Introduction to Volcanism

Volcanoes are one of the most dynamic, powerful, and visible forces on Earth. What are volcanoes and what factors cause them to form in certain areas? How are geothermal features like fumaroles and geysers related to these temperamental mountains?

Let us start by looking at the volcano itself and learn the different parts of it, the rocks associated with it, and where volcanoes form.


What is a Volcano?

Hot magma, melted rock below Earth's crust, rises and collects in a magma chamber deep below the surface. If the magma flows through a conduit up to a vent on the surface, then it may cause an eruption and form a volcano.

Gases, lava, and pyroclastic material are erupted from volcanic vents. The mountain that forms from layers of lava and tephra is called a volcano. The word "volcano" comes from the name of a Roman god, Vulcan, who was the god of fire. Magma that solidifies inside a volcano can form dikes and sills.

Volcanoes are classified as active, dormant or extinct.

The Magma Chamber

Magma is the name given to melted liquid rock below Earth's surface. It is stored below the volcano in a chamber or reservoir. During active periods, this reservoir fills with magma. After a large eruption, or during dormancy, this reservoir can drain, which may trigger the creation of a caldera.

The Magma Conduit

The magma conduit is the plumbing system of the volcano. Molten magma creates “pipes”, through the volcano. Magma then travels through them to the surface.

The Main Vent

The main vent often is located at or near the summit of the volcano. This is where most eruptive activity (lava flows, pyroclastic flows, and large gas emissions) occurs.

Lava Flows

Lava flows occur during some eruptions. Once magma reaches the vent and flows onto the surface of Earth it is called lava. Lava flows add land to the surface, and build the mountains we call volcanoes. The island of Hawaii is nearly 100% cooled and solidified lava!

Pyroclastic Flows

Pyroclastic flows occur during some eruptions. These are super-heated clouds of volcanic material ranging from ash to volcanic bombs. Pyroclastic flows travel very fast, and can destroy everything in their path.

The Volcano

Volcanoes are built from layers of lava and tephra (particles of rock, solidified lava, and ash of all different sizes). The lava cools on the sides of the volcano and hardens into rock. Pyroclastic flows and eruption clouds deposit tephra on top of lava layers, increasing the size of the mountain.

Dikes and Sills

The molten, pressurized magma intrudes into the solid volcanic rock to create dikes and sills. Dikes cut across volcanic rock layers, and sills run parallel to the layers. Eventually the magma hardens inside Earth and becomes an intrusive igneous rock.

Vulcan, Roman God of Fire

In Roman mythology, Vulcan was the god of fire. He was also known as the blacksmith of the gods.

There is a small volcanic island in the Mediterranean Sea called Vulcano. The local residents once believed the volcano on the island was the chimney of Vulcan's workshop. They thought the hot lava and smoke issuing from the mountain were products of Vulcan's work as he created thunderbolts for Jupiter, king of the gods, and weapons for Mars, god of war.

Active, Dormant, and Extinct

Volcanologists classify volcanoes based on how much activity has been recorded over time.


An active volcano is currently erupting or has erupted in recent history. Active volcanoes can have eruptions of gases, pyroclastic material, tephra, and lava.


A dormant volcano is not presently erupting, and has not erupted in recent history. There is still potential for renewed activity, because there still may be magma moving or cooling deep inside the volcano.


An extinct volcano has not erupted in recent history and is unlikely to erupt again. Wind and water have broken and smoothed the shape of the mountain. The magma has drained below the surface or cooled inside the volcano.



Magma is the word used to describe melted or molten rock inside Earth. Magma is composed of elements, minerals, and gases that were present in the rock before it melted.

The major elements in magma are those present in Earth's crust: oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), sodium (Na), magnesium (Mg), and potassium (K). These elements combine to form minerals such as magnetite, hauynite, olivine, pyroxene, hornblende, plagioclase, potassium feldspar (k-feldspar), and quartz.

Magma also contains dissolved gases like water vapor (H2O), carbon dioxide (CO2), and sulfur dioxide (SO2).

More on Magma

The composition of the magma determines the eruption style, rock type, and volcano shape. Variations in the chemical compositions and properties of the magma determine whether it will be classified as mafic, felsic, or intermediate.


- High melting point, pH, density, and Mg and Fe content
- Low viscosity
- Form basanite and basalt rocks
- Typically effusive eruptions
- Form shield volcanoes, flood basalts, cinder cones, and fissure eruptions


- Intermediate melting point, viscosity, pH, density and mineral composition
- Form andesite rocks
- Typically explosive eruptions
- Commonly form composite volcanoes


- Low melting point, pH, and density
- High viscosity and Si content
- Form rhyolite rocks
- Typically very explosive eruptions
- Form composite volcanoes, lava domes, and calderas


Types of Volcanoes

Volcanoes are openings, or vents, in the surface of Earth where gases, lava and pyroclastic material are erupted. Because of the different types of magma, and the locations where they form, volcanoes can have a wide variety of shapes and sizes.

Volcanoes are classified based on their height, shape, magma type, and eruption style. In this section, you can learn about four types of volcanoes.

The two primary types are shield volcanoes and composite volcanoes. Cinder cones and lava domes are considered to be secondary cones, because they occur on or near composite or shield volcanoes.

Shield Volcanoes

Shield volcanoes have the shape of a warrior's shield lying flat on the ground: very broad with large bases. This is due to the low viscosity of the magma. They are not as steep as composite volcanoes, but are often greater in volume.

Shield volcanoes usually have slow, gentle eruptions that produce large volumes of mafic magma (rich in iron and magnesium). Although these eruptions are usually relatively quiet, there can be large explosions when magma comes into contact with groundwater, vaporizing the water instantly. Shield volcanoes are found commonly in oceanic areas, such as Hawaii. The Big Island of Hawaii is made up of five huge shield volcanoes.

How Shield Volcanoes Form

Explore the steps involved in the formation and growth of a shield volcano.

Initial Vent Formation

A magma reservoir sits below the ocean floor. Pressure builds, pushing the magma closer and closer to the surface. Small vents and fissures open on the ocean floor, and lava escapes. Pillow lavas form when the molten rock comes in contact with the cold seawater.

Shield Building

Many successive lava flows, over thousands or even millions of years build a mountain that is shaped like a warrior's shield. Eruptions happen often, and large amounts of lava are poured out of the vent, so the mountains can grow to immense proportions.

Lava Fountains

Shield volcano eruptions are commonly gentle and effusive, with great quantities of basaltic lava flowing out of the vent. Mafic magma has low viscosity and flows easily, so lava from a shield volcano can flow great distances. Sometimes spectacular lava fountains occur when molten rock is squirted thousands of feet in the air above an erupting vent.

Eruptions and Intrusions

Magma rising from a sea-floor vent can come to the surface to cause an eruption at the main vent or at a fissure or smaller vent on the flank of the volcano. Magma can also create intrusive igneous structures like dikes and sills.

Caldera Formation

After an eruption all of the magma has either been expelled through the vents, or has drained back into a reservoir deep below the volcano. The conduits through which it flowed are left hollow and empty. Because of the heavy weight of hardened lava over the unstable, empty magma conduits, the summit of the volcano can collapse, forming a huge crater known as a caldera.

Erosion and Reef Building

Erosion from the wind and waves carves away at the volcano's summit and flanks. The volcano's height is reduced as it is eroded from the top, and the weight of the layers and layers of lava causes the volcano to subside (sink). Sandy sediments build up, and coral reefs begin to grow in the shallow water. As water depth increases, the coral die from lack of sunlight. New reefs can grow on top of the deeper ones.

Atoll Formation

Continued erosion and subsidence (sinking) of the volcano reduce its height to sea level or below. Coral reefs keep growing, building on each other in the shallow water. The reefs eventually form an atoll, which is a group of islands in the shape of a ring with a lagoon in the middle.

Seamount Formation

Eventually the volcano sinks faster than the reefs are growing. Below sea level wave erosion flattens the top of the volcano, which is now called a seamount.

Shield Volcanoes in National Parks

Explore some of America's national parks where shield volcanoes can be found.

Hawaii Volcanoes National Park
Wrangell-St. Elias National Park and Preserve

Composite Volcanoes

Composite volcanoes are also called stratovolcanoes because they are made from many layers (strata) of rock, ash, and hardened lava. In addition, volcanic mudflows (lahars) can make up some of the layers.

Composite volcanoes experience very explosive eruptions because of the intermediate to felsic magma types (high viscosity, high silica, low melting temperature). Composite volcanoes are steeper near the summit, but slope more gently near the base of the mountain. Composite volcanoes are typically found on island arcs and continents at subduction zones.

How Composite Volcanoes Form

Explore the steps involved in the formation and growth of a composite volcano.

Magma Reservoir

A magma reservoir sits below the ground in Earth's crust. Composite volcanoes form in areas where subduction occurs. Subduction happens when tectonic plates collide and one plate is pushed below the other into the interior of Earth.

The magma creating composite volcanoes is likely to have a high content of silica, making it explosive. As the volume of magma in the reservoir increases, pressure builds until a vent opens in the ground and a volcano is formed.

Lava Layers

Composite volcanoes are also called stratovolcanoes because they are composed of layers (strata) of lava flows, tephra, and mudflows.

Lava, magma that has reached Earth's surface, pours out of the main vent at the summit of the volcano, flows down its sides and hardens to form a steep mountain.

Tephra Layers

Composite volcano eruptions do not always involve lava. Some eruptions release pressurized volcanic gases in great explosions that expel tephra (ash, lapilli, cinders, bombs) into the air. This material can fall back onto the volcano, adding another layer to its height and width.

Continued Layering

Eruptions of lava flows and pyroclastic material continue, building a mountain that will be thousands of feet high. Typically, composite volcanoes erupt andesite-based lava, but they can contain lava of any composition from basalt to rhyolite.

Continued Layering

Composite volcanoes are usually active over hundreds of thousands of years. During this time, there are many eruptions with periods of dormancy between them. Generally, lava flows and pyroclastic deposits do not occur in the same eruption. The volcanoes grow to great heights, typically having a gentle slope at the base, and a steeper slope at the summit.

Explosive Eruptions

Composite volcanoes frequently erupt explosively. Magma can push its way inside the volcano to form dikes and sills, or flow out of a vent. Composite volcanoes can have multiple vents, at the summit and on the flanks of the mountain. Lahars (volcanic mudflows) can course down the side of the volcano, and hot pyroclastic flows rush downslope carrying gases, tephra, and debris at high speeds and temperatures.

Caldera Formation

During an eruption, the magma and gases that were creating high pressures inside the volcano are released. This leaves the top of the mountain very unstable. This instability can cause the summit of the volcano to collapse in on itself, forming a caldera.

Lava Dome Formation

Renewed volcanism in the caldera can lead to the formation of lava domes in the caldera. Lava domes form when viscous lava pours out of the vent. Volcanic gas and steam are still released from the caldera as the magma and pyroclastic material cool.


After many thousands or millions of years, the summit and flanks of the dormant volcano are eroded and smoothed. The layers of the volcano that were formed from tephra and mudslides erode more easily than the layers formed from lava flows. Erosion can also expose intrusive igneous rocks like dikes and sills that formed inside the volcano's layers.

Composite Volcanoes in National Parks

Explore some of America's national parks where composite volcanoes can be found.

Aniakchak National Monument and Preserve
Katmai National Park and Preserve
Lake Clark National Park and Preserve
Mount Rainier National Park
North Cascades National Park

Cinder Cones

Cinder cones are considered secondary cones because they generally form in areas of other volcanic activity, including on composite and shield volcanoes. Cinder cones are peaks formed when pyroclastic materials are ejected into the air from a vent and fall back to the ground around the vent in a cone-shaped pile resembling a mound of cinders.

The cones are small, steep-sided, and symmetrical. These volcanoes can form individually over a vent. They can also form in the crater or on the flank of another larger volcano. Wizard Island in Crater Lake National Park in Oregon formed after the summit of Mount Mazama collapsed.

How Cinder Cones Form

Explore the steps involved in the formation and growth of a cinder cone.

Vent Formation

Below the ground a magma reservoir forms. It grows in size until the pressure is too great and vents form in the crust. Cinder cones are the most common volcano, and are often found in clusters of up to 100 cones. They can align along fissures, or be parasitic cones on the flanks of a composite or shield volcano.

Cone Formation

Cinder cones typically erupt only once. The magma in the vent contains vesicles (gas bubbles) that cause lava to be thrown up into the air, where it cools and hardens before it falls back to the ground around the vent. These solid lava fragments are called cinders. They pile up in the shape of a cone around the vent.

Lava Flows

Cinder cone eruptions can also include lava flows. Rather than create a conduit through the porous, weak pile of cinders, the lava flows out at the base of the cone, from a vent called a "boca." "Boca" is Spanish for "mouth."

Endangered Volcanoes

Cinder cones can grow to be about 600 to 900 feet tall. The shortest eruptions last less than a month and the longest over 10 years. People mine cinder cones, sometimes destroying them, because the cinders can be used for road construction, road sanding in winters, and decorative "lava rocks" for grills and landscaping.

Cinder Cones in National Parks

Explore some of America's national parks where cinder cones can be found.

Sunset Crater Volcano National Monument
Capulin Volcano National Monument

Lava Domes

Lava domes, like cinder cones, are considered secondary cones. They form when magma below the surface has great upward pressure. This pressure causes the most viscous magmas to move toward the surface, forming steep-sided and bulging mountains. The domes are commonly composed of felsic magma, but can also be intermediate.

The domes grow slowly, but can be responsible for highly explosive eruptions. Lava domes can contribute significantly to the formation of composite volcanoes, and are often built at or near the summit of composite volcanoes.

How Lava Domes Form

Select the following numbers to explore the diagram and see the steps involved in the formation and growth of a lava dome.


Lava domes are formed from pasty, high-viscosity magma with dacite or rhyolite composition. Lava domes occur in the summit craters of composite volcanoes, as parasitic domes on the flanks of composite volcanoes, or as volcanoes with their own vent.

Dome Building 1

The dacite or rhyolite magma is too viscous to form a lava flow. Instead, it extrudes slowly out of the vent, piling into a dome.

Dome Building 2

Most lava domes are small compared to other types of volcanoes. The domes do not usually have craters.

Dome Building 3

The high-silica, viscous magma is injected into the dome, pushing it up and out in layers. Sometimes this increasing pressure on the hardened lava layers of the dome causes a collapse. The collapse of a lava dome can lead to pyroclastic flows.


Lava domes themselves do not typically erupt explosively, but the domes forming in the summit craters of composite volcanoes can be destroyed when the composite volcano erupts. These highly explosive and violent eruptions calm down as the volatile gases escape from the magma. The viscous magma that is left can begin to form another dome.

Successive Domes

Lava domes can be built fairly quickly, as fast as a few days, or longer. Explosive eruptions or collapses can destroy them even faster. In an active area for dome building, new domes begin to form shortly after old ones are destroyed.

Lava Domes in National Parks

Explore some of America's national parks where lava domes can be found.

Aniakchak National Monument and Preserve
Lake Clark National Park and Preserve


Volcanic Rocks

The Latin word igneous means "fire-formed." Igneous rocks are formed when molten rock cools and hardens. There are two classes of igneous rocks - extrusive and intrusive.

Extrusive igneous rocks are created when lava and/or pyroclastic materials cool at or above Earth's surface (Earth's exterior). Extrusive rocks are also called volcanic rocks.

Intrusive igneous rocks are formed when magma cools and solidifies below the surface of Earth (Earth's interior). Intrusive rocks are also called plutons. The majority of Earth's igneous rocks are plutons.

The cooling history and the chemistry of the magma will determine what kind of igneous rock will form, and how that rock will look when it is completely cooled. When magma cools slowly, there is time for crystals and grains to form in the rock. This gives the rock a coarse texture, and leaves visible crystals on it's surface. When magma cools rapidly, there is no time for crystals to form, and the rock has a very fine texture. Other rocks may have a more complicated texture, where the parts that cooled fast are fine, and the parts that cooled slower are coarser.

Extrusive Igneous Rocks

Extrusive igneous rocks, also known as volcanic rocks, are the result of lava and pyroclastic material cooling at or on the surface of Earth. Volcanic rocks can be found in a wide range of sizes, shapes, colors, and textures.

Volcanic rocks range from black to white in color. Rocks that cool quickly can have textures as fine as glass. Obsidian, volcanic glass, is a smooth, shiny rock that forms when basaltic lava cools quickly. The most common types of volcanic rocks are basalt, andesite, dacite and rhyolite. Click on the rock images to learn more about each type.


Dacite is usually light gray, but can be darker gray or black. They have a high concentration of visible crystals. Lava domes are commonly made from this rock, which is formed from intermediate magma.


Rhyolite rock is light colored. Rhyolite forms from felsic magma, which can cause explosive eruptions. The rocks formed will look different depending on the type of eruption they were ejected by.


Andesite rock is gray to black in color, with visible crystals. It is formed from intermediate magma, which can be erupted explosively.


Basalt rock is formed from mafic magma. It is usually very hard and dark gray or black in color. The islands of Hawaii are made almost entirely of basaltic lava flows.

Intrusive Igneous Rocks

Intrusive igneous rocks are formed when magma cools below Earth's surface. The magma is insulated underground, which allows it to cool slowly. The rocks that form from the magma are generally coarse-grained and have visible crystals.

Intrusive igneous rocks, regardless of their size or shape, are called plutons. Magma usually comes up from deep inside Earth and pushes its way into softer layers of sedimentary rock. The magma cools, and igneous rock formations are left inside the sedimentary rock layers. Igneous rocks are harder and resist weathering better than sedimentary rocks, so erosion eventually uncovers the intrusive igneous rock features.


The largest pluton, which has a surface area of 100 square kilometers or more. Batholiths form over long periods of time and can have irregular shapes. Many mountain ranges are batholiths that have been exposed by erosion over time.


A pluton that has a surface area less than 100 square kilometers. Stocks are similar to batholiths because they form over long periods of time, and can have similar shapes, but stocks are smaller.


Intrusions of magma that cut through sedimentary rock layers.

Sedimentary Rock Layers

Often present at the top of Earth's crust. Magma from deep below pushes through these layers. Solidified magma is harder than sedimentary rock, so when the surrounding sedimentary rock is eroded, underlying igneous rocks are exposed.


Mushroom-shaped bulges that form on sills. The bulge pushes the rock layers up above it.


Intrusions that run between or parallel to layers of sedimentary rock.


Where Volcanoes Form

There are three places that volcanism commonly occurs. These are at hot spots, spreading centers and fault zones, and subduction zones.

Hot Spots

Volcanic activity can occur in areas that are in the interior of a plate, far away from spreading centers or subduction zones. Rising magma somewhere inside the borders of a plate can create a local "hot spot." There are between 50 and 100 hot spots identified around the world, and they occur in both continental and oceanic plates.

Hot spots originate deep inside Earth, so they remain stationary while the plates above them move. That is how island chains like the Hawaiian Islands are formed. The magma associated with hot spots is mafic, so shield volcanoes are commonly formed. Hot spots are also associated with many geothermal features.

The Hawaiian Islands were formed when the Pacific Plate passed over a hot spot. Yellowstone National Park is also a result of hot spot volcanism.

Hawaii Volcanoes National Park, Hawaii
Haleakala National Park, Hawaii
Yellowstone National Park, Wyoming, Idaho, and Montana

Spreading Centers and Fault Zones

Spreading centers are places where tectonic plates are diverging (moving away from one another). As the plates separate, a pathway is created for magma to move toward the surface. Spreading centers can extend into continental plates, such as The Great Rift Valley in East Africa. Magma produced at spreading centers is mafic. Therefore, much of the oceanic crust is made of basalt, a mafic rock.

Some of the magma produced at spreading centers erupts as lava flows and pyroclastic material, but most of it cools internally below Earth's crust. Fault zones are areas where the crust is cracking, usually due to the movement of the tectonic plates. Earthquakes and volcanism are common around fault zones.

There are no areas of active sea-floor spreading found within a national park of the United States. The country of Iceland, however, continues to grow due to the phenomenon. There are a few national parks that have evidence of past volcanism in rift valleys.

Petroglyph National Monument

Subduction Zones

Subduction occurs when two tectonic plates converge, and the denser of the two plates is pushed beneath the other plate. Volcanism will be present at the leading edge of the top plate. When a continental plate and oceanic plate converge, the denser oceanic plate is subducted.

The descending plate is heated by pressure and Earth's geothermal gradient. This leads to the formation of magma. The magma rises to the surface, and a belt of composite volcanoes forms. Subduction commonly generates felsic and intermediate magmas. There is not much volcanic activity at the convergence of two continental plates because continental crust is typically not dense enough to be subducted.

The volcanic activity on the Aleutian Islands and the Alaskan Peninsula is caused by subduction. The denser Pacific Plate is being subducted below the North American Plate. Visit volcanic parks in Alaska that are associated with subduction zones.

Aniakchak National Monument and Preserve
Lake Clark National Park and Preserve
Katmai National Park and Preserve
Wrangell-St. Elias National Park and Preserve



Introduction to Volcanism

Eruptions and Hazards

Landforms and Features

Monitoring and Forecasting

Volcanisim in National Parks

Challenge Your Understanding


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