Volcanoes form when magma reaches the surface of the Earth. They can be found along plate boundaries where magma is able to reach the surface. There are over 1300 potentially active volcanoes worldwide. Volcanoes have different features like craters, vents, and magma chambers. They also go through life cycle stages of being active, dormant, or extinct. Eruptions can produce different types of lava and ash deposits that build different types of volcanic cones over time. While volcanoes provide fertile soils and minerals, their eruptions can negatively impact the environment through poisonous gases, lava flows, ash falls, pyroclastic flows, and lahars that damage infrastructure and agriculture.

1. CSEC Geography
O. Johnson
Volcanoes
The name of the Roman god of fire, Vulcan, gave rise to the English word ‘volcano’. A volcano
is a vent, or opening in the crust, from which pour molten rock, rock debris, gases and
steam. When magma penetrates the surface it is known as lava.
Most volcanoes are found along convergent and divergent plate boundaries. Here there is molten
rock or magma to supply the volcanoes. There are about 1300 potentially active volcanoes in the
world today. A small number of volcanoes are a long way from plate boundaries. These are
found at hot spots. Here the temperature at the boundary of the mantle and crust is unusually
high, and there are lines of weakness in the crust which the magma can follow to reach the
surface. The Hawaiian Islands, in the middle of the Pacific Ocean, have been formed in this way.
Hot spots can also be found beneath continents; an example is the Yellowstone Basin in the USA
Diagram showing a volcano

2. Crater: This is the opening at a top of a volcano through which lava, ash and gases emerge.
Conelet / secondary cone/ parasitic cone: This is a small cone which often forms on the side of
an existing volcano.
Pipe: The tunnel like structure which links the vent at the base of the volcano to the crater at the
top.
Magma chamber: a reservoir of magma within the earth’s crust beneath a volcano.
Vent: This is an opening in the earth’s crust from which molten rock, gases, and rock debris
escape.
Ash cloud- A cloud of ash formed by explosive eruptions.
Stages of a volcano
Volcano usually pass through three stages in their life cycle. Volcanoes are:

3.  Active when they’re currently erupting or eruptions occur at frequent intervals.
OR
Oregon State University- An active volcano is a volcano that has had at least one eruption
in the past 10,000 years. For e.g. Kick em Jenny north of Grenada
 Dormant when eruptions are infrequent and one has not occurred for some time. They are
called sleeping volcanoes.
OR one that hasn’t erupted in the past 10,000 years, but which is expected to erupt again
 Extinct when it is not expected to erupt again.
Homework: Find examples of active volcanoes in the Caribbean. Also find examples of
active, dormant and extinct volcanoes across the world.
Types of Lava/Magma
There are two basic types of lava: basic and acidic.
Basic/Basaltic Acidic
Low silica content High silica content
Darker color Lighter color
It is often very hot reaching temperatures of
10000
C-12000
C
Often has lower temperatures (8000
C-
10000
C)
Rich in Iron and Magnesium Low in Iron and Magnesium
Low viscosity (very fluid and runny e.g. water High viscosity ( very thick e.g. molasses or
toothpaste)
Flows over long distance before solidifying Flows for a short distance then solidifies
Forms gentle sided cones or volcanoes e.g.
shield volcanoes
Forms high, steep and dumpy cones
Eruptions are frequent and gentle Eruptions are infrequent and very explosive
Lava and steam eruptions Ash, rock, gases and lava ejected, pyroclastic
flow likely
It is associated with divergent plate margins
and hot spot volcanoes
It is associated with convergent plate
boundaries ( subduction zones and island
arcs)

4. Formation of volcanoes
Sometimes magma reaches the earth’s surface through a vent or a fissure (elongated crack).
When magma emerges on the surface it is called lave. If lava emerges through a vent it builds a
cone shape mound (typical volcano we are associated with). Successive eruptions overtime will
build up the cone. If magma emerges through a fissure, it builds up a lava plateau.
A volcanic cone is made of either lava, or a mixture of lava and ash, or ash and cinders only
(small fragments of lava). There are three types of volcanic cone:
 Lava cone ( basic and acidic) – (ONLY LAVA)
 Composite cone ( both lava and ash)
 Ash and Cinder Cone (ash alone)
Lava cones
Basic lava cones
Some volcanoes erupt a type of lava which flows very easily and tends to flow quite some
distance before solidifying. This is known as basic lava. This volcano is composed of basic lava
which spreads over a wide area before solidifying. This type of lava tends to form broad
volcanoes with very gentle slopes. An example is Mauna Loa on Hawaii. Since these volcanoes
resemble a warrior’s shield, they are called shield volcanoes.

5. Acidic Lava
This lava cone is made up entirely of acidic lava. Because the lava has a high viscosity it flows
for a short distance then solidifies. Acidic lava produces steep sided and dumpy cones.
Ash and Cinder Cone
In a violent volcanic eruption lava can be thrown to great heights where it cools and breaks into
small fragments of lava known as volcanic ash/cinders. The ash falls to the surface building up a
cone. Successive eruptions will build the cone overtime. An example is Paracutin in Mexico

6. Composite cones
Some volcanoes produce different types of eruptions. Sometimes there are massive ash eruptions
which produces layers of ash. At other times there are eruptions of lava which produces layers of
lava. The ash is the result of a violent eruption while the lava is produced by more gently
eruption. The result of a series of eruptions is a steep sided cone composed of successive layers
of ash and lava. This type of volcano is known as a composite cone. Composite cones are found
near convergent plate boundaries. The Soufriere Hills volcano in Montserrat is an example of a
composite cone.

7. Extrusive volcanic features
When lava cools and solidifies on the earth’s surface it forms extrusive volcanic features.
Examples include ash and cinder cone, basic lava cone, acidic lava cone, composite cone
caldera, spine, lava plateau and spine. These are all landforms are features above the
earth’s surface.
Caldera
A volcanic eruption may be so explosive that the whole top of the volcano sinks into the magma below.
A huge crater is left which may by many kilometres in diameter. Later reuptions may form new cones
inside a caldera. Lakes may form in the lowest parts of a caldera. An example is Krakatoa in Indonesia.

8. The Qualibou caldera in St.Lucia is 3.5 km x 5 km in size and was formed more than 30,000
years ago. The town of Soufriere is located inside of it.

9. Lava plateau/ Basalt plateau
Lava does not always reach the surface in a spectacular way. Sometimes large quantities of lava ooze out
slowly onto the surface from fissures (surface cracks). These fissure eruptions can cause lava to spread
out over a very wide area. Over time, a number of fissure eruptions in the same area can build up a high
plateau. Basalt plateaux are very large features, covering hundreds of thousands of square
kilometers and they usually “drown” the pre-existing landscape. In India the Deccan is a lava
plateau which covers almost 650 000 km2
Spine
Sometimes acidic lava is so viscous that it cools and solidifies in the crater to form a spine or plug.
However spines are rare because they often break up rapidly on cooling.

10. Intrusive volcanic features
Only a small amount of the magma that moves up from the mantly and through the crust reaches the
surface. Most magma cools and solidifies ( hardens) before it reaches the surface. As the magma moves
upwards it forces its way into lines of weakness in the rock. Bedding planes, joints and faults are all lines
of weakness followed by the magma. Once magma gets into a crack in the crust the huge force behind it
can cause the crack to widen.
Although intrusive volcanic features are formed underground, they may be exposed million of years later
if the rocks at the surface are eroded. Because volcanic rocks are hard they are often more resistant to
erosion than the rocks around them and they stand out in the landscape as higher ground.
 A dyke is formed when magma moving upwards towards the surface cool and solidifies. The
magma cuts across the bedding planes of sedimentary rock. Sometimes a large number of dykes,
called a dyke swarm, can occur together in an area. An example is the Moule a Chique headland
found at the southern tip of St Lucia.
 A sill is formed when magma flows horizontally between rock layers, roughly parallel to
the surface. This molten material cools forming a horizontal sheet of solid rock called a
sill. This may be horizontal or angled towards the surface.

11.  A laccolith is the result of large amounts of magma moving between rock layers causing
overlying rock strata to arch upwards.
 A bathlolith is much larger than the other intrusive volcanic features. It forms when a
giant underground reservoir of magma cools and hardens to form granite. Batholiths can
be several hundred kilometers in diameter. A batholith may form the roots of a mountain.
A good example in the Caribbean is the Tobago Batholith

12. Positive and negative effects of volcanic activity
Positive effects
Fertile Soils
Volcanic materials produce fertile soils. Ash and cinder are natural fertilizers as they are rich in
minerals. As lava cools and is eroded by the elements it also adds to soil fertility. For this reason
agriculture often thrives in volcanic regions, e.g. near Mt Vesuvius, which is an important
vegetable production are in southern Italy.
Land surface
Erupting volcanoes are thought to be the source of the first land as the hot planet cooled.
Eruptions continue in the sea creating new land. All the volcanic islands are created in this way.
There are so many islands that were created as a result of volcanic activity. The Eastern
Caribbean islands, the Aleutian Islands and the Marianas islands in the pacific are all volcanic
island arcs and were formed as a result of volcanic activity
Minerals
Many useful materials are formed directly or indirectly from volcanic activity. Building
materials such as granite and marble, precious minerals such as diamonds as well as silver and
copper are formed from magma and in and on the earth’s crust. Other mineral resources such as
gold, silver, nickel, copper, and lead are sometimes found around volcanic activity.
Geothermal energy
Geothermal energy production in Iceland is another positive of volcanic activity. Iceland is on
the Mid-Atlantic ridge (plates separating) and for this reason has a lot of volcanic activity.
Magma rises close to the surface of the crust and this heats the groundwater. This water is heated
to well beyond boiling point (up to 200 degrees Celsius) and becomes “super-heated”. Wells are
drilled into the rock and the hot water is pumped out. As this hot water reaches the surface it
does so as steam due to the intense heat. This steam is then used to drive turbines and create
electricity
Tourism

13. Volcanic peaks. Whether active or formant are natural tourist attractions in many parts of the
world. From the majestic Mt Fuji in Japan to the ever flowing Hawaiian cones, tourist visit and
wonder at this evidence of the earth’s interior. Volcanoes are also important tourist attractions.
Mount Vesuvius in Naples and Mount Etna in Sicily are major tourist attractions in southern
Italy. In Iceland, the geysers and hot springs caused by volcanic activity bring many tourists to
the island. This tourism generates jobs and money in areas that may not have many other sources
of employment. Jobs are generated in areas such as accommodation, transportation, sight-seeing
and retail (shops).Geysers are also used as tourist attractions such as the Yellowstone National
Park.
Building Construction
When lava/magma is cooled it form igneous rocks. Some igneous rocks such as granite are being
used in the construction industry for thousands of years. Granite is very durable and strong
igneous rocks that is used for all kinds of structures. Because pumice is so light it is used quite
often as a decorative landscape stone. Basalt is also quarried in some part of the world and is
also widely used in the construction industry.
Negative Effects
Poisonous gases - Although the predominant gas erupted from volcanoes is H2O vapour, other
gases are erupted can have disastrous effects on life. Some of these gases are Hydrogen Sulfide
(H2S), Sulphur Dioxide (SO2), Hydrogen Fluoride (HF), Carbon Dioxide (CO2), and Hydrogen
Chloride ( HCL). Gases such as carbon dioxide, carbon monoxide, and sulfur dioxide can travel
down a volcano and asphyxiate (suffocating) wildlife and humans.
Lava flows - A river of molten rock 1000 degrees centigrade that can travel at 40mph. lava flows
can cause extensive damage or total destruction by burning, crushing, or burying everything in
their paths. Lava flows can erupt relatively non-explosively and move very slowly (a few meters
to a few hundred meters per hour) or they can move rapidly (typically down steep slopes.

14. Tephra - All materials ejected from a volcano are called Tephra. These occurs when there is an
explosive eruption. They are classified according to their sizes. Materials the sizes of a football
can be ejected from a volcano. Ash, lapilli and volcanic bombs are tephra. The largest pieces of
tephra (greater than 64 mm) are called blocks and bombs often fall close to the volcano but
smaller size tephra such as ash and lapilli ( lapilli 2-64 mm) and ash (<2 mm) can be carried
downwind and affect areas far from the volcano. Millions of tons of ash can bury buildings.
Problems associated with tephra
- If ash builds up on the tops of roofs, it will often cause collapse. This is especially
common on flat topped buildings. Most deaths resulting from the eruption of Mount
Pinatubo in 1991 were due to collapsing roofs (Wolfe, 1992).
-Ash can disrupt electricity, television, radio, and telephone communication lines, bury
roads and other manmade structures, damage machinery, start fires, and clog drainage
and sewage systems
Ash is also a great hazard to airplanes. Ash from the 1982 eruption of Galunggung
Volcano in West Java, Indonesia caused engines in two jet airplanes to fail. Both aircraft
dropped 25,000 feet before they could get their engines to start again.
-Tephra can also destroy vegetation which can result in famine. Famines are the largest
indirect hazard produced by volcanic eruptions. In 1815, after the eruption of Tambora
which ejected 151 cubic kilometers of ash into the atmosphere, 80,000 people died due to
famine (Bryant, 1991 and Francis, 1993)
-Ash can produce poor visibility and cause respiratory problems.
Pyroclastic flows - Pyroclastic flows are very hot, fast moving clouds of gases and
tephra moving down the side of a volcano after an eruption column collapse. They are
also called nuée ardentes. They are extremely dangerous because they can travel up to
500 km/hr, reach distances of 30km and can be over 700 degrees Celsius in temperature.

15. They will burn, knock over or bury anything in their path. A pyroclastic flow from
Vesuvius volcano killed about 20,000 people in Pompeii in 79 CE.
Lahars ( resembles wet concrete) -A volcanic eruption usually leaves lots of loose
unconsolidated fragmental debris. When this loose material mixes with water from
rainfall, melting of snow or ice, or draining of a crater lake, a mudflow results. Volcanic
mudflows are called lahars. These can occur accompanying an eruption or occur long
after an eruption. Lahars are very dangerous because they do not require a volcanic
eruption yet can travel hundreds of miles. All that is required is loose pyroclastic material
on the volcano that mixes with precipitation or melting snow.
 In general, they destroy anything in their path, carrying away homes, buildings, bridges,
and destroying roads, and killing livestock and people.