factors controlling these variations.

CO 2 3 He molar ratios (4.5·10 9 –29·10 9 ) are systematically higher than the MORB average of 2·10 9 . The apparent excess of CO2 (relative to 3 He) cannot be entirely due to physical or chemical fractionation during the transfer of gases from the magmatic source to the surface. It is attributed to the effect of crustal outgassing or the contribution of a few percent of subducted sediments to the source of arc magmas. The last figure is in agreement with other geochemical tracers (Nd, Sr and pb isotopes, rare earth elements, 10 Be, etc.). The estimated CO2 flux at arcs is in the order of (1–5)·10 11 mol yr. −1 and represents <20% of the carbon flux at ridges. A mass balance of carbon at arcs shows that most sedimentary carbonates must be accreted rather than subducted. The flux of 3 He at arc volcanoes represents <13% of the total volcanic 3 He flux.


The 11 Biggest Volcanic Eruptions in History

History has seen some monstrous eruptions of volcanoes, from Mount Pinatubo's weather-cooling burp to the explosion of Mt. Tambora, one of the tallest peaks in the Indonesian archipelago.

The power of such eruptions is measured using the Volcanic Explosivity Index (VEI) a classification system developed in the 1980 that's somewhat akin to the magnitude scale for earthquakes. The scale goes from 1 to 8, and each succeeding VEI is 10 times greater than the last.

There haven't been any VEI-8 volcanoes in the last 10,000 years, but human history has seen some powerful and devastating eruptions. Because it's extremely difficult for scientists to be able to rank the strength of eruptions in the same VEI category, here we present the 10 most powerful volcanoes within the last 4,000 years (within human records) first in order of strength, then within each category, in chronological order.

But let's start with a supervolcano eruption surprisingly close to home, registering a magnitude-8, from our distant past.

1. Yellowstone eruption, 640,000 years ago (VEI 8)

The entire Yellowstone National Park is an active volcano rumbling beneath visitors' feet. And it has erupted with magnificent strength: Three magnitude-8 eruptions rocked the area as far back as 2.1 million years ago, again 1.2 million years ago and most recently 640,000 years ago. "Together, the three catastrophic eruptions expelled enough ash and lava to fill the Grand Canyon," according to the U.S. Geological Survey. In fact, scientists discovered a humongous blob of magma stored beneath Yellowstone, a blob that if released could fill the Grand Canyon 11 times over, the researchers reported on April 23, 2013, in the journal Science.

The latest of the trio of supervolcano eruptions created the park's huge crater, measuring 30 by 45 miles across (48 by 72 kilometers).

The chance of such a supervolcano eruption happening today is about one in 700,000 every year, Robert Smith, a seismologist at the University of Utah in Salt Lake City, told Live Science previously.

2. Huaynaputina, 1600 (VEI 6)

This peak was the site of South America's largest volcanic eruption in recorded history. The explosion sent mudflows as far as the Pacific Ocean, 75 miles (120 km) away, and appears to have affected the global climate. The summers following the 1600 eruption were some of the coldest in 500 years. Ash from the explosion buried a 20-square-mile (50-square-km) area to the mountain's west, which remains blanketed to this day.

Although Huaynaputina, in Peru, is a lofty 16,000 feet (4,850 meters), it's somewhat sneaky as volcanoes go. It stands along the edge of a deep canyon, and its peak doesn't have the dramatic silhouette often associated with volcanoes.

The 1600 cataclysm damaged the nearby cities of Arequipa and Moquengua, which only fully recovered more than a century later.

3. Krakatoa, 1883 (VEI 6)

The rumblings that preceded the final eruption of Krakatoa (also spelled Krakatau) in the weeks and months of the summer of 1883 finally climaxed with a massive explosion on April 26-27. The explosive eruption of this stratovolcano, situated along a volcanic island arc at the subduction zone of the Indo-Australian plate, ejected huge amounts of rock, ash and pumice and was heard thousands of miles away.

The explosion also created a tsunami, whose maximum wave heights reached 140 feet (40 meters) and killed about 34,000 people. Tidal gauges more than 7,000 miles (11,000 km) away on the Arabian Peninsula even registered the increase in wave heights.

While the island that once hosted Krakatoa was completely destroyed in the eruption, new eruptions beginning in December 1927 built the Anak Krakatau ("Child of Krakatau") cone in the center of the caldera produced by the 1883 eruption. Anak Krakatau sporadically comes to life, building a new island in the shadow of its parent.

4. Santa Maria Volcano, 1902 (VEI 6)

The Santa Maria eruption in 1902 was one of the largest eruptions of the 20th century. The violent explosion in Guatemala came after the volcano had remained silent for roughly 500 years, and left a large crater, nearly a mile (1.5 km) across, on the mountain's southwest flank.

The symmetrical, tree-covered volcano is part of a chain of stratovolcanoes that rises along Guatemala's Pacific coastal plain. It has experienced continuous activity since its last blast, a VEI 3, which occurred in 1922. In 1929, Santa Maria spewed forth a a pyroclastic flow (a fast-moving wall of scalding gas and pulverized rock), which claimed hundreds of lives and may have killed as many as 5,000 people.

5. Novarupta, 1912 (VEI 6)

The eruption of Novarupta one of a chain of volcanoes on the Alaska Peninsula, part of the Pacific Ring of Fire, was the largest volcanic blast of the 20th century. The powerful eruption sent 3 cubic miles (12.5 cubic km) of magma and ash into the air, which fell to cover an area of 3,000 square miles (7,800 square km) in ash more than a foot deep.

6. Mount Pinatubo, 1991 (VEI 6)

A stratovolcano located in a chain of volcanoes in Luzon, Philippines, created along a subduction zone, the cataclysmic eruption of Pinatubo was a classic explosive eruption.

The eruption ejected more than 1 cubic mile (5 cubic kilometers) of material into the air and created a column of ash that rose up 22 miles (35 km) in the atmosphere. Ash fell across the countryside, even piling up so much that some roofs collapsed under the weight.

The blast also spewed millions of tons of sulfur dioxide and other particles into the air, which were spread around the world by air currents and caused global temperatures to drop by about 1 degree Fahrenheit (0.5 degree Celsius) over the course of the following year.

7. Ambrym Island, 50 AD (VEI 6 +)

The 257-square-mile (665-square-km) volcanic island, part of the Republic of Vanuatu, a tiny nation in the southwestern Pacific Ocean, witnessed one of the most impressive eruptions in history, one that sent a wave of scalding ash and dust down the mountain and formed a caldera 7.5 miles (12 km) wide.

The volcano has continued to be one of the most active in the world. It has erupted close to 50 times since 1774, and has proved a dangerous neighbor for the local population. In 1894, six people were killed by volcanic bombs and four people were overtaken by lava flows, and in 1979, acid rainfall caused by the volcano burned some inhabitants.

8. Ilopango Volcano, 450 AD (VEI 6 +)

Although this mountain in central El Salvador, just a few miles east of the capital city San Salvador, has experienced only two eruptions in its history, the first known eruption was a doozy. It blanketed much of central and western El Salvador with pumice and ash, and destroyed early Mayan cities, forcing inhabitants to flee.

Trade routes were disrupted, and the centers of Mayan civilization shifted from the highland areas of El Salvador to lowland areas to the north and in Guatemala.

The summit's caldera is now home to one of El Salvador's largest lakes.

9. Mount Thera, approx. 1610 B.C. (VEI 7)

Geologists think that the Aegean Islands volcano Thera exploded with the energy of several hundred atomic bombs in a fraction of a second. Though there are no written records of the eruption, geologists think it could be the strongest explosion ever witnessed.

The island that hosted the volcano, Santorini (part of an archipelago of volcanic islands in Greece), had been home to members of the Minoan civilization, though there are some indications that the inhabitants of the island suspected the volcano was going to blow its top and evacuated. But though those residents might have escaped, there is cause to speculate that the volcano severely disrupted the culture, with tsunamis and temperature declines caused by the massive amounts of sulfur dioxide it spewed into the atmosphere that altered the climate.

10. Changbaishan Volcano, 1000 AD (VEI 7)

Also known as the Baitoushan Volcano, the eruption spewed volcanic material as far away as northern Japan, a distance of approximately 750 miles (1,200 kilometers). The eruption also created a large caldera nearly 3 miles (4.5 km) across and a half-mile (nearly 1 km) deep at the mountain's summit. It is now filled with the waters of Lake Tianchi, or Sky Lake, a popular tourist destination both for its natural beauty and alleged sightings of unidentified creatures living in its depths.

Located on the border of China and North Korea, the mountain last erupted in 1702, and geologists consider it to be dormant. Gas emissions were reported from the summit and nearby hot springs in 1994, but no evidence of renewed activity of the volcano was observed.

11. Mt. Tambora, Sumbawa Island, Indonesia - 1815 VEI 7

The explosion of Mount Tambora is the largest ever recorded by humans, ranking a 7 (or "super-colossal") on the Volcanic Explosivity Index, the second-highest rating in the index. The volcano, which is still active, is one of the tallest peaks in the Indonesian archipelago.

The eruption reached its peak in April 1815, when it exploded so loudly that it was heard on Sumatra Island, more than 1,200 miles (1,930 km) away. The death toll from the eruption was estimated at 71,000 people, and clouds of heavy ash descended on many far-away islands.


What Causes This Volcanic Activity?

There is a hot spot beneath Yellowstone. A hot spot is a persistent plume of hot material rising through Earth's mantle. This rising plume delivers heat to the area, causes forces in the crust that produce earthquakes and rarely produces a volcanic eruption. A hotspot is also responsible for the volcanic eruptions of Hawaii.

Jake Lowenstern traces some of the volcanic history of the Yellowstone area, explains recent earthquake swarms and comments on future eruptive activity.

Yellowstone geysers: Hot rock down below is what drives the geysers of Yellowstone National Park. Rain water infiltrates into the ground and enters a groundwater circulation system. Some of this water circulates deeply, is superheated and then blasted out of a geyser. Image by the National Park Service.


Contents

El Toro belongs to a formation of monogenetic volcanism, which in this part of the Central Volcanic Zone of the Andes is less well studied than the large volume dacitic eruptions that took place in this area. All these volcanic phenomena are linked to the subduction of the Nazca plate beneath the South America plate. Major north-south lineaments are also found in the area as well as other minor lineaments which may play a role in the magma supply.

The basement in the El Toro area consists of an Ordovician sedimentary layer with Paleogene-Miocene arenites. Likewise, Cretaceous sedimentary layers are found in the area. The Vizcachera Formation which underpins the volcanic field is of Eocene-Miocene age, while the volcanoclastic and sedimentary Filo Blanco sequence was formed 10.8–8.8 mya ago. The latter is of fluvial origin and also contains an ignimbrite layer.

There is a noticeable difference between the northern and southern Puna in terms of the existence of mafic volcanism, which is much more widespread in the southern Puna. Possibly, during the Miocene and Pliocene a molten layer existed within the 50–70 kilometres (31–43 mi) thick crust of the northern Puna. This layer would constitute a density barrier to the rise of mafic magma.

All these cones were breached by lava flows, which in most cones extend east and southeast of the cone. Toro 1, which also appears to be cut by a small WNW-trending structure, was breached to the west instead. Lapilli, lava bombs and scoria are also found at the cones. Large lava bombs, including some with tabular shapes, are found on Toro 1 and Toro 2. Both of the latter cones also feature dykes. The Campo Negro cone is dominated by the surrounding lava flows.

Petrology and facies Edit

There are three different rock facies in the El Toro volcanic field. The first encompasses a loose rock-breccia association with lapilli. It consists mostly of medium-sized fragments (1–6 centimetres (0.39–2.36 in)) and they form roughly stratified deposits. Average diameter of fragments decreases with increasing distance from the volcanoes. There are two types of lava bombs in the field, a more common type with flow shapes and deformation resulting from ground impact, and a type on Toro 1 and Toro 2 which are denser and more rounded. These cones also have some tabular rocks. Lava bombs may have formed from lava lakes in the cones or from colder remnant magma.

The second facies are partially welded pyroclastic rocks with fiammes which form deposits 3–6 metres (9.8–19.7 ft) thick. There are vertical variations in the grade of welding, some are only barely distinguishable from a lava flow except by the presence of clear edges around some clasts. Fiammes have brighter colours than the underlying rock matrix. Scoria layers at Toro 1 have thicknesses of 17–30 metres (56–98 ft). A third facies is also present at Toro 1 and consists of tuffs with lapilli. These layers have red-orange-brown colours and contain some palagonite. These tuffs lie directly on the basement rocks. They appear to be of phreatomagmatic origin, or by the interaction between eruption processes and a river.

Petrologically, the El Toro field rocks belong to potassium-rich andesite-basaltic andesite-basalt. Three types of rocks are distinguished by texture and mineral and phenocryst content. Other volcanoes in the area with similar petrologies include Cerros Negros de Jama and Cerro Morado, and similar to other monogenetic volcanoes in the Puna. Differences between the rocks in the various cones indicate origin of the magma at varying depths and temperatures.

The white matrix found between lapilli contains alkalifeldspath, hematite, hisingerite, montmorillonite, saponite and silica. It was probably formed by weathering of mafic minerals.

Dating Edit

The eruption products of the Toro 1 and Toro 2 cones are partially buried by an ignimbrite named the Cerro Morado Ignimbrite. This ignimbrite is rhyodacitic in composition and contains abundant crystals. Another ignimbrite layer, possibly derived from the Convento/Coyahuaima volcano, overlies the Campo Negro cone rocks. Stratigraphic relations with older ignimbrites and younger lavas indicate an age between 6.45±0.15 mya and 2.03±0.07 mya.


Ranking the 10 Most Dangerous Volcanoes, From Vesuvius to Santa Maria

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The Santiaguito volcano erupting. Martin Rietze/Getty Images

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Time to count down some dangerous volcanoes. I've gone through what might make a volcano dangerous and how I tried to rank dangerous volcanoes, developing a points system based on population, magma type, volcano type, and past large explosive eruptions. Looking at some recent articles about "dangerous volcanoes," my ranking comes to some pretty different conclusions. What my ranking boils down to is what volcano has the highest potential for mass casualties based population, style of eruption and potential for large explosive events.

I'll start with some honorable mentions that fell outside the top 10 (in order of increasing danger): Pululagua (Ecuador), Guntur, Gede-Pangrango and Semeru (Indonesia), Popocatépetl (Mexico), Colli Alban (Italy), Dieng Volcanic Complex and Tengger Caldera (Indonesia), Nyiragongo (DR Congo), and Merapi (Indonesia).

Here are the top 10 (with people living within 30 kilometers and 100 kilometers listed.)

10. Santa Maria, Guatemala (1.25 million/6.2 million): This volcano might be best known for its most active vent, Santiaguito. It has the tendency to erupt explosively with VEI 6 eruption as recently as 1902.

9. Taal, Philippines (2.38 million/24.8 million): Taal is a lake-filled caldera that produced four VEI 4 eruptions in the last 200 years and a VEI 6 eruption only

5,500 years ago (VEI stands for Volcanic Explosivity Index, and it tops out at 7). Combine that explosivity with abundant water to add to potential explosive eruptions and the large population that could be impacted by ash, and you have a very closely-watched volcano. Taal is monitored by PHIVOLCS, the Philippine volcano monitoring agency.

8. Coatepeque Caldera, El Salvador (1.2 million/6.5 million): Coatepeque is the first "dark horse" in the top 10. It gains points for erupting rhyolite and dacite, both magmas prone to large, explosive eruptions. It is also centrally-located in El Salvador, so a large eruption would likely impact the capital of San Salvador along with the city of Santa Ana. Like Taal, it is a lake-filled caldera, adding to its potential danger by potentially increasing explosivity or mudflows (lahars).

7. Corbetti Caldera, Ethiopia (1.2 million/9.8 million): Now, this is a real under-the-radar volcano. The Corbetti caldera lies within an even older caldera and has produced pyroclastic cones (explosive eruptions of lots of volcanic debris) and obsidian flows, meaning it has the right style of eruption and right composition to potentially experience a big explosive eruption. Not much is known about the Corbetti Caldera, so it is hard to constrain its recent activity. However, it is close enough to Addis Ababa that a large ash-rich eruption might cause quite a humanitarian crisis.

6. Tatun Group, Taiwan (6.7 million/9.8 million): Much like the Corbetti Caldera, Tatun is not a well-known volcano in a country most people don't associate with volcanism. However, as I wrote about recently, the Tatun Group has all the signs of a volcano that is still potentially active. It is also nestled close to Taipei, so you could imagine an eruption that produced another andesite dome could wreak havoc on the city, mainly from ash fall or mudflows.

5. Vesuvius, Italy (3.9 million/6.0 million): Did you really think Vesuvius wouldn't be in the top five? The volcano is one of the most dangerous on Earth thanks to its numerous explosive eruptions---and the city of Naples, which is slowly crawling up its flanks. The fact that it doesn't fall at the top of this list (heck, it's not even the most dangerous in Italy) betrays how hazardous the other volcanoes might be. Vesuvius has been quiet since 1944, so we're full into the "complacency" phase where most people don't remember the last eruption---never a good place to be when 6 million people could be impacted by an explosive eruption.

4. Ilopango, El Salvador (2.9 million/6.7 million): This is another caldera in El Salvador. But unlike Coatepeque, it has erupted in the last 200 years (1880 to be exact). Around 450 CE, Ilopango had a VEI 6 eruption that covered much of El Salvador with ash and brought down Mayan cities across the region. Today, San Salvador sits directly next to the this lake-filled caldera, so the significant danger from this caldera remains after 1,500 years.

3. Aira Caldera, Japan (0.9 million/2.6 million): The population around the Aira caldera might be lower than most of the top 10 volcanoes, but its frequent eruptions (from Sakurajima) and history of large eruptions means it poses a large danger to those 2.6 million people within 100 kilometers. Over the past 10,000 years of the Holocene, the Aira caldera has had a half dozen VEI 4, 5, and 6 eruptions---so don't be fooled by the constant din of smaller explosions from Sakurajima over the last decade.

2. Michoacan-Guanajuato, Mexico (5.8 million/5.8 million): Here's the thing about the Michoacan-Guanajuato (M-G) volcanic field: All three population radius values are the same: 5.8 million. Yes, almost 6 million people live within 5 kilometers of this volcanic field that has produced pyroclastic cones generated by explosive eruptions. It has produced numerous VEI 3 and 4 eruptions over the Holocene from 1,400 vents. This means it hasn't had big eruptions like some of the top 10. But the frequency, potential explosivity, and population in the widespread volcanic area makes it a high risk.

1. Campi Flegrei, Italy (3.0 million/6.0 million): If you're the sort of person who wants to worry about Yellowstone, maybe you should turn your attention to the Campi Flegrei instead. Not only is it a restless caldera with a more recent history of very large explosive eruptions, it is also smack in the middle of an area with over 6 million people . and it's partially under the Bay of Naples. All these factors mean that if the Campi Flegrei has a new bout of explosive eruptions, the hazards could exceed those of any eruption in modern history. That being said, the last eruption (Monte Nuovo in 1538) was actually a fairly-benign cinder cone.

Now, this ranking is highly subjective. There can be a multitude of ways to measure danger, so I'm sure people will disagree with this list. Volcanoes like Etna, Cotopaxi, Ruiz, Fuego, and more didn't make the top 20---mostly because I chose to put emphasis on the style and composition of magmatism.

The big important point here: This list highlights the most potentially "dangerous" volcanoes based on their past behavior (mainly) and the potentially for mass casualties. There will always be volcanoes that might only have a few thousand people living near it that could erupt and kills hundreds of people.

I think of eruptions like El Chichón in 1982 as a great example. The volcano wasn't even really recognized as a threat until it erupted and killed around 1,900 people. It is unlikely that we can eliminate all volcanic threat, and as the global population grows, the danger posed by volcanoes we can identify as hazardous---and those we don't recognize as hazardous---will only increase. Funding volcanic research and monitoring along with emergency management organizations is the only way we can hope to protect ourselves from major volcanic disasters.


9.8: Volcanoes - Geosciences

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2. Walking and study tours

You don't have to be a volcano freak to travel with us! We specialize in volcanic areas, yes, but that's also because volcanic areas (southern Italy, for instance) form some of the most beautiful and interesting environments on the planet. Each tour is accompanied by an expert in the area who personally guides you and takes care of all the details. Have a look at our range of walking and study tours, ranging from adventurous to deluxe, with a general focus on geology, volcanism, nature and culture.

3. Photo Tours
3. Private and custom tours

For individuals and groups, we can tailor custom-designed tours for your group or corporate incentive. Have a look at our existing tours and let us know your preferences.

4. Pilot tours

Exploring the unknown: we sometimes go and explore new destinations and invite a small number of people to come with us and try it out! Such trips are a reduced cost and require a higher degree of flexibility, but often offer a great deal of adventure.


9.8: Volcanoes - Geosciences

by Jessica Ball Friday, January 20, 2012

Quite a lot of volcanic activity has been reaching the news lately &mdash possible eruption in the Congo and another Sumatran volcano called Seulawah Agam that may be &ldquowaking up&rdquo after 170 years of quiet. One thing that&rsquos been mentioned in many of the news reports about these events is that the eruptions were &ldquounexpected&rdquo or &ldquosurprising&rdquo because people had always assumed the volcanoes in question were dormant. But what does dormant really mean? And what does active or extinct mean?

The answer to this question varies significantly depending on whom you ask: There are no set definitions for active, dormant and extinct volcanoes.

Generally speaking, you could say that an active volcano is one that is currently erupting, or has erupted in the last 10,000 years. Kilauea, Mount St. Helens are all considered active, as are volcanoes such as Redoubt. A dormant volcano is one that is &ldquosleeping&rdquo but could awaken in the future, such as Mount Rainier and Kohala on the Big Island of Hawaii could be considered an extinct volcano.

Extinct is probably the most exacting term in describing a volcano&rsquos eruptive potential it&rsquos a term that is very cautiously used, given that there is almost always potential for a currently inactive volcano to begin erupting again. Extinct implies that there is no magmatic, seismic or degassing activity going on at the volcano, and that it&rsquos never expected to have any in the future. That&rsquos very difficult to prove, and requires field mapping to determine the volcano&rsquos eruptive history. If it can be shown that the volcano has not erupted for periods of time much longer than its past eruptive recurrence intervals, it&rsquos probably safe to call it extinct. For example, if a volcano&rsquos eruptive history shows that it usually erupts every 10,000 years or so, and there hasn&rsquot been an eruption for a million years, it may be called extinct.

The terms dormant and active are much more arbitrary. A nonscientist who lives near a volcano might assume the volcano is dormant &mdash or may not even know it&rsquos a volcano &mdash because there have been no eruptions in living memory. (&ldquoLiving memory&rdquo means that the oldest people in a community have seen the last activity at a volcano.) Historical records may add several hundred years to a community&rsquos memory of eruptions, but that depends on people having knowledge of those records. Even if eruptions are known to have occurred in the past few centuries, a volcano may still not be considered active by the general public unless it is visibly erupting.

A volcanologist, however, has very different ideas of what constitutes a dormant or active volcano &mdash but even volcanologists don&rsquot agree on the exact definitions. Some volcanologists consider a volcano active if it has erupted within the last 10,000 years they would consider it dormant if it had erupted more than 10,000 years ago, but still has access to a magma source that could fuel future eruptions. (Sometime more than 10,000 years before present marks the beginning of the Holocene epoch, which is used as an arbitrary cutoff because it marks roughly the point in time when the last ice age ended worldwide). Other volcanologists call a volcano active only if it has erupted in &ldquohistorical time&rdquo and is considered likely to do so in the near future.

But what does historical time even mean? Whose history are they referring to? Do they count myths and legends as historical records, or do they have to be written? What about areas of the world that have no written records, but may still have oral histories going back thousands of years? &ldquoHistorical time&rdquo varies widely depending on what region of the world you&rsquore looking at and what are accepted as accurate historical records, and like the Holocene cutoff, it&rsquos an arbitrary way of describing a time period. If you see &ldquohistorical&rdquo being used to describe an eruption, it&rsquos always wise to check up on the record-keeping in that part of the world.

The lack of an overarching consensus on the meaning of these terms is a sticky spot in volcanology, and a source of confusion when it comes to dealing with nonscientists. Volcanologists may be hesitant to declare a volcano dormant for scientific purposes, but they must consider short-term concerns: Which volcanoes most urgently need to be studied and monitored? Will nonscientists support the work if they consider the volcano to be dormant? Will the public listen to outreach and education campaigns about potentially dangerous volcanoes if it doesn&rsquot seem like they will erupt?

Seulawah Agam and Sinabung haven&rsquot erupted since the 19th century (and scientists aren&rsquot even positive about that for Sinabung), which is more than enough time for people living near those volcanoes to forget that they are volcanoes. It&rsquos a dangerous situation, because being prepared for a volcanic eruption is the best way to avoid the danger.

But whether or not a volcano has been historically active, it&rsquos important to remember that natural phenomena operate on geologic as well as human timescales what seems like a quiet, forested mountain one day may become a violently exploding volcano the next &mdash though there are usually precursory signs. Volcanologists do their best to reconcile geologic and human time, scientific and layperson&rsquos terminology, and the differing perceptions of what a dormant volcano is, but it&rsquos a difficult situation and likely always will be.

For now, it&rsquos best to keep an eye on anything near you that a scientist calls a volcano &mdash because you never know when it might decide to make your life interesting.

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Watch the video: Vulkane: Das Leben eines Schichtvulkans (September 2021).