The Restless Earth – Volcanoes and Earthquakes

The Restless Earth – Volcanoes and Earthquakes

Volcanoes are hazards resulting from tectonic activity but their primary and secondary effects can be positive as well as negative. Learn about the characteristics of different types of volcanoes and discover how volcanic eruptions can be monitored and predicted.

Volcanoes occur when magma bursts through the earth’s crust. These eruptions are some of the most spectacular and hazardous natural events. When volcanoes erupt with little or no warning, they are extremely dangerous. Molten lava flows quickly down the slopes, dangerous gases are released, and they can also trigger earthquakes and tsunamis.

Sometimes volcanoes don’t simply spill forth lava and rock, but actually explode. This can lead to huge death tolls. When Mount Tambora in Indonesia exploded in 1816 around 90,000 people perished. The mountain lost a kilometre in height due to the explosion.

However, not all effects of volcanoes are negative. The soil around volcanoes is often rich in minerals and this makes it much easier to produce crops – so long as the volcano doesn’t erupt, of course!

Volcanoes can lie dormant for many thousands of years, and millions of people live in close proximity to them. In recent years, early warning systems have made predicting eruptions much easier, but it is often impossible to predict exactly when a volcano may erupt.

People who study volcanoes are called volcanologists. They are sometimes able to predict eruptions by using seismograms, which are also used to predict earthquakes. Volcanologists also use gravimeters (which measure electric currents given off by magma), satellites, tiltmeters and even the global positioning system (GPS).

Super-volcanoes are on a much bigger scale than other volcanoes and an eruption would have global consequences. What are the characteristics of a super-volcano and the likely effects of an eruption?

Although volcanoes can have enormous destructive force and be extremely dangerous, they are dwarfed by so-called ‘super-volcanoes‘. As the name implies, these can create gigantic eruptions of lava and rock of at least 1,000 cubic kilometres – thousands of times bigger than most volcanoes.

Currently it is believed there are six super-volcanoes:

  • Yellowstone, Long Valley, and Valles Caldera, United States
  • Lake Toba, North Sumatra, in Indonesia
  • Taupo Volcano, North Island, New Zealand
  • Aira Caldera, Kagoshima Prefecture, Ky?sh?, Japan

If any of these super-volcanoes erupted, the consequences for mankind could be very serious. Apart from the immediate problem of huge lava flows and possible tsunami, the huge quantities of ash pumped into the atmosphere would probably trigger a global cooling, killing crops and wildlife. Fortunately, all known super-volcanoes have been dormant for many thousands (or millions) of years, and there is no sign that they are due to erupt any time soon.


Earthquakes occur at constructive, destructive and conservative plate margins. Find out about the location and cause of earthquakes and how they can be measured using the Richter and Mercalli Scales.

An earthquake occurs when plates move against each other. They can be extremely destructive, both directly – for instance, by shaking buildings apart – but also because when they take place beneath or near water they can trigger a tsunami (covered elsewhere).

Regions of the world which sit on fault lines are most at risk from earthquakes. Countries such as Japan, China and Iran are most at risk; by contrast, they are extremely rare in the United Kingdom.

The severity of earthquakes is measured using the Richter Scale. This measurement was developed in 1935 by Charles Richter and Beno Gutenberg. The Richter Scale classifies an earthquake by the amount of energy released, and can be misleading: for instance, an earthquake measuring 5.0 on the Richter Scale is over 30 times greater than one of 4.

In contrast the Mercalli Scale quantifies an earthquake according to the effect it has on the earth’s surface, humans, objects of nature and man-made structures. The scale is from I (not felt) to XII (total destruction). The Tanghan earthquake (China, 1976) killed about 250,000 people and registered as XI on the Mercalli Scale but ‘only’ 7.5 on the Richter Scale (by comparison the 2011 Japan earthquake registered 9 on the Richter Scale but only IX on the Mercalli Scale).

The effects of earthquakes and responses to them differ due to contrasts in levels of wealth. Rich parts of the world are affected differently to poorer areas. Tsunamis are a secondary effect of earthquakes and can have devastating effects in coastal areas.

Apart from collapsing buildings, the main cause of destruction following many earthquakes is a tsunami. Often wrongly called a ‘tidal wave’, tsunamis occur when an area of the seabed is disturbed, or when a landslide falls into the water, creating huge waves. Most of the death toll following the 2011 Japan and 2004 Sumatra earthquakes occurred when vast waves, often tens of metres, high crashed into shore.