Volcanoes are holes in the earth’s crust through which molten rock and gases escape to the surface. Volcanic hazards arise from two classes of eruptions:

Explosive eruptions caused by the rapid dissolution and expansion of gas in molten rock as it approaches the earth’s surface. Explosions pose a risk by scattering rock blocks, fragments and lava at varying distances from the source.

Sudden explosions where the greater danger is the flow of material rather than explosions. The nature (mud, ash, lava) and amount of flows vary and can originate from multiple sources. Flows are governed by gravity, surrounding topography, and material viscosity.

Hazards associated with volcanic eruptions include lava flows, falling ash and projectiles, mudflows and toxic gases. Volcanic activity can also trigger other natural hazards such as local tsunamis, deformation of land, flooding when lakes erode or streams and rivers congested, and landslides that cause shaking.

A volcano is a rupture in the crust of a planetary-mass object like Earth that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface.

On Earth, volcanoes are mostly found where tectonic plates split or meet, and most are underwater. For example, a mid-ocean ridge such as the Mid-Atlantic Ridge has volcanoes caused by different tectonic plates, while the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can also form where there is stretching and thinning of the earth’s crustal plates, such as the East African Rift and the Wells Gray-Clearwater volcanic field and the Rio Grande rift in North America. Volcanism away from plate boundaries has been hypothesized to originate from diapirs rising from Earth’s 3,000 kilometers (1,900 mi) deep core-mantle boundary. This causes hotspot volcanism, of which the Hawaiian hotspot is an example. Volcanoes do not usually form where two tectonic plates slide over each other.

Historically, major volcanic eruptions were followed by volcanic winters that caused devastating famines.

Many mountains are formed by the folding, faulting, uplift, and erosion of the earth’s crust. But volcanic terrain is formed by the slow accumulation of erupted lava. The vent can be seen as a small bowl-shaped depression at the summit of a cone or shield-shaped mountain.

Volcanism and tectonic activity

Looking at volcanic landforms from space from a farther perspective, most volcanoes can be seen clustering together to form linear to curved belts on the Earth’s surface. It is now obvious that these volcanic chains are closely related to global tectonic activity. Many active volcanoes are located in what is called the “Ring of Fire”, which consists of island arcs and mountain ranges surrounding the Pacific Ocean. The concept of seafloor spreading and the broader theory of plate tectonics offer a plausible explanation for the location of most volcanoes.

Volcanoes related to plate boundaries

Topographic maps reveal the locations of major earthquakes and show the boundaries of 12 major tectonic plates. For example, the Pacific Plate is bounded by the earthquake zones of New Zealand, New Guinea, the Mariana Islands, Japan, Kamchatka, the Aleutian Islands, western North America, the Eastern Pacific Rise, and the Pacific-Antarctic Ridge.

The Earth’s tectonic plates, which move horizontally with respect to each other at a rate of a few centimeters per year, form three basic types of boundaries: convergent, divergent, and side-shifting. Japan and the Aleutian Islands are located on convergent boundaries where the Pacific Plate moves under adjacent continental plates – a process known as subduction. The San Andreas Fault system in California exemplifies a side-sliding boundary where the Pacific Plate moves northwest relative to the North American Plate—a process called strike-slip or transformation, faulting. The East Pacific Rise is representative of a divergent boundary where the Pacific Plate and the Nazca Plate (west of South America) are split apart – a process known as rifting.

Volcanoes occur along both subduction and rift zones, but are generally absent along strike-slip plate margins. Most subduction volcanoes are explosive and form stratovolcanoes, while rift volcanoes tend to be more vigorous and form shield volcanoes, but there are exceptions to these generalizations. Subduction-related volcanoes erupt basalt, andesite, dacite, and rhyolite, and andesite is the dominant rock type. Volcanoes, especially those associated with rift in the ocean floor, mainly erupt basalt.

Plate Tectonics

Map showing diverging plate boundaries and recent subaerial volcanoes

According to the theory of plate tectonics, the Earth’s lithosphere, its hard outer crust, is divided into sixteen large and several small plates. They are in slow motion due to convection in the underlying ductile mantle, and most volcanic activity on Earth takes place along plate boundaries where plates meet (and the lithosphere is destroyed) or split (and new lithosphere is created).

Divergent plate boundaries

At mid-ocean ridges, the two tectonic plates separate as warm mantle rock creeps upward under the thinned oceanic crust. Reduction in pressure in uplifting mantle rock leads to adiabatic expansion and partial melting of the rock, causing volcanism and the formation of new oceanic crust. Most different plate boundaries are at the bottom of the oceans, and so most volcanic activity on Earth is submarine, creating new seafloor. Black smokers (also known as deep-sea vents) are evidence of such volcanic activity. Volcanic islands such as Iceland form where the mid-ocean ridge is higher than sea level.

Convergent Plate Boundaries

Subduction zones are where two plates collide, usually an oceanic plate and a continental plate. The oceanic plate subducts (subducts below the continental plate) forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the melting temperature of the upper mantle wedge, thereby forming magma. This magma tends to be extremely viscous due to its high silica content, so it usually doesn’t reach the surface but cools and solidifies deep down. But when it reaches the surface, a volcano is formed. Therefore, subduction zones are bounded by chains of volcanoes called volcanic arcs. Typical examples are the Cascade Volcanoes or volcanoes in the Pacific Ring of Fire, such as the Japanese Archipelago or Indonesia’s Sunda Arc.

Hot Spots

Hotspots are volcanic areas thought to be formed by mantle plumes, presumed to be columns of hot material rising from the core-mantle boundary. As with mid-ocean ridges, rising mantle rock experiences decompression melt that produces large volumes of magma. As tectonic plates move along the mantle plume, each volcano becomes inert as it moves away from the plume, and new volcanoes are formed where the plate moves along the plume. The Hawaiian Islands are thought to have been formed in a way that, as with the Snake River Plain, the Yellowstone Caldera is now part of the North American plate above the Yellowstone hotspot. However, the mantle plume hypothesis has been questioned.

Continental Rifting

Continuous upward uplift of hot mantle rock can develop in a continent’s interior and lead to rifting. The early stages of rifting are characterized by flood basalts and may progress to the point where a tectonic plate has completely split. A divergent plate boundary is formed between the two halves of the split plate. However, rifting often fails together.

Volcanic Features

The characteristics of volcanoes are much more complex, and their structure and behavior depend on a number of factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater, while others feature landscape features such as large plateaus. Orifices that emit volcanic material (including lava and ash) and gases (mostly steam and igneous gases) can develop anywhere in the landform and give rise to smaller cones. Other types of volcanoes include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter, Saturn, and Neptune; and mud volcanoes, which are formations not usually associated with known magmatic activity. Activated mud volcanoes tend to involve much lower temperatures than those of igneous volcanoes, except where the mud volcano is actually the vent of an igneous volcano.

Fissure Holes

Volcanic rift vents are straight, linear fractures from which lava comes out.

Shield Volcanoes

Named for their broad, shield-like profiles, shield volcanoes are formed by eruptions of low-viscosity lava that can flow far away from a vent. They do not usually erupt catastrophically, but are characterized by relatively mild eruptions of effusion. Shield volcanoes are more common in the ocean than in continental environments, as low-viscosity magma is typically low in silica. The Hawaiian volcanic chain is a series of shield cones, and they are also common in Iceland.

Lava Domes

Lava Dome

Lava domes are built by the slow eruption of highly viscous lava. Helens, they sometimes form in the crater of a previous volcanic eruption, but they can also form independently, as in the case of Lassen Peak. Like stratovolcanoes, they can produce violent, explosive eruptions, but lava usually does not flow far from the outlet vent.


Cryptodomes cause the surface to swell when viscous lava is forced upwards. st. Helens eruption in 1980 was an example; The lava below the mountain’s surface formed an upward bulge that then collapsed onto the mountain’s north side.

Cinder Cones

Cinder Cones

Cinder cones are caused by eruptions of mostly small pieces of scoria and pyroclastic deposited around the culvert. These can be relatively short-lived eruptions that form a cone-shaped peak perhaps 30 to 400 meters (100 to 1300 ft) high. Most cinder cones only explode once. Cinder cones can form as side vents in larger volcanoes or form on their own. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. Caja del Rio in New Mexico is a volcanic field made up of more than 60 cinder cones.

Stratovolcanoes (composite volcanoes)

Stratovolcanoes (composite volcanoes)

Stratovolcanoes (composite volcanoes) are tall conical mountains composed of lava flows and tephra in alternating layers, these layers give rise to the name. They are also known as composite volcanoes, as they are formed from more than one structure during different types of eruptions. Classic examples include Mount Fuji in Japan, Mayon Volcano in the Philippines, and Mount Vesuvius and Stromboli in Italy.

Ash produced by the explosive eruption of stratovolcanoes has historically posed the greatest volcanic hazard to civilizations. The lava of stratovolcanoes is higher in silica and therefore much more viscous than lava from shield volcanoes. High silica lava also tends to contain more dissolved gas. The combination is deadly and causes explosive eruptions that produce large quantities of ash, as well as pyroclastic surges similar to the one that destroyed the city of Saint-Pierre in Martinique in 1902.



A supervolcano is a volcano that has experienced one or more eruptions that produce over 1,000 cubic kilometers (240 cu mi) of volcanic sediment in a single explosive event. Such eruptions occur when there is a very large magma chamber filled with gas-rich, silicic magma.

Volcanic Eruption

The molten rock below the Earth’s surface that rises in volcanic vents is known as magma, but is called lava after it erupts from a volcano. Magma consists of molten rock, crystals, and dissolved gas. After cooling, liquid magma can form crystals of various minerals until it becomes completely solid and forms an igneous or igneous rock.

Magma emanating from tens of kilometers underground is lighter than the solid rock around it. This rock, pushed towards the Earth’s surface by the buoyancy force, is lighter than the surrounding rocks and moves with the pressure of the gas inside. Magma forces it to rise and can eventually break weak areas in the earth’s crust. If so, an explosion begins.

Magma can be ejected in various ways. Sometimes molten rock is poured from the vent as liquid lava flows. It can also shoot fiercely into the air as dense boulder (tephra) and gas clouds. Larger fragments fall around the vent, and tephra clouds can move down the slope of the volcano under the force of gravity. The ash, tiny bits of tephra about the thickness of a hair, can fall for miles just by being carried by the wind. The smallest particles of ash can be scattered for miles into the sky before falling to the earth’s surface and be carried around the world by strong atmospheric winds.

Erupted Material

Material ejected in a volcanic eruption can be classified into three types:

Volcanic gases, mostly steam, carbon dioxide and a mixture of sulfur compounds (either sulfur dioxide, SO2 or hydrogen sulfide, H2S, depending on temperature)

Lava is the name given to when magma rises to the surface and flows.

Tephra, solid particles of all shapes and sizes, are thrown and thrown into the air.

Volcanic Gases

The concentrations of different volcanic gases can vary significantly from one volcano to the next. Water vapor is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide. Other major volcanic gases include hydrogen sulfide, hydrogen chloride, and hydrogen fluoride. Many minor and trace gases are also found in volcanic emissions such as hydrogen, carbon monoxide, halocarbons, organic compounds and volatile metal chlorides.

Lava Flows

Lava Flows

 Viscosity (how fluid the lava is) and the amount of dissolved gas are the most important properties of magma, both largely determined by the amount of silica in the magma. Silica-rich magma is much more viscous than silica-poor magma, and silica-rich magma also tends to contain more dissolved gas.

Types of volcanic eruptions

Eruption styles are broadly divided into magmatic, phreatomagmatic, and phreatic eruptions.

Magmatic Eruptions

Igneous eruptions are driven mainly by the release of gas due to decompression. Low-viscosity magma with little dissolved gas produces relatively gentle flood eruptions. High-viscosity magma with a high dissolved gas content produces violent explosive eruptions. The observed range of explosion styles is expressed from historical examples.

Hawaiian eruptions are typical volcanoes that spew mafic lava with relatively low gas content. They are almost completely exuberant, producing local fire fountains and highly fluid lava flows, but carrying relatively little tephra. It is named after the Hawaiian volcanoes.

Strombolian eruptions are characterized by moderate viscosities and dissolved gas levels. They are characterized by frequent but short-lived eruptions that can produce eruptive columns hundreds of meters high. Their primary product is scoria. They take their name from Stromboli.

Vulcan eruptions are characterized by higher viscosities and partial crystallization of magma, usually of moderate composition. The eruptions take the form of short bursts that destroy a central dome over the course of several hours and hurl large blocks of lava and bombs. This is followed by an enthusiastic phase of reconstruction of the central dome. Vulcan eruptions are named after Vulcano.

Peléan eruptions are more severe, characterized by dome growth and subsidence producing a variety of pyroclastic flows. They take their name from Mount Pelée.

Plinian eruptions are the most violent of all volcanic eruptions. They are characterized by continuous massive eruption columns whose collapse produces disastrous pyroclastic flows. They are named after Pliny the Younger, who chronicled the Plinian eruption of Mount Vesuvius in 79 AD.

The intensity of explosive volcanism is expressed using the Volcanic Eruption Index (VEI), which ranges from 0 for Hawaiian eruptions to 8 for supervolcanic eruptions.

Phreatomagmatic Eruptions

Phreatomagmatic eruptions are characterized by the interaction of rising magma with groundwater. They are driven by the rapid build-up of pressure that occurs in overheated groundwater.

Phreatic Eruptions

Phreatic eruptions are characterized by overheating of groundwater in contact with hot rock or magma. They differ from phreatomagmatic eruptions in that all the erupted material is provincial rock; magma does not erupt.

Materials Produced by Volcanic Eruptions

Materials released during a volcanic eruption include:

1) Lava (molten rock),

2) Ash (fine particles of volcanic material);

3) Bombs (lava rocks),

4) Slags (coarse pieces of stone formed during an explosion and hardening very quickly);

5) Gases (carbon dioxide and various compounds containing chlorine and sulfur); and

6) Steam.

Rocks and minerals associated with volcanoes are: 1) basalt, a dark, heavy type of volcano that usually comes from rift zone and hotspot volcanoes; 2) rhyolite, a type of solidified lava that is usually a pale shade of green, red, or gray; 3) pumice, a porous, hole-filled type of rhyolite formed when molten rock contains gas bubbles; and 4) obsidian. a grass-like lava produced when certain types of lava cool rapidly and individual minerals do not crystallize.

Volcanic Activity

Volcanoes vary widely in their activity levels; individual volcanic systems have eruptions that range from a few times a year to every tens of thousands of years. Volcanoes are informally defined as active, dormant, or extinct, but these terms are not fully defined.

Active Volcano

There is no consensus among volcanologists on how to define an “active” volcano. The lifetime of a volcano can range from months to several million years, making such a distinction sometimes meaningless when compared to the lifetimes of humans or even civilizations. For example, most volcanoes on Earth have erupted dozens of times over the past few thousand years, but are currently showing no signs of eruption. Given the long lifetimes of such volcanoes, they are very active. But in terms of human life, they are not.

Asleep and reactivated

Narcondam Island in India is classified as a dormant volcano by the Geological Survey of India.

It is difficult to distinguish an extinct volcano from a dormant (inactive) volcano. Dormant volcanoes are those that have not erupted for thousands of years but are likely to erupt again in the future. Volcanoes are generally considered extinct if there is no written record of their activity. However, volcanoes can remain dormant for a long time.


Extinct volcanoes are volcanoes that scientists think are unlikely to erupt again because the volcano is no longer a source of magma. Examples of extinct volcanoes are many volcanoes in the Hawaii-Emperor sea chain in the Pacific Ocean (although some volcanoes at the eastern end of the chain are active), Hohentwiel, New Mexico in Germany, Shiprock, New Mexico in the USA, Capulin in the USA. , many volcanoes such as the Zuidwal volcano in the Netherlands and Monte Vulture in Italy.

Volcanic Warning Level

Three common popular classifications of volcanoes can be subjective, and some volcanoes thought to be extinct have erupted again. To help prevent people from falsely believing that they are not at risk when living on or near a volcano, countries have adopted new classifications to describe various levels and stages of volcanic activity.

Volcanoes and geothermal energy

Geothermal energy is abundant, but geothermal energy is not. Temperatures increase below the Earth’s surface at a rate of about 30 °C per km in the first 10 km below the surface (about 90 °F per mile for the first 6 miles). This internal heat of the earth is an enormous energy store. At the top of 10 km of rock below the boundless United States, that’s 3.3×1025 joules, or about 6,000 times the energy found in the world’s oil reserves. The problem with using geothermal energy is extracting it.

The natural escape of Earth’s heat from its surface averages only 0.06 watts per square meter (0.006 watts per square meter. To make geothermal energy practical, some special circumstances must exist for concentrating the Earth’s heat energy in a small area. Underground reservoirs of steam or hot water that can be drained into a borehole provide this special case. Some geothermal steam wells can generate 25 megawatts of thermal power, an amount equal to the normal heat flow of more than 400 square kilometers (150 square miles) of land surface. The key to this concentration is heat transfer from deeper levels to the near surface by the rising magma associated with volcanism. At temperatures close to 1,200 °C (2,200 °F), magma moves to depths of only a few kilometers, where it is transferred to groundwater by heat conduction. The groundwater then circulates by convection, creating large underground reservoirs of hot water and steam. Some of this thermal water may rise to the surface as a hot spring or geyser.

Benefits of Volcanoes

Although volcanic eruptions pose significant hazards to humans, past volcanic activities have created significant economic resources.

Volcanic ash and weathered basalt produce some of the most fertile soils in the world, rich in nutrients such as iron, magnesium, potassium, calcium and phosphorus.

Formed from volcanic ash, tuff is a relatively soft rock and has been used for construction since ancient times. The Romans often used tuff for construction, which was abundant in Italy. The Rapa Nui people used tufa to make most of the moai statues on Easter Island.

Volcanic activity is responsible for the displacement of precious mineral resources such as metal ores.

Volcanic activity is accompanied by a high flow of heat through the Earth. These can be used as geothermal power.

Volcanic materials eventually break down to form some of the most fertile soils on earth, whose cultivation nurtures and sustains civilizations. People use volcanic products as building materials, abrasive and cleaning agents, and raw materials for many chemical and industrial uses. Internal heat associated with some young volcanic systems was used to generate geothermal energy. For example, electrical power generated from the Geysers geothermal field in Northern California could meet the current power consumption of the city of San Francisco.

Why are volcanic soils fertile?

Volcanic materials, when exposed to weathering, make the soil fertile and begin to break down and release their nutrients. Apart from water and carbon dioxide, plants need three essential nutrients to grow: nitrogen, potassium and phosphorus. They also need some iron to form chlorophyll, whose primary function is to absorb sunlight for photosynthesis to occur within the plant. A process possible in the presence of radiant energy (light), in which carbon dioxide and water are converted into oxygen and organic substances that can be used within the plant body. Volcanic materials can be sources of nitrogen, potassium, and phosphorus. Additionally, volcanic soil can provide small amounts of trace elements that can be scarce but are crucial and necessary to allow plants to make the right proteins and other molecules necessary for life. [Yahoo Answers]

Fertile soil is the result of the breakdown of various minerals such as olivine, pyroxene, amphibole and feldspar (main components of volcanic ash and lava), which releases iron, magnesium, potassium and other nutrients into the soil. Lava and ash are rich in potassium and iron, which is often a limiting nutrient. Some types of crystallized lava can be very porous and vesicular; this means they can hold large volumes of water, especially if they have time to weather and wear out. Depending on local magma chemistry, some lava may contain significant amounts of magnesium, silica, aluminum, sodium and chlorine.

Volcanic soils often contain non-crystalline (amorphous) minerals such as allophone and imogolite that form strong bonds with organic matter, and these minerals allow an enormous amount of plant life to take root in addition to the basic chemistry in volcanic soils. The new lava is not very productive. They need time to aerate and release their nutrients and open these pores. On Kauai’i (an ancient island where lava has time to rise into the air and break apart), Garden Isle and the Big Island (a younger, volcanically active island where the lava hasn’t had time to get up and leave). ) actually has surprisingly few places filled with vegetation.

How many volcanoes are there?

Not counting volcanoes under the oceans, there are about 1,350 potentially active volcanoes worldwide. About 500 of these have exploded in the last 100 years. Most are found around the Pacific Ocean, known as the “Ring of Fire”. In the US, volcanoes on the west coast and in Alaska (the Aleutian volcanic chain) are part of the Ring of Fire, while the volcanoes of Yellowstone and Hawaii form on a “hotspot.”


  • About Volcanoes | U.S. Geological Survey. (2022). Retrieved 13 March 2022, from https://www.usgs.gov/vhp/about-volcanoes
  • Panchuk, K. (2022). 11.2 Materials Produced by Volcanic Eruptions. Retrieved 13 March 2022, from https://openpress.usask.ca/physicalgeology/chapter/11-2-materials-produced-by-volcanic-eruptions/
  • Hays, J. (2022). VOLCANOES: TYPES AND STRUCTURE | Facts and Details. Retrieved 13 March 2022, from https://factsanddetails.com/world/cat51/sub323/item1313.html
  • Volcano | Definition, Types, & Facts. (2022). Retrieved 13 March 2022, from https://www.britannica.com/science/volcano
  • Wikipedia contributors. (2022, March 11). Volcano. In Wikipedia, The Free Encyclopedia. Retrieved 21:53, March 13, 2022, from https://en.wikipedia.org/w/index.php?title=Volcano&oldid=1076424668
Previous articleEarthquakes and Seismicity
Next articleMineral Deposits