Volcanoes: Origins, Characteristics, and Distribution

A volcano is a natural phenomenon from which molten rock material, gases, ash, and other debris escape to the Earth’s surface. They can be found along plate boundaries and in areas where the earth’s crust is thinning or under stress. Volcanic eruptions can vary in intensity and can have significant effects on the surrounding environment and communities.

Magma Composition and Eruption Characteristics

• Source of Magma: The Asthenosphere is the source from which molten rock materials or magma arise.
• Lava: When magma moves towards or breaks through the Earth’s crust, it is termed lava. 
• Composition: Materials accompanying eruptions are lava flows, pyroclastic debris, volcanic bombs, ash, dust, gases like nitrogen and sulphur compounds, and smaller quantities of chlorine, hydrogen, and argon. 
• Types of Magma: 
  • Magma types based on silica content: Increasing silica content implies a lower temperature, explosive eruption behaviour and increasing viscosity of magma. 
  • Felsic Lava: high silica (65-70%) content, explosive, & viscous.
  • Intermediate Lava: 52-63% silica content & less viscous.
  • Mafic Magma: low silica (45-52%) content, fluid and non-explosive. 
  • Ultramafic Lava: Silica <45%; it is extremely fluid.

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Causes of Volcano

• Distribution of Volcanoes: Volcanoes are found along convergent, divergent, and some continental plate boundaries.
• Convergent Boundary Volcanism: Subduction of one plate under another in cases of convergent plate boundaries results in the melting of rocks due to high temperature and pressure that rise along the fissures of rocks.

• Divergent Boundary Volcanism: In the case of divergent boundaries, thinning of the upper crust leads to a reduction in the overlying pressure of rocks, causing a decrease in rock melting points and formation of magma, which rises and erupts as lava from fissure volcanoes. 
• Intraplate Volcanism: Some continental volcanoes are located away from plate boundaries due to the stressing of plates and the creation of faults.

Distribution of volcanoes

• The distribution of volcanoes matches that of seismic activity in midy places. 
• A volcanic activity zone occurs along the divergent and convergent boundaries. 
  • Example: Mid-oceanic ridges (divergent), Pacific rim/ Rim of fire (convergent).
• Ring of Fire: It is a long chain of volcanoes and other tectonically active structures that surround the Pacific Ocean. 
• • Many of these volcanoes were created through the tectonic process of subduction.
  • Ring of fire runs up along the western coast of South and North America, runs down the eastern coast of Asia past New Zealand and into the northern coast of Antarctica
• Depth Variation in Earthquake Foci: In general, foci of the earthquakes of mid-oceanic ridges are at shallow depths, whereas along the Alpine-Himalayan belt as well as the rim of the Pacific, the earthquakes are deep-seated ones.

Conclusion

• Volcanoes result from the movement of magma from the Earth’s mantle to the surface, leading to eruptions of lava, gases, and other materials. 
• The composition and behaviour of magma, influenced by factors such as silica content and pressure, determine the type of volcanic activity and associated hazards.