Volcanos

How Do Volcanos Work?
A volcano is an opening, or rupture, in the Earth's surface or crust, which allows hot, molten rock, ash and gases to escape from deep below the surface. Volcanic activity involving the extrusion of rock tends to form mountains or features like mountains over a period of time.

There are 3 main types of volcanos. Shield, Cinder and Stratovolcanos. Rinjani is a Stratovolcano.

Stratovolcanoes are tall conical mountains composed of lava flows and other ejecta in alternate layers, the strata that give rise to the name. Stratovolcanoes are also known as composite volcanoes. Classic examples include Mt. Fuji in Japan, Mount Mayon in the Philippines, and Mount Vesuvius and Stromboli in Italy.

Volcanic cones or cinder cones result from eruptions that throw out mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 m high. Most cinder cones erupt only once. Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Paricutin in Mexico and Sunset Crater in Arizona are examples of cinder cones.

Volcano:
1. Magma reservoir
2. Country rock
3. Conduit (pipe)
4. Base
5. Sill
6. Branch pipe
7. Layers of ash emitted by the volcano
8. Flank
9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14. Crater
15. Ash cloud

Rinjani is caused by a Subduction Zone

Subduction zones are the boundaries where the earth's plates converge, one against another. In the enormous furnace of a subduction zone, the denser plate plunges beneath the other, and portions of it melt by the intense heat generated within the mantle, turning the solid rock of the crust into molten magma.

Of all visible volcanoes, 94 per cent, stand within subduction zones. A mere handful of countries – Indonesia, Japan, America, Russia, Chile, the Philippines, New Guinea, New Zealand and Nicaragua play host to most of these. These countries are home to 9 out of 10 volcanoes that are liable to erupt today or have done so in recent history.

The greatest number of volcanoes is to be found in the enormous subduction zone that stretches 5000 km from Sumatra to the Birds Head of the north western tip of the island of New Guinea, in Indonesia. In this immense factory, there are at least 87 active volcanoes.

Indonesia has more volcanoes than any other political entity in human history. On the island of Java alone there are 21 active volcanoes. Rinjani forms just one volcano in this long chain of activity throughout the archipelago.

The earth has fashioned 3 of the 5 greatest volcanoes of all time in this one subduction zone. Mount Toba in northern Sumatra erupted 74,000 years ago leaving a lake 100km long and 25 km wide. Its Volcanic Explosivity Index (VEI) was 8, the highest ever recorded.This eruption alone reduced the entire planetary human population to as few as 10,000.

The second largest was Tambora on the island of Sumbawa with a VEI of 7. This enormous eruption occurred in 1815 and it can still be seen to the east of Rinjani’s summit at sunrise. Check out our Mt Tambora Exploartion in May 2011 here.

The third was Mt Taupo in New Zealand which erupted in AD180 and the fourth is Novarupta in Alaska, erupting in 1912.

The fifth largest is Krakatoa between Java and Sumatra (Indonesia) with a VEI of 6.5 in 1883 killing 33,000 people and its sound could be heard 3000  miles away.

VEI Volcanic Explosivity Index

The VEI (created by the Smithsonian Institute in Washington) is made up of 2 factors:

1/ The amount of material ejected from the explosion

2/ The height to which these particles are thrown into the atmosphere.

Rinjani has erupted many times since 1847 and has had a high VEI of 3.

In the most recent eruption of May 2009, the plume rose to 4000m and ash fell on the nearby village of Senaru.

Volcanic Activity (Rinjani is an Active Volcano)

A popular way of classifying magmatic volcanoes goes by their frequency of eruption, with those that erupt regularly called active, those that have erupted in historical times but are now quiet called dormant, and those that have not erupted in historical times called extinct. However, these popular classifications—extinct in particular—are practically meaningless to scientists. They use classifications which refer to a particular volcano's formative and eruptive processes and resulting shapes, which was explained above.

There is no real consensus among volcanologists on how to define an "active" volcano. The lifespan of a volcano can vary from months to several million years, making such a distinction sometimes meaningless when compared to the lifespans of humans or even civilizations. For example, many of Earth's volcanoes have erupted dozens of times in the past few thousand years but are not currently showing signs of eruption. Given the long lifespan of such volcanoes, they are very active. By our lifespans, however, they are not. Complicating the definition are volcanoes that become restless (producing earthquakes, venting gasses, or other non-eruptive activities) but do not actually erupt.

Scientists usually consider a volcano active if it is currently erupting or showing signs of unrest, such as unusual earthquake activity or significant new gas emissions. Many scientists also consider a volcano active if it has erupted in historic time. It is important to note that the span of recorded history differs from region to region; in the Mediterranean, recorded history reaches back more than 3,000 years but in the Pacific Northwest of the United States, it reaches back less than 300 years, and in Hawaii, little more than 200 years. The Smithsonian Global Volcanism Program's definition of 'active' is having erupted within the last 10,000 years.

Dormant volcanoes are those that are not currently active (as defined above), but could become restless or erupt again. Confusion however, can arise because many volcanoes which scientists consider to be active are referred to as dormant by laypersons or in the media.

Extinct volcanoes are those that scientists consider unlikely to erupt again. Whether a volcano is truly extinct is often difficult to determine. Since "supervolcano" calderas can have eruptive lifespans sometimes measured in millions of years, a caldera that has not produced an eruption in tens of thousands of years is likely to be considered dormant instead of extinct.

For example, the Yellowstone Caldera in Yellowstone National Park is at least 2 million years old and hasn't erupted violently for approximately 640,000 years, although there has been some minor activity relatively recently, with hydrothermal eruptions less than 10,000 years ago and lava flows about 70,000 years ago. For this reason, scientists do not consider the Yellowstone Caldera extinct. In fact, because the caldera has frequent earthquakes, a very active geothermal system (i.e., the entirety of the geothermal activity found in Yellowstone National Park), and rapid rates of ground uplift, many scientists consider it to be an active volcano.