Volcanoes and volcanic eruptions

Volcanoes form when magma reaches the Earth’s surface, causing eruptions of lava and ash. They occur at destructive (compressional) and constructive (tensional) plate boundaries.

The immediate effects of volcanic eruptions can be devastating, but they may be beneficial in the long term.

Key facts

  • A volcano is formed by eruptions of lava and ash.
  • Volcanoes are usually cone shaped mountains or hills.
  • When magma reaches the Earth’s surface it is called lava. When the lava cools, it forms rock.
  • Volcanic eruptions can happen at destructive and constructive boundaries, but not at conservative boundaries.
  • Some volcanoes happen underwater, along the seabed or ocean floor.

The formation of volcanoes

Krakatoa in the Sunda Strait, Indonesia

Krakatoa in the Sunda Strait, Indonesia


  1. Magma rises through cracks or weaknesses in the Earth’s crust.
  2. Pressure builds up inside the Earth.
  3. When this pressure is released, eg as a result of plate movement, magma explodes to the surface causing a volcanic eruption.
  4. The lava from the eruption cools to form new crust.
  5. Over time, after several eruptions, the rock builds up and a volcano forms.

Inside a volcano

Diagram of a volcano

Diagram of a volcano


  • The magma chamber is a collection of magma inside the Earth, below the volcano.
  • The main vent is the main outlet for the magma to escape.
  • Secondary vents are smaller outlets through which magma escapes.
  • The crater is created after an eruption blows the top off the volcano.

An eruption occurs when pressure in the magma chamber forces magma up the main vent, towards the crater at the top of the volcano. Some magma will also be forced out of the secondary vent at the side of the volcano.

Different types of volcano

Volcanoes can be described in terms of activity and can be:

  • still active and erupt frequently
  • dormant (temporarily inactive but not fully extinct)
  • extinct (never likely to erupt again)

Volcanoes can also be described by their shape or type – shield or composite.

Shield volcanoes

The characteristics of a shield volcano

The characteristics of a shield volcano


  • Shield volcanoes are usually found at constructive or tensional boundaries.
  • They are low, with gently sloping sides.
  • They are formed by eruptions of thin, runny lava.
  • Eruptions tend to be frequent but relatively gentle.

Composite volcanoes

Characteristics of a composite volcano

Characteristics of a composite volcano


  • Composite volcanoes are made up of alternating layers of lava and ash (other volcanoes just consist of lava).
  • They are usually found at destructive or compressional boundaries.
  • The eruptions from these volcanoes may be a pyroclastic flow rather than a lava flow. A pyroclastic flow is a mixture of hot steam, ash, rock and dust.
  • A pyroclastic flow can roll down the sides of a volcano at very high speeds and with temperatures of over 400°C.


A supervolcano is a volcano on a massive scale. It is different from a volcano because:

  • it erupts at least 1,000 km3 of material (a large volcano erupts around 1 km3)
  • it forms a depression, called a caldera (a volcano forms a cone shape)
  • a supervolcano often has a ridge of higher land around it
  • a supervolcano erupts less frequently – eruptions are hundreds of thousands of years apart


A hydrothermal feature at Yellowstone Park, Texas, USA

A hydrothermal feature at Yellowstone


Yellowstone is one example of a supervolcano. Three huge eruptions have happened in the last 3 million years. the last eruption was 630,000 years ago, and was 1,000 times bigger than the Mount St Helens eruption in 1980.

The large volume of material from the last Yellowstone eruption caused the ground to collapse, creating a depression called acaldera. The caldera is 55 km by 80 km wide. The next eruption is predicted to have catastrophic worldwide effects.

The supervolcano at Yellowstone is formed because of a volcanic hotspot.

Every year millions of visitors come to see the related features, such as geysersand hot springs. Old Faithful is one example of a geyser.

Effects of volcanic eruptions

Helgafjell Volcano, Iceland

Helgafjell Volcano, Iceland


Volcanic eruptions can have a devastating effect on people and the environment.

However, unlike earthquakes, volcanoes can also have a positive impact on an area. These positive impacts can help to explain why people choose to live near volcanoes.

Positive and negative effects of an eruption

Positive Negative
The dramatic scenery created by volcanic eruptions attracts tourists. This brings income to an area. Many lives can be lost as a result of a volcanic eruption.
The lava and ash deposited during an eruption breaks down to provide valuable nutrients for the soil. This creates very fertile soil which is good for agriculture If the ash and mud from a volcanic eruption mix with rain water or melting snow, fast moving mudflows are created. These flows are calledlahars.
The high level of heat and activity inside the Earth, close to a volcano, can provide opportunities for generating geothermal energy. Lava flows and lahars can destroy settlements and clear areas of woodland or agriculture.
  Human and natural landscapes can be destroyed and changed forever.

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