Volcanic eruption is a natural calamity caused by volcanic activity. When a volcano erupts, it releases hot lava, volcanic ash, and gases into the environment. This can result in various destructive effects on the surrounding areas.

Causes of Volcanic Eruption:
- Volcanic eruptions occur due to the movement of tectonic plates beneath the Earth's surface. These plates are constantly shifting and when they collide or separate, it can cause magma to rise towards the surface.
- Magma is a molten rock that forms beneath the Earth's crust. When it reaches the surface, it is called lava.

Effects of Volcanic Eruption:
1. Lava Flow: During a volcanic eruption, lava flows out of the volcano and spreads over the surrounding areas. Lava flows can destroy everything in their path, including buildings, vegetation, and infrastructure.

2. Pyroclastic Flow: This is a fast-moving mixture of hot gas, ash, and volcanic rocks that rushes down the sides of a volcano. Pyroclastic flows can cause severe damage and can be deadly to living organisms.

3. Ashfall: Volcanic eruptions release large amounts of volcanic ash into the atmosphere. Ashfall can cover vast areas, blocking sunlight and causing respiratory problems for humans and animals. It can also contaminate water sources and disrupt transportation systems.

4. Volcanic Gas: Volcanoes emit various gases during an eruption, including sulfur dioxide, carbon dioxide, and hydrogen sulfide. These gases can have harmful effects on the environment and human health.

5. Volcanic Explosions: Some volcanic eruptions can be explosive, resulting in the ejection of rocks, ash, and gases into the air. These explosions can cause significant damage to the surrounding areas, including destruction of buildings and infrastructure.

Conclusion:
Volcanic eruptions are natural calamities caused by volcanic activity. They can have devastating effects on the environment and human populations. It is important for communities living near active volcanoes to be prepared for such events and have evacuation plans in place to ensure their safety.