Formation of Volcanoes

The Earth’s surface is made up of large pieces called tectonic plates. These plates slowly move over the Earth’s mantle, a layer of hot rock.

  • Volcanoes are formed when the earth’s plates move together (convergent) or apart (divergent). 
  • Molten rock called magma collects in chambers under the Earth’s surface. When magma reaches the surface, it’s called lava.
  • When the lava cools and hardens, it forms the cone-shaped mountain we think of as a volcano. 
  • Most of the world’s volcanoes are found around the edges of tectonic plates on land and in the oceans.

Eruption of Volcanoes

An eruption occurs when enough magma and gas pressure build up in the chamber under the volcano.

Cracks and Vents:

  • The pressure forces magma through cracks in the Earth’s crust, creating vents.


  • When the pressure is too high, the magma erupts through the vent and comes out as lava.

After the Eruption:

  • The volcano can continue to release gases, ash, and lava, building up over time.

Type of Eruptions

Some volcanoes explode while others gently flow lava because of the type of magma inside them.

Explosive Volcanoes:

  • Thick Magma: If the magma is thick and sticky, it traps gas bubbles.
  • High Gas Content: The trapped gas builds up pressure.
  • Big Explosion: When the pressure is too high, it causes a violent explosion.

Effusive (Gentle-Flow) Volcanoes:

  • Runny Magma: If the magma is runny, gas bubbles can escape easily.
  • Low Gas Content: Less gas means less pressure builds up.
  • Gentle Flow: The magma flows out smoothly and gently.

Fun Facts

Here are some interesting facts about volcanoes:

  • Most of the world’s volcanoes are located in the “Ring of Fire,” a region around the edges of the Pacific Ocean.
  • Underwater volcanic eruptions formed islands like Hawaii.
  • Erupting volcanoes release gases like water vapor, carbon dioxide, and sulfur dioxide. 
  • Mars has the largest volcano in the solar system, called Olympus Mons.


Let’s quickly recap what we learned about volcanoes:

  • What large pieces make up the Earth’s surface? Tectonic Plates
  • What is molten rock beneath the Earth’s surface called? Magma
  • What is the molten rock called when it erupts onto the surface?  Lava
  • What are the two types of eruptions? Effusive & Explosive
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