Question

What is a volcano
How is harmful

Answer 1

A volcano is an opening in the earth’s crust through which lava, volcanic ash, and gases escape. Volcanic eruptions are partly driven by pressure from dissolved gas, much as escaping gases force the cork out of a bottle of champagne. Beneath a volcano, liquid magma containing dissolved gases rises through cracks in the Earth’s crust. As the magma rises, pressure decreases, allowing the gases to form bubbles. How the magma (lava) behaves when it reaches the surface depends on both its gas content and chemical composition. Lavas with low silica contents have low viscosities and flow freely, allowing any gas bubbles to escape readily, while lavas with high silica contents are more viscous (resistant to flow), so that any trapped gases cannot escape gradually.
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Answer 2

Volcano is a mountain or hill, typically conical, having a crater or vent through which lava, rock fragments, hot vapour, and gas are or have been erupted from the earth crust.
Volcanoes have a serious of hazards (e.g. lava flows, ash fall,pyroclastic flows, climate changes on a global scale) that relate into different dangers or risks.

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