Type of conical volcano composed of layers of lava and tephra
Mount Rainier, a 4,390-metre-tall (14,410 ft) stratovolcano, the highest mountain in the US state of WashingtonExposed internal structure of alternating layers oflava andpyroclastic rock in theerodedBroken Top stratovolcano inOregon
Astratovolcano, also known as acomposite volcano, is a typicallyconicalvolcano built up by many alternating layers (strata) of hardenedlava andtephra.[1] Unlikeshield volcanoes, stratovolcanoes are characterized by a steep profile with a summit crater and explosive eruptions.[2] Some have collapsed summit craters calledcalderas.[3] Thelava flowing from stratovolcanoes typically cools and solidifies before spreading far, due to highviscosity. Themagma forming this lava is oftenfelsic, having high to intermediate levels ofsilica (as inrhyolite,dacite, orandesite), with lesser amounts of less viscousmaficmagma.[4] Extensivefelsic lava flows are uncommon, but can travel as far as 8 kilometres (5 miles).[5]
The termcomposite volcano is used because strata are usually mixed and uneven instead of neat layers.[6] They are among the most common types of volcanoes;[7] more than 700 stratovolcanoes have erupted lava during theHolocene Epoch (the last 11,700 years),[8] and many older, now extinct, stratovolcanoes erupted lava as far back asArchean times.[9][10] Stratovolcanoes are typically found insubduction zones but they also occur in other geological settings. Two examples of stratovolcanoes famous for catastrophic eruptions areKrakatoa inIndonesia (which erupted in1883 claiming 36,000 lives)[11] and MountVesuvius inItaly (which erupted in79 A.D killing an estimated 2,000 people).[12] In modern times,Mount St. Helens (1980) inWashington State, US, andMount Pinatubo (1991) in thePhilippines have erupted catastrophically, but with fewer deaths.[7]
The existence of stratovolcanoes on other bodies of theSolar System has not been conclusively demonstrated.[13]Zephyria Tholus is one of two mountains in theAeolis region of Mars that have been proposed as possible stratovolcanoes.[14]
Subduction zone volcanoes form whenhydrousminerals are pulled down into the mantle on the slab. These hydrous minerals, such aschlorite andserpentine, release their water into themantle which decreases itsmelting point by 60 to 100 °C (110 to 180 °F). The release of water fromhydrated minerals is termed "dewatering", and occurs at specific pressures and temperatures for each mineral, as the plate descends to greater depths.[20] This allows themantle to partially melt and generatemagma. This is calledflux melting. The magma then rises through thecrust, incorporating silica-rich crustal rock, leading to a finalintermediate composition. When the magma nears the top surface, it pools in amagma chamber within the crust below the stratovolcano.[21]
The processes that trigger the final eruption remain a question for further research. Possible mechanisms include:[22]
Magma differentiation, in which the lightest, most silica-rich magma and volatiles such as water,halogens, andsulfur dioxide accumulate in the uppermost part of the magma chamber. This can dramatically increase pressures.[23]
Fractional crystallization of the magma. Whenanhydrousminerals such asfeldspar crystallize out of the magma, this concentrates volatiles in the remaining liquid, which can lead to a second boiling that causes agas phase (carbon dioxide or water) to separate from the liquid magma and raise magma chamber pressures.[24][25]
Injection of fresh magma into the magma chamber, which mixes and heats the cooler magma already present. This could force volatiles out of solution and lower the density of the cooler magma, both of which increase pressure. There is considerable evidence for magma mixing just before many eruptions, including magnesium-richolivine crystals in freshly erupted siliciclava that show no reaction rim. This is possible only if thelava erupted immediately after mixing since olivine rapidly reacts with silicic magma to form a rim ofpyroxene.[26]
These internal triggers may be modified by external triggers such assector collapse,earthquakes, orinteractions with groundwater. Some of these triggers operate only under limited conditions. For example, sector collapse (where part of the flank of a volcano collapses in a massivelandslide) can only trigger the eruption of a very shallow magma chamber. Magma differentiation andthermal expansion also are ineffective as triggers for eruptions from deep magma chambers.[22]
Since 1600CE, nearly 300,000 people have been killed byvolcanic eruptions. Most deaths were caused bypyroclastic flows andlahars, deadly hazards that often accompany explosive eruptions of subduction-zone stratovolcanoes.[27] Pyroclastic flows are swift, avalanche-like, ground-sweeping, incandescent mixtures of hot volcanic debris, fineash, fragmentedlava, and superheated gases that can travel at speeds over 150 km/h (90 mph).[27] Around 30,000 people were killed by pyroclastic flows during the1902 eruption of Mount Pelée on the island ofMartinique in theCaribbean.[27] During March and April 1982,El Chichón in the State ofChiapas in southeasternMexico, erupted 3 times, causing the worst volcanic disaster in Mexico's history and killing more than 2,000 people in pyroclastic flows.[27]
TwoDecade Volcanoes that erupted in 1991 provide examples of stratovolcano hazards. On 15 June, Mount Pinatubo erupted and caused anash cloud to shoot 40 km (25 mi) into the air. It produced largepyroclastic surges andlahar floods that caused a lot of damage to the surrounding area.[27] Mount Pinatubo, located inCentral Luzon 90 km (56 mi) west-northwest ofManila, had been dormant for six centuries before an eruption in 1991. This eruption was the second largest in the 20th century.[29] It produced a large cloud ofvolcanic ash that affected global temperatures, lowering them as much as 0.5 °C.[29] The cloud consisted of 22 million tons ofsulfur dioxide which combined with water droplets to createsulfuric acid.[27] In 1991 Japan'sMount Unzen also erupted, after 200 years of inactivity. It's located on the island ofKyushu about 40 km (25 mi) east ofNagasaki.[27] Beginning in June, a newly formed lava dome repeatedly collapsed. This generated a pyroclastic flow that flowed down the mountain's slopes at speeds as high as 200 km/h (120 mph).[27] The 1991 eruption of Mount Unzen caused 43 deaths. In 1792, Mount Unzen was responsible for one of the worst volcanic disasters in Japan's history, killing more than 15,000 people.[30]
Theeruption of Mount Vesuvius in 79 AD is the most famous example of a hazardous stratovolcano eruption. Pyroclastic surges completely smothered the nearby ancient cities ofPompeii andHerculaneum with thick deposits of ash andpumice ranging from 6–7 meters deep. Pompeii had 10,000–20,000 inhabitants at the time of eruption.[31]Mount Vesuvius is recognized as one of the most dangerous of the world's volcanoes, due to its capacity forpowerful explosive eruptions coupled with the high population density of the surroundingMetropolitan Naples area (totaling about 3.6 million inhabitants).[32]
In addition to potentially affecting the climate,volcanic ash clouds fromexplosive eruptions pose a serious hazard toaviation.[27]Volcanic ash clouds consist of silt- or sand-sized pieces of rock, mineral,volcanic glass. Volcanic ash grains are jagged, abrasive, and don't dissolve in water.[33] For example, during the 1982 eruption ofGalunggung inJava,British Airways Flight 9 flew into the ash cloud, causing it to sustain temporary engine failure and structural damage.[34] Although no crashes have happened due to ash, more than 60, mostlycommercial aircraft, have been damaged. Some of these incidents resulted in emergency landings.[35][27]Ashfalls are a threat to health when inhaled and are also a threat to property. A square yard of a 4-inch thick volcanic ash layer can weigh 120–200 pounds and can get twice as heavy when wet. Wet ash also poses a risk to electronics due to itsconductive nature.[33] Dense clouds of hot volcanic ash can be expelled due to the collapse of aneruptive column, or laterally due to the partial collapse of avolcanic edifice orlava dome duringexplosive eruptions. These clouds are known aspyroclastic surges and in addition to volcanic ash, they contain hotlava,pumice,rock, and volcanic gas. Pyroclastic surges flow at speeds over 50 mph and are at temperatures between 200 °C – 700 °C. These surges can cause major damage to property and people in their path.[36]
Lava flows from stratovolcanoes are generally not a significant threat to humans or animals because the highlyviscouslava moves slowly enough for everyone to evacuate. Most deaths attributed to lava are due to related causes such asexplosions andasphyxiation fromtoxic gas.[37]Lava flows can bury homes and farms in thickvolcanic rock which greatly reduces property value.[37] However, not all stratovolcanoes erupt viscous and sticky lava.Nyiragongo, nearLake Kivu incentral Africa, is very dangerous because itsmagma has an unusually lowsilica content, making it much less viscous than other stratovolcanoes. Lowviscosity lava can generate massivelava fountains, while lava of thicker viscosity can solidify within the vent, creating avolcanic plug. Volcanic plugs can trapvolcanic gas and create pressure in the magma chamber, resulting in violent eruptions.[38] Lava is typically between 700 and 1,200 °C (1,300–2,200 °F).[39]
Volcanic bombs are masses of unconsolidated rock and lava that are ejected during an eruption. Volcanic bombs are classified as larger than 64mm (2.5 inches). Anything from 2 to 64mm is classified aslapilli.[40] When erupted, volcanic bombs are still molten and partially cool and solidify on their descent. They can form ribbon or oval shapes that can also flatten on impact with the ground.[41] Volcanic bombs are associated withStrombolian andVulcanian eruptions andbasaltic lava. Ejection velocities ranging from 200 to 400 m/s have been recorded causing volcanic bombs to be destructive.[40]
Lahars (from aJavanese term for volcanic mudflows) are a mixture of volcanic debris and water. Lahars can result from heavy rainfall during or before the eruption or interaction with ice and snow. Meltwater mixes with volcanic debris causing a fast moving mudflow. Lahars are typically about 60% sediment and 40% water.[42] Depending on the abundance of volcanic debris the lahar can be fluid or thick like concrete.[43]Lahars have the strength and speed to flatten structures and cause great bodily harm, gaining speeds up to dozens of kilometers per hour.[42] In the1985 eruption ofNevado del Ruiz inColombia,pyroclastic surges melted snow and ice atop the 5,321 m (17,457 ft) high Andean volcano. The ensuing lahar killed 25,000 people and flooded the city ofArmero and nearby settlements.[43]
As a volcano forms, several differentgases mix withmagma in the volcanic chamber. During an eruption the gases are then released into theatmosphere which can lead to toxic human exposure. The most abundant of thesegases isH2O (water) followed byCO2 (carbon dioxide),SO2 (sulfur dioxide),H2S (hydrogen sulfide), andHF (hydrogen fluoride).[42] If at concentrations of more than 3% in the air, when breathed inCO2 can cause dizziness and difficulty breathing. At more than 15% concentrationCO2 causes death.CO2 can settle into depressions in the land, leading to deadly, odorless pockets of gas.[44]SO2 classified as a respiratory, skin, and eye irritant if come into contact with. It is known for its pungent egg smell and role in ozone depletion and has the potential to cause acid rain downwind of an eruption.[44]H2S has an even stronger odor thanSO2 as well as being even more toxic. Exposure for less than an hour at concentrations of over 500 ppm causes death.[44]HF and similar species can coatash particles and once deposited can poison soil and water.[44] Gases are also emitted during volcanic degassing, which is a passive release ofgas during periods of dormancy.[44]
Mount Pinatubo's 1991 eruption ash cloud seen from Clark Airbase. 12 June 1991
As per the above examples, while eruptions likeMount Unzen have caused deaths and local damage, the impact of the June 1991 eruption ofMount Pinatubo was seen globally.[35] The eruptive columns reached heights of 40 km (25 mi) and dumped 17 megatons of SO2 into the lowerstratosphere.[45] Theaerosols that formed from thesulfur dioxide (SO2),carbon dioxide (CO2), and othervolcanic gases dispersed around the world. TheSO2 in this cloud combined with water (both of volcanic and atmospheric origin) and formedsulfuric acid, blocking a portion of the sunlight from reaching thetroposphere.[35] This caused the global temperature to decrease by about 0.4 °C (0.72 °F) from 1992 to 1993. Theseaerosols caused theozone layer to reach the lowest concentrations recorded at that time.[45] An eruption the size of Mount Pinatubo affected the weather for a few years; with warmer winters and cooler summers observed.[45]
A similar phenomenon occurred in the April 1815, the eruption ofMount Tambora onSumbawa island inIndonesia. This eruption is recognized as the most powerful eruption in recorded history.[35] Its eruption cloud lowered global temperatures as much as 0.4 to 0.7 °C (0.72 to 1.26 °F).[46] In the year following the eruption, most of the Northern Hemisphere experienced cooler temperatures during the summer. In thenorthern hemisphere, 1816 was known as the "Year Without a Summer". The eruption caused crop failures, food shortages, and floods that killed over 100,000 people across Europe, Asia, and North America.[46]
^Cas, R.A.F.; Wolff, J.A.; Martí, J.; Olin, P.H.; Edgar, C.J.; Pittari, A.; Simmons, J.M. (2022). "Tenerife, a complex end member of basaltic oceanic island volcanoes, with explosive polygenetic phonolitic calderas, and phonolitic-basaltic stratovolcanoes".Earth-Science Reviews.230 103990.Bibcode:2022ESRv..23003990C.doi:10.1016/j.earscirev.2022.103990.hdl:10261/267257.
