Volcanoes: Volcanoes

A volcano is an opening in the crust of the Earth through which molten rock and gases from the interior of the Earth reach the surface. The definition of volcano can include the mountain like structures that may be created from the material ejected at the volcano. Volcanoes can be created by the melting of rock at divergent and convergent plate margins, and from decompression melting at hot spots in the mantle, such as the one under the Hawaiian Islands

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.

How do volcanoes errupt?

The erruption of volcanoes is a long process and may be difficult to explain.

First a volcano makes something called magma from melted rock. The magma goes through a circulation. It has to form at the bottom of the volcano and then start its way up the main vent. The main vent is a hole that is in the volcano and when the volcano is ready to erupt the lava is at the top of the main vent. The magma goes up the main vent slowly while it is still getting hotter. When the magma is about half way up the main vent it turns into lava. Lava is a very hot liquid which burns the remaining rocks from the magma. The lava slowly continues up the main vent. While going up the lava continues to get hotter and hotter. Ash and rocks are collected and the lava is getting hotter and hotter while the lava is continuing its way up the main vent. When the lava is at the top of the main vent the volcano erupts. The lava blasts out of the volcano along with ash, rocks, and a cloud of dust that is very thick. The ash and rock crumble to the ground, but the lava is either moving down the volcano side very slowly or at a high speed. The lava burns down almost everything in its way, and it sometimes leaves bits of things burning. The lava from the volcano can cool fast, or sometimes the lava will slowly cool down from its intense heat. Lava that cools slowly forms igneous rocks. There are many types of igneous rocks. Volcanoes can damage themselves in the explosion. A volcano literally blows its top off. Volcanoes can be under water or on land. Volcanoes that are under water take a longer time than if they are on land because they are under water the water slows down the magma and lava but if the volcano is on land the lava and magma can move quicker up the main vent. It just depends on the environment how fast the volcano can make the magma the magma makes lava and the volcano makes an explosion. If the volcano is under water the cooled lava will probably make an
island. The Hawaiian Islands is an example of island made by a chain of volcanoes.

If simpler,these diagrams will explain more on how a volcano occurs;

Active and non-active volcanoes:There are volcanoes in different phases of activity:Active volcanoes, which are likely to erupt at any moment, dormant volcanoes, which lie dormant for centuries, but then erupt suddenly and violently, and extinct volcanoes - ones no longer likely to erupt.

The ordinary volcanoes can be divided in different types, relating to their forms:

1. The shield volcano:
This is a broad, shallow volcanic cone, which arises because the running lava, which is fluid and hot, cools slowly.
2. The dome volcano:
This one has a steep, convex slope from thick, fast-cooling lava
3. The ash-cinder volcano:
Throws out - besides lava - much ash into the air. Through this the volcanic cone is built up from alternate layers of ash and cinder.
4.The composite volcano:
These are also built up from alternate layers of lava and ash but, besides its main crater, it has many little craters on its slope.
5. The caldera volcano:
An older volcano with a large crater which can be 62 miles(100km) wide. In this crater many little new craters are formed.

Extinct volcanoes

Extinct volcanoes are those that scientists consider unlikely to erupt again, because the volcano no longer has a lava supply. Examples of extinct volcanoes are many volcanoes on the Hawaiian Islands in the U.S. (extinct because the Hawaii hotspot is centered near the Big Island). Otherwise, whether a volcano is truly extinct is often difficult to determine.

Dormant volcanoes

It is difficult to distinguish an extinct volcano from a dormant one. Volcanoes are often considered to be extinct if there are no written records of its activity. Nevertheless volcanoes may remain dormant for a long period of time, and it is not uncommon for a so-called "extinct" volcano to erupt again.

Effects of volcanoes

There are many different types of volcanic eruptions and associated activity: phreatic eruptions (steam-generated eruptions), explosive eruption of high-silica lava (e.g., rhyolite), effusive eruption of low-silica lava (e.g., basalt), pyroclastic flows, lahars (debris flow) and carbon dioxide emission. All of these activities can pose a hazard to humans. Earthquakes, hot springs, fumaroles, mud pots and geysers often accompany volcanic activity.

Gas emissions from volcanoes are a natural contributor to acid rain. Volcanic activity releases about 130 to 230 teragrams (145 million to 255 million short tons) of carbon dioxide each year.Volcanic eruptions may inject aerosols into the Earth's atmosphere. Large injections may cause visual effects such as unusually colorful sunsets and affect global climate mainly by cooling it. Volcanic eruptions also provide the benefit of adding nutrients to soil through the weathering process of volcanic rocks. These fertile soils assist the growth of plants and various crops. Volcanic eruptions can also create new islands, as the magma cools and solidifies upon contact with the water.

Famous volcanoes

Name :Crater lake

Location:Oregon

Facts:Last erupted 6,600 years ago with 43,000x the force of an atomic bomb.
Name:Mt. Rainier
Location:Washington

Facts:prehistoric lava flowed for 70 miles.

Name:Mt. Etna
Location:Sicily

Facts:Constantly active; still creates craters regularly

Bibliography

http://www.volcanoes.com/

http://en.wikipedia.org/wiki/Volcanos

http://cgz.e2bn.net/e2bn/leas/c99/schools/cgz/accounts/staff/rchambers/GeoBytes%20GCSE%20Blog%20Resources/Images/Plate%20Tectonics/Plate%20Tectonics/VolcanoStructure.jpg

http://www.geography.learnontheinternet.co.uk/images/nathaz/volcanoxsec.gif