8+-+Volcanoes

__
 * WHAT IS VOLCANO ** __

A volcano is a cone shaped mountain formed from **lava** or **lava and ash** which has been forced through a hole in the earth's crust.

Volcanoes are classified as


 * Active** - Volcanoes that have erupted recently
 * Dormant** - Volcanoes that have not erupted recently but have done in the last 2000yrs
 * Extinct** - Volcanoes that have not erupted for many thousands of years

__SIMPLE ANIMATED VOLCANO FORMATION__

__ **3 TYPES OF VOLCANOES** __

(1) Composite Volcanoes (2) CInder Volcanoes (3) Shield Volcanoes

**__ (1) COMPOSITE VOLCANOES __**

Composite volcanoes form when runny lava escapes through a fissure and flows a long way. Composite volcanoes are tall cone-shaped mountains that are typically steeply-sided, symmetrical cones of large dimensions.

The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the earth's crust rises to the surface.

The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash etc. are added to its slopes.

Composite volcanoes erupt in different ways at different times. These volcanoes are built in layers by multiple eruptions, sometimes recurring over hundreds of thousands of years, sometimes over a few hundred. Andesite magma (the most common but not the only magma type), tends to form composite cones.

During some eruptions, cinders, bombs and blocks form a mountain or add height to one that earlier volcanic eruptions had built. During other eruptions, lava flows cement these rocks together. Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents.

Lava either flow through breaks in the crater wall or from fissures on the flanks of the cone. Lava, solidified within the fissures, form dikes that act as ribs which help to strengthen the cone.

They may rise as much as 8,000 feet above their bases. Depending on the type of volcanic material it is composed of, some can grow to such heights that their slopes become unstable and are susceptible to collapse from the pull of gravity.

When volcanic activity ceases, erosion begins to destroy the cone. After thousands of years, the cone is stripped away and the hardened magma filling the conduit (the volcanic plug) and fissures (the dikes) become exposed, and it too is slowly reduced by erosion. Finally, all that is left is the plug or "volcanic neck" and dike complex projecting above the land surface.

Some composite volcanoes occur in chains and are separated by several tens of kilometers. There are many composite volcano chains on earth, notably around the Pacific rim, known as the "Rim of Fire".

__ Other examples of composite volcanoes and their locations are: __

-Mount St. Helens - Washington State

-Mount Rainier - Washington State

-Mount Vesuvius - Italy

__** DIAGRAM OF COMPOSITE VOLCANO **__


 * __ (2) SHIELD VOLCANOES __**

Shield volcanoes are large volcanoes that are built almost entirely of fluid lava flows.

It has broad sloping sides and is usually surrounded by gently sloping hills in a circular or fan shaped pattern, that looks like a warrior's shield.The volcano is produced by the action of the gas with heat from the earth's core.

This action melts rock turning it into magma. The pressure from the heat of the gas pushes the magma upwards till it explodes. Molten magma shoots upward from deep below the ocean floor and breaks through the plates to form shield volcanoes.

Lava flows gently and continuously out of the central volcanic vent or group of vents. This lava is very runny, and can't be piled up into steep mounds.

It gradually comes together and cools around the volcano. The volcanoes are built up slowly by the accretion of thousands of highly fluid lava flows called basalt lava.

The lava spread widely over great distances, then cools as thin gently dipping sheets. Lavas also erupt from vents along fractures (rift zones) that form on the flanks of the cone.

Some of the largest volcanoes in the world are Shield volcanoes.Shield volcanoes may be produced by hot spots which lay far away from the edges of tectonic plates.

Shields also occur along the mid-oceanic ridge, where sea floor spreading is in progress and along subduction related volcanic arcs.In northern California and Oregon, many shield volcanoes have diameters of three or four miles and heights of 1,500 to 2,000 feet.

A good example of a shield volcano is the Island of Hawaii.

The Big Island is formed of five coalesced volcanoes of successively younger ages. The Hawaiian Islands are composed of linear chains of volcanoes including Kilauea and Mauna Loa on the island of Hawaii. That is the shield volcano!

__** DIAGRAM OF SHIELD VOLCANO **__
 * __ (3) CINDER CONE VOLCANOES __**

Cinder cone volcanoes are the most common kind of volcanoes.They are steep sided cones of basaltic fragments and are smaller and simpler than composite volcanoes.

Streaming gases carry liquid lava blobs into the atmosphere that fall back to earth around a single vent to form the cone.

The volcano forms when ash, cinders and bombs pile up around the vent to form a circular or oval cone.Cinders are melted volcanic rock that cooled and formed pebble-sized pieces when it was thrown out into the air.

They are ejected from a single vent and accumulate around the vent when they fall back to earth.Bombs are melted volcanic rock that cooled and formed large pieces of rock when it was thrown out into the air before landing on the ground.

Cinder cones are chiefly formed by Strombolian eruptions. They grow rapidly and soon reach their maximum size. Cinder cones can occur alone or in small to large groups or fields.

Most have a bowl-shaped crater at the summit. The longer the eruption, the higher the cone. They rarely exceed 250 meters in height and 500 meters in diameter, although some may rise to as high as 650 meters or more.

If gas pressure drops, the final stage cinder cone construction may be a lava flow that breaks through the base of the cone. If a lot of water in the environment has access to the molten magma, their interaction may result in a maar volcano rather than a cinder cone. The shape of a cinder cone can be modified during its life.

When the position of the vent alters, aligned twin cones develop. Nested, buried or breached cones are formed when the power of the eruption varies. A example of a cinder cone is Paricutin in Mexico. In Iceland, Surter I and Surter II cinder cone volcanoes.

**__ DIAGRAM OF CINDER CONE VOLCANO __**



__** HOW DOES THE VOLCANO ERUPTS **__ The Earth's crust is made up of huge slabs called **plates**, which fit together like a jigsaw puzzle.

Between the Earth's crust and the mantle is a substance called **magna** which is made of rock and gases.

When two plates collide, one section slides on top of the other, the one beneath is pushed down. **Magma** is squeezed up between two plates.


 * __ What is the difference between lava and Magma? __**
 * Magma** is liquid rock inside a volcano.


 * Lava** is liquid rock (magma) that flows out of a volcano. Fresh lava ranges from 1,300° to 2,200° F (700° to 1,200° C) in temperature and glows red hot to white hot as it flows.


 * __ An erupting volcano can caused: __**
 * tsunamis
 * flashfloods
 * earthquakes
 * mudflows
 * rock falls.


 * __ Effects: __

Negative impacts:**

Volcanoes can have a very serious effect on the lands and people around them when they erupt.
 * Buildings are destroyed and people are made homeless.
 * People are killed.
 * Clouds of ash cover plants making them inedible.
 * Poisonous gases kill people and animals.
 * Dust causes pneumonia and illnesses to the survivors.
 * Dark skies, severe winds and heavy rains may follow an eruption for months afterwards.

One in 10 people in the world live within 'danger range' of an active volcano.
 * Positive effects or beneficial for people:**

People can get used to living near a volcano, but it is always a little dangerous.

Scientists have estimated that at least 200,000 persons have lost their lives as a result of volcanic eruptions during the last 500 years . People set up homes on the slopes of volcanoes because of the rich, fertile soil produced**.**


 * __ Diagram of a Volcano __**

__** What is the Ring of Fire? **__ Over half of the world’s volcanoes arise in a belt around the Pacific Ocean called the Ring of Fire.
 * __ Major world volcanoes __**

[|Thinkquest online volcanoes] [|National Geographic] [|Volcano World for Kids]
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