What
Causes Volcanoes?
To
understand what causes volcanoes, you need to understand how the earth is made
up. The earth has three main layers: the crust, the mantle and the core. The
crust is made up of solid rock and varies in thickness. It is more than 60km
thick under mountain chains like the Alps
and Himalayas, but just 5km under the
oceans. The mantle is a thick layer of molten rock (called magma), and the core
is made up of an outer liquid layer and a solid centre.
Temperatures inside the earth are very high – over 5000’C in the core. This means that the planet on which we live is like a huge fiery ball of hot molten rock, surrounded by a few kilometers of relatively cool, hard rock – the crust. Because heat rises, the magma in the earth’s mantle has to find a way to rise upwards though the crust above it, rather like the way that hot air rises.
An erupting volcano
A volcano erupts when magma escapes from inside the earth. As the magma is escaping from a confined space, a lot of energy is released with it, as happens with any other explosion. This is why many eruptions also produce huge quantities of gases and dust. Magma sometimes rises under enormous pressure, so it not only finds cracks in the earth’s crust, it can also create them. When magma reaches the earth’s surface it is called lava.
Tectonic plates and volcanoes
The
earth’s crust is its thinnest layer. It is broken up into large pieces, called
tectonic plates. These plates lie above the hot, liquid mantle. Each plate
contains some continental crust (land) and some oceanic crust (sea-bed). Huge
currents of molten rock circulate deep in the mantle, causing the plates to move
about very slowly on the earth’s surface.
If you look at the location of volcanoes in relation to these plates, you will notice some similarities. Many of the world’s volcanoes occur along the edges of boundaries of the plates. This is no coincidence. Plate boundaries are among the most geologically active places on earth. Here, new rock is being both created and destroyed, so this is where most of the world’s volcanic eruptions and earthquakes occur.
The Pacific ‘Ring of Fire’
More than half of all the world’s volcanoes are found in the Pacific ‘Ring of Fire’. This area forms a circle stretching down the eastern side of the Pacific Ocean, from Alaska in the north, through the Rocky Mountains of Canada and the USA, to the Andes mountains of South America. It loops back around the western side of the Pacific, up through New Zealand, Indonesia and Japan. Many of the world’s most famous volcanoes are found in this ‘Ring of Fire’: Cotopaxi in Ecuador, which last erupted in 1928; Mount St Helens in the USA, which erupted spectacularly in 1980; and Krakatoa in Indonesia, which killed 36000 people when it erupted in 1883.
Spreading centers
Some plate boundaries follow the line of the land surface. For example, the eastern edge of the Pacific Plate seems to run down the coast of the American continent. Others are found at the bottom of the world’s oceans. Here, hidden from view, lie extensive ranges of mountains, canyons and volcanoes. New rock is being created here by volcanoes spewing out lava underwater. Scientists estimate that between 2-5 cm of new crust is created each year by volcanoes along the Mid-Atlantic Ocean Ridge.
Types of volcano
There are three main types of volcano. Scientists decide what type a volcano is examining the plate movements that have caused it to form.
1.
Subduction volcanoes
Subduction volcanoes occur where plates move towards each other and collide. The areas where this happens are called destructive plate margins, because the earth’s crust is being destroyed. The diagrams on this page show that three main types of destructive plate margin, where subduction volcanoes are formed.
One plate, usually the that is heavier or more dense, is forced to dive (subduct) beneath the other into the mantle, causing it to heat up and melt. This melted material pushes its way back to the earth’s surface under great pressure, and often erupts explosively. The area where the two pieces of crust are in contact is called the subduction zone.
(1) Ocean to continent margins
Along the western coast of South America, a collision zone has formed where the Nazca Plate meets the South American Plate. The Nazca Plate sinks below the lighter continental crust of the South American Plate, leading to some violent volcanic eruptions in the Andes.
(2) Continent to continent margins
Where two plates carrying continental crust collide, for example where the Indo-Australian Plate meets the Eurasian Plate in northern India, the continental crust crumples to form fold mountains. Here, the melting crust which is forced into the mantle by the collision can again cause volcanic eruptions.
(3) Ocean to ocean margins
Sometimes, two plates which are made up of oceanic crust meet. Here again one plate subducts. When magma rises to the surface it forms a chain of volcanoes known as an island arc, like the Lesser Antilles in the Eastern Caribbean, and the Aleutians and the islands of Japan in the Pacific Ocean.
2.
Rift volcanoes
Where plates are moving apart, as in the middle of the Atlantic Ocean, magma rises through the gap between the separating plates and creates new rock. Here the volcanoes are known as rift volcanoes.
There are more rift volcanoes than subduction volcanoes, but we hear far less about them. Most of them are hidden from view on the ocean floor, and they do not often threaten human life or property. Also, they usually erupt more gently than other types of volcano, with lava that oozes rather than explodes from inside them.
Iceland, which straddles the Mid-Atlantic Ocean Ridge, and the Rift Valley of East Africa are two of the few places where rift volcanoes are found on land.
3.
Hot-spot volcanoes
Some volcanoes are found away from the edges of the plates. They are formed where rising plumes of magma force their way to the surface from deep within the mantle, a ‘hot spot’. As the plate moves, the existing volcano is carried away from the hot spot and becomes extinct. The next time the pressure builds up, new magma erupts to form another volcano in its place.
The best-known hot-spot volcanoes are in the Hawaiian Islands. The oldest in the west, such as Niihau and Kauai, contain extinct volcanoes, but the eastern islands of Maui and Hawaii itself still have active volcanoes.
Volcanic forms
The shape and size of a volcano are determined by:
-
The type of eruption
-
The type of lava which is produced in the eruption
-
The relative amounts of lava and ash which build up to form the volcano
Volcanoes come in many shapes and sizes but they can be grouped into the four main types.
Ash and cinder cone volcanoes occur where an explosive eruption hurls small, solid fragments of ash and rock from the volcano’s vent. The ash and rock build up to form volcanoes that are steep-sided but not very tall. This type of volcano is found in the Craters of the Moon area in Idaho, USA. Another example is Paricutin in Mexico.
Acid lava cone volcanoes are made up of lava which is thick or viscous. This flows very slowly, like treacle, and does not extend very far from the vent. It forms cones that have steep sides. An example is Mount Ngauruhoe in New Zealand which last erupted in 1975.
Shield
volcanoes, like
Mauna Loa on Hawaii in the Pacific Ocean, and Piton de la Fournaise on Reunion
Island in the Indian Ocean, are made up of basalt-rich lava which is thin, runny
and spreads a long way from the vent. As a result, shield volcanoes are very
large but have very gently sloping sides. Shield volcanoes are mostly made up of
lava and contain very little ash or cinder (approximately 95 per cent lava and 5
per cent ash).
Composite cone volcanoes, also known as stratovolcanoes, make up more than 60 per cent of all volcanoes on earth. They are usually quite tall. They are formed by a cycle of quiet eruptions of runny lava followed by explosive eruptions of thick lava. Stratovolcanoes have more ash than shield volcanoes. This combination of high ash content and a thick, slow-moving lava means that their sides are much steeper than shield volcanoes. Mount St Helens in the USA, Pinatubo in the Philippines and Fuji in Japan are all examples of composite cone volcanoes.
Volcanic eruptions
The exact
nature of a volcanic eruption is related to the type of lava which spills out on
to the earth’s surface. Some lava is thin and runny, and flows freely.
Eruptions of this type of lava are
usually fairly gentle. Thicker lava tends to
erupt much more explosively. It cools quickly too, and may form a solid plug
inside the main vent of a volcano.
Calderas
If the vent of a volcano is blocked, pressure builds up below the blockage. Eventually the pressure may become so great that there is an explosion which blows off the top of the volcano. This creates a larger crater at the top called a caldera. Over a long period of time, if the volcano remains inactive, the crater can fill with water, creating a crater lake. Calderas are often more than 5 km in diameter. The world’s largest caldera is at Mount Aso on Kyushu, the most southern of Japan’s four main islands. It is 22 km long and about 16 km wide.
Types of
volcanic eruption
Volcanoes can be divided according to the explosiveness of their eruptions. The least severe are known as Hawaiian eruptions – these generally produce shield volcanoes. The most severe are called Plinian eruptions – these often involve the collapse of the volcano’s cone and the formation of a caldera. Plinian eruptions were named after the Roman scholar Pliny the Elder, who lost his life, when Mount Vesuvius erupted in AD 79. His nephew, Pliny the Younger, wrote to Tacitus describing the eruption as it began.
Hawaiian
The least violent type of eruption. Large amounts of runny lava erupt and produce large volcanoes with gentle slopes.
Strombolian
Mild but fairly regular eruptions. Small sticky lava bombs, ash, gas and glowing cinders erupt.
Vulcanian
Violent eruptions shoot out very thick lava and large lava bombs.
Peleean
A violent type of eruption. Thick, sticky lava is accompanied by a burning cloud of ash, gas and pumice (a nuee ardente, which is French for ‘fiery cloud’).
Plinian
The most violent type of eruption. Cinders, gas and ash are flung explosively high into the air. The volcano cone often collapses to form a caldera.