What is a Volcano?
Volcanoes are openings, or vents, in the surface of Earth where gases, lava and
pyroclastic material are erupted. Volcanoes are Earth’s way of releasing some of its
internal heat. Molten magma, melted rock below Earth’s crust, is less dense than the
surrounding rock, and under pressure from dissolved gases. It forces its way upward
through cracks in the crust. When the hot magma reaches Earth’s surface, it erupts
as a volcano.
Hot magma rises and collects in a magma chamber (1) deep below the surface. If
the magma flows through a conduit (2) up to a vent (3) on the surface, then it may
cause an eruption and form a volcano. When hot magma reaches Earth’s surface, it
is called lava. Gases, lava (4), and pyroclastic material (5) are erupted from volcanic
vents. The mountain that forms from layers of lava and tephra is called a volcano (6).
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 (7).
Volcanoes are classified as active, dormant or extinct. Volcanologists classify
volcanoes based on how much activity has been recorded over time.
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.
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 active volcano is currently erupting or has erupted in recent history. Active
volcanoes can have eruptions of gases, pyroclastic material, tephra, and lava.
Types of Volcanoes
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 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.
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. The mountains of the
Cascade Range, including Mt. Ranier and Mt. St. Helens, are composite volcanoes.
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. An example of a cinder cone can be seen in
Crater Lake National Park in Oregon, where Wizard Island formed in the crater at the
summit of Mount Mazama after it collapsed.
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.
Where Volcanoes Form
The map below shows the outlines of Earth's crustal plates, and also the incidences of
earthquakes and volcanism associated with plate tectonics. There are three places
that volcanism commonly occurs. These are at hot spots, spreading centers
(divergent plate boundaries), and subduction zones (convergent plate boundaries).
Volcanoes (∆) and Earthquakes () occur primarily on the plate boundaries.
Magma
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, feldspar,
and quartz. Magma also contains dissolved gases like water vapor (H2O), carbon
dioxide (CO2), and sulfur dioxide (SO2).
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.
Mafic magma is characterized by low silica content and high iron and magnesium
content. The low silica gives this type of magma a low viscosity, which means it will
flow easily. This results in effusive eruptions, where runny lava pours out of the ground.
This type of magma is typical of shield volcanoes, and forms rocks called basalt.
Felsic magma has a high silica content, giving it a high viscosity. This results in highly
explosive eruptions, such as the 1980 eruption of Mt. St. Helens. Felsic magmas form
composite volcanoes and lava domes, forming rocks composed of rhyolite.
Intermediate magmas fall between mafic and felsic – with an intermediate silica
content. The eruptions are typically explosive, forming composite volcanoes, and
producing rocks called andesite.