Volcanoes erupt due to rising magma from the Earth's mantle. The viscosity and gas content of the magma determines the intensity and type of eruption. Highly viscous magma with more dissolved gases causes explosive eruptions that eject ash and rock into the atmosphere, while more runny magma results in effusive eruptions where lava flows out. Volcanic eruptions can have both negative effects like destroying landscapes and releasing ash that impacts the climate, as well as positive effects through generating nutrient-rich soils and opportunities for geothermal energy extraction.

23. • Volcanoes come in lots of shapes and sizes. It is all down to the
style of the eruptions that they produce. Some have steep slopes
and erupt with big explosions, throwing vast quantities of rock and
ash high into the atmosphere. Others have gentle slopes indicative
of their less dramatic, but mesmerizing, volcanic eruptions.
• A peaceful photo of Nakano-jima volcano, Japan

24. • All volcanic eruptions start the same, with molten rock (magma) rising up through the Earth
and breaking through the Earth’s crust. However, this is where the similarities end. Sometimes
magma spews out of the volcanic vent as red hot lava flows, but sometimes it doesn’t. Instead
it clogs up inside the vent forming a plug or lava dome. These differences are all to do with
how sticky or runny the lava is, which also holds to key to why some volcanoes erupt
explosively while others do not.
• A matter of viscosity
• Honey is thicker than water
• Viscosity is a measure of how thick a liquid is, and therefore how easily it flows. For example,
water pours more easily than honey and therefore has a lower viscosity.
• Magma is much more viscous than honey. Although it is still technically a fluid, it has an
internal structure made from silicon-oxygen tetrahedrons (silicon atoms surrounded by 4
oxygen atoms). The strength of this structure, and therefore how viscous, or thick, the magma

28. The effect of volcanic eruption
• Unlike other natural disasters such as floods, wild fires and earthquakes, volcanoes can have some positive effects, even though they can be very disastrous.
• Let us see some negative effects of volcanoes:
• volcano factEruptions occurring close to human settlements may spill and destroy lives and property. People often have to be evacuated.
• Example: Chaparrastique volcano in El Salvador started erupting on 29 Dec, 2013. Anyone living within 2 miles of the volcano is evacuated.
• Source: BBC News:www.bbc.co.uk/news/world-latin-america-25545555
• volcano factAsh discharged very high into the stratosphere can have negative consequences on the ozone layer. Read more about that here.
• volcano factLandscapes and natural sceneries can be destroyed.
• volcano factAsh and mud can mix with rain and melting snow, forming lahars. Lahars are mudflows flowing at very fast pace.
• Some positive effects of volcanoes include:
• Different types of erupting volcanoes provide extraordinary scenery, so beautiful and natural that they attract tourists to the area, bringing in some economic value.
• Places close to volcanic activities tend to have higher potential for geothermal energy, which can be an advantage to the towns and cities.
• Some ash and lava breakdown become soils that are rich in nutrients, and become good areas for crop planting activities.

29. Signs of impindings

30. What causes a volcano to erupt?
• Volcanic eruptions and earthquakes are a way for Earth to release
pressure and heat, much like a safety valve. There are three
dominating theories to explain what causes a volcano to erupt -

32. • Due to Density-difference in Magma
• As per the first theory, due to heat and pressure in the Earth's mantle, solid rocks melt, to form magma. Magma has the same mass as
the solid rock, but more volume, making it lighter and more buoyant. So, it will attempt to rise, if this magma continues to encounter
high-density material till it reaches the Earth's crust, volcanic eruption occurs. It can either be in the form of a lava-flow or may be
explosive.
• Pressure of Released Gases
• According to the second theory, magma contains dissolved substances such as water, sulfur dioxide and carbon dioxide. The solubility of
magma decreases with the decrease in pressure as it rises up towards the crust, and the gases get released in the form of bubbles. When
the volume of the gas bubbles in magma reaches around 75%, magma disintegrates into pyroclasts, a mixture of partially molten and
solid fragments. The bursting of pyroclasts is very explosive and the cause of some of the most violent eruptions on the surface of Earth.
• Injection of New Magma
• The third theory says that when new magma enters a chamber already brimming with magma, the volcano erupts due to the additional
pressure exerted by the injection of new magma. This type of eruption can be placid or violent.
• The intensity of eruption usually depends on the viscosity of magma and its gas content. High-viscosity magma usually results in bigger,
more intense eruptions, whereas magma that flows easily will have lower pressure build-up, so a less violent eruption. Highly viscous
magma is characterized by the presence of more silicates and contains less dissolved water. Another important factor is the amount of
gases present in the magma. Magma containing large amount of trapped gases will lead to a violent eruption, and less gases in magma
will result in an effusive flow.

33. volcano
• A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot
lava, volcanic ash, and gases to escape from a magma chamber below the surface.
• Earth's volcanoes occur because its crust is broken into 17 major, rigid tectonic plates that
float on a hotter, softer layer in its mantle.[1] Therefore, on Earth, volcanoes are generally
found where tectonic plates are diverging or converging, and most are found underwater. For
example, a mid-oceanic ridge, such as the Mid-Atlantic Ridge, has volcanoes caused by
divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent
• tectonic plates. Volcanoes can also form where there is stretching and thinning of the crust's
plates, e.g., in the East African Rift and the Wells Gray-Clearwater volcanic field and Rio
Grande Rift in North America. This type of volcanism falls under the umbrella of "plate
hypothesis" volcanism.[2] Volcanism away from plate boundaries has also been explained as
mantle plumes. These so-called "hotspots", for example Hawaii, are postulated to arise from
upwelling diapirs with magma from the core–mantle boundary, 3,000 km deep in the Earth.
Volcanoes are usually not created where two tectonic plates slide past one another.

34. Energy from the volcano